Symposium on Pediatric Cardiology

Disorders of Heart Rate and Rhythm Warren G. Guntheroth, M.D. *

Disorders of the heart beat are frequently classified in textbooks according to the mechanism of the disorder. This classification has the advantage of precision, based on electrophysiologic investigations in animals and man. The chief disadvantage to this approach is that it begins with the answer, whereas the physician begins with a patient, and an electrocardiogram. The presentation here will be that of the physician, beginning with regular rhythm with rate disorders, and progressing to degrees of irregularity of rhythm. 7 However, in the final diagnosis and choice of therapy, the mechanism must be considered. Accordingly, a brief discussion of mechanisms will precede the clinical segment. The study of mechanism begins with the individual myocardial cell. Figure 1 presents a typical intracellular recording of ventricular myocardium with a simultaneous ECG lead; the phases of a complete cardiac cycle are conventionally labeled as 0 for rapid depolarization, 1 for the early rapid repolarization, 2 for the plateau or slow repolarization, 3 for the late, rapid repolarization, and 4 for electrical diastole. In normal, non-pacemaker cells phase 4 is flat.

Automaticity In pacemaker cells such as cells of the sinus node, a gradual diastolic depolarization occurs in phase 4, leading to spontaneous firing when the threshold is passed (Fig. 2). This activity, automaticity, is essential to the organized heart beat, but automaticity also generates many of the ectopic arrhythmias that can be life-threatening. Normally, the sinoatrial node, the upper His bundle, and most Purkinje fibers are capable of pacing the heart, and the inherent frequency of each ·Professor of Pediatrics, and Head, Division of Pediatric Cardiology, University of Washington School of Medicine, Seattle, Washington Plidiatric Clinics of North America - Vol. 25, No.4, November 1978

869

870

WARREN G. GUNTHEROTH

4

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Figure 1. The upper record represents the ventricular, intracellular potential during a complete cardiac cycle, and the lower record represents a standard electrocardiographic lead. The numbers on the upper trace are used to designate phases in the cycle: the upstroke, the brief spike, the plateau, the rapid recovery, and electrical diastole, respectively. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

T

QRS

center is lower, in order listed. Consequently, the lower centers do not capture the pacemaking role unless the next higher center fails, or there is a block in transmission of depolarization from the higher centers, or uncommonly, the lower center has an abnormally accelerated rate. The lower pacemakers are thus normally suppressed by depolarization from the higher centers with higher frequency; the process is called overdrive suppression. Unfortunately, overdrive suppression of the sinus node can occur when an abnormally accelerated lower center captures the pacemaker. If the lower center then stops abruptly, the sinus node recovery may be slow enough to produce syncope, due to a combination of vagal/sympathetic imbalance and an excessive accumulation of sodium or calcium ions in the sinoatrial cells. 19 The rate of firing of pacemaker cells is determined by three variables that could be predicted from Figure 2. These variables are the degree of maximal polarization at the end of phase 3, the slope of phase 4, and the threshold for firing. The second of these appears to be the most important controller. Normally, temperature has a profound effect on pacemaker frequency, as do vagal and sympathetic activity. Although the effects of acetylcholine and catecholamines have been attributed to an interaction between an inward background sodium

Figure 2. Intracellular recording from sino-atrial pacemaker' tissue. Note the gradually increasing potential in phase 4 as compared with the lack of any activity in this phase in Figure 1. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

200 msec

40.

60.

DISORDERS OF HEART RATE AND RHYTHM

871

current and a decaying outward potassium current,19 there is recent evidence that, in sinoatrial cells, the inward current is carried by calcium. 6 The details of these voltage clamp experiments are not crucial here, but the phenomena may explain the inotropic effects of catecholamines and acetylcholine, by virtue of their effects of increasing and decreasing intracellular calcium, respectively.9 On the other hand, rapid depolarization in Purkinje and ventricular cells is due to a sodium current controlled by gates in the cell membrane that are ionspecific. Quinidine and procainamide reduce automaticity of ventricular ectopic pacemaker by their effects on these sodium channels. Since the transmission of current extraceUularly is directly proportionate to the rate of rise of phase 0 (rapid inward sodium current),13 reduction of the rate of rise by quinidine will tend to prevent transmission to the entire ventricle from a prematurely firing ventricular focus, as well as reducing the chances of the premature firing. In short, there are two separate ionic mechanisms, calcium and sodium, that are involved in abnormal automaticity which respond to different types of drugs.

Triggered Arrhythmias Another abnormal mechanism that is capable of producing tachycardia is early after-depolarization. 3 The affected cells are not truly automatic, since this arrhythmia requires a transmitted depolarization; these cells therefore are called triggerable cells. Definition of the phenomenon requires intracellular recordings; at present, the prevalence of this phenomenon in clinical tachycardias is unknown. The most probable example is bigeminy. The pharmacologic approach will likely be different from that of true automaticity; this may account for therapeutic failure in individual cases of arrhythmia that seem identical clinically. Re-Entry Arrhythmias Re-entry accounts for a large part of the arrhythmias seen in children, including disorders that previously had been attributed to abnormal automaticity. Re-entry includes a diverse group of arrhythmias, having in common a mismatch of recovery times; depolarization waves may be perpetuated in a complete circuit involving a succession of myocardium with slower and faster conduction. The relatively slow portion of the circle may be normal tissue, such as the upper atrioventricular node, linked to a rapidly-conducting bypass mechanism, such as the bundle of Kent in Wolff-Parkinson-White syndrome. In this syndrome, during normal rhythm, the wave of atrial depolarization bypasses the slower-conducting atrioventricular node, and causes excitation of some portion of the septum or free wall of the left or right ventricle. IS However, the atrioventricular node is also activated during normal rates, and most of the ventricles will be excited via the His bundle. The His bundle will thus be refractory to the depolarization entering from the bypass fibers. However, if premature atrial or ventricular depolarization occurs, depending on timing and location, the atrioventricular node, or the bypass tract, may still be refractory, and

