Aberrancy: electrophysiologic aspects

Intravenous administration of quinidine and metoprolol for treatment of atrial fibrillation in 2 neonatal foals

Atrial fibrillation (AF) is a rarely reported arrhythmia in otherwise healthy newborn foals, with a single case of cardioversion using procainamide administration described in the literature. Two neonatal Thoroughbred colts were presented to an equine hospital because of an irregularly irregular tachyarrhythmia and poor latching when trying to nurse. History, physical examination, and initial diagnostic testing including ECG and echocardiography confirmed AF without structural heart disease. The 1st foal converted into normal sinus rhythm after treatment with IV metoprolol and quinidine. The 2nd foal converted to normal sinus rhythm after a single IV dose of metoprolol, intended for rate control. Demeanor and nursing behavior improved markedly after conversion. The 2 foals had normal heart rates and sinus rhythm that persisted for 6 weeks until euthanasia in the 1st foal and for 2 years in the 2nd foal. Rate control and cardioversion should be considered as a treatment for persistent lone AF in neonatal foals.

Frequency and characteristics of progressive aberrancy during supraventricular tachycardia

Progressive aberrancy (defined as beats with QRS duration and morphology between that of sinus rhythm and full aberrancy) occurred in 10 of 32 episodes of atrial pacing in 5 of 11 patients. The episodes resembled fusion beats but were due to bilateral bundle branch delay.

Atrial fibrillation begets atrial fibrillation. A study in awake chronically instrumented goats

BACKGROUND

In this study we tested the hypothesis that atrial fibrillation (AF) causes electrophysiological changes of the atrial myocardium which might explain the progressive nature of the arrhythmia.

METHODS AND RESULTS

Twelve goats were chronically instrumented with multiple electrodes sutured to the epicardium of both atria. Two to 3 Weeks after implantation, the animals were connected to a fibrillation pacemaker which artificially maintained AF. Whereas during control episodes of AF were short lasting (6 +/- 3 seconds), artificial maintenance of AF resulted in a progressive increase in the duration of AF to become sustained (> 24 hours) after 7.1 +/- 4.8 days (10 of 11 goats). During the first 24 hours of AF the median fibrillation interval shortened from 145 +/- 18 to 108 +/- 8 ms and the inducibility of AF by a single premature stimulus increased from 24% to 76%. The atrial effective refractory period (AERP) shortened from 146 +/- 19 to 95 +/- 20 ms (-35%) (S1S1, 400 ms). At high pacing rates the shortening was less (-12%), pointing to a reversion of the normal adaptation of the AERP to heart rate. In 5 goats, after 2 to 4 weeks of AF, sinus rhythm was restored and all electrophysiological changes were found to be reversible within 1 week.

CONCLUSIONS

Artificial maintenance of AF leads to a marked shortening of AERP, a reversion of its physiological rate adaptation, and an increase in rate, inducibility and stability of AF. All these changes were completely reversible within 1 week of sinus rhythm.

Identification of the rate-dependent functional refractory period of the atrioventricular node in simulated atrial fibrillation

We continuously observed successive pairs of R-R intervals during atrial fibrillation and hypothesized that the shortest R-R interval for a given preceding R-R interval in a pair represents the functional refractory period of the atrioventricular node at that preceding interval. To test this hypothesis we simulated atrial fibrillation in 28 isolated cross-perfused canine hearts and obtained an R-R interval scatterplot by plotting the R-R intervals as a function of the immediately preceding R-R interval. This scatterplot enabled us to detect a series of the shortest R-R intervals for a wide range of preceding R-R intervals, and this allowed us to estimate the rate-dependent functional refractory period of the atrioventricular node in simulated atrial fibrillation. The estimated functional refractory periods correlated well with those measured by the conventional method (r = 0.93). We conclude that the proposed method makes it possible to estimate the rate-dependent functional refractory periods of the atrioventricular node in atrial fibrillation.

