Artificial pacing of the human heart: its contribution to the understanding of the arrhythmias

Frequent premature ventricular complex is associated with left atrial enlargement in patients with normal left ventricular ejection fraction

BACKGROUND

Premature ventricular complex (PVC) has been regarded as benign; however, when frequent, the arrhythmia can induce left ventricular (LV) systolic dysfunction. Meanwhile, the influence of PVCs on cardiac structural remodeling and functional change before occurrence of overt systolic heart failure has not been fully described. In this study, we attempted to identify early cardiac structural/functional manifestations of frequent PVCs in patients with normal LV systolic function.

METHODS

A total of 146 patients (age: 55 ± 15 years, 48 males) with frequent PVCs observed on 24-hour Holter monitoring (>10/h) and normal LV ejection fraction (LV EF ≥ 55% on echocardiography) were enrolled. Clinical characteristics and echocardiographic parameters of the patients were compared with those of an age-/sex-matched control group (n = 292, age: 55 ± 15 years, 96 males).

RESULTS

Patients with frequent PVCs had significantly larger left atrial volume index (LAVI [28 ± 9 mL/m(2) vs. 24 ± 7 mL/m(2) ]), along with larger LV end-diastolic dimension (49.4 ± 4.4 mm vs. 48.5 ± 3.9 mm), lower LV EF (63 ± 7% vs. 66 ± 6%), and lower peak systolic mitral annular velocity (7 ± 2 cm/s vs. 8 ± 2 cm/s; P < 0.05 for all), whereas other clinical characteristics were similar. In particular, in patients with frequent PVCs, LAVI showed linear correlation with PVC burden (R = 0.30, P < 0.001), and, in a multiple regression model, PVC burden independently estimated LAVI, even after controlling for age, sex, comorbidities, and systolic function (β = 0.309, P < 0.001).

CONCLUSION

Frequent PVC is associated with LA enlargement in patients with normal LV EF. Atrial anatomical remodeling may precede LV geometry change and systolic dysfunction in patients with frequent PVCs.

Overdrive suppression of antegrade conduction over the accessory pathway

In a patient with Wolff-Parkinson-White syndrome whose accessory pathway was primarily capable of bidirectional conduction, antegrade conduction over the accessory pathway was transiently inhibited after rapid atrial or ventricular pacing or after spontaneous termination of atrioventricular reentrant tachycardia. Pacing rate and duration of tachycardia were related to the duration of the suppression of preexcitation, while the coupling interval of the first sinus beat to the last driven or tachycardia beat was irrelevant to the phenomenon. Thus, overdrive suppression of conduction may be the most likely mechanism of this phenomenon.

AV nodal function during atrial fibrillation: the role of electrotonic modulation of propagation

The irregular ventricular rhythm that accompanies atrial fibrillation (AF) has been explained in terms of concealed conduction within the AV node (AVN). However, the cellular basis of concealed conduction in AF remains poorly understood. Our hypothesis is that electrotonic modulation of AVN propagation by atrial impulses blocked repetitively within the AVN is responsible for changes in function that lead to irregular ventricular rhythms in patients with AF. We have tested this idea using two different simplified computer ionic models of the AVN. The first ("black-box") model consisted of three cells: one representing the atrium, another one representing the AVN, and a third one representing the ventricle. The black-box model was used to establish the rules of behavior and predictions to be tested in a second, more elaborate model of the AVN. The latter ("nine-cell" model) incorporated a linear array of nine cells separated into three different regions. The first region of two cells represented the atrium; the second region of five cells represented the AV node; and the third region of two cells represented the ventricle. Cells were connected by appropriate coupling resistances. During regular atrial pacing, both models reproduced very closely the frequency dependence of AV conduction and refractoriness seen in patients and experimental animals. In addition, atrial impulses blocked within the AV node led to electrotonic inhibition or facilitation of propagation of immediately succeeding impulses. During simulated AF, using the nine-cell model, random variations in the atrial (A-A) interval yielded variations in the ventricular (V-V) interval but there was no scaling, i.e., the V-V intervals were not multiples of the A-A intervals. As such, the model simulated the statistical behavior of the ventricles in patients with AF, including: (1) the ventricular rhythm was random; and (2) the coefficient of variation (standard deviation/mean) of the ventricular rhythm was relatively constant at any given mean V-V interval. Analysis of cell responses revealed that repetitive atrial input at random A-A intervals resulted in complex patterns of concealment within the AVN cells. Consequently, the effects of electrotonic modulation were also random, which resulted in a smearing of the AV conduction curve over A-A intervals that were larger than those predicted for 1:1 AV conduction. Hence, during AF, electrotonic modulation acts in concert with the frequency dependence of AVN conduction to result in complex patterns of ventricular activation. Finally, similarly to what was shown in patients, VVI pacing of the ventricle in the nine-cell model at the appropriate frequency led to blockade of nearly all anterograde (i.e., A-V) impulses. The essential feature here was that the retrograde impulse invading the AVN cells was followed by refractoriness with slow recovery of excitability, setting the stage for electrotonic inhibition of anterograde impulses. Overall, the results provide insight into the cellular mechanisms underlying AVN function and irregular ventricular response during AF.

