330 SECTION IV Pediatric Critical Care Cardiovascular syndromes Sinus bradycardia can also be an initial manifestation of long QT syndrome in infancy and of channelopathies 4 Atrioventricular Block AV[.]
330 S E C T I O N I V Pediatric Critical Care: Cardiovascular TABLE Classification of Arrhythmias by Type and Basis 33.1 Arrhythmia Ventricular premature beats and supraventricular premature beats Sinus bradycardia, sick sinus syndrome Primary Secondary 111 111 11 11 11 111 Incomplete AV block Mobitz type I Mobitz type II Congenital third-degree AV block 11 111 Acquired third-degree AV block Paroxysmal SVT (AV reentrant tachycardia, AV nodal reentrant tachycardia) 11 11 111 Ectopic atrial tachycardia 11 Atrial flutter and intraatrial reentry 11 111 111 Atrial fibrillation Chaotic atrial tachycardia Junctional ectopic tachycardia 11 111 11 11 Torsades des pointes 1 Ventricular fibrillation 11 11 Monomorphic ventricular tachycardia Bidirectional ventricular tachycardia AV, atrioventricular; SVT, supraventricular tachycardia; 111, typical; 11, occasional; 1, rare 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 Highgrade 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) 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 Escape Rhythms and Accelerated Rhythms syndromes Sinus bradycardia can also be an initial manifestation of long QT syndrome in infancy and of channelopathies.4 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 beatto-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 In the presence of sinus bradycardia or AV block, slower escape rhythms typically emerge from the atrium, AV node, HisPurkinje system, or ventricular myocardium When slower than the appropriate sinus rate, they are referred to as escape rhythms (atrial, junctional, or idioventricular) Similar rhythms may emerge and compete with an appropriate sinus rhythm, in which case they are referred to as an accelerated (junctional or idioventricular) rhythm Such accelerated rhythms can result from increased adrenergic tone, intrinsic mechanisms (such as idioventricular rhythm), or sympathomimetic drug infusions It is important to distinguish these accelerated subsidiary rhythms from escape rhythms resulting from AV block Only rarely does an accelerated junctional or ventricular rhythm result in significant symptoms in a healthy child.5 However, in the critically ill patient, loss of AV synchrony may be poorly tolerated and atrial pacing at a slightly faster rate to reestablish AV synchrony may be beneficial The rate defining an accelerated junctional or ventricular rhythm from a corresponding pathologic tachycardia can sometimes be arbitrary but is usually determined by the similarity in rates and gradual transitions between the accelerated rhythm with the concurrent sinus rhythm Tachycardias Tachycardia Mechanisms While sinus tachycardia is the most common tachycardia among ill patients, pathologic tachycardias can occur due to three basic CHAPTER 33 Disorders of Cardiac Rhythm I aVR V1 V4 II aVL V2 V5 III aVF V3 V6 331 VI • Fig 33.1 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 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.6 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.7 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).6,7 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.8 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 332 S E C T I O N I V Pediatric Critical Care: Cardiovascular VI II V5 • 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 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) 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.6,7 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).9 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 CHAPTER 33 Disorders of Cardiac Rhythm 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 normalappearing 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,10 secondary to an underlying reentrant SVT,11 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 333 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 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.12 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 I aVR V1 V4 II aVL V2 V5 III aVF V3 V6 V1 • Fig 33.3 Sustained atrial fibrillation in a previously healthy 17-year-old placed on extracorporeal mem- brane 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 334 S E C T I O N I V Pediatric Critical Care: Cardiovascular 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 sinuscapture 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.13,14 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 • Fig 33.4 Ventricular Approach to Diagnosis Monitoring and General Assessment In the intensive care setting, there may be a trade-off between precision of diagnosis and urgency of therapy Even so, appropriate diagnosis remains key to establishing appropriate ongoing therapy When an arrhythmia develops, factors such as level of consciousness, ventilation, tissue perfusion, and acid-base status (including mixed venous saturation and lactate) govern the acuity with which treatment is required and the extent to which additional diagnostic measures can be employed before initiating therapy Minimal initial diagnostic evaluation should always include permanently recorded ECG rhythm strips along with a rapid review of drugs being administered, potential toxic exposures, respiratory and acid-base status, and known associated illnesses that might be arrhythmogenic Electrolytes, including calcium and magnesium, should be obtained along with drug screening in the patient presenting with altered mental status The details of recent cardiac surgical procedures and any recent trauma (chest and cranial) should be quickly reviewed Indwelling catheter position should be noted on radiographs for potential intracardiac location Concurrent with this brief survey, a differential diagnosis of the rhythm disturbance should be established quickly, followed by the most appropriate emergency therapy If the patient is sufficiently stable, therapy may be deferred until a 12-lead ECG is obtained and other diagnostic measures taken for more precisely characterization Surface Electrocardiogram and Bedside Monitoring The surface ECG remains the cornerstone of arrhythmia diagnosis Certainly, in patients with known cardiac abnormalities and, arguably, in all patients admitted to the ICU, a baseline ECG should be obtained at admission This ECG may provide a valuable baseline for later comparison in the event of a new arrhythmia or other changes, such as cardiac ischemia, which may predispose to arrhythmias For sustained tachycardias, a full 12-lead ECG should be obtained whenever possible because diagnostic details (such as QRS aberrancy, atrial rate, and P-wave morphology) or hidden features (such as flutter waves) may be evident only in selected leads (Fig 33.4) Because most contemporary ICU monitors provide a full disclosure feature, a strip should usually be available to assess additional details such as the onset of an arrhythmia as well as variations in rate, beat-to-beat regularity, QRS morphology and tachycardia following cardiac surgery Note similarity between QRS complexes during sinus rhythm and ventricular tachycardia in two of three recorded leads ... 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... 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... direct current cardioversion, adenosine, or pacing) are of little benefit Usually, this arrhythmia resolves within weeks or I aVR V1 V4 II aVL V2 V5 III aVF V3 V6 V1 • Fig 33.3 Sustained atrial