Cardiac Arrhythmias




The frequency and clinical significance of arrhythmias are different in children compared with adults. Although arrhythmias are relatively infrequent in infants and children, the common practice of monitoring cardiac rhythm in children requires primary care physicians, emergency department physicians, and intensive care physicians to be able to recognize and manage basic arrhythmias.


The normal heart rate varies with age: The younger the child, the faster the heart rate. Therefore, the definitions of bradycardia (<60 beats/min) and tachycardia (>100 beats/min) used for adults do not apply to infants and children. Tachycardia is defined as a heart rate beyond the upper limit of normal for the patient’s age, and bradycardia is defined as a heart rate slower than the lower limit of normal. Normal resting heart rates by age are presented in Table 24-1 .



TABLE 24-1

NORMAL RANGES OF RESTING HEART RATE





























Age Mean (range) Age (yr) Mean (range)
Newborn 145 (90–180) 4 108 (72–135)
6 months 145 (106–185) 6 100 (65–135)
1 year 132 (105–170) 10 90 (65–130)
2 year 120 (90–150) 14 85 (60–120)

From Davignon A, Rautaharju P, Boisselle E, Soumis F, Megelas M, Choquette A. Normal ECG standards for infants and children. Pediatr Cardiol 1:123-131, 1979/1980.


This chapter discusses basic arrhythmias according to the origin of their impulse. Each arrhythmia is described along with its causes, significance, and treatment.


Rhythms Originating in the Sinus Node


All rhythms that originate in the sinoatrial (SA) node (sinus rhythm) have two important characteristics ( Fig. 24-1 ). Both are required for a rhythm to be called sinus rhythm.



  • 1.

    P waves precede each QRS complex with a regular PR interval. (The PR interval may be prolonged, as in first-degree atrioventricular [AV] block); in this case, the rhythm is sinus with first-degree AV block).


  • 2.

    The P axis falls between 0 and +90 degrees, an often neglected criterion. This produces upright P waves in lead II and inverted P waves in aVR. (See Chapter 3 for a detailed discussion of sinus rhythm.)




FIGURE 24-1


Normal and abnormal rhythms originating in the sinoatrial node.

(From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 1992.)


Regular Sinus Rhythm


Description. The rhythm is regular, and the rate is normal for age. The two characteristics of sinus rhythm described previously are present (see Fig. 24-1 ).


Significance . This rhythm is normal at any age.


Management. No treatment is required.


Sinus Tachycardia


Description. Characteristics of sinus rhythm are present (see previous description). The rate is faster than the upper limit of normal for age (see Table 24-1 ). A rate above 140 beats/min in children and above 170 beats/min in infants may be significant. The heart rate usually is below 200 beats/min in sinus tachycardia (see Fig. 24-1 ).


Causes. Anxiety, fever, hypovolemia or circulatory shock, anemia, congestive heart failure (CHF), administration of catecholamines, thyrotoxicosis, and myocardial disease are possible causes.


Significance. Increased cardiac work is well tolerated by healthy myocardium.


Management. The underlying cause is treated.


Sinus Bradycardia


Description. The characteristics of sinus rhythm are present (see previous description), but the heart rate is slower than the lower limit of normal for the age (see Table 24-1 ). A rate slower than 80 beats/min in newborn infants and slower than 60 beats/min in older children may be significant (see Fig. 24-1 ).


Causes. Sinus bradycardia may occur in normal individuals and trained athletes. It may occur with vagal stimulation, increased intracranial pressure, hypothyroidism, hypothermia, hypoxia, hyperkalemia, and administration of drugs such as β-adrenergic blockers.


Significance. In some patients, marked bradycardia may not maintain normal cardiac output.


Management. The underlying cause is treated.


Sinus Arrhythmia


Description. There is a phasic variation in the heart rate caused by respiratory influences on the autonomic nervous system, increasing during inspiration and decreasing during expiration. The arrhythmia occurs, with maintenance of characteristics of sinus rhythm (see Fig. 24-1 ).


Causes. This is a normal phenomenon and is caused by phasic variation in the firing rate of cardiac autonomic nerves with the phases of respiration.


Significance . Sinus arrhythmia has no significance because it is a normal finding in children and a sign of good cardiac reserve.


Management. No treatment is indicated.


