Diagnosis and Documentation of Tachycardia

When a patient presents with a tachycardia that is documented on a 12-lead electrocardiogram and with a rhythm strip, it is often straightforward to establish the diagnosis and plan the treatment. Occasionally, the diagnosis remains unclear and then the response to vagal manoeuvres or medications will often help to clarify it. Intravenous adenosine is a potent but transient inhibitor of the atrioventricular node, with lesser and variable effects on ectopic atrial or ventricular substrates. It is important to record the electrocardiogram during vagal manoeuvres or the administration of adenosine, as transient and subtle effects can be missed when observing a monitor screen. If it is still not possible to define the arrhythmia mechanism, an electrophysiological study may then be useful in establishing the diagnosis.

Suspected Arrhythmia or Unexplained Symptoms

When a patient has symptoms such as palpitations, dizzy spells, or syncope, but the rhythm is not documented, management may be less straightforward. It is always preferable to document the clinical arrhythmia, prior to undertaking a diagnostic electrophysiological study as non-clinical arrhythmias may be induced during electrophysiological testing. An aggressive protocol of stimulation, for example, those using stimulation with short coupling intervals or including an isoprenaline challenge, can produce arrhythmias in normal subjects, although these are often not sustained or specific. ^1,2 Nevertheless, if an arrhythmia that does not mimic the clinical one is induced, management based on this finding may be inappropriate. Good examples are the use of antiarrhythmic medication or implantable defibrillators for induced ventricular arrhythmias without clinical correlation, or inappropriate ablation for dual atrioventricular nodal pathways with a risk of producing complete heart block. In adolescents and adults in whom electrophysiological studies can be performed with minimal sedation, it is possible to ask the patient during the procedure whether the symptoms produced by an induced arrhythmia are similar to those noticed clinically. In younger children deep sedation or general anaesthesia is often used, and thus this opportunity is not available. A recording of an arrhythmia can frequently be obtained by using the methods described below, but occasionally electrophysiological studies are necessary for better elucidation of arrhythmias that prove to be elusive.

Life-threatening Arrhythmias

When a patient presents with an out-of-hospital cardiac arrest or a presyncopal tachycardia that reverts before diagnosis, then it is no longer safe merely to await a spontaneous recurrence to document the rhythm, and invasive electrophysiology studies may be required. In this setting, it is more likely that any arrhythmias that are found will be clinically relevant and merit appropriate treatment.

Evaluation of Effectiveness and Safety of Drug Treatment

A major early role for electrophysiological studies was to establish which medication was effective for treating an arrhythmia. This was a particularly prominent indication in adults with ventricular tachycardia and after repair of tetralogy of Fallot and in children with supraventricular tachycardia resistant to usual therapies. ^10–14 Currently, there is a minimal role for such testing either for anti-arrhythmic or pro-arrhythmic effects ^15–17 of drugs.

Ventricular Stimulation Testing

In adults with left ventricular dysfunction, or after myocardial infarction, electrophysiological studies have been used for assessment of the risk of sudden death. When ventricular arrhythmias are non-inducible, or become so with antiarrhythmic medication, the prognosis may be better. Left ventricular dysfunction, however, is a powerful predictor in its own right, and in many such patients, it is not possible to find a drug that renders the arrhythmia non-inducible. While empirical treatment with amiodarone, despite leaving patients inducible, has been found in some studies to improve the prognosis, this is not a uniform finding and the trend is to use an automatic implantable cardioverter defibrillator. ^18–24 The landmark sudden cardiac death in heart failure trial (SCD-HFT) showed no survival benefit in adults from the use of amiodarone as a primary prevention measure; in contrast, the implantable defibrillator conferred a 23% reduction in mortality. ²⁵ In patients with dilated cardiomyopathy, clinical sustained monomorphic ventricular tachycardia can often be reproduced during electrophysiology study. It is important to exclude bundle branch re-entry in these patients as this arrhythmia is amenable to ablation. If the dilated cardiomyopathy patient has non-sustained ventricular tachycardia, or has presented with syncope, invasive electrophysiology study is, however, of no additional benefit in risk stratification. ^26,27 While there is minimal data available, some survivors of congenital cardiac surgery with ventricular dysfunction, ventricular arrhythmias on ambulatory monitoring, or inducible arrhythmias at electrophysiological study, receive drug treatment and/or implantable defibrillators. ^28,29

Alternative approaches to ambulatory monitoring and electrophysiological studies include assessment of heart rate variability over 24 hours, as a potential predictor of sudden death. Low heart rate variability suggests a worse prognosis in adult patients with left ventricular dysfunction or after myocardial infarction, and these patients may receive antiarrhythmic treatment or undergo implantation of a defibrillator. ³⁰ Whether it is appropriate to extrapolate this data to patients with congenital heart disease is not clear. There are significant differences related to age as well as acute effects from surgery, but few systematic studies. ^31,32 Signal-averaged recordings evaluated for late ventricular potentials have also been found to be a marker of a poor prognosis in adults, and are used in the diagnosis of diffuse myocardial disease such as arrhythmogenic right ventricular dysplasia. ³³ Again, even though normative data for children has been published, ³⁴ there are few studies in children with congenital heart disease, and none as yet has demonstrated a useful role in this particular population. ^35,36 (This is explored further under ventricular tachycardia, later in this chapter). Specialised exercise testing for microvolt T-wave alternans offers another non-invasive approach. In general this testing has a good negative predictive value and thus testing may be helpful in the risk stratification process. Normative data for children has been published but more paediatric follow-up data is needed. ^37–39

Evaluation of the Conduction System

Historically, invasive electrophysiology studies were used to document the level of atrioventricular block in patients with conduction system disease: block above and below the bundle of His being associated with a better and worse prognosis, respectively. Pacemaker indications are outlined in the pacing section of this chapter and no longer rely on electrophysiology study. ^40,41

Interventional Management

Overdrive pacing and programmed stimulation can be used to terminate acute arrhythmias without the need for antiarrhythmic medication or electrical cardioversion. This technique is ideal when there are already pacing leads in place, such as temporary epicardial leads after cardiac surgery or when a permanent pacemaker has been implanted. In patients with malignant recurrent arrhythmias, and/or when medication is ineffective, temporary intracardiac electrodes could be placed to avoid repeated cardioversion until a more long-term strategy is determined. A transoesophageal electrode can be used in diagnosis of arrhythmias with cryptic atrial activity on surface tracings. It can also be used to terminate atrioventricular re-entry tachycardias and atrial flutter. ⁴²

When antiarrhythmic medication is ineffective, poorly tolerated, or causes side effects in patients with chronic arrhythmias, then more aggressive management is required and electrophysiological testing may be performed to define the substrate for the arrhythmia and to plan treatment. Tachycardia that is reliably terminated with pace-stimulation but not amenable to ablation can be managed by implanting an anti-tachycardia pacemaker. If anti-tachycardia pacing accelerates the arrhythmia, or produces other arrhythmias, an implantable cardioverter defibrillator may be indicated. Antiarrhythmic surgery has been previously used to cure arrhythmias through excision or cryoablation of ectopic foci or accessory pathways. ^43–46 Prior electrophysiological testing was usually required to confirm the anatomy of the substrate and to avoid prolonged intra-operative electrophysiological mapping. Antiarrhythmic surgery laid the foundation for transcatheter ablation therapy but is now rarely indicated for most common arrhythmias. Nonetheless, antiarrhythmic surgery is still performed, often combined with other cardiac surgery, especially Fontan revision, ^47,48 and pulmonary valve replacement in patients late after repair of tetralogy of Fallot. Currently, radio-frequency ablation or cryoablation is the most common reason for undertaking an electrophysiological study in children.

