A comparison of chronaxies for ventricular fibrillation induction, defibrillation, and cardiac stimulation: unexpected findings and their implications.
BACKGROUND
A low-energy (<or= 4 J) cardioversion shock (LEC) either terminates reentrant ventricular tachycardia (VT) or accelerates it to ventricular fibrillation (VF). Optimization of the duration and amplitude of LEC shocks could improve the success rate of VT termination without VF induction.


RESULTS
In order to learn how LEC shocks may be optimized, we used an animal model to compare the strength-duration curve for VF induction and the strength-duration curve for cardiac stimulation via the shock coil. Conventional implantable cardioverter-defibrillator (ICD) leads were implanted in 12 narcotized pigs from 20 kg to 25 kg in weight. Stimulation, VF induction, and defibrillation pulses were delivered by custom-designed stimulators at preset pulse durations and amplitudes. The corresponding hyperbolic strength-duration curves were constructed using the least-squares fit method and averaged for all the animals. The mean chronaxie for stimulation via the shock coil of 0.23 ms was significantly shorter than both defibrillation (4.8 ms) and VF induction (3.1 ms) chronaxie values. At a shock duration of 0.3 ms or less, the mean VF-induction threshold amplitude exceeded 300 V.


CONCLUSIONS
It may be reasonable to study whether LEC pulses from 0.25 ms to 0.30 ms in duration and up to 250 V in amplitude would increase therapeutic yield in VT termination without VF induction in humans. Contrary to the current belief, the discrepancy between defibrillation and stimulation chronaxie is not caused by different electrode size. We postulate that the time constant of the fast sodium channel reactivation may be the underlying reason.
Lawo
Thomas
T
Medical Clinic II (Cardiology), BG University Hospital Bergmannsheil, Bochum, Germany. thomas.lawo@rub.de
Deneke
Thomas
T
Schrader
Juergen
J
Danilovic
Dejan
D
Wenzel
Beate
B
Buddensiek
Michael
M
Muegge
Andreas
A
D000818:Animals; D002907:Chronaxy; D004554:Electric Countershock / Q000009:adverse effects* / Q000379:methods*; D006329:Heart Conduction System / Q000503:physiopathology*; D033162:Incidental Findings; D008955:Models, Cardiovascular; D013552:Swine; D014693:Ventricular Fibrillation / Q000209:etiology / Q000503:physiopathology* / Q000517:prevention & control*