Hemodynamics of paced beats containing some anodal component differ from those induced by the conventional cathodal waveform in rabbits, canines, and ferrets. We investigated this phenomenon in the pig (Sus domesticus). Upstroke (dP/dt+) and relaxation (dP/dt2) of left ventricular pressure curve were measured in two 50 kg subjects paced slightly above intrinsic rate with cathodal (C) and anodal followed by cathodal biphasic (B) pacing pulses. The first animal had preexistent atrial fibrillation and developed ventricular fibrillation due to a reversion attempt with unsynchronized cardioversion. After eventual resuscitation and defibrillation, sinus rhythm resumed, although with pressure measurements significantly depressed compared to the second subject. Measurements in each animal were made over 15 min period following mode changes. Only small differences in raw data between pacing modes were observed. However, after normalization of data to express percent change from start (initial) values in each mode, we obtained graphs showing dP/dt+ and dP/dt2 readings constant or slightly increasing for B pacing. C pacing showed 7 and 8% depressions of dP/dt2 eventually returning toward initial readings over the 15 min follow-up periods. Data are most consistent with altered relaxation properties, but the issue of increased contractility is unclear since concomitant volume measurements were not done. Time dependence of hemodynamic changes in upstroke and relaxation suggests that feedback loops in the heart may be responding to initial perturbations induced by the pacing modes. Pressure-volume loops need to be performed in order to clarify the mechanism.
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