Abstract
The rapid recognition, diagnosis, and management of cardiac arrhythmias are essential skills in the intensive care unit (ICU). Outcomes can hinge on the timely initiation of arrhythmia therapy. The aim of this chapter is to serve as a useful, practical resource by: (1) describing the basic physiology of the normal cardiac rhythm followed by the common mechanisms causing rhythm disturbances, (2) describing common tools used to diagnose rhythm, (3) describing specific pediatric arrhythmias, causes, and treatments, and (4) providing a practical guide to pacemakers and pacing in the ICU setting.
Key Words
arrhythmia, electrophysiology, pacing, antiarrhythmic medications, pacemaker, heart block
The rapid recognition, diagnosis, and management of cardiac arrhythmias are essential skills in the intensive care unit (ICU). Outcomes can hinge on the timely initiation of arrhythmia therapy. The aim of this chapter is to serve as a useful, practical resource by (1) describing the basic physiology of the normal cardiac rhythm followed by the common mechanisms causing rhythm disturbances, (2) describing common tools used to diagnose rhythm, (3) describing specific pediatric arrhythmias, causes, and treatments, and (4) providing a practical guide to pacemakers and pacing in the ICU setting.
Normal Cardiac Rhythm
The normal cardiac rhythm is initiated in the sinoatrial (SA) node, which is a crescent-shaped collection of cells situated in the lateral terminal groove of the right atrium (RA) at its junction with the superior vena cava (SVC). SA nodal cells are specialized myocardial cells that depolarize automatically ( Fig. 27.1 ) and trigger a wave of electrical activation that conducts across the atria. The basal rate of spontaneous SA nodal depolarization, and thus heart rate, is modulated by autonomic nervous system input. Increased parasympathetic tone will slow SA node discharge rate, and increased sympathetic tone will correspondingly increase the rate. The myocardial cells in the remainder of the atrium can also spontaneously depolarize but do so at a slower rate than the SA node. Therefore spontaneous beats from other “ectopic” atrial sites may occur if their rate of depolarization increases or if there is significant slowing of the SA nodal discharge rate caused by damage or other factors.
The atrioventricular (AV) node is located within the muscular atrial septum at the AV junction near the midseptal portion of the tricuspid valve annulus. Its position is approximated, classically, as being at the superior apex of the triangle of Koch. Conduction through the AV node is slow and is modulated by the autonomic nervous system. Increases in vagal tone that slow heart rate will also result in conduction slowing through the AV node and prolongation of the PR interval on the electrocardiogram (ECG). Increases in sympathetic tone that result in higher heart rates will do the opposite and accelerate AV nodal conduction and shorten the PR interval. The delay between atrial and ventricular electrical activation, imposed by the AV node, provides a hemodynamic benefit by allowing time for atrial contraction to contribute to diastolic filling of the ventricles.
Electrical activation conducted through the AV node is routed to the ventricles via the His bundle at the membranous septum. The atria and ventricles are otherwise electrically insulated from one another by fibrofatty tissue planes. Within the ventricular septum the His bundle divides into left and right bundle branches. The right bundle branch runs in the deep subendocardium of the interventricular septum until it emerges in the moderator band. The left bundle branch fans out broadly shortly after emerging on the left ventricular endocardial surface. These large conduction fibers remain insulated from surrounding myocardium by sheaths of fibrous tissue until they have further divided into the Purkinje network of smaller fibers that conduct the wave of depolarization rapidly throughout the ventricular myocardium.
Mechanisms of Arrhythmia
The normal heart rhythm is the product of an elegant system with multiple layers of control at the molecular, cellular, and whole organ levels. However, despite this robust underpinning of rhythm control, an abnormality at any level can result in an arrhythmia. There are multiple factors that cause primary arrhythmias in children and lead to the need for ICU care, but there are also many factors that secondarily cause arrhythmias in the critically ill child. Despite the presence of a primary or secondary rhythm problem, the mechanisms most often responsible for arrhythmias are similar. These include (1) failure of impulse formation, (2) conduction block, (3) reentry, (4) enhanced automaticity, and (5) triggered activity. Reentry and block can be considered abnormalities of impulse conduction. Failure of impulse formation, enhanced automaticity, and triggered activity are abnormalities of impulse generation. One or more of these mechanisms underlie the specific cardiac arrhythmias.
Failure of impulse formation occurs when normal pacemaking tissue (i.e., the SA node) does not reach activation threshold, either at all (sinus arrest) or at a rate needed to meet physiologic requirements. This may occur during moments of extreme vagotonia, such as when a newborn receives nasopharyngeal stimulation, or may occur as part of sick sinus syndrome chronically following atrial surgery.
Conduction block refers to the failure of an electrical impulse to propagate in its normal direction and sequentially depolarize the myocardium. Conduction block may occur in the atrial or ventricular myocardium itself or in portions of the normal conduction system such as the AV node, His bundle, bundle branches, or Purkinje fibers. Block may occur due to an intrinsic abnormality that prevents depolarization of adjacent cells, such as metabolic derangements (e.g., ischemia), or anatomic barriers such as fibrosis or scar. Extreme vagal tone can cause conduction block within the AV node. Conduction block may also occur physiologically in response to premature impulses or extremely rapid activation. For example, conduction block within the AV node may occur in response to a premature atrial contraction or depolarization that occurs at a time when the AV node is in its refractory period. Rapid atrial activation, such as during atrial flutter or fibrillation, results in dynamic variability of AV nodal conduction such that impulses are conducted to the ventricle in a regular or irregular periodic pattern.
Reentry is the most common cause of tachyarrhythmias. Reentry is a mechanism for the self-propagation of a wavefront that repetitively travels the same conduction circuit. The conditions necessary to initiate and sustain reentrant arrhythmias include unidirectional block in one limb of the path (e.g., long refractory period at the site of block), sufficiently slow conduction around the other limb of the circuit such that the site of unidirectional block is recovered from refractoriness when the impulse returns, and capacity for retrograde conduction through the area of original block ( Fig. 27.2 ). The path may encircle inexcitable tissue such as surgical injury or scar from previous myocardial infarction or tissue fibrosis, or it may include normally excitable tissues with discrepant conduction properties such as occurs in AV nodal reentry or accessory pathway (AP)–mediated tachycardias. In most cases the initiation of reentry results from unidirectional conduction block created following a premature stimulus encountering refractory tissue (see earlier discussion). This same property can also lead to termination of reentry through timed placement of a prematurely paced beat, which creates refractoriness and blocks ongoing conduction in the reentrant circuit (see “ Treatment of Tachyarrhythmias With the Pacemaker ” later).