872

WARREN G. GUNTHEROTH

the premature depolarization will be blocked in one or the other pathway. By the time that the conducted depolarization reaches the junction of the His bundle and the bypass tract, recovery may have occurred in the alternate conduction path, and the depolarization wave will then proceed in both directions, normally (anterograde) and retrograde, causing re-entry into the chamber of origin, and completion of the circus movement, resulting in a tachycardia. The bypass tract may be involved in an atrioventricular or ventriculoatrial direction, which can be determined by the presence or absence of a delta-wave in the conventional ECG. Other forms of circus movement include atrial flutter and fibrillation; these are apt to occur in atria so large that, considering the very brief action potention of the atrial cell and the moderate velocity of atrial conduction, it is possible to have recovery of any given atrial cell by the time the wave of depolarization completes a circle of atrial tissue. (Atrial flutter can also result from rapid automatic activity from a non-sinus, atrial focus.) Atrial flutter can be produced in normal subjects by strong vagal stimulation due to further shortening of the already brief refractory period of atrial myocardium, accomplishing the necessary conditions for circus movement without lengthening the pathway.

Electrophysiology of the Atrioventricular Node Recent discoveries of the function and anatomy of the atrioventricular node have enlightened certain clinical phenomena, but have led to changes in terminology that are apt to be confusing. The primary function of the atrioventricular node, without question, is delay of conduction between the atrium and ventricle to provide optimal preload of the ventricle by the atrial contraction. This function, delay, is carried out in the upper and middle parts of the node, identified as atrio-nodal and nodal regions. 1 The most distal part of the node is the nodal-His zone. The secondary, but essential, role of the nodal apparatus is that of a pacemaker in the event of failure of the sino-atrial node, or failure of atrioventricular node conduction. In order to function as the failsafe pacemaker, the pacemaker would logically have to be distal to the sites of delay of transmission, which are the vulnerable sites. This distal pacemaker is the nodal-His or upper His bundle. Because no spontaneous phase 4 depolarization was found in the atrio-nodal or nodal parts of the atrioventricular node, Hoffman and Cranefield lO questioned whether escape rhythms should be called nodal rhythm, the term used for many years. Subsequently, there has been a bandwagon to call these rhythms "atrioventricular junctional" or even "junctional." This is regrettable, since "junctional" was applied by Erlanger4 in 1910 to the junction between the atrium and atrioventricular node (the atrio-nodal region), and could apply to the atrio-nodal or nodal-His portions of the atrioventricular node. Further, since "atrioventricular junction" could apply to the entire fibrous skeleton separating both atria and ventricles, it is more general than "atrioventricular node complex." If a change is needed from "nodal rhythm," His bundle rhythm would be far more specific.

DISORDERS OF HEART RATE AND RHYTHM

873

Another set of basic cellular studies has been of genuine value clinically. The atrio-nodal and nodal cells of the atrioventricular node have action potentials very similar to the action potential of sinoatrial cells, with a slow upstroke of phase o. This reflects the absence of a rapid inward (sodium) current, and suggests that the action potential is dependent on a slow inward (calcium) current. Evidence of the calcium-dependence of the atrioventricular node depolarization is found in the effects of a new (in this country) drug, verapamil. This drug is useful in the treatment of re-entry tachycardias by slowing atrioventricular conduction, and has a specific effect of reducing or blocking the slow inward current.

Newer Investigative Techniques In the past 10 years, clinical investigations of cardiac arrhythmias have greatly increased, primarily through the use of catheter recording of the His bundle activity.16 The electrical activity of the entire atrioventricular node does not produce sufficient extracellular current to allow an external record, and therefore an electrode catheter is necessary to record the firing of the His bundle. Further information as to the function of the "atrioventricular node can be obtained by electrical pacing of the atrium and ventricle at varying rates while recording the His bundle activity, and of course, the surface ECG. Many mechanisms that were deduced by ECG analysis in the past 50 years, with only calipers, have been confirmed by these techniques; occasionally, the use of the invasive methods are clinically justified. However, considering that drug therapy for arrhythmias is still largely empiric, His bundle recording is rarely, if ever, mandatory in the treatment of a child with a disorder of the heart beat. The medical treatment of re-entry arrhythmias will, on rare occasion, fail, and surgical division of the anomalous tract that bypasses the atrioventricular node will be necessary. This search for the anomalous connection may be assisted by careful vectorcardiography to detect the direction of the earliest part of the ventricular depolarization, the delta wave. IS In the vast majority of infants and children with disorders of the heart beat, the benign nature of the condition and the effectiveness of pharmacologic management permit a noninvasive approach, based on a careful analysis of the clinical ECG supplemented occasionally with ambulatory Holter monitoring of several hours' duration. 12

CLINICAL ELECTROCARDIOGRAPHIC APPROACH The most logical approach to a patient with a suspected disorder of the heart beat is that which begins with disorders of rate and progresses to disorders of rhythm. In addition, there is a small group of disorders with normal rate and rhythm, but an abnormal mechanism revealed only by the electrocardiogram (Table 1). Sinus Rhythm Ordinarily, the heart rate is between 60 and 100 beats per minute

