Unidirectional retrograde atrioventricular nodal block in man: determinants of reversibility by vagal antagonism

Concealed Sinus Node Dysfunction and Paradoxical Effect of Atropine during Arrhythmia Diagnostic Pharmacological Testing

A 78-year-old male patient presented with repetitive fainting episodes. His electrocardiogram showed sinus rhythm with persistent ventricular bigeminy. Concealed sinus node dysfunction (SND) with consecutive bradycardia-induced ventricular hyperexcitability was suspected. Pharmacological testing with atropine resulted in accelerated junctional rhythm along with nearly total disappearance of the ventricular ectopy. The diagnosis of SND was retained, a dual chamber pacemaker was implanted, and consequently, ventricular hyperexcitability disappeared. The junctional rhythm was a paradoxical effect of atropine, and many explanations were provided. Discussion was made accordingly taking into account relevant data from the literature.

Ventriculoatrial conduction capability and prevalence of 1:1 retrograde conduction during inducible sustained monomorphic ventricular tachycardia in 305 implantable cardioverter defibrillator recipients

Despite the advent of dual chamber ICDs, differentiation of VT (SMVT) with 1:1 VA conduction will remain a challenge. In this study, VA conduction capability and prevalence of inducible sustained monomorphic (SM) VT with 1:1 VA conduction was assessed in 305 ICD recipients. SMVT with a mean cycle length (CL) of 304 +/- 61 ms was induced in 161 (53%) patients. Twenty-six percent of the patients maintained 1:1 VA conduction to CL < or = 400 ms during incremental ventricular pacing, regardless of presenting tachyarrhythmia or presence of inducible SMVT. Among ten patients who had inducible SMVT with possible 1:1 VA conduction (based on SMVT CL comparable to the shortest CL associated with 1:1 retrograde conduction during ventricular pacing), all seven with available intracardiac tracings had documented 1:1 VA conduction during the induced SMVT--representing 4.4% of the patients with inducible SMVT (95% CI 1.2%-7.6%), and 2.3% of the entire ICD cohort (95% CI 0.6%-4.0%). We conclude that about one-fifth of ICD recipients possess 1:1 VA conduction to CL < or = 400 ms and that inducible SMVT with 1:1 VA conduction can be demonstrated in a small but nonnegligible proportion of ICD recipients. These data are relevant to the design of tachyarrhythmia-discrimination algorithms for dual chamber ICDs.

Unidirectional block between isolated rabbit ventricular cells coupled by a variable resistance

We have used pairs of electrically coupled cardiac cells to investigate the dependence of successful conduction of an action potential on three components of the conduction process: (a) the amount of depolarization required to be produced in the nonstimulated cell (the "sink" for current flow) to initiate an action potential in the nonstimulated cell, (b) the intercellular resistance as the path for intercellular current flow, and (c) the ability of the stimulated cell to maintain a high membrane potential to serve as the "source" of current during the conduction process. We present data from eight pairs of simultaneously recorded rabbit ventricular cells, with the two cells of each pair physically separated from each other. We used an electronic circuit to pass currents into and out of each cell such that these currents produced the effects of any desired level of intercellular resistance. The cells of equal size (as assessed by their current threshold and their input resistance for small depolarizations) show bidirectional failure of conduction at very high values of intercellular resistance which then converts to successful bidirectional conduction at lower values of intercellular resistance. For cell pairs with asymmetrical cell sizes, there is a large range of values of intercellular resistance over which unidirectional block occurs with conduction successful from the larger cell to the smaller cell but with conduction block from the smaller cell to the larger cell. We then further show that one important component which limits the conduction process is the large early repolarization which occurs in the stimulated cell during the process of conduction, a process that we term "source loading."

Electrophysiologic characteristics of manifest and latent retrograde conduction in dogs

Atrioventricular (AV) nodal reentry requires intact retrograde ventriculoatrial (VA) conduction. The purpose of this study was to assess the contribution of various pacing and pharmacologic techniques to uncover VA conduction during apparent unidirectional VA block, and to evaluate the role of several biologic and electrophysiologic factors in concealment of retrograde conduction. Forty healthy dogs underwent catheter-electrophysiologic studies of AV and VA conduction. Group I (20 animals) had intact VA conduction. Group II (six animals) had VA dissociation with ventricular pacing initiated during sinus rhythm, but the presence of VA conduction was confirmed by isoproterenol infusion or by premature ventricular stimulation. In group III (14 animals), the above techniques failed to uncover VA conduction. Eight of 14 group III animals underwent thoracotomy and crushing or freezing of the sinoatrial (SA) node. Ventricular pacing initiated during sinus standstill was associated with 1:1 VA conduction in each experiment. VA conduction time and retrograde Wenckebach cycle length, both in the baseline state and during isoproterenol infusion, were significantly longer in the eight animals in group III than in those in group I. Age, gender, weight, breed, sinus cycle length, and anterograde AV conduction properties were not significantly different between groups I, II, and III. The data suggest that (1) in normal dogs, complete unidirectional VA block probably does not exist; (2) in the presence of anterograde input to the AV node, even sophisticated pacing and pharmacologic maneuvers may fail to uncover the presence of VA conduction; (3) although anterograde input is essential for concealment of VA conduction, the phenomenon is more closely associated with depressed retrograde than with anterograde AV nodal characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial sensing to augment ventricular tachycardia detection by the automatic implantable cardioverter defibrillator: a utility study