Reverse alternating Wenckebach periodicity

An atrial pacing-induced reverse conduction pattern of the alternating Wenckebach periodicity was observed in 5 of 42 children (12%) during electrophysiologic study. This conduction pattern is a reverse of the usual alternating Wenckebach periodicity: During an underlying 2:1 atrioventricular conduction block there is progressive shortening of the conduction time of the conducted impulses with termination in a lower degree of block. This reverse alternating Wenckebach periodicity may be caused by a mechanism similar to that in other Wenckebach phenomena.

Tachycardia- and bradycardia-dependent atrioventricular block: observations regarding the mechanism of block

A case of paroxysmal bradycardia- and tachycardia-dependent atrioventricular (AV) block is described in a patient with right bundle branch block. The His bundle recordings demonstrated the site of the AV block to be distal to the His bundle recording site (probably in the left bundle branch). Whereas AV block distal to the His bundle occurred at an atrial paced cycle length of 700 ms, intact ventriculoatrial (VA) conduction was present up to a ventricular paced cycle length of 400 ms. Resumption of AV conduction was dependent on a critical HH or RH (in case of escapes) interval. These findings suggest that the bradycardia-dependent block is related to a time-dependent decrease in the amplitude of the current intensity of the proximal segment during late diastole. Spontaneous diastolic depolarization during late diastole resulted in impaired anterograde (AV) conduction but facilitated retrograde (VA) conduction. These findings are consistent with experimental "in vitro" observation in the sucrose gap model of AV block.

Possible role of a ventricular conduction disturbance in the electrogenesis of the ECG-VCG signs of myocardial infarction

The typical QRS patterns of myocardial infarction (MI-QRS) are commonly attributed to myocardial cellular death. However, observation of a transient appearance of MI-QRS during coronary insufficiency, the disappearance of MI-QRS after coronary by-pass surgery and the appearance of MI-QRS after intracranial hemorrhage suggest that a different electrophysiological mechanism may be at work. There is a single convincing explanation for all these observations. It seems possible, at least theoretically, that a localized conduction disturbance can generate or contribute to the generation of the MI-QRS. The results obtained in nine out of 194 cases studied by means of premature right atrial stimulation (PRAS) in our laboratory seem to confirm this hypothesis. In five of them we observed typical MI-QRS in the aberrant beats which were absent in the basal tracings. In the other four cases, MI-QRS which were present in basal tracings disappeared in the aberrant beats. In three of these a reduction in the duration of QRS was also observed, while in the fourth the duration of QRS did not change. In no case could the alterations of QRS (induction or disappearance of MI-QRS) be explained by a classical conduction disturbance, preexcitation or by a premature ventricular beat. While the induction of MI-QRS was clearly due to an aberrant conduction in the supraventricular beats, the disappearance of basal MI-QRS changes in premature supraventricular beats is more difficult to explain. One possible electrophysiological mechanism could be a supernormal phase conduction. If this is the case, the basal MI-QRS could be due to a ventricular conduction disturbance. In conclusion, our results suggest that MI-QRS can be generated, at least in our cases, by a localized conduction disturbance.

The relationship of human atrial cellular electrophysiology to clinical function and ultrastructure

Although previous studies have described the electrophysiological and ultrastructural characteristics of human cardiac fibers, no attempt has been made as yet to describe quantitatively the relationship between the ultrastructural and cellular electrophysiological derangements occurring with cardiac disease, and their clinical manifestations. In this study, we used standard microelectrode techniques to record the action potential characteristics of human atrial fibers obtained during cardiac surgery and correlated the electrophysiological parameters with clinical and ultrastructural data. Ultrastructure was studied by optical and electron microscopy. We found a multiple linear regression among maximum diastolic potential, atrial size and pressure, P wave duration and ultrastructure changes. Proliferations of Z band material, widening of intercalated discs, and degenerative changes were quantified and correlated with electrophysiological and clinical data. These studies emphasize the relationship between hemodynamic anomalies and resultant changes in both human atrial fiber structure and electrical function. Finally, the likelihood of occurrence of arrhythmias can be predicted using the analytic method described.