Automaticity is a normal property of several different types of cardiac cells and consists of a gradual depolarization of resting membrane potential during diastole (phase 4) (see Fig. 27.1 ). Once activation threshold is attained, the rate of depolarization increases steeply (phase 0), and a full action potential ensues. Cells capable of automaticity include those in the sinus node, specialized regions of the atria, the AV node, and the His-Purkinje system. The cells with the most rapid diastolic depolarization, usually in the SA node, determine the heart rate. Abnormal automaticity may develop in cells that are not normally automatic, such as atrial and ventricular myocytes. Conditions that increase the likelihood of enhanced abnormal automaticity include ischemia, electrolyte imbalance, catecholamine excess, and certain drug toxicities. An abnormal automatic rhythm often exhibits a gradual increase in rate, or “warm-up,” before becoming a regular tachycardia and a gradual deceleration before termination. Automatic rhythms can usually be transiently interrupted (overdrive suppressed) by overdrive pacing, but they cannot be reliably initiated or terminated by these methods.
Triggered activity develops in the setting of low-amplitude secondary depolarizations (afterdepolarizations) of the membrane potential during (phase 3) or after (phase 4) normal repolarization. If an afterdepolarization reaches the activation threshold potential, another action potential is triggered that is coupled closely to the first. If this second action potential is accompanied by another afterdepolarization, the process can be repetitive and give rise to a sustained arrhythmia. Early afterdepolarizations (EADs) are present during repolarization (phase 3) and arise as a result of enhanced calcium or sodium entry through sarcolemmal ion channels. Conditions associated with EADs are often present in postoperative patients and include acidosis, hypoxia, hypokalemia, and a variety of antiarrhythmic agents. The development of sustained EAD-triggered ventricular arrhythmias is bradycardia dependent, with an increased frequency of EADs observed at slow rates or following pauses in the cardiac rhythm. Delayed afterdepolarizations (DADs) develop after the membrane potential has fully repolarized (phase 4) and are caused by intracellular calcium overload and a subsequent oscillatory uptake and release of calcium from the sarcoplasmic reticulum. The prototypic arrhythmia associated with DADs is digitalis intoxication. DAD–triggered ventricular arrhythmias may also be associated with catecholamine excess and can be initiated by rapid pacing.
Electrocardiographic Monitoring in the Intensive Care Unit
The Electrocardiogram
Electrocardiography is the measurement of body surface, time-varying voltages generated by electrical activation of the heart. The ECG is a “map” of the body surface voltages based on a standardized set of recording positions. Proper interpretation of the ECG is rooted in a fundamental understanding of how it is generated, and assumptions are made as to how it has been recorded. Simple pattern reading, although potentially accurate in adults with normal cardiac structure and position, will lead to a flawed diagnosis in children with and without congenital heart disease (CHD) given the wide range of variables, including body size, cardiac position, cardiac structure, and nonstandard ECG lead positioning (especially in an ICU setting).
The standard 12-lead ECG is recorded using electrodes placed on each limb and across the chest in a reproducible pattern. Because interpretation of the ECG is usually performed by a provider who did not perform the measurement, adherence to standardized methods and correct electrode placement is critical. The standard limb leads, labeled I, II, and III, are measured from the surface potentials recorded between electrodes placed on the distal extremities: lead I left arm(+)/right arm(−); lead II right arm (+)/left leg(−); lead III left arm (+)/left leg (−). A right leg electrode acts as a ground to reduce noise and stabilize the recording baseline. The precordial leads are placed in standardized positions relative to the chest wall: V 1 — fourth intercostal space at the right sternal border; V 2 — fourth intercostal space at the left sternal border; V 3 — the midpoint between electrodes V 2 and V 4 ; V 4 —fifth intracostal space aligned at the midclavicular line; V 5 — horizontally in line with V 4 but at the anterior axillary line (or midway between V 4 and V 6 ); V 6 — horizontally in line with V 4 but at the midaxillary line. In the ICU setting it is common to place leads in nonstandard positions for bedside monitoring or with ambulatory monitors due to limitations on access to the thorax. The ECGs obtained from leads in nonstandard positions are helpful for rhythm interpretation. However, other diagnostic conclusions, such as the presence of hypertrophy, should not be made. Final diagnosis should always be made using the standardized electrode positions and lead sets.
Atrial electrical activation is manifest on the surface ECG as the P wave. The shape, duration, and orientation of the P wave is influenced by the site of origin of the activating stimulus, the size of the atria, and the rate of conduction of the electrical impulses through the atria. The PR interval, being the time elapsed from P wave onset to Q wave onset, reflects conduction time from the initial site of atrial activation summed with AV nodal and His-Purkinje conduction times. Depolarization of the ventricles produces the QRS complex on the surface ECG. QRS morphology is influenced by the origin of ventricular activation, the presence of conduction block or delay in the bundle branches, and the rate of electrical conduction within the ventricles. Repolarization of the ventricular myocardium is reflected by the ST segment and T wave. Normal ranges for the PR, QRS, and QT intervals by age are shown in Table 27.1 .
| Age | PR (ms, in Lead II) | QRS (ms, in Lead V 5 ) a | QT (ms, in lead V 5 ) |
|---|---|---|---|
| 0-1 d | 79-161 | 21-76 | 210-370 |
| 1-3 d | 81-139 | 22-67 | 223-346 |
| 3-7 d | 74-135 | 21-68 | 220-327 |
| 7-30 d | 72-138 | 22-79 | 220-301 |
| 1-3 mo | 72-130 | 23-75 | 222-317 |
| 3-6 mo | 73-146 | 22-79 | 221-305 |
| 6-12 mo | 73-157 | 25-76 | 218-324 |
| 1-3 y | 82-148 | 27-76 | 248-335 |
| 3-5 y | 84-161 | 31-72 | 264-354 |
| 5-8 y | 90-163 | 32-79 | 278-374 |
| 8-12 y | 87-171 | 32-85 | 281-390 |
| 12-16 y | 92-175 | 34-88 | 292-390 |
a The QRS duration was measured only in lead V 5 because the onset of the QRS is most sharply defined there. This measurement in a single lead underestimates the full QRS duration because in some cases the beginning or end of the QRS in lead V 5 may not deviate from the baseline and will not be detected.
