Disturbances of Rate and Rhythm of the Heart

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Chapter 429 Disturbances of Rate and Rhythm of the Heart

The term “arrhythmia” refers to a disturbance in heart rate or rhythm. Such disturbances can lead to heart rates that are abnormally fast, slow, or irregular. They may be transient or incessant, in-born or acquired, or caused by a toxin or by drugs. They may be a complication of surgical correction of congenital heart disease, a result of congenital metabolic disorders of mitochondria, or fetal inflammation as in maternal systemic lupus erythematosus (SLE). The principal risk of any arrhythmia, either slow or fast, is decreased cardiac output, or degeneration into a more critical arrhythmia such as ventricular fibrillation. Such arrhythmias may lead to syncope or to sudden death. When a patient has an arrhythmia, it is important to determine whether the particular rhythm is likely to lead to severe symptoms or to deteriorate into a life-threatening condition. Rhythm abnormalities, such as single premature atrial and ventricular beats, are common in children without heart disease and in the great majority of instances do not pose a risk to the patient.

A number of effective pharmacologic agents are available for treating arrhythmias in adults; many have not been studied extensively in children. Insufficient data are available regarding pharmacokinetics, pharmacodynamics, and efficacy in the pediatric population, and therefore the selection of an appropriate agent is often necessarily empirical. Fortunately, the majority of rhythm disturbances in children can be reliably controlled with a single agent (Table 429-1). Transcatheter ablation is acceptable therapy not only for life-threatening or drug-resistant tachyarrhythmias but also for the elective definitive treatment of arrhythmias. For patients with bradycardia, implantable pacemakers are small enough for use in premature infants. Implantable cardioverter-defibrillators (ICDs) are available for use in high-risk patients with malignant ventricular arrhythmias and an increased risk of sudden death.

429.1 Principles of Antiarrhythmic Therapy

George F. Van Hare

When considering drug therapy in the pediatric population, it is important to recognize that there are marked differences in pharmacokinetics by age and in comparison with adults. Infants may have slower absorption, slow gastric emptying, and differing sizes of drug tissue compartments affecting the volume of distribution. Hepatic metabolism and renal excretion may vary within the pediatric age group as well as in comparison to adults. When considering antiarrhythmic therapy, it is important to recognize that the likely arrhythmia mechanism may be different for the pediatric vs. adult population.

There are many antiarrhythmic agents available for rhythm control. The majority have not been approved by the U.S. Food and Drug Administration (FDA) for use in children; their use is usually considered “off-label,” Pediatric cardiologists have experience with these drugs, and there are well-recognized standards regarding dosing.

With the availability of potentially curative ablation procedures, medical therapy has become less important. Clinicians and patients accept fewer drug side effects. Intolerable side effects, as well as the potential for a proarrhythmia induced by an antiarrhythmic drug, can seriously limit medical therapy and will lead the physician and family toward a potentially curative ablation procedure.

Antiarrhythmic drugs are commonly categorized using the Vaughan Williams classification system. This system comprises 4 classes: Class I includes agents that primarily block the sodium channel, class II includes the β-blockers, class III includes those agents that prolong repolarization, and class IV are the calcium channel blockers. Class I is further divided by the strength of the sodium channel blockade (see Table 429-1).

429.2 Sinus Arrhythmias and Extrasystoles

George F. Van Hare

Phasic sinus arrhythmia represents a normal physiologic variation in impulse discharges from the sinus node related to respirations. The heart rate slows during expiration and accelerates during inspiration. Occasionally, if the sinus rate becomes slow enough, an escape beat arises from the atrioventricular (AV) junction region (Fig. 429-1). Normal phasic sinus arrhythmia can be quite prominent in children and may mimic frequent premature contractions, but the relationship to the phases of respiration can be appreciated with careful auscultation. Drugs that increase vagal tone, such as digoxin, may exaggerate sinus arrhythmia; it is usually abolished by exercise. Other irregularities in sinus rhythm, especially bradycardia associated with periodic apnea are commonly seen in premature infants.

Sinus bradycardia is due to slow discharge of impulses from the sinus node, the heart’s natural pacemaker. A sinus rate <90 beats/min in neonates and <60 beats/min in older children is considered to be sinus bradycardia. It is commonly seen in well-trained athletes; in healthy individuals, it is generally without significance. Sinus bradycardia may occur in systemic disease (hypothyroidism or anorexia nervosa), and it resolves when the disorder is under control. It may also be seen in association with conditions in which there is high vagal tone, such as gastrointestinal obstruction or intracranial processes. Low birthweight infants display great variation in sinus rate. Sinus bradycardia is common in these infants in conjunction with apnea, and may be associated with junctional escape beats. Premature atrial contractions are also frequent. These rhythm changes, especially bradycardia, appear more commonly during sleep and are not associated with symptoms. Usually, no therapy is necessary.