Overdrive prolongation of refractoriness and fatigue in the early stages of human bundle branch disease

OBJECTIVES

The aim of this study was to assess the response of refractoriness in normal and diseased human bundle branches to changes in cycle length, as well as during a long period of continuous overdrive pacing.

BACKGROUND

The anterograde refractory period of the bundle branches in patients with functional bundle branch block shortens as the rate is increased. The rate-dependent response of refractoriness in diseased bundle branches is quite different. However, this difference has not been precisely delineated, and its physiologic meaning is uncertain.

METHODS

Refractoriness of the bundle branches was measured by the extrastimulus technique in 16 patients with tachycardia-dependent bundle branch block and 10 patients with functional bundle branch block, both after basic trains of 8 atrial-paced impulses at different cycle lengths and during a 10-min period of continuous overdrive pacing.

RESULTS

The baseline refractory period in the bundle branches of patients with functional bundle branch block measured 430 +/- 32 ms (mean +/- SD) and shortened to 368 +/- 30 ms at the shortest cycle length. The maximal effect was reached within the 1st min of overdrive pacing. The baseline refractory period of the bundle branches was significantly longer in patients with tachycardia-dependent bundle branch block (611 +/- 184 ms) and demonstrated a cumulative overdrive prolongation in 15 (83%) of 18 studies with typical manifestations of fatigue. In two other studies, this occurred only after ajmaline administration.

CONCLUSIONS

A rate- and time-dependent prolongation of refractoriness frequently occurs in diseased human bundle branches. When absent, this response may be induced under the effects of sodium channel blockers. This would suggest that an abnormality in the recovery from inactivation of the sodium channel might underlie the early stages of bundle branch disease.

Bradycardia-dependent block and fatigue phenomenon of the His-Purkinje system manifested during upright tilt

Atrioventricular (AV) block occurring in the His-Purkinje system may occur sporadically and can be difficult to document. In this article, we describe two patients with spontaneous episodes of AV block, in whom the use of upright tilt during electrophysiological testing led to a diagnosis of His-Purkinje disease. In both cases, testing in the supine position only uncovered mild or no abnormalities of infra-nodal conduction. In the first case, high grade block distal to the His occurred and the mechanism appeared to be bradycardia-dependent block. In this case, increased sympathetic tone due to upright tilt may have enhanced the slope of phase 4 depolarization in abnormal His-Purkinje cells, leading to block. In the second case, high grade block distal to the His was seen with upright tilt and the mechanism of block appeared to be fatigue phenomenon in the His-Purkinje system. These cases emphasize the elusive nature and varied mechanisms of His-Purkinje block and illustrate the utility of electrophysiological testing in the upright position in patients with suspected conduction system disease.