Sinus Pause


Description. In sinus pause, the sinus node pacemaker momentarily ceases activity, resulting in absence of the P wave and QRS complex for a relatively short time (see Fig. 24-1 ). Sinus arrest is of longer duration and usually results in an escape beat (see later discussion) by other pacemakers, such as the AV junctional or nodal tissue (junctional or nodal escape beat).


Causes. Increased vagal tone, hypoxia, sick sinus syndrome, and digitalis toxicity are possible causes. Well-conditioned athletes may have bradycardia and sinus pause of greater than 2 seconds because of prominent vagal influence.


Significance. Sinus pause of less than 2 seconds are normal in young children and adolescents. It usually has no hemodynamic significance but may reduce cardiac output in patients with frequent and long period of sinus pause.


Management. Treatment is rarely indicated except in sinus node dysfunction (or sick sinus syndrome; see later discussion ).


Sinoatrial Exit Block


Description. A P wave is absent from the normally expected P wave, resulting in a long RR interval. The duration of the pause is a multiple of the basic PP interval. An impulse formed within the sinus node fails to propagate normally to the atrium.


Causes. Excessive vagal stimulation, myocarditis or fibrosis involving the atrium, and drugs such as quinidine, procainamide or digitalis.


Significance. It is usually transient and has no hemodynamic significance. Rarely, the patient may have syncope.


Management. The underlying cause is treated.


Sinus Node Dysfunction (Sick Sinus Syndrome)


Description. In sinus node dysfunction, the sinus node fails to function as the dominant pacemaker of the heart or performs abnormally slowly, resulting in a variety of arrhythmias. These arrhythmias may include profound sinus bradycardia, sinus pause or arrest, sinus node exit block, slow junctional escape beats, and ectopic atrial or nodal rhythm. Clear documentation is not always possible. Long-term recording with Holter is better in documenting overall heart rate variation and the prevalence of abnormally slow and fast rhythm.


Bradytachyarrhythmia occurs when bradycardia and tachycardia alternate. Whereas bradycardia may originate in the sinus node, atria, AV junction, or ventricle, tachycardia is usually caused by atrial flutter or fibrillation and less commonly by reentrant supraventricular tachycardia (SVT). When these arrhythmias are accompanied by symptoms such as dizziness or syncope, sinus node dysfunction is referred to as sick sinus syndrome.


Causes




  • 1.

    Injury to the sinus node caused by extensive cardiac surgery, particularly involving the atria (e.g., the Senning operation, Fontan procedure, or surgery for partial or total anomalous pulmonary venous return or endocardial cushion defect) are possible causes.


  • 2.

    Some cases of sick sinus syndrome are idiopathic, involving an otherwise normal heart without structural defect.


  • 3.

    Rarely, myocarditis, pericarditis, or rheumatic fever is a cause.


  • 4.

    Congenital heart defects (CHDs) (e.g., sinus venosus atrial septal defect [ASD], Ebstein’s anomaly, left atrial isomerism [polysplenia syndrome])


  • 5.

    Secondary to antiarrhythmic drugs (e.g., digitalis, propranolol, verapamil, quinidine


  • 6.

    Hypothyroidism



Significance


Bradytachyarrhythmia is the most worrisome. Profound bradycardia after a period of tachycardia (overdrive suppression) can cause presyncope, syncope, and even death.


Management



  • 1.

    For severe bradycardia:



    • a.

      Acute symptomatic bradycardia is treated with intravenous (IV) atropine (0.04 mg/kg IV every 2–4 hours) or isoproterenol (0.05–0.5 μg/kg IV) or transcutaneous pacing. Temporary transvenous or transesophageal pacing can be used until a permanent pacing system can be implanted.


    • b.

      Chronic medical treatment using drugs has not been uniformly successful and is not accepted as standard treatment of sinus node dysfunction.


    • c.

      Symptomatic bradycardia is treated with permanent pacing. Asymptomatic patients with heart rate under 40 beats/min or pauses longer than 3 seconds are less clear indications for permanent pacing.


    • d.

      Permanent implantation is the treatment of choice in symptomatic patients, especially those with syncope. Most patients receive atrial demand pacing. Patients with any degree of AV nodal dysfunction receive dual chamber pacemakers. Ventricular demand pacemakers may be used.