874

WARREN G. GUNTHEROTH

Table 1. Disorders of Rate, Mechanism, and Rhythm Normal Rate, Mechanism, and Rhythm Regular sinus rhythm Rapid Rate, Regular Rhythm Sinus tachycardia Atrioventricular re-entry tachycardia Ectopic tachycardias: atrial, atrioventricular nodal (His bundle), and ventricular Slow Rate, Regular Rhythm Sinus bradycardia His bundle rhythm with sinoatrial block or complete atrioventricular block Idioventricular rhythm with sinoatrial block or complete atrioventricular block 2: 1 atrioventricular block Normal Rate, Regular Rhythm, Abnormal Mechanism Ectopic atrial pacemaker with vagal control First degree atrioventricular block Accelerated His bundle pacemaker Atrial tachycardia with 2: 1 atrioventricular block Arrhythmias: Single or Infrequent Beats Extrasystole (atrial, His bundle, and ventricular) Atrioventricular dissociation with interference Second degree atrioventricular block (Mobitz II) Regular Arrhythmias Sinus arrhythmia Wenckebach phenomenon (Mobitz I) Bigeminy (atrial, His bundle, and ventricular) Trigeminy Echo rhythm Parasystole Irregular Arrhythmias Atrial flutter with variable atrioventricular block Atrial fibrillation Ventricular fibrillation Asystole Sinoatrial arrest Complete atrioventricular block with no escape

DISORDERS OF HEART RATE AND RHYTHM

875

for the adult, and each beat begins in the sinoatrial node, produces a normally oriented P-vector (to the left, inferior, and slightly anterior), and is followed after a normal P-R interval by the QRS complex. If these criteria are met, the subject has normal sinus rhythm. Although heart rates in infants are normally outside this upper range, terminology of tachycardia and bradycardia presented here will conform to the traditional. A heart rate over 100 but with characteristics of a sinus rhythm is defined as sinus tachycardia, and if the rate is under 60, sinus bradycardia.

Rapid Rate, Regular Rhythm Sinus tachycardia may run as high as 220 in infants who are stressed. At those rates, there may be overlapping of the P-waves on the T-waves, and it may be difficult to be certain that the origin of atrial activation is in the sinoatrial node. The commonest arrhythmia that involves an abnormal mechanism is paroxysmal atrial tachycardia. The characteristic clinical features of paroxysmal atrial tachycardia are an abrupt onset and termination, with a clock-like regularity during the arrhythmia (Fig. 3). The P-waves are difficult to distinguish at these rates, and some prefer to label this "supraventricular tachycardia" since the origin of the rapid rate may not be provable by noninvasive means. Some cases are truly ectopic atrial tachycardia. In those instances, there is no clear distinction between an atrial tachycardia and a regular atrial flutter, although the latter term is usually used for extremely rapid rates, and partial atrioventricular block will be probable. It is now clear that a majority of patients with paroxysmal atrial tachycardia represent a different disorder, re-entry phenomenon. In the earliest recognized condition predisposing to re-entry, Wolff-Parkinson-White syndrome, there is a short P-R interval and a wide QRS complex, with slurred upstroke (Fig. 4, below). Under certain conditions, such as an atrial premature depolarization, a circus movement may be initiated, as explained above, and a very rapid tachycardia will result (Fig. 4, above). In many normal subjects who do not have a bundle of Kent, paroxysmal atrial tachycardia is also thought to occur due to re-entry phenomenon involving other bypass tracts which are present, but usually silent. One of the most poorly tolerated forms of atrial tachycardia is that found in "sick sinus syndrome." In that disorder, the sinoatrial node does not function properly, and an atrial ectopic pacemaker without vagal control may begin firing at an abnormal rate. When the tachyrhythmia terminates, there will be overdrive suppression of both the sinus node and the atrioventricular node, and a marked bradycardia or asystole will follow. Other forms of ectopic tachycardias, including atrioventricular nodal (His bundle) or ventricular tachycardia will usually present with a somewhat slower rate than the atrial or re-entry tachycardias. Ventricular tachycardia is diagnosed on the basis of wide, bizarre QRS complexes with no fixed relationship to the P-waves (Fig. 5). The difficulty in separating out these tachycardias with different mechanisms

876

WARREN G. GUNTHEROTH

Figure 3. Electrocardiogram recorded during spontaneous conversion from paroxysmal atrial tachycardia to normal sinus rhythm. The last atrial beat is not conducted. The third beat after conversion appears to be a ventricular extrasystole, although it may represent an aberrantly conducted atrial beat. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

is increased with more rapid rate. For example, Figure' 4, top, was a re-entry tachycardia, but except for the rate, would be difficult to distinguish from a ventricular tachycardia. At rapid rates, fatigue in conduction in the His bundle may result in broadened QRS complexes, in the absence of pre-excitation, referred to as aberrant conduction.

Slow Rate, Regular Rhythm Sinus bradycardia is compatible with excellent health, such as in long-distance runners. An example of sinus bradycardia may be seen in Figure 6; in panel C the bradycardia is sufficiently slow to allow the His bundle pacemaker to escape from the control of the sinoatrial node. The escape was only partial, resulting in a fusion beat, indicating independence of the two pacemakers, firing in a coincidental fashion rather than by virtue of conduction from one to the other. After one

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Figure 4. Above, Paroxysmal atrial tachycardia in a patient with preexcitation syndrome (Wolff-Parkinson-White syndrome), more evident in the lower record, with sinus rhythm. The upper record is practically indistinguishable from ventricular tachycardia. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

DISORDERS OF HEART RATE AND RHYTHM

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Figure 5. Ventricular tachycardia. There are two nonnal QRS complexes in aVF. In general, there is independence of the atrial and ventricular beats. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

beat, the sinoatrial node again captures as the dominant pacemaker. This is a form of atrioventricular dissociation. This term should be reserved for dissociation of the atrial and ventricular beats due to the rate of the His bundle exceeding that of the sinus node, and should not be used for failure of atrioventricular conduction (atrioventricular block). Atrioventricular dissociation is unlikely with very slow, regular rhythm since, if the conduction is intact, the sinus node is almost certain to capture occasionally and produce a mild irregularity of the rhythm, whereas complete atrioventricular block is characterized by a