Reflected reentry in depolarized foci with variable conduction impairment in 1 day old infarcted canine cardiac tissue

A recent study in 1 day old infarcted canine cardiac tissue demonstrated that variations in entrance and exit characteristics of depolarized spontaneous foci could profoundly affect their expression, resulting in behaviors such as modulated parasystole and entrainment, second and third degree exit block "autoentrainment" and annihilation (abrupt termination) of spontaneous activity. Foci with exit and entrance delay should also allow the occurrence of reflected reentry. To test this, the left circumflex coronary artery was ligated and, after 1 day, simultaneous microelectrode impalements were made in infarcted and uninfarcted portions of isolated ventricular preparations. Preparations were stimulated from the uninfarcted portions. Reflected reentry was demonstrated in 5 of 11 preparations. It occurred when exit conduction delay resulted in reexcitation of the focus or, alternatively, when entrance conduction delay resulted in reexcitation of extrafocal tissue. Reflection occurred in which the action potentials were reexcited during phase 2 or 3, resulting in prolongation of action potential duration (type I), and in which reexcitation occurred after full or nearly full repolarization, resulting in a closely coupled extrasystole (type II). Electrotonic modulation and reflection could coexist, the type of behavior depending on the phase relation between focal and extra-focal action potentials. An example is illustrated in which type I reflection occurred only when preceding driven activity induced overdrive suppression of exit conduction from the focus. The results suggest that, in infarcted tissue, entrance and exit conduction delays to depolarized foci may form the basis for the occurrence of reflected reentry.

Effect of right ventricular pacing on ventricular rhythm during atrial fibrillation

In 13 patients with atrial fibrillation, the effect of right ventricular pacing at various rates on spontaneous RR intervals was studied. Five hundred consecutive RR intervals were recorded and measured before and during varying right ventricular pacing rates. As anticipated, all RR intervals longer than the right ventricular pacing intervals were abolished. However, RR intervals shorter than the right ventricular pacing intervals were also eliminated. It is difficult to explain the elimination of RR intervals shorter than the pacing intervals with the accepted concepts concerning the mechanisms governing the rate and rhythm of the ventricular response to atrial fibrillation. An alternative explanation may be that during atrial fibrillation the atrioventricular node behaves as a nonprotected pacemaker that is electrotonically modulated by the chaotic atrial electrical activity. The result is a random ventricular rhythm. With right ventricular pacing, the automatic focus is depolarized by the retrogradely concealed conducted ventricular impulses, the short RR intervals are not generated as a consequence and the rhythm becomes pacemaker dependent.

Fatigue of the His-Purkinje system during routine electrophysiologic studies

In this report we describe fatigue of the His-Purkinje system during retrograde stimulation of the His bundle by ventricular programmed stimulation. The patient underwent electrophysiologic evaluation for syncope. Antegrade conduction and supraventricular studies were normal with the exception of baseline left bundle branch block. During programmed ventricular stimulation, the patient developed intra-Hisian and infra-Hisian block with symptomatic 3:1 atrioventricular heart block requiring insertion of a permanent pacemaker. This case demonstrates the need for careful study of both antegrade and retrograde conduction properties of the His bundle and atrioventricular node when performing standard His bundle studies in evaluation of syncope.

Overdrive suppression of conduction at the canine Purkinje-muscle junction

We have shown previously that overdrive suppression of conduction in depolarized His-Purkinje tissue requires conduction asymmetry. In this study we examined whether overdrive suppression of conduction can occur at the Purkinje-muscle junction, where natural asymmetry of conduction exists. Canine Purkinje-muscle preparations were superfused with hyperkalemic Tyrode's solution (KCl 8 to 12 mM), and action potentials were recorded from Purkinje, junctional, and muscle cells. Initially, the Purkinje fiber was paced at the shortest cycle length at which 1:1 anterograde Purkinje-muscle conduction occurred. The papillary muscle then was paced for 10 to 50 beats at shorter cycle lengths during which, because of conduction asymmetry at the Purkinje-muscle junction, 1:1 retrograde muscle-Purkinje conduction also occurred. After overdrive papillary muscle pacing, Purkinje fiber pacing at the same cycle length that previously resulted in 1:1 conduction now produced transient Purkinje-muscle conduction block (overdrive suppression of conduction). The degree and duration of overdrive suppression of conduction were proportional to the rate and duration of overdrive pacing. After overdrive pacing, Purkinje cell action potential amplitude and Vmax recovered within 300 msec, yet conduction block persisted for up to 7 sec. In contrast, excitability in papillary muscle cells near the Purkinje-muscle junction increased continuously after overdrive pacing. These data suggest that rapid activation of Purkinje cells during overdrive pacing was not required for overdrive suppression of conduction and that restoration of conduction after overdrive pacing was determined primarily by recovery of excitability in papillary muscle cells. Transient Purkinje-muscle conduction block after periods of rapid ventricular rates might account for overdrive-induced conduction disturbances normally attributed to bundle branch block.

