Appropriate versus normal heart rate

Since normal heart rate ranges vary tremendously during childhood as a function of age and autonomic tone, “appropriate” heart rate is a more useful concept than “normal” heart rate. Thus, any inappropriately fast or slow rate for a given clinical circumstance warrants evaluation for factors affecting the sinus rate, such as pain, agitation, respiratory insufficiency, oversedation, anemia, or acidosis, as well as the potential for other arrhythmias resembling sinus.

Sinus bradycardia and sinus pauses

Causes of sinus bradycardia include high vagal tone, hypothermia, acidosis, increased intracranial pressure, drug toxicities, or direct surgical trauma to the sinoatrial (SA) node. Primary sinus node dysfunction in childhood is rare but has been described. Transiently profound sinus bradycardia or prolonged sinus pauses of several seconds’ duration may be caused by intense vagal episodes, such as those occurring during neurocardiogenic syncope, apnea, or endotracheal suctioning. When clearly correlated with a vagal stimulus, pacing can usually be deferred. However, the hemodynamically tenuous patient with persistent or recurring bradycardias may warrant vagolytic, sympathomimetic, or pacing therapies.

Atrial premature beats that fail to conduct over the AV node can be mistaken as sinus pauses (when isolated) or sinus bradycardia (when in a bigeminal pattern). The blocked P wave may be obscured when buried in the preceding T wave; searching for changes in T-wave morphology or for intermittently conducted P waves may reveal the diagnosis. In cardiac patients, sinus node dysfunction may be the result of surgical injury or heterotaxy syndromes. Sinus bradycardia can also be an initial manifestation of long QT syndrome in infancy and of channelopathies.

Atrioventricular block

AV block is characterized as first-degree, second-degree, and third-degree, according to whether there is conduction delay, intermittent block, or complete loss of AV conduction, respectively. As with sinus bradycardia, AV conduction delay may be the result of intense vagal tone, metabolic derangements, drug toxicity, or direct injury to the AV node. When transient AV block is the result of vagal tone, there is usually concurrent slowing of the sinus rate.

Second-degree block can be further characterized as Mobitz type I or Mobitz type II. In Mobitz I (Wenckebach conduction), there is progressive prolongation of the PR before block. It may be best recognized by comparing the last PR interval before block with the next conducted PR. Mobitz type I usually represents block in the AV node and is less likely to progress suddenly to high-grade block. The periodicity is the result of progressive beat-to-beat delay within the AV node until conduction fails, allowing the AV node to recover on the ensuing cardiac cycle. In some settings, such as in well-conditioned athletes, Mobitz type I block may be benign.

Mobitz type II AV block is characterized by abrupt failure to conduct without prior lengthening of the PR interval. It usually is due to block below the AV (within the His bundle) and may portend a greater potential for sudden progression to complete AV block. The PR and QRS duration are usually prolonged in type II block. Type II block may be more ominous in that there may be abrupt loss of conduction of multiple consecutive atrial impulses, resulting in ventricular asystole. As such, Mobitz type II and may require more aggressive and preemptive intervention with pacing.

Higher grades of second-degree AV block are best characterized by the ratio of atrial to ventricular depolarizations (2:1, 3:1, 4:1, and so on), as the level of block cannot be inferred. High-grade AV block during sinus rhythm is usually pathologic, whereas it can represent a normal physiologic response within the AV node in the setting of a rapid atrial tachyarrhythmia, such as atrial flutter. Importantly, vagally mediated AV block may result in transient high-grade block, usually with concurrent slowing of the sinus rate. This should not be misconstrued as Mobitz type II AV block, as pacing would not usually be warranted.

Complete or third-degree AV block represents complete loss of AV conduction, usually with a junctional or idioventricular escape rhythm. In complete AV block, the escape rhythm is usually very regular, whereas AV block with variation in the RR interval usually indicates intermittent conduction during second-degree block ( Fig. 33.1 ) or a junctional rhythm with slower atrial rate and intermittent AV conduction (sinus capture complexes).

Complete atrioventricular (AV) block, presumably congenital, in an asymptomatic 9-year-old with slow resting heart rate. Note the regular RR interval, which confirms complete rather than incomplete (second-degree) AV block.