  • 2.

    For symptomatic tachycardia:



    • a.

      Antiarrhythmic drugs, such as propranolol or quinidine, may be given to suppress tachycardia, but they are often unsuccessful.


    • b.

      Digoxin may help to decrease AV conduction of rapid tachycardia.


    • c.

      Catheter ablation of arrhythmia substrates (often requiring concomitant surgical revision of previous surgeries) may be indicated.


    • e.

      Patients with tachycardia–bradycardia syndrome may benefit from antitachycardia pacemakers.






Rhythms Originating in the Atrium


Rhythms that originate in the atrium (ectopic atrial rhythm) are characterized by the following ( Fig. 24-2 ):



  • 1.

    P waves have an unusual contour, which is caused by an abnormal P axis or an abnormal number of P waves per QRS complex.


  • 2.

    QRS complexes are usually of normal configuration, but occasional bizarre QRS complexes caused by aberrancy may occur (see later discussion).




FIGURE 24-2


Arrhythmias originating in the atrium.

(From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 1992.)


Premature Atrial Contraction


Description. The QRS complex appears prematurely. The P wave may be upright in lead II when the ectopic focus is high in the atrium. The P wave is inverted when the ectopic focus is low in the atrium (so-called coronary sinus rhythm). The compensatory pause is incomplete; that is, the length of two cycles, including one premature beat, is less than the length of two normal cycles (see Fig. 24-2 ).


An occasional premature atrial contraction (PAC) is not followed by a QRS complex (i.e., a nonconducted PAC ; see Fig. 24-2 ). A nonconducted PAC is differentiated from a second-degree AV block by the prematurity of the nonconducted P wave (P′ in Fig. 24-2 ). The P′ wave occurs earlier than the anticipated normal P wave, and the resulting PP′ interval is shorter than the normal PP interval for that individual. In second-degree AV block, the P wave that is not followed by the QRS complex occurs at the anticipated time, maintaining a regular PP interval.


Causes. PAC appears in healthy children, including newborns. It also may appear after cardiac surgery and with digitalis toxicity.


Significance. PAC has no hemodynamic significance.


Management. Usually no treatment is indicated except in cases of digitalis toxicity.


Wandering Atrial Pacemaker


Description. Wandering atrial pacemaker is characterized by gradual changes in the shape of P waves and PR intervals (see Fig. 24-2 ). The QRS complex is normal.


Causes. Wandering atrial pacemaker is seen in otherwise healthy children. It is the result of a gradual shift of the site of impulse formation in the atria through several cardiac cycles.


Significance. Wandering atrial pacemaker is a benign arrhythmia and has no clinical significance.


Management. No treatment is indicated.


Ectopic (or Autonomic) Atrial Tachycardia


Description. There is a narrow QRS complex tachycardia (in the absence of aberrancy or preexisting bundle branch block) with visible P waves at an inappropriately rapid rate. The P axis is different from that of sinus rhythm. When the ectopic focus is near the sinus node, the P axis may be the same as in sinus rhythm. The usual heart rate in older children is between 110 and 160 beats/min, but the tachycardia rate varies substantially during the course of a day, reaching 200 beats/min with sympathetic stimuli. This arrhythmia is sometimes difficult to distinguish from the re-entrant AV tachycardia, and thus it is included under SVT. It represented 18% of SVT in one study.


Holter monitoring may demonstrate a characteristic gradual acceleration of the heart rate, the so-called warming up period, rather than abrupt onset and termination seen with reentrant AV tachycardia. The P waves of ectopic atrial tachycardia may not conduct to the ventricle, especially during sleep, when parasympathetic tone is heightened.


Causes. Ectopic atrial tachycardia originates from a single focus in the atrium. This arrhythmia is believed to be secondary to increased automaticity of nonsinus atrial focus.



  • 1.

    Most patients have structurally normal heart (idiopathic).


  • 2.

    Myocarditis, chronic cardiomyopathy, AV valve regurgitation, atrial dilatation, atrial tumors, and previous cardiac surgery involving atria (e.g., Senning operation, Fontan procedure) may be the cause.


  • 3.

    Occasionally, respiratory infections caused by mycoplasma or viruses may trigger the arrhythmia.