Tissue Effects of Radio-frequency Energy

During delivery of radio-frequency energy, the tissue in contact with the ablation catheter is heated. ⁶³ At temperatures of 50°C and greater, there is permanent denaturation of the cell membranes and tissue dehydration. After further heating, there is coagulation necrosis of the tissue with surrounding haemorrhage and inflammation. The lesion retracts acutely because of dehydration, and the coagulation necrosis heals to form a well-demarcated fibrotic scar. The size of the lesion depends on the power of the radio-frequency energy delivered, the size of the ablation catheter, the duration of heating and the temperature achieved. ^64–66 Small electrode tips result in a rapid rise in temperature with boiling at the tissue interface and charring of blood, with a rise in impedance, a decrease in heat transfer and a smaller lesion. Longer electrode tips, with less contact with tissue, have a reduced current density so that the temperature rise is curtailed. This allows use of higher powers and a longer delivery of energy without boiling and charring, and it creates deeper lesions. A standard 7 French ablation catheter with a 4-mm tip will produce lesions 5 to 6 mm in diameter and 2 to 3 mm deep. After delivery of energy is discontinued, the tissue in contact with the electrode is hotter than the surrounding tissues and continued heat transfer occurs. This thermal latency explains why lesions can continue to increase in size after cessation of delivery of energy. ⁶⁷ Catheters with cooled tips allow the use of even greater power without charring and create even deeper lesions. ⁶⁸ Scar tissue takes longer to heat and produces smaller lesions, thus making lesion creation in scarred regions challenging. ⁶⁹

During the healing phase, tissue on the border of the lesion may recover if permanent denaturation was not achieved. Progressive necrosis may also occur at the border of a lesion if sufficient inflammation and microvascular damage occurred during delivery of energy. ^63,70,71 Therefore, it is possible for an acutely successfully ablated arrhythmia to recur, or a partially successful ablation to become completed. The latter progression is seen less often clinically. In neonatal lambs, delivery of radio-frequency energy resulted in a progressive increase in the size of lesions in the atrial and ventricular myocardium, but not in the atrioventricular groove. ⁷²

Procedure

Technique

Catheter ablation is usually performed immediately subsequent to diagnostic electrophysiology. It is important to discontinue antiarrhythmic medication, if at all possible, prior to the procedure as the effects may render arrhythmias non-inducible and make it impossible to find the site for ablation. In some laboratories, systemic heparinisation is used during the procedure regardless of ablation location; others only heparinise for left-sided ablations or if arterial access is required.

Once the arrhythmic substrate has been determined and mapped, an ablation catheter is advanced via an additional site of access, usually the femoral vein or artery, to the appropriate site for delivery of energy. Energy can be delivered during sinus rhythm, during tachycardia, or during atrial or ventricular pacing. In radio-frequency ablation, a short test application of 10 seconds or so, which usually does not cause irreversible changes, will be continued for 30 to 90 seconds if the test causes the desired effect, such as loss of the delta wave, termination of tachycardia, or loss of retrograde conduction. ⁷⁶ Ineffective ablation may occur as a result of inexact localisation of the site, poor contact between the ablation catheter and the endocardium, or build-up of coagulum and char on the electrode tip. These problems will need to be corrected before ablation is successful. Moving the catheter to a new site guided by the electrograms is often sufficient but, if there is repeated failure, then different catheter shapes and approaches to enhance contact between catheter and endocardium should be utilized. Long sheaths with a variety of shaped tips are available to aid stability and positioning. Monitoring the impedance and temperature of a radio-frequency catheter allows delivery of the minimal energy to raise the temperature at the tip to 60° to 70°C without creating charring, which then impedes effective delivery of energy to the tissue. If necessary, the catheter is removed from the body and the char and coagulum wiped off with a saline swab. Further attempts are made until success is achieved.

Following a successful ablation, attempts are made to re-induce the tachycardia. If the ablation appears to be successful, repeat attempts to induce are made 20 to 30 minutes later, as occasionally there is recovery of the ablated area.

Placement of Electrophysiologic Catheters

Bipolar or quadripolar catheters for pacing and sensing are usually placed in the high right atrium, in the right ventricular apex, and through the antero-superior part of the tricuspid valve to record the electrical activity of the His bundle ( Fig. 19-2 ). A multi-polar catheter is placed in the coronary sinus to record left atrial and left ventricular electrograms around the left atrioventricular groove. At least four different sheaths for venous access are required to place these electrodes. These are usually inserted through three or four sites of femoral venous puncture and/or a puncture in the subclavian or jugular vein for access to the coronary sinus. In some cases, additional electrode catheters are inserted to increase the number of sites for recording and pacing, or to serve as anatomical landmarks. Electrodes are not routinely placed in the left ventricle or left atrium unless this is indicated during the course of the study.

Catheter electrodes positioned in the high right atrium, right ventricular apex, site of the His bundle, and coronary sinus (poles labelled). The fluoroscopic views are frontal ( A ), left anterior ( B ), and right anterior ( C ) orientations.

Conduction Intervals

The PA interval is a measurement of the intra-atrial conduction time, the AH interval is a measurement of the atrioventricular nodal conduction time, and the HV is a measurement of the His–Purkinje conduction time ( Fig. 19-3 ). Conduction intervals are generally measured in sinus rhythm and then during atrial pacing.

Intracardiac and surface electrograms. CS, coronary sinus poles 1–10; HRA, high right atrium; H, His (proximal and distal); RV, right ventricle.

Tachycardia Mapping

Mapping of the site of origin of the tachycardia, or of sites critical to maintenance of a re-entry circuit, is necessary for planning interventions to interrupt the tachycardia. One or more of the following methods are used for this purpose.

Electroanatomic Mapping

In recent years, electroanatomic mapping systems have become more widely used, in conjunction with traditional fluoroscopy and activation mapping. These systems utilise electromagnetic fields to identify the location of a catheter in three-dimensional space. Using these systems the anatomy of the area of interest can be mapped, and location of catheters tracked. Ideally these systems will offer better definition of anatomic substrate, and reduce fluoroscopy times. Once the anatomy is defined, colour-based timing maps can be created and superimposed on the anatomical map ( Fig. 19-4 ).

Electroanatomical map of an ectopic atrial tachycardia. The white area indicates the earliest timing at the roof of the left atrium, while colours red to purple indicate later and later timing. The yellow ball shows the site of successful ablation.

Administration of Drugs during Catheterisation

When it is difficult to induce an arrhythmia in the basal state, sympathomimetics, such as isoprenaline (isoproterenol) given as a bolus or an infusion, are used to initiate or reduce the threshold for the arrhythmia. Further information on the mechanism of the arrhythmia may be obtained by administering drugs during pacing and/or tachycardia. Adenosine is a short-lived inhibitor of atrioventricular nodal conduction and is particularly useful. Verapamil is used to produce more sustained atrioventricular block. Atropine and propranolol are given together to produce autonomic blockade. Disopyramide is used to expose impairment of atrioventricular nodal conduction, ajmaline or procainamide to abolish conduction down an accessory pathway, and flecainide to terminate atrial fibrillation. ^82,83 These antiarrhythmic agents, however, have more sustained effects than adenosine. While providing additional information, they may interfere with the remainder of the study. Some of these medications are also used in the diagnosis of inherited arrhythmias, described previously in this chapter.