Bundle branch block patterns occur when impaired conduction in the specialized intraventricular conduction system results in delayed right or left ventricular depolarization, resulting in an aberrant widened QRS complex. Bundle branch block and AV block sometimes represent normal physiologic responses to abrupt shortening of the cycle length (as with premature atrial systoles or supraventricular tachycardia initiation) or may result from drug effects, surgical injury, or primary disease within the specialized conduction tissue.

Tachycardia mechanisms

While sinus tachycardia is the most common tachycardia among ill patients, pathologic tachycardias can occur due to three basic mechanisms: reentry, automaticity, or triggered. Reentry accounts for most forms of supraventricular tachycardia (SVT), including atrial flutter and most sustained ventricular tachycardias (VTs). Reentrant tachycardias display an abrupt onset and termination, usually maintaining a relatively fixed rate. In contrast, automatic tachycardias arise from ectopic foci within the atrium, AV node, or ventricles and tend to display more gradual changes in rate with warm up and cool down at onset and offset. Triggered tachycardias result from abnormal secondary depolarizations (afterdepolarizations). They tend to occur as repetitive bursts or salvos of tachycardia and may be recognized by their dependence on underlying heart rate for initiation. Triggered activity is especially important in several specific situations, such as digoxin toxicity (delayed afterdepolarizations), congenital and drug-induced long QT syndromes (early afterdepolarizations), and other hereditary arrhythmia syndromes, such as catecholaminergic polymorphic ventricular tachycardia (CPVT). Many early postoperative atrial arrhythmias and chaotic (multifocal) atrial tachycardias display behavior suggestive of a triggered mechanism. The dependence of triggered activity on underlying heart rate can sometimes be exploited therapeutically because either raising or lowering the underlying rate may suppress the tachycardia. Though it can be difficult to discern among the various tachycardia mechanisms, it is important to recognize that automatic and triggered arrhythmias will not respond favorably to electrical cardioversion.

Supraventricular tachycardias

Supraventricular tachycardia is often used to describe the typical electrocardiographic phenotype of a regular, narrow QRS tachycardia without discernible P waves that starts and stops abruptly (also described as paroxysmal atrial tachycardia [PAT] or paroxysmal supraventricular tachycardia [PSVT]). However, SVTs include a more diverse array of tachycardia mechanisms, including atrial flutter and fibrillation and junctional tachycardias, many of which may, at times, display a typical SVT phenotype. As such, depending on the specific SVT mechanism, they may display regular, irregular, or wide QRS patterns. These mechanisms may respond very differently to treatments with adenosine, pacing, cardioversion, or other antiarrhythmic drugs. Therefore, in approaching a patient with SVT, it is best to understand the broader differential diagnosis and to identify the most likely mechanism, to the degree possible, in order to choose the most appropriate therapy.

Atrioventricular reciprocating tachycardias (AV reentry)

AV reciprocating tachycardias represent the most common SVT in infants and young children. They result from reentry between the atria and ventricles, using an accessory AV connection (pathway) and the AV node. As such, they must display a fixed 1:1 AV relationship, since any break in this relationship will terminate the tachycardia. Usually, antegrade conduction from atria to the ventricles is over the AV node–His conduction system and retrograde conduction is via the accessory connection, referred to as orthodromic reciprocating tachycardia (ORT). ^, Atrial activation closely follows ventricular activation, such that P waves (if discernible) immediately follow each QRS complex in the ST segment.

Permanent junctional reciprocating tachycardia (PJRT) is a unique variant of ORT in which the accessory pathway displays slow, decremental (AV node-like) conduction. The result is typically a slower, more incessant long RP tachycardia displaying a short or normal PR interval. In a single-lead rhythm strip, PJRT may resemble sinus tachycardia, although a 12-lead electrocardiogram (ECG) reveals atypical P-wave morphology with P-wave inversion in the inferior limb leads. Repetitively spontaneous termination and prompt reinitiation are common features aiding in recognition of PJRT ( Fig. 33.2 ).

Supraventricular tachycardia resulting from the permanent form of junctional reciprocating tachycardia. Note the slow rate, long RP and short PR interval, and inverted P waves in electrocardiogram leads II and III. At rest, this rhythm often shows incessantly repetitive termination (with retrograde block), followed by immediate reinitiation (after single, isolated sinus complexes). With exercise, this rhythm is often faster and sustained, rendering it electrocardiographically indistinguishable from the atypical form of nodal reentry.