A

B

c Figure 6. Electrocardiograms from three nonnal premature infants. A, Sinus arrhythmia. B, Sinus tachycardia. C, Sinus bradycardia with nodal escape (n), also referred to as junctional or His bundle escape. The complex with the escape is a fusion beat, since atrial excitation had begun just prior to the independent firing of the His bundle. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

878

WARREN G. GUNTHEROTH

Figure 7. Complete atrioventricular block, and (below) pressure record from the left ventricle, demonstrating the effects of atrial contraction. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

Figure 8. A particular type of second degree atrioventricular block, 2:1 block. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

remarkably constant rate, With no interference from the sinus pacemaker (Fig. 7). An even slower rate is produced by the relatively uncommon idioventricular rhythm, with a pacemaker somewhere in the Purkinje system, with a very low frequency. The QRS's are broad and bizarre in configuration, and are not regularly preceded by P-waves. Figure 8 presents another form of slow rate and regular rhythm, 2: 1 atrioventricular block, with an atrial rate of 150, and a ventricular rate of 75. This is only one form of second degree atrioventricular block. Normal Rate, Regular Rhythm, but Abnormal Mechanism Since the rate and rhythm are normal, these disorders are diag- . nosed almost exclusively by the ECG. The category includes the relatively benign ectopic atrial pacemaker, with abnormal P-axis which may shift during the recording. The pacemaker is usually under vagal control, like the sinoatrial node, and therefore not subject to the tachybrady problems of the "sick sinus syndrome." A very common disorder, first degree atrioventricular block, is diagnosed on the basis of a P-R interval prolonged beyond the normal limits for age and rate. (Table 2). The majority of individuals with this

879

DISORDERS OF HEART RATE AND RHYTHM

Table 2.

P-R Interval, with Rate and Age: Average (and Upper Limits of NormalY

0·1/12 YEAR

RATE

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0,lO(I),12 ) (O]S) 0,IO(O,U5) Q,IO({).II) 0,105(0,14) (l,ll (0,14) 0,]2(0,14) i (),]I(O,14) O,OfJ(O.ll ) 0,10 (0.13) 0.]()S(O,1")E ().IO(O.ll ) 0,]0 (0,12) 0,10 (0,12) 1),10(0,12) {).(m O,O\) ((1.11) (l.lO (Oil)

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0.16 (0.18) 0.16101g) 0.105 (lUi) 1 0.1.'l(OI8) 0,145({),16) O.I.5(O,li) (),145({),1.5) 0.15({),16) 0,14 ({),15)

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Co., 1965.

finding have no heart disease, although it may serve as a subtle indication of myocardial disorder in a patient with a previously normal P-R interval. This may occur in rheumatic fever, but also in common infectious diseases. P-R prolongation is a very useful early sign of digitalis intoxication in children, and for this reason, a control ECG tracing is mandatory before digitalization. Accelerated nodal (His bundle) pacemaker may produce a relatively normal heart rate. Similarly, an atrial tachycardia with 2: 1 atrioventricular block could result in a relatively normal heart rate and regular rhythm.

Arrhythmias: Single or Infrequent Beats Extrasystoles are very common, probably a universal experience. If the premature contraction begins with a P-wave, with a reasonably normal P-R interval, the extrasystole would be termed an atrial premature depolarization (Fig. 9). Since these usually originate from a site other than the sinoatrial node, the P-wave will usually be different from the ordinary P-wave, and, depending upon the degree of prematurity, the QRS complex also may be altered, representing aberrant conduction. With atrial extrasystoles, the atrium and sinus node are depolarized, which re-cycles these pacemaker cells, so that the next normal beat occurs at a normal R-R interval. Nodal or His bundle premature depolarization will not be preceded by a P-wave, and mayor may not have a normal appearing QRS complex, depending on whether or not aberrant conduction occurs. The His bundle premature beat may proceed retrogradely through the atrioventricular node, but by that time, the atrium will usually have been depolarized independently from the sinoatrial node. There will thus be an unusually long interval until the next firing of the sinoatrial node. This interval is referred to as a compensatory pause, and is also found in ventricular extrasystoles (Fig. 9). Atrioventricular dissociation produces an infrequent irregularity of rhythm due to interference between two pacemakers. Sometimes the phenomenon of atrioventricular dissociation accompanies marked sinus arrhythmia (Fig. lOA). In Figure lOB, the basic rhythm is regular with an R-R interval of .53 seconds, and a single premature beat occurs with an R-R interval of .42 seconds. This premature beat is

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c Fig. 10 Figure 9. Ahove, Frequent premature atrial contractions (pa). The succeeding sinoatrial beat is only slightly delayed, and there is no compensatory pause. In the first atrial extrasystole, the QRS is somewhat broadened, indicating aberrant ventricular conduction. In the lower record, a premature ventricular beat (pv) is the first complex, followed by a compensatory pause prior to the next, normal sinoatrial beat. The beat labeled "pn" is a premature nodal beat from the His bundle with a compensatory pause. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.) Figure 10. Atrioventricular dissociation. A, Sinus arrhythmia with His bundle or nodal escape (ne). B, Nodal or His bundle tachycardia producing atrioventricular dissociation with interference by sinoatrial capture for one beat (sa). C, 2: 1 atrioventricular block with tachycardia is the basic rhythm; most atrial beats are conducted (c), but occasionally the His bundle is sufficiently early to take over (n). (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

