Cardiac electrophysiological experiments in numero, Part III: Simulation of arrhythmias and pacing

The antiarrhythmic effect of verapamil on atrioventricular re-entry in the Wolff-Parkinson-White syndrome: a computer model study

Verapamil is supposed to suppress the initiation of circus movement supraventricular tachycardia by affecting the atrioventricular node. In electrophysiological tests, programmed stimulation is usually performed by using the same location for pacing and premature stimulus. Spontaneous ectopic activity starts from a different location than the sinus node and can therefore find altered re-entry conditions. In this study a 3D computer model based on Huygen's principle is used for simulation of the spread of excitation in the human heart in combination with a posterobasal, right or left lateral accessory pathway (AP). The effect of verapamil on properties of the atrioventricular node were modelled by prolonging the effective refractory period and basal conduction time. For each of the three APs, ectopic foci at the atrial base and between sinus node and AP were modelled at various coupling intervals for investigating re-entrant activation. In the control state (without verapamil) only orthodromic echoes were found. The maximum echo zone (EZ) range was found near the AP. If stimuli were selected further away from the AP on the atrial basis, the EZ range decreased until no EZ was found. The EZ range decreased from it's maximum value near the AP, towards the difference of the effective refractory periods between AP and AV-node near the sinus node Verapamil abolished the EZ in case of a posteroseptal AP. For a lateral AP the administration of verapamil resulted in an orthodromic and antidromic EZ depending on the atrial premature activation site. A maximum orthodromic EZ was found for premature stimuli near the AP. As stimulus site moved away from the AP, the EZ range first decreased continuously to zero leading eventually to an antidromic EZ. These findings suggest the important influence of the site of premature stimuli with respect to the accessory pathway and AV-node on the inducibility of atrial re-entry.

A computer model study of the ventricular fibrillation vulnerable window: sensitivity to regional conduction depressions

The cardiac vulnerable window is typically defined to be that portion of the cardiac cycle during which ectopic stimuli can induce ventricular fibrillation (VF). We have used cardiac electrophysiological computer modeling to investigate how the size and shape of the cardiac vulnerable window is affected by regional conduction velocity depressions (RCVDs), as might be found in ischemia. Computer simulations were performed on a three-dimensional finite-state, discrete-element model of the ventricles of a dog heart, with simulated RCVDs of 0% (normal) to 100% (infarcted) isolated to the apical region of this heart. Using a programmed electrical stimulation protocol, vulnerability was quantified as the number of ectopic stimuli necessary to induce VF. We observed a nonlinear and nonmonotonic relation between increases in RCVD and the vulnerability of the heart to ectopic stimuli. The size and shape of the vulnerable window remained stable through RCVDs of 30%, expanded rapidly thereafter through RCVDs of 90%, and then contracted significantly at the RCVD of 100%. These increases in vulnerability were manifest as both a lowering of the overall stimulus thresholds necessary for fibrillation (window "deepening"), and an increase in the fraction of the cardiac cycle susceptible to these ectopic stimuli (window "widening"). In all cases of induced VF, the mechanism of induction was through reentry facilitated by temporary functional block. Moreover, the ability to form such a block--and thus the likelihood of subsequent VF--was enhanced as the RCVD increased. Taken together, these results demonstrate the complex relation between vulnerability and RCVD and, at the very least, suggest that the entire shape of the vulnerable window, rather than just its minimum threshold, is important when determining a heart's electrical stability. These conclusions are supported by results obtained from an experimental study--which utilized a similar programmed stimulation protocol--of normal and ischemic canine hearts.