Bedside Telemetry
Continuous bedside electrocardiographic monitoring is performed in critically ill children in the ICU. In addition to real-time ECG monitoring from one or more leads for immediate analysis at bedside, ICU telemetry systems provide continuous archiving of data for review. An important skill in the management of postoperative cardiac patients and those requiring intensive care is the review of these recordings. In general, providers should review a patient’s telemetry at least daily and with any concern for arrhythmia or change in condition. Important events and useful diagnostic information tend to be found at inflection points, peaks , and troughs of the heart rate trend graph. Thus in most modern telemetry systems, this graph should serve as the starting point for telemetry review. A straightforward approach to telemetry review is as follows:
- 1.
Review the heart rate trend graph. This graph can typically be set to display 12 or 24 hours of heart rate at a time, although shorter time intervals can usually be set for more detailed review. Attention should be focused on overall heart rate trend —has the heart rate been increasing or decreasing? Is there mainly bradycardia or tachycardia? Is there normal heart rate variability? Most patients after orthotopic heart transplant will lack normal heart rate variability due to poor innervation of the graft. Patients after the Fontan procedure or after more extensive atrial surgery, such Mustard or Senning atrial switch procedure, will often have bradycardia and/or a diminished ability to raise heart rate (chronotropic incompetence) due to sinus node dysfunction. Heart rate trend can be a good indication of patient recovery or response to therapy.
- 2.
Heart rate peaks and troughs should be identified and rhythm at these points examined in greater detail, looking at the telemetry strips at those times. Is the patient in sinus rhythm? If not, is the rhythm an expected response to change in heart rate, such as junctional escape rhythm in the setting of sinus bradycardia? Is apparent bradycardia caused by frequent ectopy that is either not conducted (blocked premature atrial complexes [PACs]) or followed by a compensatory pause? Is the telemetry indicative of a tachyarrhythmia such as supraventricular tachycardia or junctional ectopic tachycardia?
- 3.
Inflection points should be examined in detail. A finding of abrupt or rapid increase in heart rate on the graphic trend followed by a new heart rate plateau can be suggestive of tachyarrhythmia, especially when the heart rate plateaus or varies slightly around a supranormal heart rate. Dysrhythmias often terminate abruptly, but gradual offset can be seen in the setting of automatic rhythms or due to sympathetic stimulation–related sinus tachycardia at termination of tachyarrhythmia.
- 4.
If an arrhythmia is identified, the provider should especially examine the onset, termination, and any points of change or interruption during the arrhythmia. As described in the section on tachyarrhythmias in this chapter, the mode of onset and termination, the relationship of atrial to ventricular signals, and the effect of PACs, premature ventricular complexes (PVCs), or changes in atrio-ventricular/ventriculo-atrial (A-V/V-A) conduction are important in the identification of type and mechanism of arrhythmia. Widening or narrowing of the QRS complex (e.g., loss of ventricular preexcitation at the initiation of orthodromic AV reciprocating tachycardia [AVRT] in a patient with Wolff-Parkinson-White [WPW] syndrome, change in the pattern of conduction delay at the onset of ventricular tachycardia [VT] in a patient with tetralogy of Fallot, widening of QRS at the onset of VT or supraventricular tachycardia [SVT] with aberrant conduction in a patient with baseline normal QRS complex), can often be helpful in identifying type of arrhythmia. Proper identification allows targeting of therapy for termination and prevention of the arrhythmia.
- 5.
Review alarms . Although much superfluous information will be provided by automatically generated alarm strips, careful review can help focus the provider’s attention on areas of telemetry that might otherwise be overlooked.
- 6.
Distinguish true signals from artifact . Abnormal telemetry signals can be caused by lead interference, patient movement, or artifacts such as those caused by respiratory care/chest physiotherapy. It is important to distinguish true abnormal rhythms from artifact. Correlation of heart rhythm with other monitoring signals (which can usually be displayed simultaneously with heart rhythm on telemetry), such as arterial line tracing or pulse oximetry tracing, can be helpful. Also, even in the setting of significant artifact, an R or S wave can often be found that marches out with the QRS complexes preceding and following the period of possible artifact, with some slight leeway given for normal heart rate variation. Examination of the relationship of the abnormal signals in question to P waves or QRS complexes (does the signal appear to be driving the QRS complexes or is there no clear relationship between the two signals) can also help in distinguishing artifact from dysrhythmia.
Atrial and Ventricular Electrograms
In the postoperative patient, bipolar recordings from each pair of epicardial wires may be attached to standard ECG recording devices or to ICU monitors as follows: attach one electrode connecting wire (or use alligator clamps if necessary) to the monitor’s cable corresponding to “right arm” and the other electrode to the monitor’s cable corresponding to “left arm.” Be certain that the monitor’s leg cables are appropriately attached to the patient ( Fig. 27.3A ). Lead I will represent a bipolar electrogram from the chamber in continuity with the epicardial pair of wires. If the chamber being viewed is atrial, there may be minimal ventricular signal, making a simultaneous surface lead desirable; the surface QRS can serve as a ventricular reference. If the lead I, II, III montage is available, leads II and III will represent electrical fusion between the atrial electrogram and QRS, which will serve the same purpose. Alternatively, either or each of the epicardial wires can be connected to a precordial “V” lead input to obtain unipolar recordings of atrial or ventricular activation. Useful activation timing information will be obtained from either of these methods. In addition, there are other methods of obtaining discrete cardiac electrograms using temporary pacing systems ( Fig. 27.3B ). Examples of bipolar and unipolar atrial electrograms with simultaneous ventricular references appear in Fig. 27.4 . These methods are invaluable for discriminating postoperative junctional ectopic tachycardia (JET) from sinus tachycardia or SVT, sinus bradycardia from nonconducted atrial bigeminy, and atrial flutter with 2 : 1 AV conduction from sinus tachycardia, to name a few. Simultaneous use of pharmacologic agents, such as adenosine, makes this technique even more powerful.