DISORDERS OF HEART RATE AND RHYTHM

881

preceded by a P-R interval of .21 seconds, whereas the other beats present a random relationship between the P and QRS complexes. All but one of these beats are produced by a nodal or His bundle tachycardia, and the premature beat represents capture by the sinoatrial node, with a slower rate of 107 compared with the ventricular rate of 113 (Fig. lOB). This arrhythmia is defined as interference dissociation, and can occur either with marked slowing of the sinoatrial node, or with acceleration of the His bundle pacemaker. Figure laC represents a more complex arrhythmia, with a fundamental rhythm of 2: 1 atrioventricular block, but occasionally the atrioventricular node (n) or His bundle escapes and becomes the pacemaker. (The first of the two beats marked "n" is a fusion beat, resulting from superimposition of the P and QRS complexes.) Occasionally, a single atrial beat will be blocked in a special form of second degree atrioventricular block called Mobitz Type II, and in this disorder, there is no progressive prolongation of the P-R interval prior to the dropped beat in contrast to Wenckebach phenomenon. Although the disorder by itself is not dangerous, it is of considerable clinical significance, since it may indicate a disease of the common His bundle, and may precede complete heart block, with an idioventricular pacemaker.

Regular Arrhythmias The most common of all arrhythmias is sinus arrhythmia (see Fig. 6). The rate accelerates with inspiration and slows with expiration, and its presence indicates that the sinoatrial node is operating under vagal dominance. A cyclical arrhythmia which is usually pathologic is the Wenckebach (Fig. 11). In this phenomenon there is a progressive increase in the P-R interval, culminating in a blocked atrial contraction, with no ventricular response, Following the dropped beat a relatively normal P-R interval ensues, and the cycle begins over. This arrhythmia, also known as a Type I Mobitz block, is less ominous than Type II, and can be found in normal athletes. Usually, Wenckebach phenomenon occurs in the upper atrioventricular node. An additional characteristic of this disorder which is less well known is the progressive shortening of the R-R interval with each beat after the dropped beat. Clearly, this form of second degree atrioventricular block represents a rate-related conduction disorder which can be produced even in a healthy heart with a sufficiently rapid atrial rate or increased vagal tone. In this phenomenon, the conduction of the atrioventricular node fatigues in a cumulative fashion, resulting in the complete failure of conduction, which then permits an interval sufficient for recovery, thereby allowing the cycle to commence again with a normal P-R interval. An easily recognizable, regular arrhythmia is ventricular bigeminy, produced by a regular alteration between a normal sinus beat and a ventricular extrasystole (Fig. 12). Bigeminy or coupling can also occur from atrial or His bundle foci. Ventricular bigeminy is an indicator of digitalis intoxication in the adult, but in children it is rarely re-

882

WARREN G. GUNTHEROTH

lated to digitalis. The mechanism is thought to be triggered due to after-potentials. :1 When extrasystoles occur every third beat, another form of regular coupling is found which is sometimes referred to as trigeminy, but this term should be reversed for the situation in which two extrasystoles follow a single normal beat, a much more alarming state. (From a semantic point of view, "three twins" is an unfortunate term at best.) An arrhythmia that may resemble bigeminy on auscultation is echo rhythm (Fig. 13). In this record, the patient has sinoatrial block as a result of the "sick sinus syndrome," and the fundamental pacemaker is in the His bundle. Most beats are followed by a coupled atrial beat, or return extrasystole. The mechanism of echo rhythm varies from one case to another, and includes phenomena such as re-entry, although this does not appear to be a likely situation in Figure 13, since the atria seem to be excited in a relatively normal sequence. An alternate possibility is that mechanical stimulation of the atria occurs secondary to the ventricular contraction. Pollack has suggested that this type of mechanical-electrical sequence may even be a fundamental characteristic of the sinoatrial node and atrioventricular node. 14 Parasystole is a regular arrhythmia that almost never occurs in children, and will not be discussed here. Irregular Arrhythmias Completely irregular arrhythmias involve either chaotic electrical activity (fibrillation), or very rapid, regular pacemaker activity with varying atrioventricular conduction. Atrial fibrillation is seen in Figure 14. With this extremely rapid bombardment of the atrioventricular node, it is fortunate that the conduction fatigues and does not conduct on a 1: 1 basis since, at that rate, the ventricles would not fill adequately during diastole. Although the rate present in the patient in Figure 14 does not have a disastrously high ventricular response, the ventricular rate is sufficiently high that it is inefficient, and the patient will usually benefit by the administration of a dosage of digitalis which could be considered toxic. The ability of the digitalis to prolong atrioventricular conduction will allow a higher degree of block in atrial fibrillation, or flutter, with resulting improvement due to this dromotropic effect, in contrast to the usual use of digitalis to improve the inotropic state (contractile state). Atrial flutter is demonstrated in Figure 15. Flutter will frequently produce a regular ventricular response at one-half to one-fourth of the atrial frequency, but a more complex relationship may occur when. there is a type of Wenckebach phenomenon (Fig. 15, above). (In the lower tracing, the rate is in fact regular, because there is complete atrioventricular block at the time of that tracing.) Atrial flutter may occur as the result of two different mechanisms. In some patients the flutter is due to the rapid firing of a single ectopic pacemaker in the atrium, which is almost certainly the case in the patient in Figure 15. The other type of flutter is caused by circus motion. The very brief phase 2 of the atrial action potential allows a very rapid recovery rela-

DISORDERS OF HEART RATE AND RHYTHM

883

Fig. 12

Fig. 13 Figure 11. Wenckebach phenomenon, a special form of second degree atrioventricular block. Every fourth or fifth atrial beat is not conducted. (FJ'Om Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.) Figure 12. Ventricular bigeminy or coupling. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.) Figure 13. Sinoatrial block with reciprocal rhythm or return extrasystoles. This form of coupling occurs here after the second, third, and fifth atrioventricular nodal beats. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

tive to the atrioventricular node or the ventricle. Consequently, it is possible to have the atrial cell recover and be re-excitable by the time a wave of excitation traverses the entire atrium, so that the circus motion will be maintained. This possibility will be enhanced if the pathway is longer, due to atrial dilatation, or if the transmission is slowed by abnormal myocardium. The ultimate arrhythmia, ventricular fibrillation, is seen in Figure 16. It should be immediately apparent that the chaotic electrical activity of the ventricles is not compatible with an effective cardiac output, and the resultant lack of coronary perfusion limits the possibility of spontaneous recovery.