Wandering atrial pacemaker (Fig. 429-2) is defined as an intermittent shift in the pacemaker of the heart from the sinus node to another part of the atrium. It is not uncommon in childhood and usually represents a normal variant; it may also be seen in association with sinus bradycardia in which the shift in atrial focus is an escape phenomenon.

Extrasystoles are produced by the premature discharge of an ectopic focus that may be situated in the atrium, the AV junction, or the ventricle. Usually, isolated extrasystoles are of no clinical or prognostic significance. Under certain circumstances, however, premature beats may be due to organic heart disease (inflammation, ischemia, fibrosis) or to drug toxicity, especially from digoxin.

Premature atrial contractions are common in childhood, usually in the absence of cardiac disease. Depending on the degree of prematurity of the beat (coupling interval) and the preceding R-R interval (cycle length), premature atrial complexes may result in a normal, a prolonged (aberrancy), or an absent (blocked premature atrial complex) QRS complex. The last occurs when the premature impulse cannot conduct to the ventricle due to refractoriness of the AV node or distal conducting system (Fig. 429-3). Atrial extrasystoles must be distinguished from premature ventricular contractions (PVCs). Careful scrutiny of the electrocardiogram for a premature P wave preceding the QRS will either show a premature P wave superimposed on, and deforming, the preceding T wave, or a P wave that is premature and has a different contour from that of the other sinus P waves. Atrial premature complexes usually reset the sinus node pacemaker, leading to an incomplete compensatory pause, but this feature is not regarded as a reliable means of differentiating atrial from ventricular premature complexes in children.

PVCs may arise in any region of the ventricles. They are characterized by premature, widened, bizarre QRS complexes that are not preceded by a premature P wave (Fig. 429-4). When all premature beats have identical contours, they are classified as uniform, suggesting origin from a common site. When PVCs vary in contour, they are designated as multiform, suggesting origin from more than 1 ventricular site. Ventricular extrasystoles are often, but not always, followed by a full compensatory pause. The presence of fusion beats, that is, complexes with morphologic features that are intermediate between those of normal sinus beats and those of PVCs, proves the ventricular origin of the premature beat. Extrasystoles produce a smaller stroke and pulse volume than normal and, if quite premature, may not be audible with a stethoscope or palpable at the radial pulse. When frequent, extrasystoles may assume a definite rhythm, for example, alternating with normal beats (bigeminy) or occurring after 2 normal beats (trigeminy). Most patients are unaware of single premature ventricular contractions, although some may be aware of a “skipped beat” over the precordium. This sensation is due to the increased stroke volume of the normal beat after a compensatory pause. Anxiety, a febrile illness, or ingestion of various drugs or stimulants may exacerbate PVCs.

It is important to distinguish PVCs that are benign from those that are likely to lead to more severe arrhythmias. The former usually disappear during the tachycardia of exercise. If they persist or become more frequent during exercise, the arrhythmia may have greater significance. The following criteria are indications for further investigation of PVCs that could require suppressive therapy: (1) 2 or more ventricular premature beats in a row, (2) multiform PVCs, (3) increased ventricular ectopic activity with exercise, (4) R on T phenomenon (premature ventricular depolarization occurs on the T wave of the preceding beat), and (5) most importantly, the presence of underlying heart disease, a history of heart surgery, or both. The best therapy for benign PVCs is reassurance that the arrhythmia is not life threatening, although very symptomatic individuals may benefit from suppressive therapy. Malignant PVCs are usually secondary to another medical problem (electrolyte imbalance, hypoxia, drug toxicity, cardiac injury, or an intraventricular catheter). Successful treatment includes correction of the underlying abnormality. An intravenous lidocaine bolus and drip is the 1st line of therapy, with more effective drugs such as amiodarone reserved for refractory cases or for patients underlying ventricular dysfunction or hemodynamic compromise.

429.3 Supraventricular Tachycardia

George F. Van Hare

Supraventricular tachycardia (SVT) is a general term that includes essentially all forms of paroxysmal or incessant tachycardia except ventricular tachycardia. The category of SVT can be divided into 3 major subcategories: re-entrant tachycardias using an accessory pathway, re-entrant tachycardias without an accessory pathway, and ectopic or automatic tachycardias. Atrioventricular reciprocating tachycardia (AVRT) involves an accessory pathway and is the most common mechanism of SVT in infants. Atrioventricular node re-entry tachycardia (AVNRT) is rare in infancy but there is an increasing incidence of AVNRT in childhood and into adolescence. Atrial flutter is rarely seen in children with normal hearts, whereas intra-atrial re-entry tachycardia (IART) is common in patients following cardiac surgery. Atrial and junctional ectopic tachycardias are more commonly associated with abnormal hearts (cardiomyopathy) and in the immediate postoperative period following surgery for congenital heart disease.