Significance. The chronic nature of the arrhythmia at a relatively low heart rate (<150 beats/min) can escape detection, and CHF is common at presentation. There is a high association of this tachycardia with tachycardia-induced cardiomyopathy.


Management. It is refractory to medical therapy and cardioversion. Drugs that are effective in reentrant atrial tachycardia (e.g., adenosine) do not terminate the tachycardia. Cardioversion is ineffective because the ectopic rhythm resumes immediately.



  • 1.

    Drugs to slow the ventricular rate are not usually successful. There are conflicting reports regarding the efficacy of digoxin and beta-blockers in slowing down the ventricular rate. Class IC (e.g., flecainide) and class III (e.g., amiodarone) antiarrhythmic agents are generally most effective (up to 75%).


  • 2.

    Radiofrequency ablation may prove to be effective in 95% to 100%. In children, a majority of the foci are found in the left atrium near the pulmonary veins and the atrial appendage in contrast to the right atrium found in adults.



Chaotic (or Multifocal) Atrial Tachycardia


Description. This is an uncommon tachycardia characterized by three or more distinct P-wave morphologies. The PP and RR intervals are irregular with variable PR intervals. The arrhythmia may be misdiagnosed as atrial fibrillation (AF).


Causes. Most patients with the condition are infants; it is very rare after 5 years of age. About 30% to 50% have respiratory illness. Myocarditis and birth asphyxia have been described. Structural heart disease may or may not be present.


Significance. The mechanism of this arrhythmia has been poorly defined. Cardiac enlargement or reduced left ventricular (LV) systolic function may be present at the diagnosis. Sudden death has been reported in up to 17% while on therapy. Spontaneous resolution frequently occurs.


Management. This arrhythmia is refractory to cardiac pacing, cardioversion, and adenosine.



  • 1.

    When there is no evidence of cardiac dysfunction, observation on a regular basis may be reasonable.


  • 2.

    Drugs that slow AV conduction (propranolol or digoxin) and those that decrease automaticity (e.g., class IA or IC or class III) have not been very effective.


  • 3.

    Concurrent illness should be treated.


  • 4.

    If the patient has cardiac dysfunction, medical therapy with amiodarone should be begun. Amiodarone appears to be the current treatment of choice.



Atrial Flutter


Description. The pacemaker lies in an ectopic focus, and “circus movement” in the atrium is the mechanism of this arrhythmia.



  • 1.

    Typical atrial flutter is characterized by an atrial rate (F wave with “sawtooth” configuration) of about 300 (ranges, 240–360) beats/min, a ventricular response with varying degrees of block (e.g., 2:1, 3:1, 4:1), and normal QRS complexes (see Fig. 24-2 ).


  • 2.

    Another form of atrial flutter may be seen in children who have undergone atrial surgery with multiple suture lines. Atrial flutter is secondary to a reentry mechanism within the scarred atrial muscle (called incisional intraatrial reentrant tachycardia ). In this situation, the atrial rates are commonly 250 beats/min or slower, and the P-wave morphology is variable without the usual sawtooth F waves, and the P wave is often difficult to detect. Either 2:1 or 1:1 AV conduction is present.



Causes. Atrial flutter usually suggests a significant cardiac pathology, although most fetuses and neonates with atrial flutter have normal hearts, and spontaneous conversion is common. Structural heart disease with dilated atria, acute infectious illness, myocarditis or pericarditis, digitalis toxicity, and thyrotoxicosis are possible causes. Previous surgical procedures involving atria (e.g., Senning operation for transposition, Fontan operation, and other CHDs) may cause incisional intraatrial reentrant tachycardia.


Significance. The ventricular rate determines eventual cardiac output. With a reasonable ventricular rate, the arrhythmia is well tolerated for a long time. A too-rapid ventricular rate may decrease cardiac output and result in heart failure. Thrombus formation may lead to embolic events. Uncontrolled atrial flutter may precipitate heart failure. The flutter may be associated with syncope, presyncope, or chest pain.


Management. Management of atrial flutter is divided into acute conversion, chronic suppression of the arrhythmia, control of ventricular rate, prevention of recurrences, and for refractory cases.



  • 1.

    Acute situation



    • a.

      Adenosine does not convert the arrhythmia to sinus rhythm, although it may be helpful in confirming the diagnosis of atrial flutter by temporarily blocking AV conduction.


