The role of ultrasound operation mode for safely interfering in the heart rate

Diagnostic ultrasound applies low intensity acoustic waves to noninvasively investigate biological tissues. Higher intensities can alter tissue characteristics, and this is of interest for therapeutic ultrasound, when the occurrence of bioeffects is — to a certain extent — desirable for tissue healing. Relative to cardiology, diagnostic ultrasound is well established, whereas there is an unexplored potential for therapeutic applications. Ultrasound is an alternative source of energy that has different characteristics when compared to electrical energy and so its interference in the cardiac activity might be useful for treating arrhythmias. The objective of this study is to investigate the role of operation mode (continuous / pulsed ultrasound) for interfering in the heart rate without damaging the tissue. Nine Sprague-Dawley rats were anesthetized with isoflurane and exposed to high-intensity, 1MHz ultrasound. One animal was submitted to continuous wave application, which produced thermal damage. Two groups of four animals were submitted to different pulsed schemes (single / variable pulse repetition frequency). Post-therapy values were divided by pre-therapy values, resulting in normalized values for heart rate and respiratory rate. These values were compared between both pulsed schemes, resulting in a statistically significant difference (p<0.05) only for the heart rate. When comparing the post-therapy and pre-therapy absolute values within the variable pulse repetition rate group, there was a heart rate drop (p<0.05), whereas there was no significant effect on the ejection fraction and end diastolic volume, meaning that no major damage was produced. The insonification scheme used in this study excludes temperature effects, so the observed effect results from nonthermal mechanisms, possibly from radiation force.