The anterior displacement of the QRS loop as a right ventricular conduction disturbance. Electrophysiologic and vectorcardiographic study in man

Anterior displacement (AD) of the QRS horizontal loop (Frank VCG method) was induced by programmed right atrial stimulation (PRAS) in 15 cases. When AD occurred we noticed changes of the terminal QRS vectors and of the T loop similar to those observed in incomplete right bundle branch block (RBBB). The increasingly anticipated extrastimuli induced progressively the AD and then progressive degrees of RBBB. The anterior shifting of the efferent limb never appeared after the induction of RBBB. A left conduction disturbance never appeared after the AD. In cases of supposed incomplete left bundle branch block (i.e. left ventricular hypertrophy) the QRS duration decreased when the AD was induced. Therefore, the AD induced by PRAS and probably those observed in some clinical cases are due to a right ventricular conduction disturbance.

Carotid sinus massage induced elimination of rate related bundle branch block during paroxysmal atrial tachycardia: a simple method of proving bypass tract participation in the tachycardia

Four cases of paroxysmal atrial tachycardia are described in whom rate related left bundle branch block (LBBB) was often present which persisted indefintely and showed no signs of spontaneous disappearance. Transient slowing of the tachycardia by carotid sinus massage in each case eliminated LBBB and this led to tachycardia acceleration. The tachycardia acceleration was traceable to a shortening in ventriculoatrial conduction. These observations proved the participation of a left sided bypass tract in the tachycardia circuit in each of these cases.

[The adaptation of AV-nodal conduction time on gliding increase and decrease of atrial frequency before and after autonomic blockade (author's transl)]

In 19 patients with healthy AV-nodes the adaptation of the intranodal conduction time (A-H time) to gliding increase and decrease in atrial frequency and to the blockade of the autonomic nervous system was investigated using His bundle electrograms. The measurements were performed during right atrial stimulation with three frequencies, each with a duration of one minute, before and after blockade of the parasympathetic (8 pat.; 1 mg atropine i.v.) and the sympathetic (11 pat.; 0.4 mg Visken i.v.) nervous system. Gliding increase and decrease in atrial frequency results in a staircase pattern of A-H-adaptation in 18 of the patients. The height of the steps was identical in both phases of stimulation in each individual patient. One patient showed functional dissociation of intranodal conduction which was different during increase and decrease of atrial frequency. With parasympathetic blockade the staircase behavior of the A-H time basically remained unchanged with the exception of shorter A-H intervals resulting in lower steps. Atropine abolished the functional dissociation of intranodal conduction; thus the drug might prevent reentrytachycardias due to functional dissociation in the AV-node. Sympathetic blockade lengthens the intranodal conduction time; thus shifting the staircase pattern of the A-H time to higher levels. The results are discussed with respect to the electrophysiological characteristics of AV nodal cells as slow response fibers, and to the changes caused by atrial stimulation, acetylcholine and adrenaline.

Lenegre's disease in youth

The case of a 22-year-old white male without known heart disease who presented with activity related lightheadedness at age 19 and dizziness and fatigue at age 21 is described. Standard electrocardiograms (ECG's) revealed intermittent complete trifascicular block. Rapid progression of symptoms over the succeeding eight months resulted in increasing incapacity. Holter monitoring demonstrated that symptoms were related to development of second and higher degrees of A-V block. Normal A-H interval and markedly prolonged H-Q interval on His bundle electrograms indicated that block was infranodal and localized to bundle branch system. Conduction problems aside, clinical and laboratory evaluation, including echocardiograms and cardiac catheterization, were unremarkable. Progression of bilateral bundle branch disease in a young patient without other demonstrable heart lesions and a negative family background conforms with criteria for Lenegre's disease. To our knowledge, this represents the youngest reported patient with this entity. Possible electrophysiologic basis of block and of exercise induced improvement in A-V conduction also are considered.

Tachycardia and bradycardia-dependent bundle branch block alternans: clinical observations

Eleven patients with tachycardia-dependent, bradycardia-dependent, or "pseudobradycardia-dependent" bundle branch block (BBB) alternans were studied. This classification is based on the following criteria: 1) When alternans is initiated by a sudden acceleration in ventricular rate, or it appears with aberration of the second beat after a pause, the alternans is tachycardia-dependent and results from a 2:1 bidirectional block in the affected bundle branch. 2) When alternans begins with the aberrant complex terminating a pause it is bradycardia-dependent; such an alternans results from alternating bundle branch cycle lengths and refractoriness, possibly produced by alternating transseptal retrograde penetration of the affected bundle branch. 3) In cases referred to as "pseudobradycardia-dependent BBB" alternans, a change from alternans to persiscardia-dependent BBB" alternans, a change from alternans to persistent BBB occurs as the cycle lengthens; however, the disappearance of BBB with further increase of the cycle length proves the tachycardia-dependence of the conduction defect.