Indications for Ablation

The indications for ablation have evolved since its introduction, with expansion of the arrhythmic substrates to which it is applied. This has led to a significant fall in the use of anti-tachycardia pacemakers and antiarrhythmic surgery. The threshold for intervention has also fallen, and patients who were previously well or could be controlled on medication, now undergo radio-frequency ablation in preference to chronic antiarrhythmic medication. ⁸⁴ It is still not considered by many as first-line therapy in children weighing less than 15 kg, because the rate of complications is higher, and many of these children will lose the tendency to continued tachycardias with aging. ⁸⁵

Atrioventricular Re-entry Tachycardia

Atrioventricular re-entry tachycardia due to an accessory atrioventricular pathway is the commonest cause of tachyarrhythmia in the fetus, and in early childhood. ⁸⁶ A minority are associated with hydrops fetalis, fetal death, or heart failure in the neonatal period. Infants presenting with supraventricular tachycardias should be evaluated for possible tachycardia-induced cardiomyopathy, as the duration of the arrhythmia is generally unknown; it is essential to confirm cardiac function prior to initiation of medications with potential negative inotropic effects, such as many of the antiarrhythmic medications. In addition echocardiography should be performed in all patients who have supraventricular tachycardia to identify structural lesions associated with accessory pathways. Some of these may be clinically silent and include Ebstein’s malformation, congenitally corrected transposition, myocardial tumours, and hypertrophic and dilated cardiomyopathy.

In older children, symptomatic tachycardias are the usual presentation. Approximately one-third will have evidence of pre-excitation, with a delta wave on the surface electrocardiogram, indicating an overt accessory pathway. In the remainder the pathway is concealed. In these patients only retrograde conduction occurs, from the ventricle to the atrium, thus no change in depolarisation of the ventricle is seen in sinus rhythm. In the majority of children, the tachycardia is easily controlled with drugs. Many neonates and infants will be free from arrhythmias by the age of 1 year, without the need for medication. However, 30% to 50% do not outgrow the tendency to recurrent tachyarrhythmias, and of those who do, a small proportion have a recurrence in later childhood. ⁸⁷ When tachycardias persist over the age of 5 years, about four-fifths continue to experience intermittent symptomatic tachycardias into adulthood. ⁸⁸

Arrhythmic Substrate

The accessory pathway permits retrograde conduction from the ventricle to the atrium; consequently a re-entry circuit can be established ( Fig. 19-5 ). An early atrial extrasystole is conducted slowly down the atrioventricular node to the His–Purkinje system and ventricle; it returns to the atrium over the accessory pathway. If the atrioventricular node has recovered and is no longer refractory, then a circus movement tachycardia, also known as re-entry tachycardia or reciprocating tachycardia, can be established. A ventricular extrasystole can also trigger tachycardia in this situation, via retrograde conduction up the accessory pathway into the atrium, with the retrograde P wave then being conducted anterograde across the atrioventricular node. Following ventricular depolarisation, there is conduction back up the accessory pathway, and the tachycardia is initiated.

Substrates for arrhythmias that may be amenable to ablation: (1) unidirectional concealed accessory pathway; (2) slowly conducting accessory pathway, such as permanent junctional reciprocating tachycardia; (3) atrioventricular nodal re-entrant tachycardia; (4) focal atrial tachycardia; (5) intra-atrial re-entrant tachycardia; (6) fascicular tachycardia; and (7) ventricular tachycardias

Histology of the accessory pathways shows that they are composed of abnormal myocardium that crosses the atrioventricular groove. They have been found around the left atrioventricular groove at the level of attachment of the mural leaflet of the mitral valve, and around the right atrioventricular groove at the level of attachment of all three tricuspid valvar leaflets. There is no atrioventricular junction around the aortic leaflet of the mitral valve where it joins the aortic valve. The right atrioventricular groove is thicker because of infolding of the atrial myocardium at the tricuspid valvar origin. In one-tenth of patients, there are multiple accessory pathways. Accessory pathways may be described as right-sided, left-sided, or septal, and as superior, inferior, or lateral ( Fig. 19-6 ). ⁸⁹

Diagram of the locations of accessory pathways comparing ( A ) the traditional surgical perspective and ( B ) an attitudinally correct nomenclature, as proposed by Cosio and colleagues.

Ablation

Left-sided pathways are accessed either via an antegrade or retrograde approach. The antegrade approach uses a patent oval foramen or requires a trans-septal puncture to place lesions on the atrial side of the atrioventricular ring. The retrograde transarterial approach requires the catheter to cross the aortic valve and be looped up under the leaflets of the mitral valve for delivery of lesions on the ventricular side of the ring. Using this route, it is also possible to cross the mitral valve and apply the lesions on the atrial side. Ablation is guided anatomically and electrographically by placing a multi-polar electrode catheter in the coronary sinus. This allows the ablation catheter to be aimed at the poles in which the earliest activation is detected ( Fig. 19-7 ).Right-sided and septal pathways are usually approached from the inferior caval vein, though occasionally approach via the superior caval vein is required. Lesions are usually placed on the atrial side of the atrioventricular ring or in the mouth of the coronary sinus, but it may be necessary to loop the catheter in the right ventricle to ablate on the ventricular side of the atrioventricular ring. A diverticulum of the mouth of the coronary sinus is occasionally found, and an accessory pathway may run in the floor of the diverticulum, which can be identified with angiography. The ablation catheter is placed on the atrioventricular ring and moved around its circumference to identify the earliest activation. In difficult cases, where delivery of energy at the site of the earliest activation is ineffective, an intracoronary electrode catheter can be used. The catheter defines the right coronary artery, and thereby the right atrioventricular groove, and also provides electrograms so that a stable reference electrogram at the site of earliest activation can be identified to guide the ablation catheter ( Fig. 19-8 ). The existence of additional accessory pathways that can sustain arrhythmias may only become apparent after successful ablation of the first pathway; these will also be ablated at the same procedure if possible. Mapping is always more challenging when multiple pathways are present, due to the potential for fused or figure-of-eight conduction (in which the impulse travels alternately between the two pathways, sometimes suggested by alternating tachycardia cycle length).

Ablation of a left-sided accessory pathway. Fluoroscopy in left anterior ( A ) and right anterior ( C ) oblique projections. B , Electrograms recorded during ablation with abolition of conduction across the accessory pathway. CS, coronary sinus; HRA, high right atrium; RF, radio-frequency; RV, right ventricle.

A , 2 French quadripolar electrode in the right coronary artery (RCA) through a coronary guide catheter to assist ablation of a right-sided manifest accessory pathway after a previous attempt had failed. B , Electrograms from surface and intracardiac leads show that the shortest atrioventricular conduction time is at the distal pair of electrodes on the catheter within the right coronary artery. The ablation catheter shows continuous atrioventricular signals at the site of successful ablation. CS, coronary sinus; HRA, high right atrium; RV, right ventricle.

Ablation of left-sided pathways has a success rate of more than 90% in most centres, and the success rate increases with experience. Right-sided pathways are technically more challenging. Placement of catheters on the right atrioventricular junction is less stable and some pathways are epicardial. Nonetheless, success rates are generally above 80%. The use of long sheaths to stabilise the catheter may be helpful. Septal and paraseptal pathways can also be ablated with a high degree of success, but this does carry a risk of complete heart block owing to inadvertent damage to the atrioventricular node and bundle of His. Some centres advocate the usage of cryoablation in these situations, due to the potential reversibility of lesions. A high rate of success has also been reported in infants and neonates with intractable re-entry tachycardias, but with higher rates of complications, including major complications. ^91–94 Ablation in the infant should be reserved for those with medically refractory life-threatening arrhythmias likely to respond to ablation.