Common Intensive Care Unit Rhythm Issues and Their Management
Using the mechanistic approach described earlier, the most common cardiac arrhythmias encountered in the ICU are listed in Table 27.2 . This categorization allows for a more rational approach to the selection of pharmacologic or interventional therapies. This section addresses these heart rhythm issues, focusing on the clinical setting in which the rhythm issue is encountered, diagnosis of the arrhythmia, and treatment options. Before discussing individual clinical entities, a brief review of available medications and other techniques for the management of cardiac rhythm issues is touched upon.
| Bradycardias | Failure of impulse formation |
| Sinus bradycardia | |
| Sinus node arrest | |
| Conduction block | |
| Sinus node exit block | |
| AV node block | |
| Bundle branch block | |
| Tachycardias | Reentry |
| Atrial flutter (macroreentry) | |
| Atrial fibrillation | |
| Sinus node reentry tachycardia | |
| AV node reentry tachycardia | |
| AV reciprocating tachycardia (including permanent form of junctional reciprocating tachycardia) | |
| Some ventricular tachycardia (including torsades de pointes) | |
| Enhanced automaticity | |
| Some atrial ectopic tachycardias | |
| Junctional ectopic tachycardia | |
| Accelerated junctional rhythm | |
| Accelerated idioventricular rhythm | |
| Some ventricular tachycardias | |
| Triggered activity | |
| Some atrial ectopic tachycardias | |
| Digitalis toxicity | |
| Initiating beat of torsades de pointes | |
| Some ventricular tachycardias | |
| Other mechanisms causing premature beats | |
| Supernormal conduction | |
| Parasystole |
Pharmacology
Pharmacologic Treatment of Tachyarrhythmias
The Vaughan-Williams classification is a widely used antiarrhythmic medication schema based on common actions of different agents ( Table 27.3 ). These medications predominantly affect ion currents responsible for various phases of the cardiac action potential (see Fig. 27.1 ). Some have their effect on the sympathetic and parasympathetic balance contributing to the persistence of cardiac arrhythmias. Several medications are effective through multiple mechanisms. The same mechanisms responsible for the antiarrhythmic effects may also be responsible for the side effects seen with antiarrhythmic medications.
| Class | Subclass | Drug | Pharmacologic Effect |
|---|---|---|---|
| I | Moricizine | Depression of rate of increase of action potential | |
| IA | Quinidine | Increased action potential duration, and increased atrial and ventricular ERP; increased JT interval; vagolytic action | |
| Procainamide | |||
| Disopyramide | |||
| IB | Lidocaine, mexiletine, tocainide | Decreased action potential duration but increased ventricular ERP; unchanged QRS complex; unchanged JT interval | |
| IC | Propafenone flecainide (encainide) | Depressed rate of increase of action potential, causing widening of QRS complex; unchanged action potential duration, but increased atrial and ventricular ERP; unchanged JT interval | |
| II | Beta-blockers | Inhibition of beta-adrenergic receptors | |
| III | Amiodarone | Increased action potential duration | |
| Sotalol | Increased JT interval | ||
| Bretylium | |||
| IV | Verapamil, diltiazem | Blockade of Ca ++ channels |
Class I drugs inhibit inward depolarizing sodium channels, resulting in a slowed upstroke of the cardiac action potential and secondary alteration of action potential duration and refractoriness of excitable tissues. Class II agents block beta-adrenergic receptors altering the sympathetic influence on electrophysiologic properties of cardiac cells. Class III medications predominantly block potassium channels, resulting in prolonged phase 3 repolarization of the cardiac action potential. Class IV antiarrhythmics block calcium channels, which are the depolarizing channels in the sinoatrial and AV nodes.
Class IA Antiarrhythmic Medications (Quinidine, Procainamide, and Disopyramide).
Procainamide is the Class IA agent most often used in the ICU setting in the United States due to its availability in parenteral form. The primary action of procainamide is to block the inward sodium channel, with lesser repolarizing potassium channel blockade and vagolytic effects. It can be expected to prolong the cardiac action potential and refractory period of atrial, ventricular, and His-Purkinje cells. Conduction velocity is slowed, and automaticity is decreased. The major effect of these agents on the ECG is prolongation of the corrected QT interval (QTc). The PR interval will prolong in the presence of preexisting His-Purkinje system disease. Likewise, procainamide slows the heart rate in the presence of sinus node dysfunction. Class IA drugs are most useful in treating all reentrant supraventricular tachycardias and some VTs.
The major noncardiac side effects limiting use of procainamide in the acute setting are nausea, vomiting, and central nervous system symptoms. These agents all have negative inotropic properties, especially disopyramide, and due to slight alpha-adrenergic blocking effects, they may also cause hypotension. Due to its vagolytic property, procainamide used in the setting of atrial fibrillation or rapid atrial tachycardia will accelerate AV nodal conduction, resulting in a rapid ventricular response. In those instances a beta-blocker or calcium channel blocking drug is also necessary. Prolongation of the QTc may predispose patients with an unstable ventricular myocardium to develop torsades de pointes (TdP).
Class IB Antiarrhythmic Medications (Mexiletine, Lidocaine).
Lidocaine is a Class IB agent that is available in intravenous form in the United States and had classically been part of Advanced Cardiac Life Support treatment algorithms for VT and ventricular fibrillation (VF). Its primary effects are to slow ventricular conduction and to raise the threshold for VF. It has little effect on atrial and nodal tissue. Lidocaine particularly depresses conductivity in injured or hypoxic myocardium, accounting for its efficacy immediately following myocardial infarction. It has mild negative inotropic effects. Heart block and decreased myocardial function are potential complications of lidocaine therapy in the postoperative period, although it is generally well tolerated. Central nervous system toxicity in the form of seizures, disorientation, drowsiness, agitation, and paresthesias can be expected at supratherapeutic blood levels, especially at levels in excess of 10 to 15 mcg/mL.