An analysis of post-pacing R-R intervals during atrial fibrillation

Bursts of ventricular pacing at cycle lengths of 350-260 ms were introduced during atrial fibrillation in nine patients, and the post-pacing R-R intervals were compared to the R-R intervals of spontaneous QRS complexes. In eight of nine patients, the mean post-pacing R-R interval was 126-199 ms longer than the mean spontaneous R-R interval (p less than 0.005). Spontaneous runs of aberrantly conducted supraventricular complexes were recorded during atrial fibrillation in one patient. The mean R-R interval following the runs of aberrantly conducted supraventricular complexes was significantly longer than the mean R-R interval of spontaneous narrow QRS complexes (p less than 0.001), but not significantly different than the mean post-pacing R-R interval. The findings of this study suggest that the R-R interval that follows a wide-complex tachycardia during atrial fibrillation is unlikely to be of value in differentiating ventricular tachycardia from aberrantly conducted supraventricular complexes. Analysis of R-R intervals that follow bursts of ventricular pacing suggests that there is likely to be considerable overlap between the R-R intervals that follow runs of ventricular tachycardia and the spontaneous R-R intervals during atrial fibrillation. Furthermore, even when the post-tachycardia R-R interval clearly exceeds the longest spontaneous R-R interval during atrial fibrillation, this is still of little diagnostic value, because a long pause may occur after either a run of ventricular tachycardia or a run of aberrantly conducted QRS complexes of supraventricular origin.

Rate-related suppression and facilitation of conduction in isolated canine cardiac Purkinje fibers

Previous studies have shown that antegrade conduction through damaged His Purkinje tissue may be suppressed following rapid ventricular pacing (overdrive suppression of conduction). We studied this phenomenon using isolated Purkinje fibers placed in a three-chamber bath. Superfusates for the left, middle, and right segments of the fiber were altered to produce action potentials that resembled those of normal bundle branch, damaged His bundle, and normal His bundle, respectively. To produce anisotropic conduction, the left segment of the fiber was adjusted to be three to four times longer than the right segment. Pacing the right segment at intermediate rates produced maximal action potential amplitude in the middle segment and 1:1 right-to-left conduction, whereas pacing at faster or slower rates reduced action potential amplitude and produced block. Pacing the left segment at fast or slow rates also reduced action potential amplitude in the middle segment, but conduction was maintained (anisotropy). After rapid or slow left segment pacing, action potential amplitude in the middle segment remained low during subsequent right segment pacing at intermediate rates, and transient block occurred (overdrive or underdrive suppression of conduction). With time, action potential amplitude normalized and conduction resumed. In other more severely depressed preparations, conduction block occurred even at intermediate right segment pacing rates prior to left segment pacing. Under these conditions, pacing the left segment at intermediate rates increased action potential amplitude in the middle segment and temporarily permitted 1:1 conduction at intermediate right segment pacing rates (overdrive facilitation of conduction).(ABSTRACT TRUNCATED AT 250 WORDS)

Overdrive suppression of conduction in the canine His-Purkinje system after occlusion of the anterior septal artery

The purpose of these experiments was to determine whether overdrive suppression of conduction (OSC), i.e., transient worsening of conduction or development of atrioventricular block after cessation of rapid pacing, could be produced in the canine His-Purkinje system damaged by ligation of the anterior septal coronary artery and to investigate the responsible mechanism. We found that OSC occurred in vivo after rapid ventricular and His bundle pacing but not after atrial pacing, and that it occurred in vitro after rapid pacing from the left bundle branch but not after pacing from the proximal His bundle. OSC was related to the duration and cycle length of pacing. Lidocaine increased while verapamil reduced the duration of OSC in vivo. The mechanism responsible for the unidirectionality of OSC is not clear but is probably related to the geometry of the atrioventricular junction and the anterograde versus the retrograde activation sequence. Changes in regional myocardial blood flow, autonomic tone, hemodynamic variables, or ventricular function do not appear to be required to produce OSC, based on the demonstration of the phenomenon in vitro. The data suggest a time- and rate-dependent change in factors affecting conduction such as excitability or cell-to-cell coupling, possibly due to accumulation of intracellular cations such as calcium.