Other Types of Accessory Pathways

Wolff–Parkinson–White Syndrome

Electrocardiographical evidence of manifest accessory atrioventricular pathways is found in up to 3 of every 1000 of the population. ^105,106 The incidence is increased in family members, and in those with a congenitally malformed heart, especially Ebstein’s malformation and congenitally corrected transposition. ^107,108 These allow pre-excitation of parts of the ventricle before the normal impulse can arrive via the His–Purkinje system. When more myocardium is pre-excited, a wider QRS complex and more overt delta wave is seen. The pattern of the delta wave and QRS complexes can be used to identify into which portion of the ventricle the accessory pathway inserts. A number of algorithms have been developed. These algorithms correlate reasonably, though not absolutely, with the findings at electrophysiologic study. ^109–112 When multiple pathways are present, or pre-excitation is only partial or intermittent, the electrocardiographic pattern can vary, making localisation from the surface recordings impossible. Right-sided pathways are more common in patients with congenital heart disease, and the algorithms are much less accurate in these patients. ^113,114

Atrioventricular re-entry tachycardia is possible because of retrograde conduction across the pathway, but not all Wolff–Parkinson–White pathways are capable of sustaining such tachycardias, due to weak or nonexistent retrograde conduction. Many patients are therefore asymptomatic, and the diagnosis is often established as an incidental finding when an electrocardiogram is performed for another reason. Patients with a delta wave and symptoms of palpitations or documented tachycardia have Wolff–Parkinson–White syndrome, while those with an asymptomatic delta wave have only the electrocardiographic phenomena of Wolff–Parkinson–White.

By definition, the manifest pathways are capable of antegrade conduction from the atrium to the ventricle. Occasionally, an antidromic circus movement tachycardia is set up with antegrade conduction down the accessory pathway and retrograde conduction via the atrioventricular node or a second accessory pathway. Pre-excitation of the ventricles in this rhythm results in a broad complex tachycardia, due to all ventricular activation occurring via the accessory pathway. Those patients who demonstrate this form of tachycardia usually also have typical orthodromic tachycardia.

Risk of Sudden Death

Wolff–Parkinson–White patients are at higher risk than the general population of developing atrial fibrillation, which may be conducted rapidly to the ventricle bypassing the atrioventricular node. This results in a broad complex irregular tachycardia. If the atrial fibrillation is conducted at a very rapid rate, then the ventricles may not be able to maintain cardiac output and even more rapid conduction can cause the ventricle to fibrillate. These patients can present with syncope or out of hospital arrest and sudden death. ¹¹⁵ Pre-excited atrial fibrillation or Wolff–Parkinson–White with syncope is an indication for ablation, as these patients are known to be at higher risk of ventricular fibrillation and sudden death.

Controversies in Management

There are no absolute predictors of a benign or malignant prognosis in asymptomatic patients with Wolff– Parkinson–White syndrome. Intermittent episodes, with sudden loss of the delta wave on exercise or after administration of class I antiarrhythmic drugs, suggests a benign prognosis, as pathways with this feature tend not to be able to conduct very rapidly. ^82,116 The onset of symptoms, palpitations, or tachycardia suggests that the risk of sudden death is increased. When syncope is a symptom, then there is a very real risk of sudden death. Sudden death or out-of-hospital cardiac arrest can, however, be the first symptom in the previously asymptomatic patient. For this reason, attempts have been made to identify those asymptomatic patients at increased risk.

During an electrophysiological study, rapid atrial pacing is used to induce atrial fibrillation. If atrial fibrillation can be induced, the shortest interval between two pre-excited beats is measured. Intervals less than 220 msec, indicating a rate of 270 beats per minute, are defined as very rapid conduction over the pathway, thus identifying high-risk pathways. If atrial fibrillation is not able to be induced, then the antegrade effective refractory period is measured during rapid atrial pacing, though this may be less predictive than that measured during atrial fibrillation ¹¹⁵ ( Fig. 19-9 ). Most patients who present with syncope or cardiac arrest demonstrate very rapid conduction over the accessory pathway. Asymptomatic patients, or those with tachycardia without syncope or cardiac arrest, may also demonstrate such rapid conduction. So, while slow conduction may identify a group at low risk, rapid conduction does not necessarily identify the patients at high risk. ¹¹⁶ Nevertheless, routine electrophysiological testing to distinguish between rapid and slow conductors is performed in many laboratories. Those found to have rapid conduction can be offered an ablation, accepting that not all are at high risk. It is also important to note that accessory pathway conduction rates under sedation or anaesthesia may not completely reflect those seen while awake or during exercise. When the pathway is located away from the septum, this can be performed at low risk of the complication of heart block and with a high rate of success. Ablation of pathways that are close to the septum carry a small but definite risk of complete atrioventricular block and the need for a permanent pacemaker. Pathways in the inferior paraseptal area, however, are also thought to potentially carry a greater risk of sudden death. ¹¹⁷

Pre-excited atrial fibrillation with extremely rapid conduction to the ventricle, consistent with a high risk Wolff–Parkinson–White accessory pathway.

The risk of sudden death is around 1 per 1000 patient-years of follow-up with the syndrome, while the mortality of ablation is similar ( Table 19-1 ). ^118,119 Therefore, the argument is finely balanced, and some centres do not offer electrophysiological study and ablation to asymptomatic patients. Electrophysiological study and ablation are easier to recommend in individuals undertaking extreme exertion or in those requiring a license to fly, or other occupations in which those with pre-excitation are excluded. In asymptomatic children, an electrophysiological study is often recommended in adolescence, but sudden death has also been reported in young children. ^120,121 In some centres, early electrophysiological studies are performed, using a transoesophageal electrode. There is still no universally accepted policy, and the controversy remains. ^122,123

TABLE 19-1

RESULTS OF THREE LARGE MULTI-CENTRIC STUDIES OF RADIO-FREQUENCY ABLATION

	MERFS ∗	AMIG †	PES ‡
Patients
Age group	Adult	Adult and paediatric	Paediatric
Number of patients	4398	1050	3277
Number of procedures	n/a	1136	3653
Success Rates
Accessory pathways	n/a	93%	91%
Nodal re-entry tachycardia	n/a	97%	96%
Complications
Death	5 (0.11%)	3 (0.3%)	4 (0.11%)
Complete heart block	57 (1.6%) §	10 (1%)	25 (0.7%)
Tamponade or effusion	44 (1.0%)	26 (2.5%)	24 (0.7%)
Thromboembolism	28 (0.6%)	7 (0.7%)	8 (0.22%)

 

n/a, not available

∗ MERFS, Multicentre European Radiofrequency Survey. ¹³² Complication rates published with reference to success rates.

† AMIG, ATAKR Multicentre Investigators Group. ¹³³

‡ PES, Pediatric Electrophysiology Society of North America. ^84,85 Patients were aged below 21 years, with supraventricular tachycardia and no structural heart disease.

§ Excludes 900 patients who underwent ablation of the atrioventricular node to produce complete heart block Includes a significant proportion of ablations of the fast pathway for nodal re-entry techycardia.

Atrioventricular Nodal Re-entry Tachycardia

Atrioventricular nodal re-entry tachycardia is an uncommon arrhythmia in the first year of life; it becomes more frequent with increasing age thereafter.

