345CHAPTER 33 Disorders of Cardiac Rhythm loss of pacemaker capture—have also been reported with dexme detomidine infusion 42,43 Ivabradine Ivabradine is a novel antiarrhythmic drug that works by inhi[.]
CHAPTER 33 Disorders of Cardiac Rhythm loss of pacemaker capture—have also been reported with dexmedetomidine infusion.42,43 Ivabradine Ivabradine is a novel antiarrhythmic drug that works by inhibiting cardiac pacemaker current (If) in normal sinus node as well as in AV node and bundle of His.44 Unlike b-blockers, it exerts its negative chronotropic action independent of the neurohumeral system Its most common usage is in the treatment of inappropriate sinus tachycardia However, its use has been reported in treatment of both congenital and postoperative junctional ectopic tachycardias.45,46 Adverse events have not been reported in this setting Cardioversion and Defibrillation Cardiovascular collapse or failure of mechanical and pharmacologic interventions for tachycardias may warrant cardioversion For tachycardias with discrete QRS complexes, synchronization with the QRS should be confirmed (the default mode for most defibrillators is nonsynchronized, and most revert to nonsynchronized shocks after each shock is delivered) Proper synchronization may require changing the ECG lead configuration to achieve an upright QRS complex Several factors may determine the success of cardioversion and defibrillation Energy requirements may vary from 0.25 to J/kg for SVTs to greater than J/kg for VTs Newer defibrillators with a biphasic rather than monophasic waveform have reduced defibrillation energy requirements Electrode (paddle) location is an important variable If conversion is not achieved with low- or moderate-energy levels, consideration should be given to changing electrode position before using higher-energy levels Automatic tachycardias are characteristically refractory to cardioversion and may account for treatment failure Finally, some antiarrhythmic drugs, particularly sodium channel–blocking drugs (see later discussion), increase defibrillation energy requirements and pacing thresholds, whereas other drugs (QT-prolonging drugs) appear to have a favorable effect.47 Approach to Therapy Extrasystoles In general, isolated extrasystoles not require treatment unless they are sufficiently frequent to impair hemodynamics or they serve as frequent initiating events for tachycardias In otherwise healthy children and adolescents, extrasystoles are a benign finding In other settings, complex ventricular extrasystoles may identify patients at increased risk for cardiac arrest Even in such situations, prophylaxis may not decrease and may actually increase the risk Effort should instead be directed at identifying possible causes and correcting any predisposing factors, which include ischemia, electrolyte disorders, acidosis, pericarditis, or direct trauma from recent cardiac surgery, blunt or penetrating chest trauma, and intracardiac catheter-induced irritation Numerous drugs—including digoxin, catecholamines, or drugs associated with the acquired long QT syndrome—may produce extrasystoles Sustained Tachycardias Most sustained tachycardias observed in the intensive care setting warrant immediate attention and intervention Sinus tachycardia may indicate the need for additional sedation and analgesia or may reflect hemodynamic compromise as a consequence of 345 anemia, hypovolemia, or impaired myocardial function Sinus tachycardia as a consequence of hyperthermia may be poorly tolerated in children already critically ill, especially following cardiac surgery Sinus tachycardia may reflect an underlying neuroendocrine process, such as hyperthyroidism or pheochromocytoma, requiring acute medical intervention (b-blocker) while instituting therapy for the underlying disorder Nonsinus tachycardias in patients with primary rhythm disturbances may warrant therapy to prevent life-threatening events, prevent the development of myocardial dysfunction as a consequence of chronic (incessant) tachycardia, or simply alleviate acute tachycardia-related symptoms The acuity of the situation dictates the approach to therapy Tachycardias occurring secondary to other abnormalities (structural heart disease, metabolic derangements, drug toxicity) should always be regarded as high risk for serious hemodynamic deterioration Unstable Patients The approach to patients with tachycardia is determined largely by the degree of hemodynamic compromise (see Table 33.3) Patients who are hemodynamically unstable or in cardiovascular collapse resulting from sustained tachycardia almost always warrant prompt cardioversion or defibrillation Antiarrhythmic medications (and other supportive measures) should replace cardioversion in the unstable patient only when tachycardia is known to be incessant or is unresponsive to cardioversion (e.g., JET, atrial ectopic tachycardia, chaotic tachycardia, and PJRT) Cardiopulmonary resuscitation should always be instituted in the absence of a pulse or blood pressure, as is typically the case for polymorphic VT