Bundle branch block after ventricular tachycardia: a manifestation of "fatigue" or "overdrive suppression"

Three cases of bundle branch block due to ventricular tachycardia are presented. Although the exact mechanism remains obscure, this phenomenon may be related to previously described acceleration-induced atrioventricular and bundle branch block.

Electrocardiography of arrhythmias: from deductive analysis to laboratory confirmation--twenty-five years of progress

Before the advent of the microelectrode, His bundle recording and direct cardiac pacing, electrocardiographic interpretation of arrhythmias in human beings was based on presumed mechanisms derived by deductive analysis. This indirect approach was forced by the fact that the surface electrocardiogram does not directly record the behavior of the specialized tissue that is the site of origin of most arrhythmias. In the past 25 years it has become possible to record directly from the specialized tissue. The result has been experimental confirmation of several underlying concepts of the electrocardiographic interpretation of arrhythmias and progress evidenced by the conversion of concept to fact.

Depression of distal AV conduction following ventricular pacing

A 61-year-old man presented with symptomatic 2 degree Mobitz II AV block in association with bifascicular block. Ventricular pacing resulted in temporary depression of AV conduction. The extent of pacing-induced AV block varied directly with the duration and rate of ventricular pacing. Intracardiac recordings proved that the site of spontaneous and pacing-induced AV block was distal to the His bundle. Possible mechanisms by which this phenomenon may arise and several implications of practical importance are discussed.

The "compensatory pause" of atrial fibrillation

A long pause after an abnormal heat during atrial fibrillation has been called a "compensatory pause" and has been used to identify premature ventricular complexes (PVCs) and to differentiate them from supraventricular beats with aberration. The diagnostic value of the compensatory pause is controversial and has not been tested systematically with programmed stimulation and intracardiac recordings. In this study we used these methods to determine if PVCs induced during atrial fibrillation were followed by compensatory pauses. Five patients were studied who had ECGs with a normal PR interval and a normal QRS duration and morphology during sinus rhythm. Atrial fibrillation was induced by rapid atrial pacing. PVCs were induced by coupling a ventricular extrastimulus to every eighth or tenth QRS complex during atrial fibrillation. The coupling interval of the PVC was changed in 10- or 20-msec increments until the entire cardiac cycle was scanned. The mean duration of recorded atrial fibrillation was 15.7 minutes. Fifty-seven to 163 PVCs were induced in each patient. The mean cycle after the induced PVC was calculated and compared with the mean control cycle. The mean cycle after the PVC was 107-136 msec longer than the mean control cycle (p < 0.001 in every patient). This study confirms the presence of a compensatory pause after stimulated PVCs in atrial fibrillation.

Escape rhythm in complete A-V block. The recovery phase after overdrive suppression from artificial ventricular suppression from artificial ventricular pacing

In 60 patients with third degree A-V block, recovery of escape rhythm from overdrive suppression after ventricular pacing has been studied. Implanted unipolar VVI pacemakers were inhibited by chest wall stimuli. A total of 165 rhythmograms were studied. In 37, the rate was irregular, in the other 128 the escape rate increased gradually, following an exponential curve until stabilization after 3 minutes. In 29 of these rhythmograms, a possible exit block of the first escape impulse was observed. In 99 rhythmograms without exit block, escape rhythm recovery time was an average 1.45 times basal escape RR intervals. Overdrive suppression was most marked in patients with a slow escape rhythm.

Overdrive pacing for supraventricular tachycardia: a review of theoretical implications and therapeutic techniques

Rapid atrial pacing for treatment of supraventricular arrhythmias has been demonstrated to be safe and effective. Virtually any supraventricular tachycardia with the exception of atrial fibrillation, Type II atrial flutter, and probably sinus tachycardia can be treated successfully with pacing techniques. The recognition of the advantages of cardiac pacing over drug therapy or DC cardioversion has resulted in its widespread use, especially after open-heart surgery. Although the response to overdrive pacing may not reliably identify the underlying mechanism of supraventricular tachycardia, the response of the arrhythmia to pacing (i.e., whether it is interruptable or noninterruptable), is most useful in the approach to management of the individual patient.