Arrhythmic Substrate

The anatomic substrate is the presence of dual atrioventricular nodal pathways, which allow a re-entry circuit to form in the atrioventricular node, for example, after an atrial extrasystole. Typical nodal re-entry occurs when antegrade conduction across the atrioventricular node is blocked in a more rapid conduction region, called the fast pathway, and then conducts slowly down a region of slower conduction, called the slow pathway. By the time the impulse has crossed the atrioventricular node, the fast pathway has recovered and is no longer refractory. The impulse is then able to conduct back up the atrioventricular node retrograde to the atrium. The slow pathway has then recovered and conducts antegrade down the atrioventricular node ( Fig. 19-10 ), and thus the tachycardia is propagated. These pathways are well defined electrophysiologically, but not so clearly histologically. The slow pathway is associated with the junction of working atrial myocardium, atrial transitional cells, and the inferior extension from the compact atrioventricular node in the area of the septal isthmus at the base of Koch’s triangle. ^124,125 The fast pathway is usually located more anteriorly near the apex of the triangle of Koch and is associated with transitional atrial cells in that area. The arrhythmia is usually well-tolerated haemodynamically, but can be recurrent and symptomatically poorly tolerated. Atrioventricular nodal blocking drugs, such as digoxin, beta blockers, and verapamil, will usually suppress the arrhythmia, but the ability to cure it by ablation is appealing. ^126,127

Induction of atrioventricular nodal re-entry tachycardia using a drive train (S1) and a single atrial extrastimulus (S2). The solid bar indicates conduction down the fast pathway and the hollow bar indicates conduction down the slow pathway. During the drive train, conduction form the atrium to His bundle is via the fast pathway. After the extrastimulus (S2), the fast pathway is blocked and conduction proceeds down the slow pathway to the His bundle (AH jump). At this point the fast pathway has recovered and conducts retrograde so that the A and V are superimposed in the His bundle electrode and a retrograde A is detected in the high right atrium and coronary sinus leads. Antegrade conduction is via the slow pathway and a re-entry tachycardia follows. CS, coronary sinus; HRA, high right atrium; RV, right ventricle.

Ectopic Atrial Tachycardia

Ectopic atrial tachycardias can arise from foci in either atrium. It may present at any age from fetal life onwards. It is a relatively uncommon arrhythmia but when incessant can cause a rate-related tachycardiomyopathy.

Chaotic Atrial Tachycardia

Chaotic atrial tachycardia is an atrial tachycardia related to abnormal impulse formation where not one, but many atrial foci fire prematurely (at least three atrial foci are required to make the diagnosis). It can be an incessant tachycardia, and thus monitoring for tachycardia-mediated cardiomyopathy is necessary. It has a different natural history than the similar multi-focal atrial tachycardia, which is an adult arrhythmia commonly in association with pulmonary disease. In children it can be seen in those with structurally normal hearts as well as those with congenital cardiac anomalies and hypertrophic cardiomyopathy. It can be a severe arrhythmia, which may be difficult to control even with multiple antiarrhythmic medications. However, following the acute phase of the disease (variable but often 1 to 2 years) spontaneous resolution can be seen in more than half the patients. ¹⁴⁰ Ablation is not a good option, due to the multiple foci, and rate control, rather than achieving sinus rhythm, should be the goal of therapy. ⁷⁸

Permanent Junctional Reciprocating Tachycardia

An important pattern to note on the surface electrocardiogram in some supraventricular tachycardias is a long RP interval. In permanent junctional reciprocating tachycardia, the P wave is of an abnormal morphology (a retrograde P wave) and closer to the succeeding R wave than the preceding R wave. This is different from nodal re-entry tachycardia, where the P wave is buried or very close to the QRS complex, or atrioventricular re-entry tachycardia, where the P wave is closer to the preceding R wave than to the successive R wave ( Fig. 19-11 ). The long RP pattern results from a slowly conducting concealed accessory pathway, giving the persistent form of junctional reciprocating tachycardia. The differential diagnosis of these long RP tachycardias also includes atypical nodal re-entry where antegrade conduction is down the fast pathway and retrograde conduction is via the slow pathway, or an ectopic atrial tachycardia. However, the most common mechanism in children is the persistent or permanent reciprocating tachycardia caused by a slowly conducting concealed accessory pathway with decremental properties. Such pathways are most commonly found in an inferior paraseptal position and give rise to inverted P waves in the inferior leads of the surface electrocardiogram. ^141,142 Occasionally there may be multiple pathways.

A run of permanent junctional reciprocating tachycardia with typical long RP interval and retrograde atrial activation, which breaks to sinus on the second-last beat of the tracing.

Permanent junctional reciprocating tachycardia is often incessant. When the rate is not very fast, it can be mistaken for a sinus tachycardia unless note is taken of the abnormal morphology of the P waves. The rate-related tachycardiomyopathy may then be incorrectly ascribed to myocarditis or a post-viral dilated cardiomyopathy. While antiarrhythmic medication can suppress this tachycardia, it is often difficult to control, requires multiple medications, and usually recurs after withdrawing medication. Radio-frequency ablation is especially rewarding as it effects a cure and allows cardiac function to return to normal in those patients with tachycardia-mediated cardiomyopathy. ^94,143

Atrial Flutter

Atrial flutter is an uncommon tachycardia in childhood, though it can be seen in fetal life and the newborn period. In the neonate, after return to sinus rhythm, it rarely recurs. Conversion to sinus rhythm can be accomplished via oesophageal overdrive atrial pacing, which is highly successful in the infant. ⁴² In rare cases it will terminate spontaneously, and even less often will require direct cardioversion. Atrial flutter can also be seen as part of the spectrum of chaotic atrial tachycardia. In older children with structurally normal hearts, atrial flutter is an unusual arrhythmia, and may be associated with sick sinus syndrome. In congenital heart disease causing right atrial dilation, it may occur either before or after surgery and can cause additional symptoms and morbidity in its own right. Atrial flutter often responds poorly to prophylactic medication.

Intra-atrial Re-entry Tachycardias

Intra-atrial re-entry tachycardias are also called incisional re-entry tachycardias. They may occur when surgical scars have created areas of slow conduction and block around which the wavefronts may pass. Examples are atriotomy scars, atrial septal patches, intra-atrial patches, and baffles. There are frequently multiple circuits in patients with congenital heart disease, and therefore, successful ablation of one may not always result in clinical cure. This is particularly common after the Fontan operation with a dilated and scarred right atrium that may be under higher pressure. In this setting, more extensive electrophysiological mapping is required using multiple catheters and entrainment techniques to delineate the circuit and areas of slow conduction that might be targets for ablation. This is facilitated by using the electroanatomic mapping systems. Lines of block between two scars, or from a scar to a patch or a caval vein, are often required. ^156–159 A further constraint is the fact that new circuits may emerge after a successful ablation. While in some series there have been encouraging results acutely, more extended follow-up has shown recurrence rates of up to 50% at 2 years. ^153,154,159 Particularly in these patients, who may need multiple procedures, the newer non-contact balloon-mounted electrode arrays and non-fluoroscopic techniques are finding a role in improving the ability to map out the tachycardia circuit without excessive exposure to radiation. Use of irrigated catheters also appears to improve success rates. ¹⁵³