Mexiletine is an orally active agent effective at suppressing PVCs and ventricular arrhythmias, especially in patients who have undergone ventriculotomy for repair of congenital heart defects and are having abnormal hemodynamics (e.g., tetralogy of Fallot). Mexiletine is increasingly used in patients with long QT syndrome type 3 (LQTS3). It does not depress ventricular function.
Class IC Antiarrhythmic Medications (Flecainide, Propafenone).
Flecainide and propafenone are Class IC agents, which decrease the phase 0 slope and slow conduction velocities throughout the myocardium. They are available only in oral form in the United States. They are efficacious in treating sustained and resistant ventricular tachyarrhythmias and refractory reentrant SVT. Propafenone also has calcium channel and beta-blocking activity. It has been reported to be useful in treating JET. These drugs can increase the PR interval and QRS duration. They have been associated with malignant proarrhythmia, so caution is required when using them, especially in children with impaired left ventricular function or previous cardiac surgery. Proarrhythmia may take the form of worsening of the index VT or SVT or induction of an incessant, difficult-to-terminate VT. Proarrhythmia is usually related to higher doses, and careful monitoring of the rhythm and QRS duration is warranted. Patients initiated on flecainide are typically monitored for at least 72 hours in hospital.
Class II Antiarrhythmic Medications (Beta-Blockers).
Class II agents are direct antagonists of beta-adrenergic receptors. They decrease the slope of phase 4 of pacemaking tissues, thus reducing automaticity and lowering heart rates. Beta-blockers increase the refractory period of the AV node and are used as chronic therapy to prevent the SVTs that use the AV node as a part of the tachycardia circuit. They are also used chronically to limit the ventricular rate response to atrial flutter and atrial fibrillation, and intravenous preparations are used for immediate reduction in ventricular rate. They may also be useful in suppressing PVCs and VT in persons having no other heart disease.
Propranolol is the prototypic drug in this class, having both beta 1 and beta 2 receptor–blocking effects. Atenolol and nadolol are also in common use, with nadolol the most efficacious beta-blocker in treatment of long QT syndrome (LQTS). Esmolol, an intravenous agent that is ultra–short acting, has a plasma half-life of 8 minutes and is an excellent alternative when initiating beta-blockade therapy in postoperative or cardiomyopathic patients. It can be delivered in small IV boluses or as a rapidly accelerating IV infusion. When using these drugs in the acute care setting, side effects include depression of ventricular function, vasodilation and hypotension, and bradycardia. Extreme caution should also be used in patients with SA node dysfunction or advanced AV nodal disease. In patients with asthma, beta-blockers may precipitate small airway bronchospasm, and beta 1 -specific blocking agents such as metoprolol are preferred.
Class III Antiarrhythmic Medications (Amiodarone, Sotalol).
The Class III antiarrhythmic agents act principally by potassium channel blockade. Thus they prolong repolarization and refractoriness of most cardiac tissues. Amiodarone and sotalol have additional beta-adrenergic blocking effects, and amiodarone has lesser sodium and calcium channel blocking effects. The ECG effects of these drugs are to increase the PR and QT intervals without affecting the QRS duration. Oral sotalol and amiodarone are useful agents for chronic prevention of otherwise medically resistant VT and most SVT. They have unique efficacy in treating automatic tachycardias, such as atrial ectopic tachycardia (AET).
Intravenous amiodarone load is administered as 1 mg/kg over 12 minutes or 5 mg/kg over 60 minutes and may be repeated to achieve the desired effect to a maximum of 20 mg/kg/day. An infusion is needed to maintain the effect. Although there are no direct negative inotropic effects of amiodarone, alpha-adrenergic blocking properties and resultant hypotension may occur. Intravenous volume expansion or calcium chloride is used to treat secondary hypotension. Significant prolongation of the QTc may occur, resulting in the development of TdP, and patients should be evaluated with serial ECGs. Additionally, amiodarone decreases conductivity of nodal tissue and may produce significant sinus bradycardia, sinus arrest, or variable AV block. Long-term systemic side effects of amiodarone include phototoxicity, corneal deposits, altered thyroid function, and depressed liver function. Pulmonary interstitial fibrosis rarely occurs with prolonged chronic use, but shock lung may rarely occur in the acute setting. Treatment for these side effects consists of discontinuing the antiarrhythmic medication and supportive therapy. Unfortunately, amiodarone is very lipophilic and has an extremely long elimination half-life, making treatment of chronic side effects a problem.
Sotalol has combined beta-adrenergic and potassium channel blockade, making it a useful agent in the therapy of supraventricular and ventricular arrhythmias. Sotalol is an oral drug (available in IV form in some centers) with similar electrophysiologic effects as amiodarone without the systemic toxicities. Sotalol causes QT prolongation in a dose-related manner, increasing the risk of TdP to a greater extent than amiodarone. In addition, sinus bradycardia, sinus arrest, or AV block may occur. The beta-blocking effects are responsible for other side effects such as decreased ventricular function, fatigue, dizziness, and syncope.
Class IV Antiarrhythmic Medications (Calcium Channel Blockers).
The Class IV drugs are calcium channel inhibitors. Verapamil is the most frequently used drug of this class in children, although it has more negative inotropic properties than other subclasses of calcium channel blockers, such as nifedipine and diltiazem. These agents slow conduction in calcium channel–rich tissues such as the SA and AV nodes.
Before the availability of adenosine, intravenous verapamil was widely used to terminate AV nodal reentrant tachycardia (AVNRT) and AVRT. However, due to reports of hypotension, bradycardia, and cardiac arrest in infants, the use of IV verapamil is to be avoided in infants younger than 1 year of age. Although all calcium channel blockers cause some degree of negative inotropy and peripheral vasodilation, diltiazem appears to be safer than verapamil and still provides an excellent negative dromotropic effect. Both agents are excellent choices to slow the ventricular response in the presence of atrial fibrillation or flutter. The exception is in patients with WPW syndrome—the ventricular response during atrial fibrillation may actually be enhanced via the AP, due to relative block in the AV node, and therefore it is not indicated in this setting. As discussed later, verapamil is the drug of choice in fascicular VT. Complications of calcium antagonists in the ICU setting are peripheral vasodilation, decreased myocardial contractility, sinus bradycardia, and AV block, any of which may lead to hypotension and shock. The half-life of IV verapamil is extremely short (minutes).