    • b.

      Immediate synchronized DC cardioversion is the treatment of choice for atrial flutter of short duration if the infant or child is in severe CHF.


    • c.

      Transesophageal atrial overdrive pacing may be used for the same purpose. Pacing stimuli are delivered at rates 20% to 25% faster than the flutter rate until the flutter circuit is captured.


    • d.

      In children, IV amiodarone (class III) or IV procainamide (class IA) may be effective.



  • 2.

    For chronic cases: For long-standing atrial flutter or fibrillation (of 24–48 hours) or those with an unknown duration, thrombus formation may lead to cerebral embolic events, especially when the atrial arrhythmia is terminated.



    • a.

      It is essential to rule out atrial thrombi, preferably by echocardiography. Transesophageal echocardiography may define atrial thrombi better than transthoracic echocardiography.


    • b.

      If a thrombus is found or its absence is uncertain, anticoagulation with warfarin (with an international normalized ratio between 2.0 and 3.0) is started and cardioversion delayed for 2 to 3 weeks. After conversion to sinus rhythm, anticoagulation is continued for an additional 3 to 4 weeks.



  • 3.

    For rate control: For control of ventricular rate, calcium channel blockers appear to be the drug of choice. Propranolol may be equally effective. In the past, digoxin was popular for this purpose.


  • 4.

    For prevention of recurrences: Class I and class III antiarrhythmic drugs have been shown to be successful in preventing recurrences in some cases. However, class IA drugs (procainamide, quinidine, disopyramide) also have anticholinergic effects that may produce a faster conduction through the AV node, worsening the situation. Therefore, they should be used with drugs that offset the anticholinergic effect, such as digoxin, beta-blockers, or diltiazem. Amiodarone and ibutilide (class III) have also been shown to be effective in treating atrial flutter. For a quick review of antiarrhythmic drugs, readers should see Table A-4 , Tables A-5 in Appendix A .


  • 5.

    For refractory cases: For incisional intraatrial reentry tachycardia, radiofrequency ablation to interrupt the flutter circuit may be indicated. The success rate for this condition is not as high as in typical atrial flutter (with an acute success rate of 75% and a recurrence rate as high as 50%).



Atrial Fibrillation


Description. Atrial fibrillation (AF) is the most common arrhythmia seen in adults, but it is rare in children and is less common than atrial flutter in children. The mechanism of this arrhythmia is “circus movement,” as in atrial flutter. AF is characterized by an extremely fast atrial rate (f wave at a rate of 350–600 beats/min) and an “irregularly irregular” ventricular response with narrow QRS complexes (see Fig. 24-2 ).


Causes. AF usually is associated with structural heart diseases with dilated atria, such as seen with mitral stenosis and regurgitation, Ebstein’s anomaly, tricuspid atresia, ASD, or previous intraatrial surgery. Thyrotoxicosis, pulmonary emboli, and pericarditis should be suspected in a previously normal child who develops AF.


Significance. The rapid ventricular rate, in addition to the loss of coordinated contraction of the atria and ventricles, decreases the cardiac output, as occurs in atrial tachycardia. AF usually suggests a significant cardiac pathology.


Management. Some part of medical management for AF is similar to that described under a trial flutter (see previous discussion).



  • 1.

    If AF has been present for more than 48 hours, anticoagulation with warfarin for 2 to 3 weeks is recommended to prevent systemic embolization of atrial thrombus if the conversion can be delayed. Anticoagulation is continued for 3 to 4 weeks after the restoration of sinus rhythm. If cardioversion cannot be delayed, IV heparin should be started and cardioversion performed when activated partial thromboplastin time (aPTT) reaches 1.5 to 2.5 times control levels (in 5–10 days), with subsequent oral anticoagulation with warfarin. An alternative to anticoagulation is transesophageal echocardiography to rule out atrial thrombus.


  • 2.

    Propranolol, verapamil, or digoxin may be used to slow the ventricular rate.


  • 3.

    Class I antiarrhythmic agents (e.g., quinidine, procainamide, flecainide) and the class III agent amiodarone may be used, but the success rate in rhythm conversion is disappointingly low. Table A-4 , Table A-5 in Appendix A provide a quick review of antiarrhythmic drugs.


  • 4.

