Local effects of electrical and mechanical stimulation on the recovery properties of the canine ventricle

The effects of electrical stimulation on local recovery properties of the canine ventricle were studied. Ventricular excitability was examined by an analysis of unipolar or bipolar strength-interval curves, and the effective refractory period was derived from the steep portion of the curve. Conduction times of all propagated responses to testing stimuli were recorded. When ventricular driving and testing sites were the same, effective refractory periods were significantly shorter (probability [p] < 0.001), conduction times longer (p < 0.001) and anodal dip thresholds lower (p < 0.001) than during atrial drive. However, as ventricular driving and testing sites were separated, effective refractory periods increased and conduction times shortened. At interelectrode distances of greater than 15 mm, both effective refractory periods and conduction times remained constant and similar to those observed during atrial drive. Effective refractory periods measured at the ventricular drive electrode shortened by 8.2 percent (p < 0.001) when the driving stimulus was twice threshold and by 15.4 percent (p < 0.001) when 10 times threshold. At a distance of 15 mm from the driving sites, the effective refractory period was not significantly shortened when testing stimuli of twice threshold intensity were used and was reduced by only 8.1 percent (p < 0.002) when stimuli 10 times threshold were used. Shortening of the effective refractory period near the ventricular driving site was accentuated by short coupling intervals. When ventricular drive was accomplished using mechanical impulses delivered to the epicardium, effective refractory periods during atrial and ventricular drive were virtually the same, regardless of the proximity of ventricular driving and testing sites. Thus, shortening of the effective refractory period and prolonged conduction in the vicinity of the driving electrode are a function of stimulus intensity, distance from the driving site and time. These local alterations in the recovery properties of the ventricle provide conditions that may be favorable for the induction of reentrant arrhythmias.