Other Antiarrhythmic Medications
Adenosine.
Adenosine is an endogenous nucleoside that in high doses produces sinus bradycardia and transient conduction block in the AV node. This effect is mediated through specific membrane A 1 adenosine receptors coupled to adenylate cyclase and specific sarcolemmal potassium channels. The transient effects are due to its rapid uptake and deamination by red blood cells, which results in a very short half-life (less than 10 seconds). The effects of adenosine are dose dependent and time dependent. AV block usually develops within 10 to 30 seconds after an IV bolus injection. In general, AP conduction is not influenced by adenosine.
Clinically, IV adenosine will abruptly terminate approximately 90% of cases of reentrant tachycardias that involve the AV node, including AVRT and AVNRT. Adenosine may also be useful for determining the mechanism of unknown arrhythmias. Transient AV block by adenosine may reveal atrial flutter or other atrial tachycardias by blocking the ventricular response, without affecting the primary arrhythmia mechanism. The failure of adenosine to terminate a wide-complex tachycardia suggests that the arrhythmia is VT or a preexcited atrial tachycardia rather than an aberrantly conducted SVT. Some forms of right ventricular outflow tract VT in the otherwise normal heart are also terminated by adenosine.
Adenosine is initially administered as a 100 mcg/kg (to a maximum of 6 mg) rapid bolus into a large peripheral vein. Failing the initial dose, increasing doses up to 300 mcg/kg (to a maximum of 12 mg) may be administered every 1 to 2 minutes as needed. Following tachycardia conversion, the initial escape rhythm may include PVCs, marked sinus bradycardia, AV block, and, rarely, atrial fibrillation. This drug should not be administered unless an external defibrillator is available. Systemic side effects are common but usually mild and short-lived; they include dyspnea, flushing, chest discomfort, bronchospasm, coughing, headache, and hypotension. In children with impaired contractility and uncertain volume status, cautious monitoring of blood pressure is necessary. Adenosine should be used with caution in patients with asthma. Adenosine is avoided when possible in patients post orthotopic heart transplant because its effect can be prolonged.
Digoxin.
The use of digitalis glycosides is time honored. Its primary action is inhibition of membrane-bound Na + -K + ATPase, with resultant intracellular calcium loading. Its primary cardiac electrophysiologic effect is AV conduction delay, related to its effect on calcium traffic, and by enhancing vagal influences. The glycosidic portion of the digoxin molecule enhances carotid sinus baroreceptor reactivity, which leads to increased vagal tone and decreased sympathetic tone. In addition, it appears to have a central parasympathetic influence. In ventricular muscle, digoxin shortens the action potential and decreases the VF threshold, thus explaining the tendency for digoxin to induce ventricular tachyarrhythmias. Digoxin slows the normal sinus rate, increases the PR interval, and causes visible alteration (coving) of the ST segments, even in the absence of toxicity. QRS interval duration is unaffected, even at toxic doses. The QT interval may be shortened as a result of hastened ventricular repolarization.
Similar to calcium channel blockers, the major applications of digoxin are for treatment and prevention of reentrant supraventricular tachyarrhythmias that involve the sinoatrial or AV node, and ventricular rate control in the presence of atrial flutter or fibrillation. Unlike verapamil, however, its AV node–blocking effects are countered by physiologic periods of vagolysis or enhanced adrenergic states, as may occur during exercise. Systemic loading, even when administered parenterally, requires at least 12 to 16 hours; thus its use in the ICU setting may be limited. Digoxin also may shorten AP refractoriness in a patient with WPW, and thus digoxin is considered contraindicated in this setting.
Systemic signs of toxicity include visual disturbances, disorientation, anxiety, drowsiness, abdominal pain, hyperkalemia, nausea, and vomiting. Cardiac signs of toxicity are exclusively proarrhythmias: advanced SA and AV block in younger patients and a variety of ventricular or atrial tachyarrhythmias. An SVT with AV conduction blockade is the classic sign of digoxin toxicity, and failure to recognize this and administration of additional doses of digoxin may be catastrophic. Neonates may have artificially elevated levels due to maternal digoxin-like substances. Furthermore, the pharmacokinetics of digoxin are altered by multiple agents, including phenytoin, lidocaine, quinidine, and amiodarone, and the dose of digoxin should be decreased in patients concomitantly receiving verapamil or amiodarone.
Recognizing digoxin toxicity is important because the emergent use of digoxin-specific Fab fragments (Digibind) may be lifesaving. Indications for the use of Digibind include hyperkalemia, the new occurrence or worsening of bradycardia, or the occurrence of a tachyarrhythmia in a patient in whom digoxin ingestion is known or strongly suspected. Serum concentrations may be misleading and should not be considered steady state unless the ingestion occurred at least 6 hours previously. Especially if there is a delay in Digibind administration, arrhythmias should be aggressively treated. AV block should be treated with atropine or a temporary pacing catheter, and VT with intravenous phenytoin or lidocaine. Hyperkalemia should be treated by standard means, potentially excluding intravenous calcium. The exclusion of calcium in this circumstance was historically due to concern for the development of “stone heart,” which is an irreversible noncontractile state due to impaired diastolic relaxation from calcium–troponin C binding. This concern has not been consistently reproduced in the literature. Further concern for calcium repletion in this circumstance may be due to development of calcium excess with repletion, which may predispose to dysrhythmia by causing delayed afterdepolarizations. The dose of Digibind is calculated based on total body load of digoxin. If the amount of digoxin received is known, then 1 vial (40 mg) per 0.5 mg digoxin is given intravenously over 15 to 30 minutes. If a steady-state serum concentration is known, then dosing is based on total body load as calculated from serum concentration. If digoxin toxicity is highly suspected and dose or level of digoxin is unknown, then 10 vials (400 mg) should be administered. A bolus injection may be given should cardiac arrest be imminent. Cardioversion may be necessary should the patient be unstable, although there is additional risk because digoxin lowers the VF threshold. Prophylactic administration of lidocaine is advisable in those cases.