    In patients with chronic AF, anticoagulation with warfarin should be considered to reduce the incidence of thromboembolism. In chronic cases, rate control, rather than conversion, is increasingly used.


  • 5.

    In the Cox maze procedure (or the “cut-and-sew-maze”), multiple surgical incisions are made in the right and left atria that are then repaired in an attempt to minimize the formation of reentrant loop. The procedure showed greater than a 96% cure rate 10 years after the surgery in adult patients. Freedom from stroke has generally been reported as exceeding 99% for Cox maze procedures.


  • 6.

    Radiofrequency ablation to electrically isolate the pulmonary veins from the left atrium or directly ablating the ectopic focus within the pulmonary veins has shown better results than pharmacologic agents in rhythm control in adults.



Supraventricular Tachycardia


Description


Supraventricular tachycardia (SVT) is a general term that refers to any rapid heart rhythm originating above the ventricular tissue. In general, SVTs are caused by two separate mechanisms. The first mechanism is reentry, and the second is automaticity. The majority of SVTs are caused by reentrant (or reciprocating) AV tachycardia rather than rapid firing of a single focus in the atria. Examples of reentry (reciprocating) SVT include AV reentrant (or reciprocating) tachycardia and nodal reentrant AV tachycardia. Examples of automatic types of SVTs are atrial ectopic tachycardia and junctional ectopic tachycardia (JET). These arrhythmias share a number of clinical, electrocardiographic, and therapeutic similarities. Most of the discussion here focuses on the reentry type of SVTs; others are discussed under specific headings.


Reentrant (Reciprocating) Type of SVT


In the reentry type of SVT, the heart rate is extremely rapid and regular (usually 240 ± 40 beats/min). The P wave usually is invisible. When visible, the P wave has an abnormal P axis and either precedes or follows the QRS complex (see Figs. 24-2 and 24-3 ). The QRS complex is usually normal (narrow), but occasionally, aberrancy increases the QRS duration, making differentiation from ventricular tachycardia (VT) difficult (see later discussion).




FIGURE 24-3


Diagram showing the mechanism of reciprocating atrioventricular tachycardia (RAVT) in relation to electrocardiographic (ECG) findings. A, Orthodromic accessory RAVT is the most common mechanism of supraventricular tachycardia in patients with Wolff-Parkinson-White syndrome. Antegrade conduction through the normal, slow atrioventricular (AV) node produces a normal QRS complex, and the retrograde conduction through the bypass tract creates inverted P waves after the QRS complex (with a short RP interval). B, In antidromic accessory RAVT, the antegrade conduction through the bypass tract produces a wide QRS complex. Retrograde P waves precede the wide QRS complex with a short PR interval (and a long RP interval). C, In orthodromic nodal RAVT (common form), the retrograde P waves are usually concealed in the QRS complex of normal duration. The ECG is similar to that of orthodromic accessory RAVT, and differentiation between these two is possible only when the tachyarrhythmia terminates by the presence of preexcitation in the accessory RAVT. D, In antidromic nodal RAVT (uncommon), narrow QRS complexes are preceded by retrograde P waves, with a short PR interval. The ECG is similar to that of ectopic atrial tachycardia. LV, left ventricle; RV, right ventricle.

(From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 1992.)


AV reentrant (or reciprocating) tachycardia is not only the most common mechanism of SVT but also the most common tachyarrhythmia seen in the pediatric age group. This arrhythmia was formerly called paroxysmal atrial tachycardia (PAT) because the onset and termination of this arrhythmia were characteristically abrupt.


In SVT caused by reentry, two pathways are involved; at least one of these is the AV node, and the other is an accessory pathway. The accessory pathway may be an anatomically separate bypass tract, such as the bundle of Kent (which produces accessory reciprocating AV tachycardia [RAVT]; see Fig. 24-3 , A and B ), or only a functionally separate bypass tract, such as in a dual AV node pathway (which produces nodal RAVT; see Fig. 24-3 , C and D ). Patients with accessory pathways frequently have Wolff-Parkinson-White (WPW) preexcitation.