Junctional Ectopic Tachycardia

The congenital form of junctional ectopic tachycardia (previously known as His bundle tachycardia) is the least common sustained arrhythmia in childhood. If uncontrolled, it can lead to left ventricular dysfunction and a tachycardiomyopathy with heart failure. Congenital junctional ectopixc tachycardia may occur in siblings of affected patients. It has been proposed that maternal anti-SSA and anti-SSB antibodies may play a role in development of congenital junctional ectopic tachycardia, suggesting a wider spectrum of AV nodal effects of maternal autoimmune disease. ¹⁶⁰ This would also make sense, given the recognition that some patients go on to develop heart block, with or without antiarrhythmic medications. Children with the arrhythmia often require many years of medication to control the rate, although they may outgrow the tendency to arrhythmia in the second decade of life. ¹⁶¹ The site of the ectopic focus, at the atrioventricular node, carries a risk of complete heart block after ablation, but a number of successful reports have appeared. Indeed, this was one of the earliest arrhythmias for which radio-frequency ablation was applied in children. ¹⁶²

The more common form of junctional ectopic tachycardia is seen after cardiac surgery as a transient arrhythmia in the setting of a sick child. Although it can be seen after many congenital heart repairs, typical surgeries involve stretch or suturing close to the atrioventricular node, such as tetralogy of Fallot or perimembranous ventricular septal defect repair. It requires full supportive treatment in addition to treatment of the arrhythmia. Historically this arrhythmia was one of the causes of post-operative mortality due to haemodynamic deterioration, but with improved management this is no longer the case. Body cooling, medications (typically amiodarone or procainamide), and atrial pacing are usually able to control the arrhythmia, which does not recur once the acute post-operative phase has passed. ¹⁶³

Ventricular Tachycardia

In the setting of a structurally normal heart, ventricular tachycardia is an uncommon arrhythmia in childhood. Some cases may be a result of myocarditis or a Purkinje cell tumour, usually presenting in the first few years of life. In these types of cases, after medical control has been achieved, it is often possible to withdraw treatment some years later without a recurrence of the arrhythmia, suggesting regression of the arrhythmic substrate. ¹⁶⁴

Benign Ventricular Tachycardia

Arrhythmic Substrate

In the structurally normal heart, there are two common sites for an automatic focus, though they may occur at any site. Tachycardias originating in the right ventricular outflow tract are the most common and demonstrate a pattern of left bundle branch block on the surface electrocardiogram with an inferior axis. They may be sensitive to adenosine. More recently, subtle changes have been identified on resonance imaging scanning in some of these patients. ¹⁶⁵ Left posterior fascicular tachycardias have a pattern of right bundle branch block with a superior axis. They are usually intermittent but can sometimes be incessant. Left anterior fascicular tachycardias have a pattern of right bundle branch block with an inferior axis. They are much less common. These tachycardias are often mistaken for supraventricular tachycardias and are responsive to verapamil.

Electrophysiological Findings

Timed ventricular extrastimulation may not be able to start or stop the right ventricular outflow tract tachycardia reproducibly. An infusion of isoprenaline is usually required for initiation if it is not spontaneously present. Ventricular pacing close to the site of the tachycardia origin is sometimes helpful in initiating tachycardia. Like most automatic foci, this can be challenging to induce under general anaesthesia.

Rapid atrial or ventricular pacing may be used to initiate a left fascicular tachycardia, and isoproterenol is also often helpful.

Radio-frequency Ablation

Radio-frequency ablation requires the catheter to be directed to the site of onset of the focus. Ventricular pacing at the site of origin will produce electrocardiographical recordings identical to that of the spontaneous arrhythmia. During ventricular tachycardia, an electrode catheter at the site of origin will detect ventricular activation that precedes the QRS complex in all of the surface electrocardiographical leads and any intracardiac reference electrodes ( Fig. 19-12 ). A Purkinje potential may precede the ventricular activation at the site of a fascicular tachycardia. ^166–168 Radio-frequency ablation can be performed with reasonable success. ^169–171

Activation mapping of a left fascicular ventricular tachycardia. The earliest activation on the ablation electrode is 64 msec ahead of the onset of the QRS complex in any of the surface ECG leads at the site of successful ablation. HRA, high right atrium; RV, right ventricle.

Long QT Syndromes

Resulting from abnormalities in ion channel function, there are now multiple genes associated with the long QT syndromes. Ion channel abnormalities lead to abnormal repolarisation of cardiac muscle, creating vulnerability for malignant ventricular arrhythmias and sudden death. Standard therapy includes β-blockade for most types of long QT, and if patients remain symptomatic, pacing, left cardiac sympathetic denervation, and implantable defibrillator placement may be considered. ^185–187 Specific indications for device management are discussed in the pacing section of this chapter.

Brugada Syndrome

Brugada syndrome is characterised by ST segment elevation in the right precordial leads, right bundle branch block, and risk of sudden death due to malignant ventricular arrhythmias. Some patients have the Brugada QRS phenotype present on their electrocardiogram at baseline, while in others it is only evident with fever or through drug provocation with ajmaline, procainamide, or flecainide. ¹⁸⁸ Therapy is most commonly placement of an implantable cardioverter defibrillator, although timing and risk stratification remain challenging. ¹⁸⁹

Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy

Arrhythmogenic right ventricular dysplasia, like long QT syndrome, has been shown to be caused by several different mutations, leading to desmosomal dysfunction. Clinically it is characterised by fibro-fatty replacement of the right (and in advanced disease, the left) ventricular myocardium, causing myocardial dysfunction and ventricular arrhythmias. Implantable cardioverter defibrillators are used frequently in this disease as well when life-threatening arrhythmias are seen. ^190,191 The use of ventricular stimulation studies for risk stratification remains controversial. ¹⁹²

Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia is a disease characterised by ventricular arrhythmias (typically bidirectional ventricular tachycardia which may degenerate into ventricular fibrillation) triggered by exercise or emotion. It is thought to be caused by mutations in the ryanodine receptor. ¹⁹³ It is treated primarily with β-blockade, and then implantable defibrillator implantation in refractory cases. ¹⁹⁴ Implantable defibrillator use in this population can be particularly difficult due to the possibility of electrical storm, where the pain and trauma of a shock reinduces the ventricular arrhythmia.

Atrioventricular Nodal Ablation

In some patients with supraventricular arrhythmias, curative radio-frequency ablation is not possible. Good examples are some patients with atrial fibrillation, multi-focal atrial tachycardia, or multiple atrial re-entry tachycardias. In these settings, atrioventricular nodal ablation will prevent the atrial rate from being conducted to the ventricles. While this necessitates permanent pacing using either ventricular or dual chamber with mode switching, it allows a more stable ventricular rate. Although this was the original indication for ablation in adults, it has only a small role in paediatric practise. There is also an incidence of sudden death after successful catheter ablation of the atrioventricular junction despite implantation of a pacemaker. This may be because most of the patients in this group will have structural heart disease and/or ventricular dysfunction. ⁵³ Ablation is performed from a venous approach placing the ablation catheter in a position to record a His bundle spike and then withdrawing it to increase the atrial signal. If this fails, then ablation can be performed via a transaortic approach on the left side of the septum. ¹⁹⁵

Atrial Fibrillation

In adults with atrial fibrillation, two approaches have been taken. In some with focal fibrillation, ablation of a focus in one of the pulmonary veins has been effective. ¹⁹⁶ In others, long lines of block, similar to those in atrial flutter, are created in the left and right atriums, or the pulmonary veins are isolated. ¹⁹⁷ In recent years, success rates of atrial fibrillation ablation have increased, and it usage has increased. ¹⁹⁸ Atrial fibrillation is uncommon in children and young patients with congenital heart disease. It becomes more frequent in older patients, particularly with untreated or palliated conditions. After the Fontan operation, multiple atrial arrhythmias may be present. It is conceivable that some of the strategies being developed for atrial fibrillation will be applicable in these patients.