Overdrive suppression of implanted pacemakers in patients with AV block

Patients being permanently paced for symptomatic AV block were studied by overdrive suppression of the QRS-inhibited pacemaker, in order to observe the underlying heart rhythm. The chest wall stimulation method was used. In complete AV block the escape rhythm recovery time proved highly reproducible on repeated testing on the same day, and in many patients remained so over months or years. Occasionally, a doubling of the escape rhythm recovery time was seen, suggesting initial exit block of the escape focus. Resetting of the escape rhythm usually followed an exponential curve until stabilisation after about 3 minutes. An early escape rhythm with a recovery time of less than 4 seconds was found on every occasion in 21 of 58 patients with complete AV block, and inconstantly in 23 more; in 14 it was never observed. Accidental pacing failure was seen in 15 patients. The overdrive suppression test was helpful in selecting pacemaker dependent patients.

Recurrent paroxysmal supraventricular tachycardia: a complication of ventricular pacing in a patient with occult Wolff-Parkinson-White syndrome

A 60 year old man suffering from syncope believed to be due to the sick sinus syndrome was treated with a permanent demand ventricular pacemaker. This led to almost continous bouts of paroxysmal supraventricular tachycardia (SVT) over the ensuing two years, mistakenly believed to be part of the sick sinus syndrome. Careful study showed that this man had a type A Wolff-Parkinson-White accessory atrioventricular connection which consistently conducted retrogradely, but only rarely antegradely, during applications of carotid sinus massage. Episodes of SVT were repeatedly induced whenever ventricular-paced impulses captured the atria retrogradely. All episodes of SVT stopped when the ventricular pacemaker was removed. Following insertion of an atrial pacemaker, the patient had no episodes of SVT or syncope over a nineteen month follow-up period. This case illustrates the care required in selecting a proper site for protective pacing in patients who suffer from paroxysmal SVT.

Parasystolic property of the ectopic focus

Among 4476 ECGs recorded from May 1972 to April 1973, 206 ECGs showed various arrhythmias. All ECGs showing any kind of arrhythmia were recorded for 2 to 4 minutes in order to obtain more than 3 premature beats in one tracing. Of 141 ECGs with premature beats from 126 cases, more than 2 interectopic parasystolic intervals could be measured in 122 cases (96.8% of those with premature beats and 2.7% of all the patients examined). More than 3 coupling intervals could be measured in 117 patients with premature beats. In only 13 of these 117 cases (11.1%), the coupling interval varied less than 0.04 second and less than 0.02 second in 2 cases (1.7%). 1) The parasystolic cycle length ranged between less than 0.20 and more than 2.51 seconds with 82.8% of them being in a range between 0.21 and 1.00 second. 2) The fluctuation rate of the parasystolic cycle length ranged between less than 1.0 and more than 15.1. It was between less than 1.0 and 5.0 in 56,6%, between 5.1 and 10.0 in 30.0% and between 10.1 and more than 15.1 in 13.1%. 3) Fusion beats were observed in 44 of 122 cases in which the interectopic parasystolic cycle length could be determined (36.1%). Of these 44 fusions 14 were found in atrial, 14 in A-V junctional and 16 in ventricular parasystole. 4) Theories of re-entry and ectopic enhancement were discussed.

Effect of extrasystoles on idioventricular rhythm. Clinical and electrophysiologic correlation

When premature ventricular contractions (PVCs) occur in the idioventricular rhythm of patients with complete heart block (CHB), one of several rhythm alterations may occur: the postextrasystolic or returning cycle (RC) may be prolonged, unaltered, or even shortened without any apparent reason. To elucidate the mechanisms by which these alterations occur, the effect of spontaneous and of electrically induced PVCs was studied in five patients with CHB. Over 500 observations were obtained. PVCs occurring late in the basic cycle, i.e., after the T wave of the last spontaneous idioventricular beat, almost uniformly resulted in lengthening of the first RC. In sharp contrast, PVCs elicited early in the cycle, i.e., during the T wave, were followed by a shortened RC in 95% of instances. The underlying electrophysiologic mechanisms responsible for the relationship of RC length to degree of prematurity of the PVC are discussed.