In many patients with ORT, the accessory pathway can only conduct retrograde and thus is clinically evident only during tachycardia or during ventricular pacing; the QRS is normal during sinus rhythm. However, if the accessory connection conducts antegradely during sinus rhythm, preexcitation results in a delta wave: shortened PR interval and slurring of the QRS upstroke, the hallmark of Wolff-Parkinson-White (WPW) syndrome. Many patients with congenital heart disease may display a slurred QRS upstroke due to intraventricular conduction delay, which can be readily distinguished from WPW if the PR is normal or prolonged.

Antidromic reciprocating tachycardia (ART) is a much less common form of AV reentry in which the circuit is reversed—antegrade conduction from atrium to ventricle is over the accessory connection, resulting in a very wide (maximally preexcited) QRS. ART can be difficult to distinguish from VT, and because it is a potentially more dangerous rhythm than ORT, it is most appropriately managed as VT in the acute setting. However, patients experiencing ART will display the WPW pattern upon restoration of sinus rhythm. A specific form of antidromic AV reentry uses an atriofascicular connection as the antegrade limb of the tachycardia and AV node (or a second accessory connection) as the retrograde limb. The atriofascicular connection (essentially an accessory AV node) traverses the lateral tricuspid valve annulus and inserts into the right ventricular fascicular system. It results in wide QRS tachycardia resembling a typical left bundle branch block pattern and is particularly common among patients with Ebstein anomaly. Owing to the decremental nature of the fiber, overt preexcitation is often subtle or altogether absent during sinus rhythm. Though usually responsive acutely to maneuvers interrupting AV node conduction, electrophysiologic study is required to confirm the diagnosis and distinguish it from VT or other forms of SVT with left bundle branch block aberrancy.

Atrioventricular nodal reentrant tachycardia

AV nodal reentrant tachycardia (AVNRT) is the second most common cause of SVT in older children and young adults without WPW syndrome or structural heart disease. It is seen less commonly in infants. ^, This tachycardia is attributed to so-called dual AV nodal physiology in which two or more separate inputs into the AV node (slow and fast pathways) conduct into and out of the AV node, providing the substrate for reentry.

Classically, two electrocardiographically distinct forms of AVNRT may occur. In typical AV node reentry, antegrade and retrograde conduction occurs over the slow and fast AV node inputs, respectively; retrograde P waves are obscured by the preceding QRS complex. In the atypical form of AV node reentry, the circuit is reversed, resulting in a long RP and short PR with an inverted P wave. Thus, typical AVNRT closely resembles ORT, whereas atypical AVNRT resembles PJRT (see previous section). Like PJRT, atypical AVNRT can also be mistaken for sinus tachycardia as well as other atrial tachycardias (see next section). AVNRT with 2:1 conduction and other complex variations can be defined only with an intracardiac electrophysiologic study.

Primary atrial tachycardias

Atrial tachycardias present with diverse ECG patterns and behaviors and may be the result of reentrant, automatic, and likely triggered mechanisms. ECG patterns may include discrete and regular P waves (ectopic atrial tachycardia, intraatrial reentry), sawtooth flutter waves (atrial flutter), or disorganized atrial activity (atrial fibrillation, chaotic atrial tachycardia). Conduction to the ventricles can be variable, depending on atrial rate and AV conduction. Thus, the resultant ventricular rate, rather than the atrial rate, determines the severity of symptoms. Irregularity in the ventricular rate and loss of AV synchrony also contribute to symptoms. Sometimes, it may be difficult to discern atrial tachycardias from sinus, particularly when 1:1 conduction is present and if the P-wave axis is normal. Likewise, a 2:1 conduction pattern can be difficult to recognize, but an unusually short or long PR during tachycardia should raise suspicion of a second hidden P wave. Observing for transient variations in the conduction pattern or using adenosine or vagal maneuvers to create transient AV block may reveal the faster, ongoing atrial rhythm. Direct recordings of atrial activity (via esophageal, epicardial, or intraatrial recordings) are also useful, particularly in revealing 2:1 AV conduction. Interrogation of a permanent pacemaker will also allow determination of the atrial rhythm. Despite the potential electrocardiographic similarities of the various primary atrial tachycardias, the varying mechanisms confer important differences in clinical behavior and management.

