Investigation of the possibilities of cardiac defibrillation by ultrasound.

In 15 experiments on dogs ultrasound with a frequency of 500 kHz and an intensity of 10 W/cm2, exerted a defibrillatory and antiarrhythmic effect and stopped ventricular fibrillation in 28% of the animals. Electrophysiological studies on isolated right ventricles of rabbits in oxygenated chambers demonstrated that the antiarrhythmic action of the ultrasound acted by prolonging the refractory period of the myocardial cells. Ultrasound with an intensity of 4 W/cm2 and 30 s duration increased the refractory period up to 37%. Any further increase of intensity and duration of the ultrasound action resulted in a shortening of the refractory period and inhibition of the electrical activity of the myocardial cells. Investigations of the harmful effects of ultrasound with a frequency of 500 kHz and an intensity of 10 W/cm2 showed that ultrasound for up to 2 min did not cause evident histological changes.