Magnesium.
Intravenous magnesium may be helpful to treat TdP and ventricular arrhythmias associated with LQTS. Side effects of magnesium include hypotension and hypotonia, which may lead to respiratory complications. This drug should be considered for patients with ventricular arrhythmias and prolonged QT syndrome.
Pharmacologic Agents for Treatment of Bradyarrhythmias
A wide array of pharmacologic and device treatment options exist for bradycardia encountered in the intensive care setting (see also “ Pacing in the Cardiac ICU ” later). It must be remembered that in infants and all patients with diminished systolic function, chronotropic support is particularly important due to relatively reduced capacity for inotropic recruitment during periods of hemodynamic stress.
Pharmacologic augmentation of heart rate is accomplished though the use of either vagolytic or beta-adrenergic agents. Atropine is a vagolytic agent that substantially increases sinoatrial rate and improves AV conduction in most patients. However, impaired AV conduction due to surgical trauma or edema of the AV node may not be as responsive to atropine. Also, atropine is not expected to be effective treatment for impaired AV conduction caused by pathology below the level of the AV node (i.e., the bundle of His and bundle branches). Immediately following cardiac transplant, bradycardia may not be responsive to atropine, because the heart has been at least temporarily denervated.
Of the adrenergic agents, isoproterenol is the most widely used for pure heart rate augmentation, although epinephrine and norepinephrine also cause varying degrees of beta-receptor stimulation. Isoproterenol is a nonselective, pure beta-adrenergic agonist. As such, it increases both the chronotropic and the inotropic state of the heart. It also lowers systemic vascular resistance and diastolic blood pressure and, in certain settings of pulmonary arteriolar hypertension, may reduce pulmonary vascular resistance. Its major drawbacks include increase of myocardial and total body oxygen consumption, hyperglycemia, tachyarrhythmias, and increase of metabolic requirements by the injured myocardium.
Treatment of Tachyarrhythmias Using Vagal Maneuvers
An abrupt increase in parasympathetic tone generated by a variety of vagal maneuvers can result in transient AV block that will terminate AVRT and AVNRT and rare other tachycardias. The maneuvers in Table 27.4 are most effective when used soon after the onset of symptoms, before sympathetic tone rises to a high level. Failing these maneuvers, pharmacologic intervention often becomes necessary.
| Enhancement of vagal tone to the AV node | Exposure of upper half of face to ice water or ice in washcloth |
| Finger in throat (gag maneuver) | |
| Right carotid sinus massage | |
| Sudden volume/pressure changes to the right heart | Valsalva maneuver (has vagal component) |
| Bearing down | |
| Squatting | |
| Gentle pressure to abdomen in an infant until he or she resists | |
| Turning patient upside down |
Cardioversion and Defibrillation
For unstable patients with hypotension or altered mental status due to atrial or ventricular tachyarrhythmias, prompt cardioversion or defibrillation is indicated. Electrical energy is applied between two paddles or adhesive electrode patches placed on the chest or on the chest and back. The success of this procedure depends on its ability to fully depolarize the heart, thereby terminating most reentrant tachyarrhythmias and allowing sinus rhythm to be restored. Automatic tachyarrhythmias persist despite cardioversion and may actually accelerate due to the release of endogenous catecholamines.
Cardioversion refers to a shock, usually in the range of 0.25 to 4 J/kg, that is delivered synchronously with the QRS complex of the surface ECG. Energy delivery synchronous with the QRS complex reduces the risk of conversion of the tachycardia to VF. The lower energy range is generally used for atrial arrhythmias, and the upper energy range for ventricular arrhythmias. Defibrillation is a high-energy shock, usually 2 to 4 J/kg, delivered asynchronously for the treatment of VF.
Electrophysiology Study
The electrophysiology (EP) study is the most provocative means of tachyarrhythmia induction and analysis. Typically the study involves the placement of either a single esophageal catheter or several multipolar catheters into the right heart from the femoral and/or internal jugular and subclavian veins. In the intracardiac EP study, catheters are placed with electrodes in the high right atrium, right ventricular (RV) apex, coronary sinus (to record left atrial and left ventricular electrograms), and adjacent to the His bundle. They are used to both record signals and pace the heart to evaluate the conduction system and determine the mechanism of any provoked arrhythmias. An additional steerable catheter can be added to map the arrhythmia substrate and potentially burn (radiofrequency ablation) or freeze (cryoablation) the mapped target.
Ectopy
Premature Atrial Complexes
PACs are common in the intensive care setting, especially in acutely ill patients or following cardiac surgery. A PAC typically originates because of abnormal automaticity or triggered activity outside the sinus node. A PAC can conduct normally, demonstrating the same QRS morphology as in sinus rhythm, though P wave and PR interval may be different than for a normal sinus beat. If the PAC occurs at a short interval after a normal beat (short coupling interval ), a portion of the conduction system may be in its refractory period from the prior activating beat. In this situation the PAC may demonstrate aberrant conduction, resulting in a wide QRS complex. It is important to carefully assess for the premature P wave in the ST segment or T wave of the preceding beat to help differentiate the wide QRS complex as aberrant, rather than being generated by the ventricular myocardium itself (PVC, see later). If the coupling interval is short enough, the AV node or more distal conduction system may be completely refractory when the premature impulse reaches it, and the PAC will be blocked (there will be an abnormal P wave, often within the preceding ST segment, but no QRS following it). A blocked PAC can still reset atrial timing, so a compensatory pause will be noted after the PAC occurs. Frequent blocked PACs can result in ventricular bradycardia, especially if occurring in a pattern of bigeminy, in which every other atrial impulse does not conduct to the ventricle. In this situation, if the P wave of the PAC is not discovered, the ECG may be misinterpreted as marked sinus bradycardia. Therefore careful inspection of the ECG in all bradycardia is warranted to make the proper diagnosis and initiate appropriate treatment as necessary.
