EFFECTS OF PULSED ULTRASOUND ON THE FROG-HEART .1. THRESHOLDS FOR CHANGES IN CARDIAC-RHYTHM AND AORTIC PRESSURE

High intensity pulsed ultrasound at 1.2 MHz is shown to change the cardiac rhythm and aortic pressure of frog hearts in vivo. Threshold levels for these effects occur at acoustic pressure amplitudes of the order of 10 MPa for 5 ms pulse lengths. Depending upon the phase of the heart cycle, a pulse of ultrasound either may cause a premature ventricular contraction, a reduction in the strength of contraction as measured by the aortic pressure, or an enhanced relaxation of the heart muscle. There is an increase in the effectiveness of the ultrasound with increase in pulse length in the range from 1 to 5 ms.