Ultrasound intensity and contractile characteristics of rat isolated papillary muscle.

The effect of graded intensities of continuous wave ultrasound on the contractile performance of isolated papillary muscle of rat was tested. Under isometric conditions rat left ventricular papillary muscles (n = 48) were electrically stimulated to contract at rates of 30, 60, 120 and 240 beats per minute. Muscles were perfused with a Tyrode solution at 30 degrees C under normoxic conditions. Ultrasound at intensities of 0.25, 0.50, 1.0 and 2.0 W/cm2 spatial average temporal average (SATA) at 963 kHz was applied to the muscles while recording muscle contractile characteristics. The analog data were digitized and stored on disk for analysis by computer. This revealed a significant (p less than 0.001) increase in peak developed force (F), peak rate of force development (+dF/dt) and peak rate of myocardial relaxation (-dF/dt) that was linearly related to ultrasound intensity. The muscles were more sensitive to ultrasound at 240 contractions per minute. Resting force was significantly decreased by ultrasound. Although bath temperature increased according to the ultrasound intensity, control studies in papillary muscles (n = 24) on the correlation between contractile parameters and temperature revealed that bulk heating could not account for the positive inotropic action with ultrasound. These data confirm the inotropic effect of continuous wave ultrasound on myocardial tissue and point to the possibility of applying this phenomenon therapeutically.

Effect of therapeutic level ultrasound on visual evoked potentials in the hypoxic cat.

The effect of 1.0 MHz ultrasound at an intensity of 1.0 W/cm2 SATA on brain function of anaesthetized cats was assessed using Visual Evoked Potentials (VEPs). Ultrasound alone did not significantly modify the averaged VEP signals. However, acute hypoxia depressed the VEP response. When the brain was exposed to ultrasound during the hypoxic episode, the VEP response remained normal. Raising brain temperature by whole body heating could not mimic the beneficial effect of ultrasound on the VEP response during hypoxia. This suggests that therapeutic ultrasound may have a protective effect on hypoxic tissues and may have clinical applications.

31P NMR studies of the metabolic status of pig hearts preserved for transplantation.

31P NMR spectroscopy has been used to evaluate the metabolic status of cardioplegically arrested pig hearts. Hearts were stored with Plegisol for up to 12 hours at either 5 degrees C or 12 degrees C. Results indicated that the ATP content of hearts could be maintained (greater than 70% of initial values) for up to 5 hours in the ischemic storage state. The ATP loss was greater at 12 degrees C. PCr was lost exponentially under the same conditions. Functional testing by reperfusing the stored hearts in vitro indicated a good correlation between the ATP content and survivability of the preparations. Twenty-four hour preservation of pig hearts using slow perfusion with a modified cardioplegic solution (Wicomb) allowed for preservation of both PCr and ATP, in all cases, reperfusion of hearts revealed a loss of NMR- visible ATP and PCr.

Ultrasound-enhanced diffusion through isolated frog skin.

The effect of ultrasound on the transport of oxygen across excised frog abdominal skin has been studied. Samples were mounted in an exposure chamber in which the Ringer's solution on one side was saturated with oxygen while the other side of the skin had a low initial oxygen concentration. They were treated with ultrasound at 1, 1.5 and 2 W cm-2 SATA c.w., respectively, and increases in the rate of oxygen transport were observed at all intensities. These increases ranged from 38 +/- 4% at 1 W cm-2 to 55 +/- 8% at 2 W cm-2. Variation in the pulse lengths from 25 to 200 ms and a constant average intensity did not affect the rate of transport significantly provided that the temporal intensity was constant. Since the peak acoustic pressure within the pulse increased with decreasing pulse length and increasing acoustic pressure increases the probability of cavitation occurring, the mechanism responsible for this phenomenon is probably not cavitation.

The effects of ultrasound on the mechanical properties of rat cardiac muscle.

The mechanical properties of cardiac muscle during ultrasonic irradiation have been studied in vitro. Left anterior papillary muscle from normal rats was suspended in buffered lactated Ringers solution equilibrated with 95% O2, and 5% CO2 and maintained at 20 degrees C. The muscles were stimulated to contract isometrically three times per minute at the length which produced maximum tension. Each muscle was irradiated with a MHz ultrasound at an average power of 2.4 Wcm-2 for a period of 10 min with a 10 min recovery period. Irradiation caused an average increase in temperature of the muscle of 1.7 +/- 0.2 degrees C (mean +/- SEM). Irradiation caused the resting tension (1.46 +/- 0.13g) to decrease by 17.8 +/- 4.7% and the developed tension (3.33 +/- 0.61g) to decrease by 4.1 +/- 0.9%. Since changes in contractile properties have been reported with temperature the bath temperature was raised and changes in contraction observed. When compensated for effects of temperature, the changes in resting tension became - 13.3 +/- 4.1% while the change in developed tension became + 1.6 +/- 2.3%. The change in resting tension is highly significant (p less than 0.05 paired t-test) while the change in developed tension is not. Thus 1 MHz ultrasound at an intensity of 2.4 Wcm-2 appears to affect resting tension of cardiac muscle without affecting the active tension. Since changes in cardiac mechanics of this type have not been described previously the effects of ultrasound appears to be unique.