P4E-2 On the Mechanisms of Ultrasound Contrast Agents Induced Arrhythmias

Contrast ultrasound imaging is a noninvasive imaging modality using gas microbubbles. However disorders of the heart rate expressed by the appearance of premature ventricular contractions (PVCs) occur in few patients. We have recently shown through electrophysiological studies that ultrasound activated microbubbles oscillate and induce a mechanical stress on the cell membrane resulting in the activation of stretch activated channels (SAC). These channels, described initially in the muscular cells, are ionic channels expressed in all cell types. In the heart, their activation can trigger arrhythmias. The aim of this study is to demonstrate, in vivo and in vitro, the relationship between PVC and SAC opening by ultrasound contrast agents (UCA) during ultrasound (US) exposure. Three sets of experiments were conducted: two on animal models with microbubbles and mechanical stress and one in vitro. 5 anesthetized rats were used. PVC's were created in vivo using SonoVuetrade microbubbles at a rate of 0.5 ml/min. The microbubbles were injected intravenously through the tail vein. US consisted of 40-cycle waveforms at 1 MHz and peak negative pressures up to 300 kPa. A PVC animal model based on a manually-induced mechanical stretch was used. The model consisted of stimulations by a flexible catheter introduced into the rat aorta and pushed until the left ventricle. PVC's were quantified through ECG measurements. In-vitro, patch Clamp technique in ruptured-patch whole cell configuration was used to monitor the changes in the electrical potential on HL-1 heart cell line during manual stimulation. In vivo, UCA and US at 300 kPa induced modification of rat's ECG while pressure below 300 kPa did not induce any PVC. US alone did not induce PVC at any acoustic pressures. Similar PVC's were also observed in the animal model but ceased immediately after the stimulation. In vitro, electrophysiological recordings showed that the mechanical stretch engendered a cell membrane depolarization due to SAC opening. The cell membrane potential depolarized as a consequence of the manual stimulation and returned to its initial value when the stimulation was released. This effect might trigger additional action potential and by that the generation of extra contractions such as PVC's. We presume that PVC's are triggered through the opening of stretch activated ion channels. This study indicates that PVCs induced by UCA are related to mechanical interaction between cell membrane and oscillating microbubbles.