PACs are generally benign and well tolerated but can reflect an underlying metabolic derangement. If PACs are frequent, attention should be paid to electrolyte imbalance, choice of medications, hypoxia, and body temperature. Mechanical stretching of the atrial wall, such as from the tip of a central venous catheter, can induce local injury and automaticity resulting in PACs or runs of atrial tachycardia. In these cases a chest radiograph may be diagnostic, and withdrawing the line slightly can be therapeutic.
Premature Ventricular Complexes
PVCs are also common in the intensive care setting. Like PACs, they are generally caused by abnormal automaticity or triggered activity. PVCs can occur in the setting of metabolic derangement, hypoxia, strain, as a result of medications, as a response to catecholamines, or by direct stimulation due to a catheter or a device touching the myocardium. Following aborted cardiac arrest, PVCs may be a harbinger of underlying pathology, such as catecholaminergic polymorphic ventricular tachycardia (CPVT) (increasing PVCs with increasing heart rate), arrhythmogenic right ventricular cardiomyopathy (ARVC) (multiform or polymorphic PVCs with left bundle branch block [LBBB] morphology), or myocarditis (see “ Clinical Entities Frequently Associated With Arrhythmias ” later). Electrolyte levels should be evaluated, in particular potassium, ionized calcium, and magnesium. In the period following cardiac surgery if no other reversible cause is found for frequent PVCs, an echocardiogram should be performed to assess cardiac function and the integrity of the surgical repair and for the presence of a pericardial effusion.
PVCs can occur as singles, couplets (two PVCs strung together), or triplets and runs. PVCs can be monomorphic (single morphology) or polymorphic (also known as multiform), suggesting multiple abnormal foci or multiple exit points from a reentrant circuit. Up to 2% of normal children have PVCs on routine ECG.
Bradyarrhythmias
Sinus Bradycardia
Normal heart rate range in children is age dependent. In the ICU, sinus bradycardia is commonly observed and has multiple potential causes, including increased vagal tone, medication side effects, and sinus node dysfunction due to surgical injury or scar. Sinus node dysfunction can occur acutely or develop later after complex atrial surgical procedures, including atrial switch (Mustard or Senning operation) and Fontan palliation. Medications are a common cause for sinus bradycardia, especially antiarrhythmics such as beta-blockers, amiodarone, and sedatives such as dexmedetomidine. In sinus bradycardia, P-wave morphology should be normal (upright in ECG leads I, II, and aVF) or reflect a rhythm from low in the sinus node (upright in I, negative in II and aVF). Note that the recording axis of limb lead III tends to be nearly perpendicular to the direction of atrial activation for sinus rhythm, and small deviations in the location of sinus node output could result in a negative P wave in this lead during normal sinus rhythm or sinus bradycardia. If there is significant sinus bradycardia, a junctional escape rhythm may be observed. In general, sinus bradycardia does not require treatment; however, symptomatic sinus node dysfunction may require pacing. Further, following heart transplant, often the new heart cannot mount a sufficient heart rate response for the chronotropic needs of the early posttransplant period. Isoproterenol infusion or temporary atrial pacing may be required to give adequate chronotropy for effective cardiac output.
Sinus Arrest
Sinus arrest results from failure of impulse generation in the sinus node. It is manifested as a pause in the rhythm for a duration that is not a multiple of the sinus cycle length. If sinus arrest is prolonged, another automatic focus in the atria, AV node, or ventricles may become active and generate an escape rhythm that continues until sinus node function recovers. Pauses greater than 3 seconds warrant careful assessment and in some cases manifest as an indication for permanent pacemaker implantation.
Junctional Escape Rhythm
When the sinus rate is significantly slow or abruptly decreases, an ectopic rhythm from a site distal in the conduction system may take over. Junctional escape beats or a junctional escape rhythm should emerge, with the same QRS morphology and duration as in sinus rhythm but without a preceding P wave. If there is intact retrograde conduction during this rhythm, a retrograde P wave may be observed within or immediately following the QRS complex. If there is no retrograde conduction, the rhythm may appear somewhat irregular because sinus impulses occasionally conduct as a sinus capture beat. If a sinus capture beat is closely coupled to the preceding junctional complex, the sinus beat can conduct with aberrant conduction (much like a closely coupled PAC) and generate a wide QRS complex. A sustained junctional rhythm should never be misconstrued as JET given that JET may require aggressive treatment as a clinically significant arrhythmia.
Atrioventricular Block
AV block describes various degrees of slowing or failure of conduction within the AV node or His bundle. First-degree AV block manifests as a prolonged PR interval for age and is generally benign and well tolerated. It can occur in rare disease states such as Lyme disease or acute rheumatic fever but is also seen as a normal variant in conditioned athletes. First-degree AV block can also be seen with high vagal tone and generally resolves with exercise or increased sympathetic tone.
Second-degree AV block describes intermittent failure of an impulse to conduct to the ventricles. Type I second-degree AV block (Wenckebach) is common and manifests as gradual prolongation of the PR interval before a dropped QRS complex. Careful examination for sinus slowing should be performed, in which case the nonconducted beat may be secondary to a vagal response rather than progressive conduction slowing across the AV node. Sometimes prolongation of the PR interval is clearly seen only in the first to second beat of the Wenckebach sequence. The Wenckebach phenomenon is commonly seen during sleep on Holter monitors in healthy children and is generally benign and not considered evidence of conduction system disease.
Type II second-degree AV block is considered high grade, tends to occur more distal in the AV node or within the His bundle, and is considered evidence of potentially significant conduction system disease ( Fig. 27.5 ). A stable PR interval is present before the dropped QRS in the absence of sinus slowing. Type II second-degree AV block may be a sign of progression to complete heart block (CHB) and may be an indication for permanent pacing. Patients who have recurrent or persistent type II AV block and a low ventricular escape rate for longer than 7 to 10 days postoperatively may require permanent pacemaker implantation. Note that when 2 : 1 AV block is observed, the cause may be either type I or type II. Therefore the clinical circumstances are included in determining the correct diagnosis because type I 2 : 1 AV block may be benign, whereas type II may warrant further treatment.
