A relationship between ultrasonic intensity and changes in myocardial mechanics

The effects of ultrasound on the mechanical properties of isometrically contracting rat papillary muscle have been studied as a function of the intensity of the ultrasonic irradiation. Each muscle was subjected to irradiation at 1.1., 2.2, and 3.3 W/cm2 at a frequency of 2.3 MHz. Ultrasonic irradiation caused heating in the vicinity of the muscle; thus it was necessary to separate the pure thermal and ultrasonic effects. After irradiation the bath temperature was increased so that the muscle reached the same temperature which occurred during ultrasonic irradiation. The mechanical parameters measured during the equivalent thermal procedure were compared with the parameters during irradiation. There was a significant (p less than 0.05) decrease in resting force which was significantly different from the equivalent thermal intervention and could be related to the ultrasonic intensity. Thus, the effect of ultrasound on rat papillary consists of two components, a thermal component which affects all parameters studied and a nonthermal component which affects only the diastolic force.