Acoustic Propagation Properties of Normal, Stunned, and Infarcted Myocardium

Abstract

Background Identification of viable but stunned myocardium remains a major problem. Since stunned myocardium results in impairment of myocardial function without any structural damage and infarcted myocardium causes major structural disruption, we postulated that acoustic properties could distinguish between the two insults. Methods and Results Anesthetized open-chest dogs underwent a total occlusion of the left anterior descending coronary artery for 15 minutes (stunned, n=7) and 90 minutes (infarcted, n=8), followed by reperfusion for 3 hours. Circumflex coronary artery perfusion territory (n=15) served as normal control tissue. Regions of myocardium were quantitatively evaluated with a scanning laser acoustic microscope operating at 100 MHz and a research ultrasound system operating at 4 to 7 MHz. Four ultrasonic parameters were determined: attenuation coefficient (an index of loss per unit distance), speed of propagation, a spatial variation of propagation speed called the heterogeneity index (HI), and ultrasonic backscatter at 5 MHz (IBR5). Myocardial water, lipid, and protein contents of normal, stunned, and infarcted myocardium were also determined. The attenuation coefficient of normal myocardium (179±20 dB/cm) was significantly greater than that of stunned (136±7 dB/cm, P <.001) and infarcted (130±8 dB/cm, P <.001) myocardium. The propagation speed of normal myocardium (1597±6 m/s) was similar to that of stunned (1600±6 m/s) and significantly higher than that of infarcted (1575±7 m/s, P <.001) myocardium. The HI for specimen thicknesses of 75 to 100 μm showed an increase of 33% between normal (5.0±0.8 m/s) and stunned (7.5±2.3 m/s, P <.05) myocardium. However, for the infarcted myocardium (5.8±2.0 m/s), the HI was essentially the same as that of the normal myocardium (5.0±0.8 m/s). The IBR5 of normal (−47.1±1.0 dB) was not significantly different from that of stunned myocardium (−46.8±0.9 dB). The IBR5 of infarcted myocardium (−42.4±1.0 dB) was significantly greater than that of normal myocardium. Myocardial water and protein contents were similar in the normal and stunned myocardium. Water content in the infarcted myocardium (80.8±2%) was significantly greater ( P <.05) than in the normal (72.7±1.3%), and protein content of 18.5±0.7% was significantly lower ( P <.05) than the normal (21.4±0.8%). Lipid content was increased in the stunned (8.5±0.5%) and virtually absent in the infarcted myocardium (0.8±0.3%) compared with normal (5.5±0.6%). Conclusions We conclude that acoustic propagation properties can identify stunned and infarcted myocardium and may be related to biochemical/morphological differences.