Ectopic Beats

There are isolated reports of ablation for frequent, symptomatic ectopic beats in both the atriums and ventricles in patients with a structurally normal heart, using similar approaches to that for sustained arrhythmias at these sites. ¹⁹⁹ The long-term utility of these strategies is unclear.

Inappropriate Sinus Tachycardia

In some patients who are highly symptomatic, the only arrhythmia documented is an inappropriate sinus tachycardia with an excessive rate response during mild exercise. When this is unresponsive to high doses of β-blockers and/or verapamil, ablation has been used in the past to reduce the resting heart rate and rate response to mild exercise, but this is no longer accepted at some centres. ²⁰⁰ There is a risk of causing sinus bradycardia and the need for an atrial pacemaker, and this is a very unusual diagnosis in children. Consideration needs to be given to the important and more common differential diagnosis of postural orthostatic tachycardia syndrome, which has treatment other than ablation. ²⁰¹ All efforts should be made to rule out a subtle ectopic atrial tachycardia, possibly including detailed mapping of the atrial focus.

Issues of Ablation in Congenital Heart Disease

Patients with structural congenital heart disease may present additional challenges for ablation strategies. Knowledge of the underlying anatomy, and of any previous surgery, is critical in planning and undertaking radio-frequency ablation. Despite some of the difficulties that may be encountered, it is important to consider radio-frequency ablation prior to surgery. Arrhythmias that are well tolerated pre-operatively may be less well tolerated after cardiopulmonary bypass. Inotropic support in the peri- and post-operative periods may accelerate and maintain haemodynamically unstable arrhythmias, and peri-operative atrial flutter may conduct rapidly (1:1) to the ventricle. In addition, access may be lost to relevant cardiac chambers; for example, venous access to the left atrium might be lost following completion of a Fontan. Anatomy, tissue scarring, and patient size are all sources for difficulties in ablation in congenital heart disease. ^{108,114,154,202–204}

Vascular Access

Even in small children, it is possible to place multiple electrode catheters using both femoral veins as well as jugular and/or subclavian veins. In those who have undergone repeated cardiac catheterisation procedures, some of the points for central venous access may have become occluded. After the Glenn or hemi-Fontan operation, use of the neck and subclavian veins is no longer possible. This may mandate use of alternative sites, for example, a transhepatic approach, or necessitate using more catheters from one site, fewer catheters overall, or a transoesophageal catheter for atrial sensing and pacing. The development of smaller catheters, which will allow more catheters from one femoral vein, is an advance. It may be necessary to recanalise occluded veins using intravascular stents prior to ablation, with the assistance of intervention catheterisation colleagues.

Venous Anatomy

Azygos continuation of an interrupted inferior caval vein is usually associated with isomerism of the left atrial appendages. This may make manipulation of catheters more difficult, if not impossible, on the venous side of the heart when approaching through the femoral vein. Use of the superior caval vein, or hepatic veins, may facilitate manipulation. ¹⁰⁸ A left-sided superior caval vein draining to the coronary sinus is an infrequent finding in patients with a structurally normal heart, but is more common in those with congenital heart disease. The resultant coronary sinus is dilated. While this is easier to catheterise, it may not provide electrograms from as close to the left atrioventricular valve ring as normal. In addition, passage of the catheter into the terminal portion of the coronary sinus to map accessory pathways in the left lateral and supero-lateral positions is more difficult. In patients with isomerism, the coronary sinus is often absent. Indeed, this is the rule in right isomerism. This removes a useful anatomical and electrical reference. Coronary angiography may help to localise the plane of the atrioventricular ring and confirm absence of the coronary sinus. Placement of a steerable 0.018-inch electrode into the left circumflex coronary artery can delineate the left atrioventricular groove anatomically and provide electrograms to help with mapping.

Cardiac Anatomy

Certain malformations impose constraints on the procedure. In Ebstein’s malformation, the septal and mural leaflets of the tricuspid valve do not arise at the atrioventricular junction. The concomitant lack of valvar tissue makes stability of the catheter at the atrioventricular ring less secure. Frequently in this setting, there are multiple accessory pathways around the right atrioventricular ring. It is usually necessary to ablate all of them to abolish the tendency to arrhythmias. Similar problems occur when Ebstein’s malformation occurs in the setting of discordant atrioventricular connections. The retrograde arterial approach may be more difficult, as placing the ablation catheter below the leaflets of the left atrioventricular valve will not be close enough to the atrioventricular ring. A trans-septal approach will enhance placement of the ablation catheter on the left atrioventricular ring. ^108,205 In patients with isomeric hearts, the plane of the atrioventricular ring may not fall into the usual fluoroscopy views. Transoesophageal echocardiography and angiography may help to locate the tip of the catheter and to define the plane of the atrioventricular ring. ²⁰⁶ Where an atrial septal defect is present, access to the left atrioventricular ring is possible from a femoral venous approach. If surgery is planned, it is appropriate to perform the ablation prior to surgery to retain the additional route to the left atrioventricular ring. Similarly, if a Fontan operation is planned, then prior ablation can be performed via the femoral venous route. After surgery, the systemic atrioventricular valve can only be approached via the retrograde arterial approach and fewer mapping catheters can be placed.

Conduction System

In order to avoid inadvertent atrioventricular block during the procedure, it is vital to know the position of the bundle of His. The His bundle catheter helps both during analysis of the arrhythmic substrate and as an anatomical marker. In patients with atrioventricular septal defects, the bundle of His is located more inferiorly than usual, towards the coronary sinus, and is more at risk when ablating inferior paraseptal accessory pathways. In patients with discordant atrioventricular connections, the bundle of His is located more anterosuperiorly, and is at risk during ablation in the superior paraseptal area. In left isomerism, there may be no discrete bundle of His, while in right isomerism there may be two atrioventricular nodes that form the substrate for twin atrioventricular re-entry tachycardias. ²⁰⁷

Complications

The risks of the ablation procedure include all those seen with routine cardiac catheterisation and diagnostic electrophysiology but there are also a number of specific risks. ^{121,204,208,209}

Complete Heart Block

The atrioventricular node is vulnerable during ablation of dual atrioventricular nodal pathways or accessory pathways in the parahisian areas. In the Paediatric Radiofrequency Catheter Ablation Registry, which carries records of almost 2000 ablations, the incidence of atrioventricular block was 1.6% for procedures for modification of atrioventricular nodal re-entrant tachycardia, 2.7% for superior paraseptal, 10.4% for septal, and 1.0% for inferior paraseptal accessory pathways. ²¹⁰ These results improved slightly in the Pediatric Prospective Ablation Registry (radio-frequency ablations only), where AV block was uncommon overall at 1.2%; occurring in 2.1% of atrioventricular nodal tachycardia ablations, and 3.0% in septal accessory pathways ablations. ²⁰⁸ There is a lower incidence when ablating ectopic atrial or ventricular focuses or accessory pathways in other areas unless the catheter moves during delivery of energy. When ablating in the para-Hisian areas, it is important to observe closely for any signs of atrioventricular block, either antegrade or retrograde, and to stop delivery of energy immediately when this occurs. ²¹¹ In patients with life-threatening arrhythmias or disabling symptoms, the risk of creating complete heart block may occasionally be justifiable. This is likely not acceptable in small children with atrioventricular re-entry tachycardia, or in patients with Wolff–Parkinson–White syndrome, who are asymptomatic or at low risk. While complete heart block is usually apparent immediately, it may occasionally only become evident during the few weeks after the ablation. ^71,212 It is prudent to observe before proceeding directly to implantation of a permanent pacemaker in asymptomatic patients, as recovery is not uncommon in the first few days and can occur even as late as a month after the procedure. ²¹⁰