Asystole Asystole could refer to either atrial or ventricular, although the latter is the more important. Sinoatrial arrest is compatible with life by virtue of the automaticity of the lower pacemakers in the His bundle and Purkinje cells of the ventricles. If these lower centers do not escape from sinoatrial arrest, or from complete atrioventricular block

884

WARREN G. GUNTHEROTH

Figure 14. Atrial fibrillation in a 15 year old girl with advanced rheumatic mitral stenosis. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

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Figure 15. Above, Atrial flutter with variable ventricular response. Below, Same patient as above, but with much slower atrial rate and with a slow, regular ventricular rate indicating complete atrioventricular block. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

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Figure 17. Complete atrioventricular block with atrial tachycardia, and no nodal or ventricular escape. Ventricular asystole is interrupted by external cardiac massage producing the QRS complexes at the end of the strip. (From Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Co., 1965.)

DISORDERS OF HEART RATE AND RHYTHM

885

(Fig. 17), death will ensue unless resuscitative efforts are immediately effective. The patient in Figure 17 was resuscitated by cardiac massage, which produced the two QRS complexes seen at the end of the strip. This is an example of an electrical response of the organ to mechanical stimulation. However, the effectiveness of cardiac massage does not depend entirely on the electrical response.

THERAPY The therapeutic approach to an infant or child with a disorder of the heart beat should be conditioned by the relatively benign nature of many of these problems. In many instances, no intervention whatsoever may be the safest and most sensible management. Experience with arrhythmias in adults with coronary artery disease who have little margin of safety cannot be transferred to the youthful heart. For example, ventricular tachycardia in an adult is commonly lifethreatening, and attempts to convert to sinus rhythm are urgently required. In the rare child with chronic ventricular tachycardia, conversion is rarely of lasting benefit, and the most reasonable goal is to keep the heart rate at a reasonable level, under 150, rather than an urgent assault with the goal of complete suppression of the ectopic ventricular focus. Second, before instituting relatively toxic antiarrhythmic drugs, scrutiny of the patient's other medical problems may reveal that the arrhythmia is a secondary manifestation, and treatment of the primary disorder will be more effective. Conduction disturbances and ectopic tachycardias may reflect hypoxia secondary to ventilation problems, acid-base disorders, electrolyte disturbances (particularly Ca++ and K+), or even toxicity from other cardiac drugs such as digitalis or quinidine. In fact, changes in heart rate can be useful in monitoring the primary disorder. Apnea will reliably produce bradycardia; therefore heart rate monitoring is simpler, less expensive, and has fewer false alarms than an apnea monitor. H The apnea monitor may not detect obstructive apnea, since chest movement continues in spite of the obstruction.

Pharmacologic Approach For most patients, as well as their physicians, noninvasive approaches to disorders of the heart beat are preferable for the initial therapy unless the patient is in shock. The choice of drug is guided by the ECG diagnosis and the characteristics of the drug. There have been attempts to classifY the antiarrhythmic drugs, and at least one system has achieved acceptance. 15 Class I drugs decrease the amplitude of depolarization and increase the action potential duration, and originally included quinidine and procainamide. Subsequently, propranolol was placed in Class I, although it tends to decrease the duration of the action potential. Class II drugs have no effect on the amplitude of the action potential but decrease its duration. Lidocaine and

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WARREN G. GUNTHEROTH

phenytoin were the "charter members" of Class II. All five of the above drugs decrease the slope of diastolic depolarization, and thus should be effective against abnormal automaticity. Newer drugs do not readily fit into these classes, particularly those that affect the inward calcium current, such as verapamil. Digitalis is another antiarrhythmic drug which is not classifiable by this system, but is the m o s t commonly used drug in the pediatric age group, as the drug of choice in paroxysmal atrial tachycardia (atrioventricular re-entry tachycardia). In that disorder, the in tropic effect of the drug is a useful but secondary feature in relation to the therapeutic goal of reversion to sinus rhythm. The two primary effects of digoxin are both potentially important, prolongation of atrioventricular nodal conduction and reduction of the refractory period of the accessory pathway;20 the desired result is, of course, interruption of the circus movement. In addition, digoxin "sensitizes" the vagus, which enhances vagal stimulation and makes conversion to sinus rhythm more likely (for dosage, see Table 3). The safest means of vagal stimulation is gagging with a tongue blade or massage of one carotid sinus. Phenylephrine works in a similar manner through the baroreceptor response after elevating the blood pressure. Although all of the available antiarrhythmic drugs may be required in a sequential trial in controlling a chronic and debilitating disorder in adults, that would be extremely rare in the pediatric age group. For similar reasons, it is unusual for a pediatrician or even pediatric cardiologist to develop much experience in the use of a long list of drugs. It would thus seem prudent to stick to a very few in the management of disorders of the heart beat. For the rare acute problem of ventricular tachycardia, spontaneous or secondary to digitalis intoxication, intravenous lidocaine is probably the drug of choice. Phenytoin is also highly effective for digitalis-induced arrhythmias. For the rare chronic ventricular tachycardia, quinidine is very effective in controlling the rate to a tolerable point in infants and children; although it may not abolish this serious arrhythmia, we have found nothing more effective. Procainamide is also effective, but requires more frequent administration, and we prefer quinidine. These two drugs are also at least as effective, or better, than propranolol, phenytoin, or lidocaine in the control of the His bundle (atrioventricular nodal or junctional) tachycardias, atrial fibrillation and paroxysmal atrial tachycardia. Quinidine is also effective against frequent premature beats, whether of atrial, His bundle, or ventricular origin, but these are rarely a problem in the pediatric population; even when present, children usually do not complain of them, even when "coached," and drug treatment accomplishes little. Although the disorder is more of a nuisance than a life-threatening disorder, the treatment of paroxysmal atrial tachycardia can be challenging. Digoxin is usually continued for months or years after the episode, but does not necessarily reduce the frequency of attacks; it does permit more rapid reversion to sinus rhythm. Reduction of the frequency of attacks has been achieved by propranolol (Table 3) or reserpine in addition to the digoxin. Reserpine (0.1 to 0.25 mg daily) is in-