and ventricular fibrillation Although hemodynamic and ventilatory support should be initiated immediately and maintained following tachycardia termination as needed, cardioversion (with bag and mask ventilation initially) should take precedence over other interventions.48 Underlying factors contributing to the tachycardia should be sought, including hypoxia, infection (cardiac or systemic), drug toxicities (see later discussion), and electrolyte derangements Once tachycardia is terminated, acute therapies may focus on either suppressing recurrences or terminating them when they recur Although antiarrhythmic medications eventually may be necessary to suppress recurrences, most have negative inotropic or vasodilating effects, particularly when administered intravenously Often, it is preferable to delay specific therapy after initial termination until ventricular function improves Most recurrences of SVTs can be safely treated with temporary pacing or adenosine rather than with repeated cardioversions In the event of frequent recurrences, a transesophageal catheter may be left in place for this purpose, or a transvenous atrial pacing catheter may be warranted in selected patients Similarly, temporary ventricular pacing may be useful in some circumstances for recurrent VT Although adenosine can be administered repeatedly because of its short halflife, the resulting vasodilation may be poorly tolerated in patients with tachycardia mechanisms unresponsive to adenosine Treatment Failure When seemingly appropriate electrical and pharmacologic interventions fail to terminate tachycardias, three possibilities should be considered: erroneous diagnosis, unrecognized tachycardia termination and reinitiation, or a technical error in the termination technique 346 S E C T I O N I V Pediatric Critical Care: Cardiovascular Errors in Diagnosis As noted earlier, automatic atrial tachycardias, JET, and occasionally chaotic atrial tachycardia might be mistaken for tachycardias with reentrant mechanism (ORT, AVNRT, and primary atrial reentry) Each of these conditions is usually refractory to electrical termination (either pace termination or cardioversion), yet the diagnosis may be subtle if atrial activity is obscured Confusion between VTs and SVTs with prolonged QRS probably remains the most frequent diagnostic error Occasionally, the presumption of VT may lead to ineffective treatments For example, following cardiac surgery, incessant, monomorphic wide QRS tachycardia that is refractory to cardioversion may actually be JET with postsurgical bundle branch block Brief atrial pacing at a faster rate may be necessary to confirm the diagnosis Certain tachycardias, such as torsades de pointes related to long QT syndromes (congenital or drug-induced) or bidirectional VT resulting from digoxin toxicity, must be recognized to provide more appropriate and specific therapies to prevent recurrences following cardioversion Unrecognized Termination and Reinitiation Unrecognized reinitiation may occur following medical termination, pace termination, or cardioversion In some tachycardias (as in PJRT or other incessant forms of SVT), reinitiation is expected However, it also may occur inadvertently as the result of continued pacing beyond the point of termination or may be facilitated by sinus pauses, junctional beats, or ectopic beats following adenosine or cardioversion Again, measures to decrease the factors favoring reinitiation (e.g., shorter pacing bursts, antibradycardia pacing, and coadministration of an antiarrhythmic drug) should be used rather than further increases in energy or dose of the terminating therapy With frequent terminations and reinitiation of tachycardia, multiple repeated cardioversions are likely ineffective and may cause myocardial injury Improper Technique Appropriate administration of adenosine is accomplished through an IV catheter (peripheral or central) by rapid push followed immediately with ample flush Because of rapid metabolism by erythrocytes, arterial administration may be ineffective in terminating tachycardia (yet still produce vasodilation) Errors in cardioversion or pacing technique are generally attributable to insufficient energy or improper electrode (or paddle) placement For pace termination, the stimulator must be capable of sufficient output for the pacing modality being used (see section on temporary pacing) ICU personnel should be familiar with the defibrillation devices available in the ICU, including adjustment of electrocardiographic gain and lead selection to allow synchronous cardioversion when appropriate However, in ventricular fibrillation or polymorphic tachycardia, asynchronous countershock is necessary Use of excessive energy may damage the myocardium and, when repeated, may lead to preterminal bradycardias, which are refractory to all pacing modalities and progress to complete electromechanical dissociation if hypoxia and acidosis are not corrected Specific Arrhythmias Primary Arrhythmias Some arrhythmias require unique therapeutic approaches or are seen with sufficient