Multifocal ventricular parasystolic tachycardia

Two cases of spontaneous multifocal ventricular parasystolic rhythm are described. One case showed double and the other fivefold parasystole. All seven foci had an enhanced rate of discharge, ranging from 57 to 102/min and at least three of them showed exit block. Though beats from three foci displayed very short coupling intervals, occasionally interrupting the terminal part of the preceding T wave, in none of the cases was repetitive firing or ventricular fibrillation seen. Both patients had organic heart disease and both are still alive six months after the arrhythmia was first recorded. Problems in the diagnosis of multiple parasystole and some mechanisms which may be responsible for irregular interectopic intervals are discussed. It is concluded that multiple parasystole is probably not a very rare arrhythmia if long strips of simultaneously recorded multiple leads are available.

Combined effects of rate membrane potential, and drugs on maximum rate of rise (Vmax) of action potential upstroke of guinea pig papillary muscle

We studied the effect of increasing the rate of stimulation on the maximum rate of rise of the action potential upstroke (Vmax) in guinea pig papillary muscles at various resting membrane potentials and after the addition of quinidine and lidocaine to the perfusate. Increasing rate caused a decrease in Vmax due to interaction of three factors: (1) a metabolic factor, presumably resetting of the Na-K pump, which caused a decrease in Vmax at all levels of resting potential between -90 and -60 mV, (2) a transient decrease in resting potential which influenced Vmax when the resting potential was less negative than approximately -80 mV, and (3) the recovery characteristics of Vmax which contributed to the decrease in this variable when rate was faster than 5/sec. As a result of these factors the steady state curve relating membrane potential to Vmax was itself rate-dependent. Lidocaine and quinidine exaggerated the rate-dependent decrease in Vmax; however, their effects differed. The effect of quinidine was consistent with its known depressant effect on the Na-K pump. The lidocaine effect was consistent with a slowing of recovery of Vmax. Our results help to explain the effects of an increase in rate on Vmax and conduction velocity in normal, partially depolarized, and drug-treated fibers

Double ventricular parasystole. Supernormal phase of conduction as a mechanism of intermittent parasystole. Report of a case

A rare case of spontaneous double ventricular parasystole was studied in depth, together with a critical review of similar cases in the literature. The discussion was focused on 1) the variation of the shortest interectopic interval (SIEI), 2) entrance block and its failure, 3) supernormality as a mechanism of intermittence, and 4) effects of lidocaine and atropine on such an arrhythmia. In double ventricular parasystole a greater than usual variation in the SIEI tended to occur in one of the two parasystolic groups. If, however, such variations were too great in the face of otherwise parasystolic rhythm, presence of intermittence was confirmed. A temporary loss of the entrance block was deemed primarily responsible for the intermittency. That is to say, invasion, discharge, and resetting of one parasystolic focus by another parasystolic impulse during the supernormal phase of the ventricle was considered the cause of an intermittence. In a strict sense, this is the first report in the literature in which the supernormality was clearly indicated as one mechanism of intermittent ventricular parasystole. The advantage of the concept of double ventricular parasystole as compared to single parasystole in defining such a mechanism is stressed.

His bundle electrogram in trifascicular disease: report of a case studied with His bundle electrograms

His bundle electrograms were performed on a 75 year old female with trifascicular block and digitalis induced junctional block. The usefulness of this technique in understanding the patients' electrocardiographic abnormalities and the relationship to phase-3 and phse-4 block is discussed.