Clinical Manifestations

Re-entrant SVT is characterized by an abrupt onset and cessation; it usually occurs when the patient is at rest, although in infants it may be precipitated by an acute infection. Attacks may last only a few seconds or may persist for hours. The heart rate usually exceeds 180 beats/min and may occasionally be as rapid as 300 beats/min. The only complaint may be awareness of the rapid heart rate. Many children tolerate these episodes extremely well, and it is unlikely that short paroxysms are a danger to life. If the rate is exceptionally rapid or if the attack is prolonged, precordial discomfort and heart failure may occur. In children, SVT may be exacerbated by exposure to over-the-counter decongestants or by bronchodilators.

In young infants, the diagnosis may be more obscure because of the inability to communicate their symptoms. The heart rate at this age is normally higher than in older children and it increases greatly with crying. Infants with SVT on occasion initially present with heart failure, because the tachycardia may go unrecognized for a long time. The heart rate during episodes is frequently in the range of 240-300 beats/min. If the attack lasts 6-24 hr or more, heart failure may be recognized, and the infant will have an ashen color, and be restless and irritable, with tachypnea and hepatomegaly. When tachycardia occurs in the fetus, it can cause hydrops fetalis, which is the in utero manifestation of heart failure.

In neonates, SVT is usually manifested as a narrow QRS complex (<0.08 sec). The P wave is visible on a standard electrocardiogram in only 50-60% of neonates with SVT, but it is detectable with a transesophageal lead in most patients. Differentiation from sinus tachycardia may be difficult, but is important, as sinus tachycardia requires treatment of the underlying problem (e.g., sepsis, hypovolemia) rather than antiarrhythmic medication. If the rate is >230 beats/min with an abnormal P-wave axis (a normal P wave is positive in leads I and aVF), sinus tachycardia is not likely. The heart rate in SVT also tends to be unvarying, whereas in sinus tachycardia the heart rate varies with changes in vagal and sympathetic tone. AV reciprocating tachycardia uses a bypass tract that may either be able to conduct bidirectionally (Wolff-Parkinson-White [WPW] syndrome) or retrograde only (concealed accessory pathway). Patients with WPW syndrome have a small, but real risk of sudden death. If the accessory pathway rapidly conducts in antegrade fashion, the patient is at risk for atrial fibrillation begetting ventricular fibrillation. Risk stratification, including 24 hr Holter monitoring and exercise study, may help differentiate patients at higher risk for sudden death from WPW. Syncope is an ominous symptom in WPW and any patient with syncope and WPW syndrome should have an electrophysiology study and likely catheter ablation.

The typical electrocardiographic features of the Wolff-Parkinson-White syndrome are seen when the patient is not having tachycardia. These features include a short P-R interval and slow upstroke of the QRS (delta wave) (Fig. 429-5). Though most often present in patients with a normal heart, this syndrome may also be associated with Ebstein anomaly of the tricuspid valve, or hypertrophic cardiomyopathy. The critical anatomic structure is an accessory pathway consisting of a muscular bridge connecting atrium to ventricle on either the right or the left side of the AV ring (Fig. 429-6). During sinus rhythm, the impulse is carried over both the AV node and the accessory pathway; it produces some degree of fusion of the 2 depolarization fronts that results in an abnormal QRS. During AVRT, an impulse is carried in antegrade fashion through the AV node (orthodromic conduction), which results in a normal QRS complex, and in retrograde fashion through the accessory pathway to the atrium, thereby perpetuating the tachycardia. In these cases, only after cessation of the tachycardia are the typical ECG features of WPW syndrome recognized (see Fig. 429-5). When rapid antegrade conduction occurs through the pre-excitation pathway during tachycardia and the retrograde re-entry pathway to the atrium is via the AV node (antidromic conduction), the QRS complexes are wide and the potential for more serious arrhythmias (ventricular fibrillation) is greater, especially if atrial fibrillation occurs.

AV nodal re-entrant tachycardia (AVNRT) involves the use of 2 pathways within the AV node. This arrhythmia is more commonly seen in adolescence. It is one of the few SVTs that is occasionally associated with syncope. This arrhythmia is usually amenable to antiarrhythmic therapy, such as β-blockers, or to catheter ablation therapy.