frequency to warrant a brief review (Box 33.1) • BOX 33.1 Rhythm Disturbances Primary Rhythm Disturbances Paroxysmal supraventricular tachycardias (atrioventricular nodal reentrant tachycardia) • Congenital AV block • Congenital long QT syndrome, Brugada syndrome • Other genetic arrhythmias • Ventricular tachycardias resulting from Purkinje hamartoma • Verapamil-sensitive ventricular tachycardias • Accelerated ventricular rhythm Secondary Rhythm Disturbances Early Postoperative Arrhythmias JET • Postsurgical AV block • Early primary atrial tachycardia Late Postoperative Arrhythmias Ventricular arrhythmias (postoperative tetralogy of Fallot) • Sick sinus syndrome Metabolic Derangements Electrolyte disturbances • Endocrine derangements (thyroid) • CNS injury • Hypothermia, hyperthermia • Acute hypoxia (newborns) • Acute myocardial infarction Drug Toxicity, Proarrhythmia Digoxin Cocaine Tricyclic antidepressants Antiarrhythmic drugs Quinidine/sotalol Flecainide/encainide Organophosphates Infectious Lyme disease • Myocarditis, endocarditis AV, Atrioventricular; CNS, central nervous system; JET, junctional ectopic tachycardia Orthodromic Reciprocating Tachycardia in Infancy Infants with ORT can present with tachycardia in utero, at birth, or within the first weeks to months of life Postnatally, tachycardia sustained beyond a few hours may result in CHF that may progress to shock, acidosis, and complete cardiovascular collapse.49 In the latter situation, ORT may be terminated during resuscitation efforts such that its causative role remains unrecognized Thus, SVT should be considered in the differential diagnosis of neonatal shock, along with other conditions such as sepsis, aortic coarctation, and congenital adrenal hyperplasia, in which sinus tachycardia associated with cardiovascular collapse would be expected Initial conversion can typically be achieved with either vagal stimulation such as ice-to-face, IV adenosine or, in dire circumstances, DC cardioversion With successful conversion, initial oral therapy with digoxin or a b-blocker appear similarly effective in infants.39 However, when initial conversion is followed by immediate reinitiation, repetitive doses of adenosine tend to promote incessant tachycardia IV agents useful for initial control of incessant tachycardia include esmolol, procainamide, sotalol, or amiodarone Second-line oral agents include sotalol, flecainide, propafenone, and amiodarone CHAPTER 33 Disorders of Cardiac Rhythm I aVR V1 V4 V3R II aVL V2 V5 V4R III aVF V3 V6 V7 347 V4R • Fig 33.12 Chaotic atrial tachycardia Note two discrete P-wave morphologies before conversion to sinus rhythm Tachycardia-Induced Cardiac Dysfunction Although most SVTs are paroxysmal or episodic, chronic SVTs pose a unique problem Many are minimally symptomatic and are recognized only by the inappropriately fast rate However, with time, varying degrees of CHF become evident and ventricular dysfunction may be severe Even then, the diagnosis may not be immediately evident Consequently, chronic tachycardia must be considered in any patient presenting with gradually progressive CHF In one series, chronic atrial tachycardia was present in 37% of patients initially diagnosed with idiopathic cardiomyopathy and listed for heart transplantation.50 In patients with structurally normal hearts, the most common incessant SVTs are PJRT and ectopic atrial tachycardia These conditions may occur throughout infancy, childhood, and adolescence The rates (often ,200 beats/min) and normal PR interval during tachycardia may lead to an erroneous diagnosis of sinus tachycardia secondary to the hemodynamic compromise (see earlier section, Approach to Diagnosis) An abnormal P-wave axis on 12-lead ECG and Holter monitoring to look for interruptions in the tachycardia with changes in P-wave morphology are helpful Electrophysiologic study may still be necessary to establish the diagnosis In infants, incessant VTs and the rare congenital form of JET also are seen In each of these entities, it is important first to recognize the primary role of the tachycardia in producing secondary congestive symptoms and to recognize the futility of acute therapies such as adenosine, pace termination, and cardioversion Most are catecholamine dependent so that inotropic agents may aggravate the situation and compromise the efficacy of antiarrhythmic regimens, whereas b-blocking agents may be useful despite the presence of heart failure Once the diagnosis is established, chronic antiarrhythmic therapy is instituted to control or limit the tachycardia Uncontrolled, severe cardiac symptoms may result, but ventricular dysfunction improves once tachycardia is suppressed medically or treated by catheter ablation.51 Despite the severity of heart failure, antiarrhythmic medications that depress ventricular function are usually well tolerated Chaotic Atrial Tachycardia Chaotic atrial tachycardia is a primary atrial tachycardia characterized by three or more different P-wave morphologies and irregular, rapid atrial rates (Fig 33.12) Although atrial flutter