Many atrial tachycardias encountered in the pediatric intensive care setting are due to atrial reentry and are commonly referred to as atrial flutter regardless of whether the classic sawtoothed pattern (as in neonatal atrial flutter) is observed. In older patients with congenital heart disease (CHD), the term intraatrial reentrant tachycardia (IART) is often used to describe the slower, scar-dependent atrial reentry, which can result in more discrete and sometimes normal-appearing P waves (rather than classic flutter waves). Because of the slower atrial rate, 1:1 AV conduction may be especially common, further confounding the diagnosis. Likewise, administration of an AV node–blocking agent may result in 2:1 AV conduction, leading to the erroneous conclusion that the tachycardia has been converted to sinus rhythm with first-degree AV block. Thus, this arrhythmia should be considered in any patient with postoperative CHD presenting with a monotonous and inappropriate tachycardia, otherwise suggestive of sinus tachycardia on ECG.

Early after congenital heart surgery, atrial tachycardias often occur as repetitive, self-limited bursts. Many of these are likely due to automatic or triggered behavior. While they may require therapy in the short term, they may not portend ongoing susceptibility to long-term IART.

Atrial fibrillation , a common arrhythmia among the elderly, is far less common in pediatric patients. It may occur as the result of atrial myocarditis, secondary to an underlying reentrant SVT, or mechanical stimulation by central venous catheters ( Fig. 33.3 ). In patients with WPW syndrome, rapid conduction over the accessory pathway during atrial fibrillation can be life-threatening. When atrial fibrillation occurs in a young patient, the underlying basis should be sought because the long-term implications and treatment measures are much different from those in adult patients. A significant proportion of teens and adolescents presenting with lone atrial fibrillation are found to have an underlying SVT when taken for electrophysiology study. Thus the long-term therapy of atrial fibrillation in this age group is much different from that administered to the elderly.

Sustained atrial fibrillation in a previously healthy 17-year-old placed on extracorporeal membrane oxygenation for pneumonia and acute respiratory distress syndrome. Review of stored telemetry revealed atrial ectopy soon after cannulation, eventually initiating atrial fibrillation. Following direct current cardioversion to sinus rhythm, frequent ectopy persisted until the venous cannula was repositioned. Several months later, the patient remained free of further arrhythmias.

Ectopic atrial tachycardia (EAT) is an automatic arrhythmia that typically presents as an incessant and chronically elevated atrial rhythm that may be mistaken for sinus tachycardia, especially when the automatic focus is in the right atrium. First-degree AV block may be seen as a physiologic response to the inappropriately accelerated atrial rate. Patients with EAT often experience no overt palpitations but present instead with ventricular dysfunction and sometimes frank congestive heart failure (CHF). Because adenosine may transiently inhibit the automatic focus, termination with adenosine is nondiagnostic in distinguishing EAT from sinus tachycardia. Instead, observation of the heart rate behavior over periods of several hours and careful scrutiny of the P-wave morphology on 12-lead ECG, especially with transitions between the EAT and sinus, are necessary to establish the diagnosis.

Chaotic (multifocal) atrial tachycardia is an uncommon arrhythmia that is usually observed in infants and toddlers, often in association with a viral respiratory illness. The hallmark features are a rapid and irregular atrial rate, often exceeding 300 beats/min, and presence of multiple P-wave morphologies. The resulting ventricular response is irregularly irregular, simulating atrial fibrillation. However, this rhythm is probably the result of multiple triggered foci within the atria. Thus, acute termination measures (i.e., direct current cardioversion, adenosine, or pacing) are of little benefit. Usually, this arrhythmia resolves within weeks or months of presentation. Treatment is based on the severity of symptoms, which may range from negligible to life-threatening.

Junctional ectopic tachycardia

Junctional ectopic tachycardia (JET) probably arises from an abnormal automatic focus or a protected microreentrant circuit in the region of the AV node or proximal His bundle. Antegrade conduction is usually over the normal His-Purkinje system with a narrow QRS. Commonly, there is retrograde (ventriculoatrial [VA]) block with complete VA dissociation; the resulting sinus-capture complexes aid in the recognition of this mechanism. Sometimes, antegrade AV block coexists with JET. Variants include the common transient postsurgical JET, a congenital chronic JET, and paroxysmal JET described primarily in adults. ^, As in postoperative atrial tachycardias, direct atrial recordings aid the diagnosis. In some cases, JET is associated with 1:1 VA conduction, in which case additional pacing or pharmacologic maneuvers may be necessary to distinguish it from other mechanisms of SVT.