Table 3.

Drug Therapy for Arrhythmias

INTRAVENOUS DOSAGE'"

DRUG

ORAL DOSE

EFFECTIVE

(MAINTEN ANCE)

BLOOD LEVELS

Atropine

Adult Children

1.0 mg .02 mg/kg

Digoxin

Adult Children 2 yrs.

.03 mg/kg into 3 doses

.005 mg/kg twice daily

Phenytoin

Adult Children

100 mg, repeat in 5 min. 2 mg/kg

300 to 400 mg daily 6 to 8 mg/kg daily

Isoproterenol

Adult Children

.25 mg, or drip 1 to 5 /Lg/kg, or drip

Lidocaine

Adult Children

75 mg bolus, or drip 1 to 5 mg/kg

Phenylephrine

Adult Children

.5 mg, repeat prn .01 mg/kg, repeat pm

'Procainamide

Adult Children

500 mg bolus slowly, or drip 3 to 10 mg/kg slowly

Propranolol

Adult Children

Quinidine

'I mg/min 10 to 100 /Lg/kg slowly LV. not recommended

1 to 2 ng/ml 1.5 to 3.5 ng/ml

10 to 20 /Lg/ml 5 to 18 /Lg/ml

2 to 5 /Lg/ml 1 to 5 /Lg/ml

250 to 500 mg at 4 hours intervals 5 to 10 mg/kg at'4 hour intervals

4 to 10 /Lg/ml 3 to 10 /Lg/ml

40 to 320 mg total daily .05 to 2 mg/kg at 6 hour intervals

20 to 100 ng/ml

Adult: 300 to 400 mg at 6 hour intervals Child: 5 to 15 mg/kg at 6 hour intervals

4 to 6 /Lg/ml 2,5 to 6 /Lg/ml

"For conversion to body surface area, multiply adult dose by .6 for dose per m '. Blood levels are taken from McCall" for adults and Gelband and Rosen' for children.

888

WARREN G. GUNTHEROTH

expensive, and can be taken once a day, whereas propranolol must be taken more often, a problem for the school-aged child. Sudden bradycardia of profound degree that appears to be associated with abnormal vagal tone will respond to intravenous atropine. Isol?roterenol has an added advantage of positive inotropism in this !iituation. New drugs are appearing in the literature at a surprising frequency, but only one has been approved by the Food and Drug Administration - disopyramide. It is classed in Group I, and is similar to quinidine, has relatively frequent atropine-like side effects, and occasionally substantially depresses the inotropic state. Other new and "unofficial" medications include some that have been used in Europe for some time, such as verapamil, which is very effective in terminating paroxysmal atrial tachycardia. However, it is not known to be effective orally as prophylaxis against attacks. Three drugs that are similar to lidocaine, but which are effective orally, are tocainide, which lasts 8 to 12 hours, aprinidine, and mexiletine. Ethmosin is also similar to lidocaine, and seems quite promising because of the paucity of side effects and its ability to induce a mild euphoria. It is effective against both supraventricular and ventricular arrhythmias. Amiodarone, a new Class I drug, is effective in" prolonging the refractory period of the accessory pathway in Wolff-Parkinson-White syndrome, and is generally well-tolerated, but has an unfortunate tendency to cause corneal deposits. Finally, acebutolol is a new cardioselective beta-blocker, potentially useful in children with asthma who cannot tolerate propranolol.

Electrical Cardioversion There are obvious life-threatening situations requiring the use of cardioversion, such as ventricular fibrillation. However, I cannot agre~ with assertions that ventricular tachycardia requires "swift" cardiaversion, since our experience with this disorder has revealed its chronicity, transient response to cardioversion, and generally adequate response to quinidine. When cardioversion is required with vascular collapse, cardioversion should be accomplished with minimal trauma, since excessive power can produce myocardial necrosis. The starting level should not exceed one watt sec per pound. For elective cardioversion of atrial flutter or fibrillation, one-half that level should be tried, and the patient prepared by stopping digitalis preparations 48 hours in advance and by being placed on quinidine. Unless the atrial arrhythmia is recent and not associated with large atria, cardioversion is un-" likely to have significant lasting benefit. Pacemaker Therapy The use of temporary pacemakers during or after surgery is wellestablished as a safety precaution during a high-risk interval of a few days' duration. Permanent implantation, on the other hand, should not be undertaken lightly, since it will usually require numerous minor and some major surgical procedures in children. We believe that per-

