Original contributionMyocardial Injury Induced by Ultrasound-Targeted Microbubble Destruction: Evidence for the Contribution of Myocardial Ischemia
Introduction
Current ultrasound (US) contrast agents typically consist of a suspension of gas-filled microbubbles that have been specially designed to interact with diagnostic US and produce image enhancement (Tsutsui et al. 2005). Because these microbubbles are compressible, they alternately contract and expand in the acoustic field. At low acoustic pressure, they usually grow and shrink rhythmically and symmetrically around their equilibrium size. At higher acoustic pressure, however, their expansion and contraction usually become unequal and markedly exaggerated, leading to their destruction, a process often referred to as ultrasound-targeted microbubble destruction (UTMD) (Wei et al 1997, Skyba et al 1998, Dayton et al 1999, Shi et al 2000, Bouakaz et al 2005, Amni et al 2006).
There is growing evidence that UTMD can produce microscale damage to organs containing the microbubbles, such as demonstrated ex vivo in isolated rabbit hearts (Ay et al. 2001) and in vivo in the mouse (Miller et al. 2000), rat (Chen et al 2002, Miller et al 2005a, Miller et al 2005b, Vancraeynest et al 2006), dog (Miller at al. 2006) and human (Vancraeynest et al. 2007) myocardium. The most frequently observed UTDM-induced tissue alterations usually involve microvascular ruptures and tissue hemorrhage. However, UTMD has also been shown to produce cardiomyocytes injury (Miller et al 2005a, Miller et al 2006, Vancraeynest et al 2006) and to transiently impair left ventricular (LV) function (Ay et al 2001, Vancraeynest et al 2006).
Although there is little doubt that UTMD-induced microvascular ruptures are directly caused by the UTDM process itself, the mechanisms by which UTMD affects cardiomyocytes' integrity and function remains uncertain. In previous works, the observation of a close relationship between both the extent and amount of vascular ruptures and the severity of cardiac dysfunction nonetheless suggests that UTMD-induced myocyte alterations might be partially ischemic in origin (Ay et al 2001, Vancraeynest et al 2006). The present study was therefore designed to test whether UTMD causes myocardial ischemia. Specifically, we sought to determine whether exposure of rat hearts to UTMD resulted in specific ischemia-related electrocardiogram (ECG) and morphologic changes, such as ST-segment elevation and myocyte contraction band necrosis.
Section snippets
Methods
This study was approved by the Animal Research Committee of the Université Catholique de Louvain and conformed to the American Heart Association Guidelines for Use of Animals in Research.
Effects of UTMD on survival and arterial pressure
Several animals died during US exposure. As shown in Fig. 1, mortality increased with time and with peak negative pressure. At a peak negative pressure of 1.8 MPa, five of six rats exposed to UTMD for 30 min died during US exposure. Mortality was significantly less at a peak negative pressure of 1.2 MPa because only one of six rats died during insonation. Finally, there were no deaths in rats exposed to a peak negative pressure of 0.6 MPa, or in control rats. In all cases, death was because of
Discussion
Our data indicate that exposure of rat hearts to UTMD in vivo induces bio-effects that are consistent with myocardial ischemia, including transient ST-segment elevation, regional contractile dysfunction and myocyte contraction band necrosis. Our data further indicate that the extent and magnitude of these changes parallel those of UTMD-induced microvascular ruptures.
Acknowledgements
This work is supported in part by grants Nos. 3 to 4563–98 and 3 to 4504–03 from the “Fonds National de la Recherche Scientifique et Médicale” and by the “Action de Recherche Concertée” No. 01/06–271. DV is supported by the “Fondation St-Luc” and the “Foundation Damman,” Louvain-la-Neuve, Belgium. P.R. is from Philips Medical Systems, Andover, MA.
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