Figure 24-3 shows the mechanism of RAVT in relation to electrocardiographic (ECG) findings. If a PAC occurs, the prematurity of the extrasystole may find the accessory bundle refractory, but the AV node may conduct, producing a normal QRS complex; when the impulse reaches the bundle of Kent from the ventricular side, the bundle will have recovered and allows reentry into the atrium, producing a superiorly directed P wave that is difficult to detect. In turn, the cycle is maintained by reentry into the AV node, with a very fast heart rate. When there is an antegrade conduction through the AV node (slow pathway); the rhythm is called orthodromic reciprocating atrioventricular tachycardia (see Fig. 24-3 , A ).


Less common is a widened QRS complex with antegrade conduction into the ventricle via the accessory (fast) pathway and retrograde conduction through the (slower) AV node ( antidromic reciprocating AV tachycardia; see Fig. 24-3 , B ). A premature ventricular contraction (PVC) could initiate this arrhythmia if the recovery time of the two limbs is ideal for the initiation of the reentry.


Dual pathways in the AV node are more common than accessory bundles, at least as functional entities. For SVT to occur, the two pathways would have to have, at least temporarily, different conduction and recovery rates, creating the substrate for a reentry tachycardia. When the normal, slow pathway through the AV node is used in antegrade conduction to the bundle of His (orthodromic), the resulting QRS complex is normal with an abnormal P vector, but the latter is unrecognizable because it is superimposed on the QRS complex (see Fig. 24-3 , C ). The resulting tachycardia could be the same as that seen with SVT associated with WPW syndrome. The two can be differentiated only after conversion from the SVT; after conversion, the patient with accessory bundle would have WPW preexcitation. In antidromic nodal reentrant AV tachycardia (see Fig. 24-3 , D ), which is uncommon, the fast tract of the AV node transmits the antegrade impulse to the bundle of His, and the normal, slow pathway of the AV node transmits the impulse retrogradely. The resulting SVT demonstrates normal QRS duration, a short PR interval, and an inverted P wave.


In general, nodal RAVT is more influenced by increased sympathetic tone than accessory RAVT. Nodal RAVT is more likely triggered by physical activity, emotional stress, and abrupt changes in body position. In addition, nodal RAVT is less likely to be incessant (and therefore rarely causing tachycardia-induced cardiomyopathy). SVT, seen in the first year of life but few afterward, is more likely to have accessory RAVT, and an adolescent who has first SVT is more likely to have nodal RAVT.


Any type of AV block is incompatible with reentrant tachycardia; AV block would abruptly terminate the tachycardia, at least temporarily. This is the reason that adenosine, which transiently blocks AV conduction, works well for this type of arrhythmia.


Automatic Type of SVT


Ectopic (or nonreciprocating) atrial tachycardia is a rare mechanism of SVT in which rapid firing of a single focus in the atrium is responsible for the tachycardia (see previous section). Unlike in reciprocating atrial tachycardia, in ectopic atrial tachycardia, the heart rate varies substantially during the course of a day, and second-degree AV block may develop. In contrast, in reentrant tachycardia, second-degree AV block terminates the SVT. Nodal ectopic tachycardia may superficially resemble atrial tachycardia because the P wave is buried in the T waves of the preceding beat and becomes invisible but the rate of nodal tachycardia is relatively slower (120–200 beats/min) than the rate of ectopic atrial tachycardia. These two arrhythmias are further discussed under separate headings.


Causes




  • 1.

    WPW preexcitation is present in 10% to 20% of cases, which is evident only after conversion to sinus rhythm. Approximately 10% of WPW patients have multiple (two to four) accessory pathways.


  • 2.

    No heart disease is found in about half of patients. This idiopathic type of SVT occurs more commonly in young infants than in older children.


  • 3.

    Patients with some CHDs (e.g., Ebstein’s anomaly, single ventricle, congenitally corrected transposition of the great arteries) are more prone to this arrhythmia.


  • 4.

    SVT may occur after cardiac surgeries.



Significance




  • 1.

    Many infants tolerate SVT well. If the tachycardia is sustained for 6 to 12 hours, signs of CHF usually develop in infants. Clinical manifestations of CHF include irritability, tachypnea, poor feeding, and pallor. When CHF develops, the infant’s condition can deteriorate rapidly.


  • 2.

    Older children may complain of chest pain, palpitation, shortness of breath, lightheadedness, and fatigue. A pounding sensation in the neck (i.e., neck pulsation) is fairly unique to the reentrant-type SVT and considered to be the result of cannon waves when the atrium contracts against a simultaneously contracting ventricle.