D1SORDERS OF HEART RATE AND RHYTHM

889

manent pacing as a preventive measure, that is not based on an actual event in that patient, is not justified. The risks in patients being paced is substantial, and sudden failure of wires, electrodes, or other components may be less tolerable to a child whose cardiac function is accustomed to the more rapid rate of the electrical pacemaker. The indications for pacing are the same as they have been for many years: congestive heart failure related to a slow rate, or syncope. Even syncope should be carefully evaluated, since ordinary vasovagal fainting is frequent in the normal population, and fainting in the child with complete heart block is not an indication for pacing. In general, we have been impressed with the tolerance children have for low heart rates; symptoms usually indicate the presence of a second cardiac disorder, such as a major shunt. The choice of pacemaker type has not been critical, because the durability of batteries or electronic components is less of an issue than problems with wires and electrodes. It is our impression that for very active children, transvenous pacemakers are not satisfactory because of dislodgement. Sur~cal Therapy of Arrhythmias Obviously, surgical therapy is rarely indicated in most childhood disorders of the heart beat. The seriousness of the symptoms and the ease of medical management will dictate the need for further steps. For example, intracardiac tumors are rare, but for serious arrhythmias, they should probably be ruled out since a true "cure" is possible. 2 In the long QT syndrome with syncope or proven episodes of ven~ tricular fibrillation, surgical removal of the left stellate ganglion can be done without thoracotomy and provides permanent protection. 17 Although propranolol in modest doses has been remarkably effective in our experience, the reliability and freedom from a four-a-day medication schedule makes the surgery seem reasonable. Stellate ganglionectomy has also been successful in adults with serious arrhythmias not associated with the long QT syndrome in whom medications were im:ffective. The surgical interruption of an accessory bundle bypassing the atrioventricular node may occasionally be required, particularly if it is associated with other pathology, such as Ebstein's anomaly. The procedure should be done only with the support of an experienced team capable of electrophysiologic mapping. 18

SUMMARY The clinical approach to a child with a disorder of the heart beat is stiiJ. that of a careful history, physical examination, and an ECG with a long rhythm strip. A diagnostic approach is presented which is based on rate, rhythm, and mechanism. Newer diagnostic methods are presented as well as advances in the basic cellular electrophysiology. Disorders of automaticity, triggerable cells, and re-entry are analyzed. Fi-

890

WARREN G. GUNTHEROTH

nally, therapy is revised for conventional drugs, and new and nonapproved drugs are listed. Indications and limitations for cardioversion, electrical pacing, and surgery are presented.

REFERENCES 1. Anderson, R. H., Janse, M. J., van Capelle, F. J. L., et al.: A combined morphological

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

and electrophysiological study of the atrio-ventricular node of the rabbit heart. Circ. Res., 35:909, 1974. Caldwell, P. D., Ricketts, H. J., Dillard, D. H., et al.: Ventricular tachycardia in a child: An indication for angiocardiography? Am. Heart J., 88:771, 1974. Cranefield, P. F.: Action potentials, afterpotentials, and arrhythmias. Circ. Res., 41 :415, 1977. Erlanger, J., and Blackman, J. R.: Further studies in the physiology of heart block in mammals: Chronic auriculoventricular heart block in the dog. Heart, 1: 177, 1910. Gelband, H., and Rosen, M. R.: Pharmacologic basis for the treatment of cardiac arrhythmias. Pediatrics, 55:59, 1975. Giles, W., and Noble, S. J.: Changes in membrane currents in bullfrog atrium produced by acetylcholine. J. Physiol., 261 :103,1976. Guntheroth, W. G.: Pediatric Electrocardiography. Philadelphia, W. B. Saunders Company,1965. Guntheroth, W. G.: Sudden infant death syndrome (crib death). Am. Heart J., 93:784, 1977. Guntheroth, W. G.: The heart: Electrical activity. In Abel, F. L., and McCutcheon, E. P. (eds.): Cardiovascular Function: Principles and Applications. Boston, Little, Brown and Co., in press. Hoffman, B. F., and Cranefield, P. F.: The physiologic basis of cardiac arrhythmias. Am. J. Med., 37:670, 1964. McCall, D.: Pharmacology of anti-arrhythmic drugs. In Fowler, N. O. (ed.): Cardiac Arrhythmias. Edition 2. Hagerstown, Harper and Row, 1977, p. 4. Morgan, B. C., Deane, P. G., and Guntheroth, W. G.: Long-term continuous electrocardiographic recording in pediatric patients. Pediatrics, 36:792, 1965. Noble, D.: The Initiation of the Heartbeat. Oxford, Clarendon Press, 1975. Pollack, G. H.: Cardiac pacemaking: An obligatory role of catecholamines? Science, 196:731, 1977. Rosen, M. R., and Hoffman, B. F.: Mechanisms of action of antiarrhythmic drugs. Cire. Res., 32:1, 1973. Scherlag, B. J., Lau, S. H., Helfant, R. H., et al.: Catheter technique for recording His bundle activity in man. Circulation, 39:13,1969. Schwartz, P. J., Periti, M., and Malliani, A.: The long Q-T syndrome. Am. Heart J., 89:378, 1975. Tonkin, A. M., Wagner, G. S., Gallagher, J. J., et al.: Initial forces of ventricular depolarization in the W-P-W syndrome. Circulation, 52:1020, 1975. Vassalle, M.: Cardiac Physiology for the Clinician. New York, Academic Press, 1976. Wellens, H. J. J., and Durer, D.: Effect of digitalis on atrioventricular conduction and circus movement tachycardias in patients with Wolff-Parkinson-White syndrome. Circulation, 47: 1229, 1973.

Department of Pediatrics University of Washington School of Medicine Seattle, Washington 98195

Disorders of heart rate and rhythm.

Symposium on Pediatric Cardiology Disorders of Heart Rate and Rhythm Warren G. Guntheroth, M.D. * Disorders of the heart beat are frequently classif...
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