The triggering of automatic implantable cardioverter defibrillator (AICD) discharges by supraventricular tachycardias, despite the presence of a probability density function algorithm, remains a limitation of an otherwise highly effective device. We systematically investigated the diagnostic utility which theroretically could derive form the addition of atrial sensing capability to the AICD in 25 patients with 30 inducible sustained monomorphic ventricular tachycardias (VTs) at clinically relevant rates (greater than or equal to 150 beats/min). Patients were included only if they were not taking medication capable of depressing ventriculoatrial (VA) conduction for at least 5 half-lives prior to electrophysiological testing. We tested the simple criterion for VT that ventricular cycle length (CL) be shorter than the atrial CL (not met in sinus or most other supraventricular tachycardias). Mean VT CL was 283 +/- 47 ms (range 210 to 370). In 25 (83%) VTs, the VT criterion was consistently satisfied. Of the five cases in which the criterion was not met, 1:1 VA conduction during VT was present in four, three of which initially manifested 2:1 VA conduction lasting from 14 to 28 s and therefore would have transiently fulfilled the VT criterion. The remaining patient who failed to satisfy the VT criterion had ongoing atrial flutter during a relatively slower sustained VT, but this circumstance could be recognized because of the varying AV interval. The absence of 1:1 VA conduction at CLS less than or equal to 400 ms during ventricular pacing accurately predicted the absence of 1:1 VA conduction during VT in 95% of patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of upright posture on anterograde and retrograde atrioventricular conduction in patients with coronary artery disease, mitral valve prolapse or no structural heart disease

To assess the effects of posture on anterograde and retrograde atrioventricular conduction, electrophysiologic testing was performed in 25 patients in both the supine and 45 degrees upright positions on a tilt table. Retrograde conduction was present during ventricular pacing in 17 patients in the supine position; all 17 continued to manifest retrograde conduction in the upright position. In all patients with absent retrograde conduction while supine, retrograde conduction could not be demonstrated while upright. Upright posture significantly (p less than 0.05) shortened the sinus cycle length (from 808 +/- 34 to 678 +/- 26 ms, mean +/- standard error of the mean), AH interval during sinus rhythm (78 +/- 6 to 69 +/- 6 ms), and AH interval during atrial pacing at cycle length 500 ms (123 +/- 13 to 91 +/- 9 ms). Total atrioventricular conduction time during atrial pacing shortened significantly (from 169 +/- 13 to 136 +/- 10 ms), as did ventriculoatrial conduction time during ventricular pacing (from 192 +/- 9 to 178 +/- 7 ms). Upright posture also significantly shortened both anterograde block cycle length (390 +/- 20 to 328 +/- 17 ms) and retrograde block cycle length (466 +/- 27 to 354 +/- 18 ms). However, the effect of upright posture on retrograde block cycle length was significantly greater than on anterograde block cycle length: a 21% decrease retrograde vs a 14% decrease anterograde (p less than 0.05). These effects may produce clinically important changes in characteristics of arrhythmias that depend on the properties of anterograde and retrograde conduction.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer simulation of cardiac arrhythmias

A mathematical model of the cardiac conduction system has been developed. The mechanisms of cardiac arrhythmias are described mathematically, and the heart is modeled as a network, where each element is defined by a unique set of time parameters from the action potential. The mathematical description is separated from the network structure, thus making it possible to run the model with different network sizes. Simulated ECG curves are produced in each case. This model is especially suited for rhythm studies, and a variety of different cardiac arrhythmia mechanisms has been simulated such as reentry, reflection, modulated parasystole, and different kinds of block.

Effect of pharmacologic autonomic blockade on ventriculoatrial conduction

To determine the influence of autonomic tone on retrograde ventriculoatrial (VA) conduction, incremental atrial and ventricular pacing was performed before and after pharmacologic autonomic blockade in 28 patients. VA conduction during ventricular pacing was demonstrated, with highest frequency in patients capable of 1:1 atrioventricular (AV) conduction at atrial paced cycle lengths of 300 ms or less (7 of 7, 100%). In subjects with 1:1 AV conduction at minimum cycle lengths of 300 to 500 ms, 14 of 21 (67%) demonstrated VA conduction in the control state; however, only 12 of 21 (57%) did so after autonomic blockade. The lowest frequency was observed in those capable of 1:1 AV conduction at minimum cycle lengths of 505 ms or more before and after autonomic blockade (2 of 7, [29%], p less than or equal to 0.02 compared with values in the first group). No change in the mean minimum ventricular paced cycle length at which 1:1 VA conduction could be maintained was demonstrated after autonomic blockade. In individual subjects, incremental change in this cycle length after autonomic blockade correlated positively with the corresponding change in minimum atrial cycle length at which 1:1 AV conduction could be maintained (r = 0.62, p less than 0.005), and was concordant in direction in 18 of 21. In conclusion, the sympathetic and parasympathetic modulation of VA conduction is balanced and concordant in direction to the effect on AV nodal conduction.