Electrocardiographic observations in bradycardia and tachycardia-dependent atrioventricular block. Relationship to supernormal phase of intraventricular conduction

This report describes the clinical course of a patient with bradycardia and tachycardia-dependent atrioventricular block. Bradycardia dependent A-V block (phase 4 block) was transient and precipitated by spontaneous slowing of the sinus rate, atrial and ventricular extrasystoles; The degree of slowing (critical RP interval) required to induce A-V block increased progressively over a three-day period. Bradycardia-dependent A-V block was terminated mostly by critically times spontaneous or paced ventricular escape beats, but normally conducted atrial impulses also appeared to restore A-V conduction on several occasions. The tachycardia-dependent component was manifested by an unusual fatigue phenomenon in the His-Purkinje system seen only at an atrial pacing rate of 150 per minute. These observations document the presence of both bradycardia and tachycardia-dependent A-V block in the presence of a normal H-V time and also illustrate the dynamic nature of both phase 4 block and the period of "supernormal" intraventricular conduction.

Three variants of concealed bigeminy

Long electrocardiographic strips were analyzed from five patients who exhibited periods of typical "concealed bigeminy," i. e., recurrent unifocal extrasystoles which were separated from one another by odd numbers of normally conducted sinus beats. However, in each of these patients, there were periods in which one of three different variants of concealed bigeminy was observed. Three patients displayed an "even number" variant; i. e., there were large numbers of consecutive extrasystoles which were separated exclusively or preponderantly by even rather than by odd numbers of sinus beats. One other patient exhibited an "interpolated extrasystole" variant: those interectopic intervals which were initiated by an interpolated extrasystole contained an even number of sinus beats, whereas all other interectopic intervals contained an odd number. In the fifth patient, the distribution of the numbers of sinus beats separating extrasystoles was such as to suggest a periodic fluctuation between the classical forms of concealed bigeminy and concealed trigeminy; i. e., a "combined bigeminy and trigeminy" variant.

Experimental production of rate-dependent bundle branch block in the canine heart

The main ventricular conducting fascicles were slightly injured in anesthetized dogs by gently scratching them with a blunt needle introduced through the ventricular wall. Initially the bundle branch block that resulted was rate independent (stage 1). When conduction returned to normal, both premature atrial and vagal stimulation reproduced the bundle branch block (stage 2). During stage 2 (5-15 minutes), three conduction ranges were documented: an early (phase 3) block range, a late (phase 4) block range, and an intermediate normal conduction range. The normal conduction range was narrow at the beginning but widened progressively, mostly at the expense of the phase 4 block range. Another period during which only phase 4 bundle branch block occurred (stage 3) preceded total normalization (stage 4). The escape beats that arose from the injured fascicle were most abundant and had the shortest coupling during stage 1, they were less common and their coupling became longer during stages 2 and 3, and they disappeared in stage 4. The tachycardia-dependent or phase 3 bundle branch block was related to a prolongation of refractoriness; the bradycardia-dependent or phase 4 bundle branch block was attributed to slight hypopolarization, enhanced spontaneous diastolic depolarization, and a shift in the threshold potential toward zero. These abnormalities were assumed to be secondary to the hypopolarization, which was probably the basic derangement. This hypothesis satisfactorily accounts for the observation that phase 3 and phase 4 bundle branch block commonly coexist in the same injured fascicle.

Role of positive feedback in the atrioventricular nodal Wenckebach phenomenon

In the atrioventricular (AV) nodal Wenckebach phenomenon, there is a progressive increase in the P-R interval and a concomitant reduction in the R-P interval. Two series of experiments were conducted on anesthetized dogs to ascertain whether the changes in the P-R interval depended on the associated changes in the R-P interval. In one series of experiments, the atrium was paced at various basic cycle lengths, and a single test cycle was interposed about every 100 beats. When the test cycle was very short, the increase in the P-R interval sometimes exceeded the associated reduction in the R-P interval. During the subsequent basic cycle, although the P-P interval was much greater than it was during the test cycle, there was a further reduction in the R-P interval. When this change in the R-P interval was appreciable, AV conduction failed despite the increase in cycle length. In the second series of experiments, the AV nodal Wenckebach phenomenon was induced by rapid atrial pacing. When the stimulation mode was suddenly altered to maintain a constant R-P interval, the Wenckebach rhythm immediately ceased, although the mean heart rate had not changed significantly. Therefore, it appears that above a critical heart rate a positive feedback loop is established in which an increase in the P-R interval evokes a corresponding decrease in the R-P interval. In turn, the curtailed R-P interval that reflects a less complete recovery of AV nodal excitability elicits a still greater increase in the P-R interval. This cycle continues until an impulse is ultimately blocked. Clamping the R-P interval opens the feedback loop and terminates the arrhythmia.