Management


Acute Treatment of SVT




  • 1.

    Vagal stimulatory maneuvers (unilateral carotid sinus massage, gagging, pressure on an eyeball) may be effective in older children but rarely effective in infants. Placing an ice-water bag on the face (for up to 10 seconds) is often effective in infants (by diving reflex). In children, a headstand often successfully interrupts the SVT.


  • 2.

    If the vagal maneuver is ineffective, adenosine is considered the drug of choice. It has negative chronotropic, dromotropic, and inotropic actions with a very short duration of action (half-life <10 seconds) and minimal hemodynamic consequences. Adenosine is effective for almost all reciprocating SVT (in which the AV node forms part of the reentry circuit) of both narrow- and wide-complex regular tachycardia. It is not effective for irregular tachycardia. It is not effective for non-reciprocating atrial tachycardia, atrial flutter or AF, and VT, but it has differential diagnostic ability. Its transient AV block may unmask atrial activities by slowing the ventricular rate and thus help clarify the mechanism of certain supraventricular arrhythmias (see Fig. 24-4 ).




    FIGURE 24-4


    Adenosine can uncover the mechanism of supraventricular tachycardia. A 3-month-old infant developed an extremely fast, narrow QRS complex tachycardia and a heart rate of 220 beats/min after insertion of a central line through a jugular vein. Adenosine produced a transient atrioventricular block and unmasked very rapid atrial fibrillation waves (570 beats/min).


    Adenosine is given by rapid IV bolus followed by a saline flush, starting at 50 μg/kg, increasing in increment of 50 μg/kg, every 1 to 2 minutes. The usual effective dose is 100 to 150 μg/kg with maximum dose of 250 μg/kg. Adenosine is 90% to 100% effective.


  • 3.

    If the infant is in severe CHF and adenosine is not readily available, emergency treatment is directed at immediate cardioversion. The initial dose of 0.5 joule/kg is increased in steps up to 2 joule/kg.


  • 4.

    IV administration of propranolol may be used to treat SVT in the presence of WPW syndrome. IV verapamil should be avoided in infants younger than 12 months of age because it may produce extreme bradycardia and hypotension in infants. Esmolol, other beta-adrenergic blockers, verapamil, and digoxin also have been used with some success.


  • 5.

    Overdrive suppression (by transesophageal pacing or by atrial pacing) may be effective in children who have been digitalized.



Prevention of Recurrence of SVT




  • 1.

    In infants without WPW preexcitation, oral propranolol for 12 months is effective.


  • 2.

    Verapamil can also be used, but it should be used with caution in patients with poor LV function and in young infants.


  • 3.

    In infants or children with or without WPW preexcitation on the ECG, beta-blockers such as atenolol or nadolol are often the medication of choice in the long-term management. In the presence of WPW preexcitation, digoxin or verapamil may increase the rate of antegrade conduction of the impulse through the accessory pathway and therefore should be avoided.


  • 4.

    For children who have infrequent episodes of SVT that result in little hemodynamic compromise, observation is indicated. They should be taught how to apply vagal maneuvers (e.g., gagging, headstands). If not effective, adenosine is used to correct the rhythm. Alternatively, the use of a beta-blocker or calcium channel blocker can be effective in slowing and terminating the SVT.


  • 5.

    Radiofrequency catheter ablation or surgical interruption of accessory pathways should be considered if medical management fails or frequent recurrences occur. Ablation therapy is controversial for asymptomatic patients with WPW preexcitation. Ablation is not recommended in infants 1 to 2 years of age because of a possibility of spontaneous resolution of SVT.



Radiofrequency ablation can be carried out with a high degree of success and a low complication rate. The success rate of the procedure for accessory pathway ablation is between 90% and 95%; the highest success rates are found in patients with left-sided accessory pathways. Patients with para-Hisian pathways have the lowest success rate because of more cautious applications (fearing risk of AV block). A risk of heart block is 1.2% with a risk as high as 10.4% for patients with a midseptal ablation site. The overall risk of complication is 3% to 4%.

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Apr 15, 2019 | Posted by in CARDIOLOGY | Comments Off on Cardiac Arrhythmias
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