Profibrillatory Effects of Multiple Extrastimuli

Quatrefoil reentry in myocardium: an optical imaging study of the induction mechanism

INTRODUCTION

The "critical point hypothesis" for induction of ventricular fibrillation has previously been extended to infer the coexistence of four critical points, and hence four simultaneous spiral reentries or a quatrefoil reentry, resulting from only one premature stimulus delivered to the same location as the pacing stimulus. An optical imaging technique was used to explore its existence and to study the induction mechanism of this peculiar reentry pattern.

METHODS AND RESULTS

In 16 isolated, Langendorff-perfused rabbit hearts, high-speed optical imaging at 133 or 267 frames/sec was performed to observe the induced response with a unipolar point electrode. A novel quatrefoil-shaped reentry pattern consisting of two pairs of opposing rotors was created by delivering long stimuli during the vulnerable phase. Successful induction occurred in a narrow range of coupling intervals. A dogbone pattern of virtual electrodes was established during the premature stimulus. Propagating wavefronts launched from the virtual anodes immediately after the termination of S2. The alternating blocking and conducting effects of the virtual electrodes, as well as the boundary between virtual cathode and virtual anode, provided the necessary pathways for quatrefoil reentry. Propagation directions of the reentrant spiral wavefronts reversed with a reversal in S2 polarity. Quatrefoil reentries were not sustained and lasted 1 to 4 complete cycles.

CONCLUSION

The initiation of quatrefoil reentry followed anodal- or cathodal-break stimulation as a result of local symmetrical enhancement of the dispersion of tissue excitability. The "critical point hypothesis" provides the minimum topology required for this type of reentry; the "graded response hypothesis" can be viewed as providing a more detailed explanation of how this topology is actually realized. Triggering mechanisms due to the "break" mode of stimulation also posits a new mechanism for defibrillation.

Induction of ventricular fibrillation versus monomorphic ventricular tachycardia during programmed stimulation. Role of premature beat conduction delay

BACKGROUND

Premature stimuli can cause ventricular fibrillation (VF) during electrophysiological testing. The electrophysiological correlations associated with the onset of VF were evaluated in 40 patients who had this rhythm induced during programmed ventricular stimulation. These parameters were compared with those observed in 51 patients who had inducible sustained monomorphic ventricular tachycardia (VT) and 45 patients who had no inducible sustained ventricular tachyarrhythmias.

METHODS AND RESULTS

Shortest premature coupling intervals for S2, S3, and S4 at induction of tachycardia or before achieving refractoriness, corresponding conduction latencies (defined as the time from the premature stimulus to the upstroke of the depolarization wave front recorded 35 mm away from the stimulation site), and ventricular activation times (defined as the time from the premature stimulus to the end of the depolarization wave) were compared. The mean coupling intervals were longest in the inducible VT patients: 300 +/- 30, 254 +/- 57, and 228 +/- 32 msec for S2, S3, and S4, respectively. In the inducible VF group, the coupling intervals were 260 +/- 37, 208 +/- 20, and 213 +/- 30 msec. In the group with no inducible VT or VF, these coupling intervals were 251 +/- 24 (p less than 0.01 versus inducible VT group), 209 +/- 27 (p less than 0.001 versus inducible VT group), and 194 +/- 21 msec (p less than 0.05 versus inducible VT and VF groups). The coupling interval of the last premature extrastimulus was above 200 msec in 70% of the patients in whom VF was induced. The largest increases in latency and activation times were recorded in patients in whom VF was induced. The cumulative increase in latency, defined as increased conduction time from baseline, summed for all the premature stimuli was also the greatest at initiation of VF. In contrast, the smallest increases in these parameters were noted in the patients with no inducible VT or VF. Measurements of total activation time yielded similar results as those recorded for latencies. The most important parameters distinguishing the VT patient population from the other two groups were the low ejection fractions and the longer coupling intervals at which VT was induced, whereas in the VF group, the most important discriminating factor was cumulative activation time. Sixty-three percent of the inducible VF patients presented with abnormal hearts (myocardial infarction or cardiomyopathy), whereas 88% of the inducible VT patients had abnormal hearts. In contrast, only 25% of the patients in whom no arrhythmia was induced presented with abnormal hearts. Mean ejection fraction was 32 +/- 15% for the inducible VT group, 45 +/- 13%* for the inducible VF group, and 51 +/- 17%* for patients with no inducible VT/VF (*p less than 0.001 versus VT).

CONCLUSIONS

The results suggest that 1) initiation of ventricular tachycardia during programmed ventricular stimulation occurs with minimal conduction latency; 2) because of the large overlap in coupling intervals where VF or VT were induced, a single coupling interval cannot be recommended to adequately separate these groups; and 3) induction of VF was preceded by increased latency and prolongation of the local activation time. These parameters should not be allowed to prolong if VF is to be avoided during programmed stimulation. In addition, 4) the initiation of VF during electrophysiological studies is often associated with the presence of structural heart disease; such structural disease may promote conduction latency and the development of VF.