may be associated with it, episodes are usually self-limited and cardioversion is neither indicated nor effective Asymptomatic patients with slow or intermittent tachycardia may not require treatment Occasionally, digoxin is used to limit AV conduction when atrial rates are excessive or to enhance contractility in the setting of tachycardia-induced cardiomyopathy.52 An association with respiratory syncytial virus has been described in some patients.12 Various agents have been used in symptomatic cases; amiodarone and propafenone are the most effective.52,53 Long QT Syndromes The long QT syndromes (LQTSs) are a diverse group of disorders, both congenital and acquired, in which individuals are at risk for torsades de pointes and sudden death because of abnormalities in ventricular repolarization In both congenital and acquired forms, the rate-corrected QT intervals usually exceed 0.46 second and, more typically, are greater than 0.48 to 0.50 second Associated anomalies of T-wave morphology—including T-wave alternans, bifid T waves, and prominent U waves—are common and T-wave morphology may offer clues to genotype in congenital LQTSs It can sometimes be difficult to establish the diagnosis of congenital long QT syndrome because QT prolongation may sometimes be modest in affected individuals and there may be considerable overlap in QTc ranges with the normal, healthy population.54 However, congenital LQTS should be strongly considered in all patients with any degree of QT prolongation and a history of 348 S E C T I O N I V Pediatric Critical Care: Cardiovascular • Fig 33.13 Congenital long QT syndrome presenting with 2:1 atrioventricular block The corrected QT interval is greater than 560 ms, resulting in functional block of every other sinus beat in the His-Purkinje system • Fig 33.14 Torsades des pointes in a teenage patient with long QT syn- sustained, and even prolonged episodes may terminate spontaneously Torsades de pointes that degenerates to ventricular fibrillation requires defibrillation Because the adrenergic response to defibrillation may trigger recurrent arrhythmias, defibrillation should be performed in an unconscious or sedated patient Treatment of immediate recurrence of torsades de pointes is challenging and includes magnesium sulfate, increasing the heart rate with temporary pacing or isoproterenol, and sedation.56 For most acquired long QT syndromes (and some congenital forms), increasing the heart rate using isoproterenol or by pacing shortens the QT interval, though isoproterenol may exacerbate some forms of the congenital long QT syndrome.57 Therefore, isoproterenol should be used only when there is underlying bradycardia and cardiac pacing cannot be started immediately Correcting hypokalemia, hypomagnesemia, or hypocalcemia and removing potentially causative agents may be important in the ICU setting syncope, polymorphic VT, cardiac arrest, or a family history of unexplained sudden death The diagnosis should also be considered in any child presenting with unexplained seizures, particularly if experienced during physical exertion or other conditions of adrenergic stress Conversely, incidental QT prolongation in an asymptomatic child (with a negative family history) warrants further scrutiny and evaluation for other factors contributing to QT prolongation Finally, infants with LQTS may present with functional 2:1 AV block due to prolonged cardiac repolarization (Fig 33.13).4 Patients with symptoms or arrhythmias associated with QT prolongation require careful evaluation for secondary causes, which include CNS injury, hypocalcemia, hypokalemia, and drugs that prolong the QT interval The list of drugs that prolong the QT interval is extensive and includes antiarrhythmic and noncardiac drugs, most of which block IKr, the rapid component of the delayed rectifier potassium current.55 Continually updated lists of these drugs are available at www.crediblemeds.org Torsade de pointes is the specific arrhythmia associated with long QT syndromes and is responsible for the symptoms (Fig 33.14) This characteristic arrhythmia is recognized by progressive undulation in the QRS axis, resulting in a twisting appearance, and is usually associated with a specific initiation with a VPB following a pause (often following a previous VPB) Many episodes are not Idiopathic Ventricular Tachycardias in Healthy Patients Accelerated ventricular rhythm is observed occasionally in neonates in the first few days of life at rates only slightly faster than the appropriate sinus rates The rhythm competes with the sinus mechanism, and alternation between sinus and ventricular rhythm with fusion beats is common The rhythm is self-limited, does not usually result in hemodynamic compromise, and carries a good prognosis No specific therapy is necessary unless rates are excessive.58 A similar rhythm is seen in older children and usually has a similarly benign course.59 Two other characteristic VTs may be seen in otherwise healthy children and adolescents One arises from the right or left ventricular outflow tracts, resulting