Rate dependent aberrancy

Forty patients with "rate dependent aberrancy" (RDA) were studied. This large group of patients permitted a clear definition of the syndrome and recognition of a number of features not previously described. These proved to have a significant bearing on the recognition and differential diagnosis of RDA and non-rate dependent aberrancy.

It was found that a small change in cycle length, perhaps too small to be recognized in the surface ECG, can result in RDA. Consequently, if a critical shortening of cycle length is to be recognized, it is necessary to record not only the onset of aberrant rhythm but also sufficiently long strips with normal intraventricular conduction preceding and following the RDA. In some patients there was no recognizable sudden change in cycle length and the onset of aberrancy was a function of the duration of the accelerated rate. In others only the first cycle of the rhythm with RDA was shortened and the remaining R-R intervals were paradoxically longer than the R-R cycle which initiated the RDA.

The aberrancy in RDA occurred at relatively slow heart rates (in 26 of the 40 patients the rate was below 80), and was frequently independent of any significant changes in the duration of the immediately preceding cycle length. There was a striking prevalence of left bundle branch block, and in 35 of the 40 patients obvious organic heart disease was documented.

Tachycardia-dependent left bundle branch block associated with bradycardia-dependent variable left bundle branch block

This report describes tachycardia- and bradycardia-dependent left bundle branch block in a patient who exhibited normal intraventricular conduction with impulses intermediate in timing. During bradycardia, longer cycles were associated with increasing degrees of incomplete left bundle branch block. These clinical observations support the existence of slow Phase 4 depolarization of latent pacemaker cells as one of the mechanisms for abnormalities of conduction.

Digitalis intoxication in patients with atrial fibrillation

The characteristic arrhythmias induced by digitalis glycosides in patients with atrial fibrillation are illustrated, and their mechanisms are described. The two effects of the drug which are most important in the genesis of such disturbances are: production of A-V nodal block and arousal of subsidiary pacemakers. These properties account for slow ventricular responses, escape beats, and nonparoxysmal junctional tachycardia. Less commonly observed arrhythmias include exit block from junctional pacemakers, and bidirectional tachycardia which may reflect blocking actions of digitalis in subnodal tissues. The recent development of His bundle electrocardiography and the immunoassay method of blood level determination permit more accurate appraisal of the clinical status of patients in whom digitoxicity is suspected. Treatment of junctional rhythms due to digitalis intoxication is usually passive. The occasional use of drugs, pacing, or cardioversion is discussed.

Relation of electrolyte disturbances to cardiac arrhythmias

While a number of electrolytes play a role in the genesis of the transmembrane action potential (AP), the changes in the action potential most clearly related to arrhythmias are dependent to a large extent on K + . Potassium gradient is a major determinant of the magnitude of transmembrane resting potential (TRP), and secondarily the rate of rise (dV/dt) of phase 0, and consequently the speed of conduction. The cell membrane conductance for K + , or a decrease therein, is most likely the major determinant of spontaneous slow depolarization during phase 4. Thus K + has a pronounced effect on both conduction and automaticity. Furthermore, these electrophysiologic properties are altered within levels of K + encountered in clinical medicine, a situation which, with rare exceptions, is not seen with Ca ++ , Mg ++ , or Na – . These latter ions affect the action potential and induce experimental arrhythmias at concentrations which are unphysiologic and frequently incompatible with life. Consequently, of all the electrolytes, disturbed K + metabolism accounts for the vast majority of clinical arrhythmias. For the same reasons, with the exception of the ability of Na + and Ca ++ to reverse the K + -induced depression of conduction, K + is the only electrolyte with clinacally significant antiarrhythmic properties.