Impedance changes during the electrical and mechanical activities of a bullfrog heart

Electrical impedance of a bullfrog ventricle was recorded epicardially by the V-I method and the bridge method together with an electrocardiogram (ECG) and a contraction curve, in terms of shortening. Impedance change during one cardiac cycle of a contracting ventricle corresponded completely to the contraction curve in its time course: impedance decreased during systole and attained minimum at the peak of contraction, and the durations of both curves were almost identical. The ratio of impedance just before T-wave to that just after R-wave ([ZT]/[ZR]) of an in situ heart was increased when the ventricle contracted isovolumetrically by means of mineral oil: the ratio was far less than 1.0 in control ventricle, while it was more than 1.0 in oil-filled one (2/3 cases). Upon isometric condition or treatment of cadmium the motion restricted ventricle caused an increase in the impedance ratio ([ZT]/[ZR]) to higher than 1.0, as in the case of oil-filled ventricle. The epicardial recording of ventricular impedance seems to consist of two components: contraction(shortening-dependent impedance change, which reflects the membrane activity. Vigorous contraction will probably mask the latter and reveal only the former.