Coronary Arterial Injury

The coronary arteries can be damaged by unintended or unrecognised entry while trying to cross the aortic valve, and myocardial infarction has been reported. ^84,213 A trans-septal approach will avoid these aortic complications. Delivery of energy on the atrioventricular ring may also cause heating of the wall of the coronary artery. Usually the rapid flow in the artery is able to dissipate heat and the wall is not permanently damaged, though mild stenoses have occurred in an experimental model. ²¹⁴ Rarely, infarction has been recognised clinically. ^215,216

Thromboembolism

When ablating on the left side of the heart, there is a risk of the thrombosis on the ablated area embolizing. Because of this, anticoagulants should always be used during the procedure and many centres give heparin overnight following the procedure. Some of these emboli may only occur in the days or weeks after the procedure. It is a routine, therefore, to give prophylactic aspirin for 4 to 6 weeks after a procedure on the systemic side of the circulation. ^213,217 There is very little data on which to base post-ablation anticoagulation, and thus a fair degree of variation in practise. On the right side of the heart, small pulmonary emboli are unlikely to be clinically apparent; in spite of this some also use heparin during the procedure and aspirin afterwards. This is mandatory when large areas are ablated, as when creating lines of block, or in patients with decreased flow, for example, after the Fontan operation.

Valvar Damage

Damage has been reported to the aortic valve and may be more common than after diagnostic catheterisation as the ablation catheter is stiffer than routine diagnostic catheters. A trans-septal approach is indicated if there is significant aortic valvar disease. The mitral valve can also be damaged during manipulation to place the ablation catheter under the valvar leaflets on the atrioventricular ring, and possibly by delivery of energy. In one echocardiographic study, mild new aortic regurgitation was reported in one-third of patients, and mitral regurgitation in one-eighth. ²¹⁸ More commonly, mild valvar regurgitation occurs with a frequency of less than 2%. ²¹⁹ If a patent oval foramen is present, then a trans-septal approach avoids problems of arterial access and reduces the incidence of possible valvar damage. Some have argued for routine trans-septal puncture in children to avoid the retrograde arterial approach. ²²⁰ Trans-septal puncture carries risks of its own, and air embolism during changes of catheter has been reported. ²²¹ There have been two non-randomised comparative studies of the retrograde and anterograde approaches, with a similar incidence of complications. ²²² Lower fluoroscopy times, however, were reported with the trans-septal approach. ²²³ In these studies, it was recognised that the two approaches could be complimentary, as failure by one route did not invalidate a successful procedure using the other route. Damage to the tricuspid valve is less common as it is rarely necessary to place the catheter below the valvar leaflets.

Cardiac Perforation and Tamponade

Tamponade due to perforation can present during or shortly after the procedure occurs. ²²⁴ Perforation can occur during delivery of energy or manipulation of catheters. Excessive temperature and repeated applications at one site are recognised risk factors. Perforation is more common when ablating in the coronary sinus than around the atrioventricular groove. The stiffer ablation catheters themselves may be responsible for perforation during intracardiac manipulation in infants, thus producing a higher incidence of pericardial effusions. ⁹³ Percutaneous drainage is usually sufficient, but open drainage with repair of the bleeding site is sometimes required. A slow leak into the pericardial space is also possible. Because of this, many centres routinely perform an echocardiogram the following day. Late pericarditis and postpericardiotomy syndrome have also been reported. ²²⁵

Exposure to Radiation and Non-fluoroscopic Mapping

Exposure during the earliest procedures was generally considerably longer than for routine diagnostic cardiac catheterisation, with times generally exceeding 1 hour, but not dissimilar to complex interventional catheterisation. ²²⁶ Isolated reports of radiation skin injury were associated with these early procedures using older fluoroscopy equipment, and are still seen with particularly long procedures such as atrial fibrillation ablations. ²²⁷ Early estimations were of a 0.1% per patient lifetime additional risk for a malignancy from fluoroscopy for 1 hour. ²²⁸ This is to be viewed against a 20% risk per lifetime of an individual developing a malignancy naturally. Subsequent studies suggest this additional risk is closer to 0.03%. ²²⁹ This small risk is outweighed by the benefits in avoiding a lifetime of medication, admissions to hospital, and the risks of antiarrhythmic surgery. In addition, it is highly cost effective. ^230–236 Using modern fluoroscopic systems with optimised fluoroscopy settings, beam collimation, pulsed fluoroscopy, and improved procedure times, the amount of radiation has been reduced. Most procedures now require less than 1 hour of fluoroscopy. ⁸⁴ It may be better to abandon a procedure and return on another day with fresh and/or additional hands and minds and different equipment than persevering when the procedure is becoming increasingly difficult. Usage of electroanatomic mapping systems may assist in decreased amount of fluoroscopy required, and may also assist with creation of improved substrate maps. ^{152,237–239}

Results of Radio-frequency Ablation

A number of single- and multi-centre studies have shown that the overall risk of radio-frequency ablation is low and the success rate is high. Currently, the success rate is more than 96% for nodal re-entry tachycardia, and more than 90% for accessory pathways. A learning curve of the order of 100 cases per institution has been noted. Centres that perform more procedures have a lower complication rate during shorter procedures, with shorter times required for fluoroscopy, and increased rates of success. ^{205,240–243} This applies to radio-frequency ablation in both adults and children. ²⁴⁴ Structural heart disease, a body weight of less than 15 kg, and the presence of multiple arrhythmic targets increase the risk of complications.

The multi-centre series (see Table 19-1 ) all have different criterions for inclusion and follow-up. In 1993, Hindricks reported the Multicentre European Radiofrequency Survey of almost 5000 adults treated up to 1992. ¹³² Calkins and colleagues in 1998 reported on the ATAKR Multicentre Investigators Group of just over 1000 adults and children undergoing radio-frequency ablation of supraventricular arrhythmias including ablation of the atrioventricular node. ¹³³ The Pediatric Electrophysiology Society of North America first reported in 1994 the acute results of ablation in patients aged less than 21 years. They included those with structural heart disease and those undergoing ablation for ventricular tachycardia. ⁸⁵ A later follow-up of the patients with a structurally normal heart undergoing ablation for supraventricular tachycardia was reported in 1997. ⁸⁴ In these studies, the death rate was 0.1% to 0.3%, complete atrioventricular block occurred in 0.6% to 1% and pericardial tamponade or effusion in 0.7% to 2.5% (see Table 19-1 ). A prospective study of radio-frequency ablation of accessory pathways in children showed similar results, with overall acute success rates of 95.7%, highest for left free-wall pathways (97.8%) and slightly lower for right free-wall pathways at 90.8%. ^205,208

Although the acute rates of success and complications have been well documented, with similar results for adults and children, there have been few studies of the longer-term outcome. Calkins et al reported recurrence rates of 5% after ablation for nodal re-entry, and 8% for accessory pathways. The median time to recurrence was 35 days. Studies from individual centres have reported recurrences after successful ablation of accessory pathways in about one-tenth of patients after follow-up of about 6 months. ^{217,245–247} The majority occurred within the first few days of the procedure, and most patients underwent a successful second ablation. The most comprehensive follow-up study to date is from the Pediatric Electrophysiology Society. ^84,205 Surprisingly, given the impressive acute results, there was a significant incidence of late recurrent arrhythmias.