in a left bundle branch block pattern with inferior QRS axis This pattern of arrhythmia often displays incessant, self-limited salvos, in which case the term repetitive monomorphic VT has been used A more paroxysmal pattern is seen less often Patients are often asymptomatic, though palpitations, chest pain, and occasionally syncope can result Treatment is dictated largely by symptoms The other idiopathic VT arises from the posterior fascicle of the left-sided conduction system, producing a right bundle branch block pattern with leftward QRS axis (Fig 33.15) This tachycardia has been called fascicular or verapamil-sensitive VT Interestingly, the acute response to verapamil in the past sometimes resulted in the misclassification as SVT with aberrant conduction Clinically, this arrhythmia can be managed much like SVT in that life-threatening decompensation or progression to ventricular fibrillation is uncommon drome This arrhythmia is associated with no pulse and results in syncope It often terminates spontaneously but otherwise rapidly degenerates to ventricular fibrillation CHAPTER 33 Disorders of Cardiac Rhythm I aVR V1 V4 II aVL V2 V5 III aVF V3 V6 349 • Fig 33.15 Idiopathic ventricular tachycardia in an otherwise healthy 12-year-old Note right bundle branch block pattern with superior axis, typical of origin within the left posterior fascicle region of the left ventricular septum Although the mechanisms underlying outflow tract VTs (triggered activity) and posterior fascicular VT (micro-reentry) are dissimilar, both may respond acutely and chronically to calcium channel blockers Chronic therapies can include b-blockers or (for RVOT) flecainide However, ultimately, catheter ablation is usually the treatment of choice in symptomatic individuals Ventricular tachycardias not displaying the characteristic features of outflow tract VT or posterior fascicular VT should prompt an aggressive search for occult heart conditions, including cardiomyopathies, myocarditis, cardiac tumors (rhabdomyomas, fibromas, hamartomas) or arrhythmogenic right ventricular cardiomyopathy.60 Evaluation should include magnetic resonance imaging with contrast and, often, electrophysiology study Bidirectional Polymorphic Ventricular Tachycardias CPVT is an inherited condition that bears some similarity to the long QT syndrome Individuals may present with syncope or cardiac arrest during exertion or emotional stress However, at baseline, the ECG may be entirely normal and the condition may become evident only when challenged with exercise or catecholamine infusion The characteristic arrhythmia is bidirectional VT, in which there is an alternating QRS axis during salvos of VT Often, patients are entirely asymptomatic during sometimes long, incessant runs of bidirectional VT, and acute therapies, particularly cardioversion, are not warranted Patients may also experience concurrent atrial arrhythmias Defects in multiple genes that alter calcium release from the sarcoplasmic reticulum can underlie CPVT, but most patients carry mutations in the cardiac ryanodine receptor (RYR2) Once the diagnosis has been established, chronic therapies generally include a b-blocker, sometimes in combination with flecainide Cervical sympathetic denervation may also enhance chronic arrhythmia suppression Secondary Rhythm Disturbances Certain arrhythmias characteristically follow operative treatment of CHD Among those observed in the early postoperative period are complete heart block, JET, and primary atrial tachycardias Late postoperative arrhythmias include ventricular arrhythmias following tetralogy of Fallot repair and atrial arrhythmias following the Mustard/Senning and Fontan procedures Postoperative Arrhythmias Postsurgical Atrioventricular Block Inadvertent damage to the AV conduction system may occur with cardiac surgery, especially after closure of ventricular septal defects (particularly associated with l-loop ventricles), during resection of septal tissue, or after insertion of prosthetic valves in the tricuspid, aortic, or mitral position Bradycardia from AV block can be initially managed using isoproterenol to accelerate the ventricular rate or with epicardial temporary pacing wires placed at surgery Although temporary pacing frequently is necessary for rate support, permanent pacemaker implantation should usually be delayed to 14 days to allow for potential recovery of AV conduction Most patients who recover AV conduction so within days of surgery.61 ... The other idiopathic VT arises from the posterior fascicle of the left-sided conduction system, producing a right bundle branch block pattern with leftward QRS axis (Fig 33.15) This tachycardia... conduction Clinically, this arrhythmia can be managed much like SVT in that life-threatening decompensation or progression to ventricular fibrillation is uncommon drome This arrhythmia is associated... 349 • Fig 33.15 Idiopathic ventricular tachycardia in an otherwise healthy 12-year-old Note right bundle branch block pattern with superior axis, typical of origin within the left posterior