Ventricular tachycardias

VTs arise exclusively within the ventricle(s); by definition, the QRS duration is always aberrant and prolonged for a given age and heart rate. The QRS morphology may be either uniform or changing (bidirectional, polymorphic). Classically, VTs are associated with VA dissociation (atrial rhythm at a slower rate). Thus, VA dissociation, when present, is helpful, but when absent (or uncertain) does not exclude VT as the underlying mechanism. The presence of periodic fusion complexes (QRS morphology intermediate between tachycardia morphology and sinus morphology) is diagnostic since it implies VA dissociation.

In infants and young children, recognition of VT may sometimes be difficult. Acutely, VT may be mistaken for SVT due to the relatively narrower QRS complexes and 1:1 VA conduction due to robust AV node conduction. However, the QRS should be different from the baseline QRS during sinus. JET with aberrancy, like VT, is often characterized by sinus-capture complexes but without QRS fusion. The QRS with sinus capture or with sustained AV conduction during overdrive atrial pacing should remain unchanged in JET.

Like SVTs, VTs are diverse in pattern, mechanism, and severity, and they may result from each of the tachycardia mechanisms previously discussed (reentry, automaticity, and triggered activity) with important therapeutic implications. The clinical setting, electrocardiographic pattern, and severity of symptoms dictate acute treatment approaches.

Bradycardias

Perhaps the most important diagnostic issues in bradycardias are determination of whether AV conduction occurs and recognizing the presence and rate of any underlying intrinsic escape rhythms. This should usually be straightforward when the atrial rate exceeds the ventricular rate during second-degree or third-degree AV block. In complete AV block, the resultant escape rhythm is usually regular; in second-degree AV block, the ventricular intervals vary (see Fig. 33.1 ). This is especially helpful when sinus node disease and AV block coexist; variation of the RR interval typically implies some degree of AV conduction. When atrial pacing is feasible (as in patients with recent cardiac surgery), AV conduction can be more directly characterized.

The distinction between bradycardia resulting from AV block and sinus node dysfunction may have important therapeutic implications, particularly with respect to pacing. If AV nodal conduction is fully intact, it is usually desirable to pace the atrium only (see AAI mode) rather than perform dual-chamber pacing. In contrast, isolated AV block is best managed by sensing and tracking the intrinsic atrial rate (see DDD mode) and pacing the ventricle. Other variations in pacing modes are discussed elsewhere in this chapter.

Extrasystoles

Extrasystoles, or premature beats, are defined as supraventricular (supraventricular premature beats or premature atrial or junctional complexes) or ventricular (premature ventricular complexes, ventricular premature beats [VPBs]) in origin. True junctional extrasystoles are uncommon.

Isolated premature QRS complexes with prolonged QRS duration may represent either ventricular extrasystoles or aberrantly conducted atrial extrasystoles. Distinguishing the two may be difficult from a single rhythm strip. When the extrasystole results in an early QRS with normal morphology and duration, a supraventricular extrasystole may be presumed. Usually, an early P wave can be discerned, but it may be obscured by the preceding T wave in certain leads. The ensuing sinus beat is usually advanced by the atrial extrasystole, but entrance block can often result in a full compensatory pause, which is usually more characteristic of ventricular extrasystoles. It is important to view multiple leads, since a ventricular extrasystole may resemble the normal QRS in one lead but appear totally dissimilar and broader in others. The ECG can also help discern unifocal versus multifocal PVCs and also help localize the site of PVC origin.

The ECG features favoring ventricular extrasystoles over aberrantly conducted atrial extrasystoles include (1) wide QRS morphology, (2) a full compensatory pause prior to the ensuing sinus beat, (3) presence of fusion beats, and (4) absence of a discernible premature P wave. A full compensatory pause indicates failure of the sinus node to be reset by the ventricular depolarization, though a very premature beat may occasionally reset the sinus node because of retrograde (VA) conduction over the AV node to the atrium. Fusion indicates a QRS morphology intermediate between a fully anomalous QRS and the normal QRS. Fusion is also seen in patients with ventricular preexcitation (WPW) and in whom PACs often result in a widened QRS due to selective delay in conduction of the premature impulse through the AV node but not the accessory pathway.

