[SHORT REPORTS]Effects of Countershock-Type Field Stimulation on the Electro-Mechanical Properties of Guinea Pig Verntricular Muscles:VI. Influence of the Electrical Axis of Stimulation

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The influence of electrical axis of stimulation on the post-shock cardiac dysfunction was investigated in guinea pig papillary muscles. Ten msec shocks were imposed on the preparations in a longitudinal (L) or transverse (T) direction with respect to their fiber orientation. Diastolic threshold for excitation by L-shocks (1.2±0.2 V/cm) was significantly lower than T-shocks (1.2±0.2 V/cm). Shocks>20 V/cm caused dose-dependent "after-effects"lasting 1-3 min consisting of a decrease in the resting potential(RP), a shortening of the action potential duration, and an increase in the conttactiale force. With shocks>40 V/cm, an oscillation of membrane potential leading to repetitive spontaneous acitivity (RSA) was often elicited. The after effects by L-shocks were significantly less than T-shocks. With shocks at 62V/cm, RP reduction was 18±2mV(L) vs. 31±4 mV(T); RSA was 0.1±0.1 beats (L)vs. 2.9±0.5 beats (T)(n=10, p<0.05).These findings suggest an advantage of L-shocks over T-shocks in terms of higher stimulating efficacy and less detrimental after effects.
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