The distinction between atrial and ventricular extrasystoles may be somewhat academic in otherwise asymptomatic individuals because neither generally warrants therapy. However, either might be a harbinger for myocardial irritability and should prompt a search for underlying causes. Occasionally, measures to suppress ectopy may appear to improve cardiac output by regularizing filling time in an otherwise tenuous patient. The relative advantages and risks of any such measure, whether achieved with medications or temporary pacing, need to be considered individually.

Tachycardias with normal QRS

In otherwise healthy infants, children, and adolescents, SVT usually represents AV reentrant tachycardia or AVNRT ( Fig. 33.7 ). Further distinction between these two mechanisms has little impact on acute management. However, in the ICU setting, primary atrial tachycardias (including sinus tachycardia) and junctional tachycardias are considerably more prevalent (particularly following cardiac surgery). The finding of abnormal P-wave morphology (determined by 12-lead ECG), a PR interval greater than 50% of the RR interval, or completely obscured P waves favors a nonsinus mechanism. Finally, ensuring normal QRS duration for age, rather than simply relying on adult standards, is essential in discriminating from VTs.

Supraventricular tachycardia resulting from atrioventricular nodal reentry in an infant. Although considerably less common than orthodromic reciprocating tachycardia in this age group, the P wave on the terminal portion of the QRS complex results in a pseudo rSr′ pattern. During sinus rhythm, this terminal deflection on the QRS was absent; the transesophageal recording confirmed the mechanism.

Distinguishing sinus tachycardia from various types of SVT may be difficult. In young patients, intraarterial reentry, atrial flutter, and atrial fibrillation are usually seen following surgical treatment for congenital heart defects involving the atrium (atrial septal defects, atrial repair of transposition of the great arteries, or the Fontan operation). The term intraatrial reentrant tachycardia is often used in this setting when a reentrant tachycardia displays discrete P waves rather than the usual sawtooth flutter waves. Because the atrial rate is often relatively slow in comparison with typical atrial flutter, a high index of suspicion is essential in distinguishing this rhythm from sinus rhythm or sinus tachycardia, especially when fixed 1:1 conduction or 2:1 conduction (with blocked P waves obscured by the QRS or T wave). Again, direct atrial recordings using transesophageal electrocardiography or temporary epicardial atrial pacing wires usually facilitate the diagnosis and characterize the AV relationship (see Fig. 33.6 B), as can vagal maneuvers, administration of adenosine to temporally interrupt AV conduction, or—in ambulatory patients—simply changing position from supine to standing to assess for changes in the ventricular response.

Tachycardias with prolonged QRS

Tachycardia with prolonged QRS can represent VT, an SVT with aberrancy, or, occasionally, a preexcited tachycardia in a patient with ventricular preexcitation (WPW, Mahaim fiber). VA dissociation, the hallmark and most specific ECG feature of VT, may not be seen in childhood because of rapid retrograde conduction over the AV node (see Fig. 33.5 ). The distinction between VT and SVT with aberrant conduction can be difficult and may ultimately require invasive electrophysiologic study. Other features favoring VT are the presence of fusion complexes (implying AV dissociation), a superior QRS axis, or a concordant QRS axis across the precordium (i.e., pure R waves or pure S waves). However, these criteria may be unreliable in patients with CHD where normal conduction axes may be abnormal at baseline.

Generally, tachycardias with prolonged QRS should be presumed to be VTs until or unless evidence of an alternative diagnosis is clearly demonstrated. Prolonged attempts to differentiate SVT from VT by noninvasive means may simply delay treatment, and the wrong conclusion may prove disastrous: acute treatment based on a presumed diagnosis of VT is rarely deleterious even when the mechanism subsequently proves to be supraventricular. However, an erroneous presumption of SVT with aberrant conduction may result in rapid clinical deterioration. When feasible, a full 12-lead ECG may aid in the diagnosis, particularly when a baseline ECG in normal rhythm is available for comparison. Apparent hemodynamic stability should not be mistaken as evidence of SVT over VT whether in an otherwise healthy child or in a patient with known cardiac disease.