*Original Contribution THE RELATIONSHIP BETWEEN MYOCARDIAL INTEGRATED BACKSCATTER, PERFUSION PRESSURE AND WALL THICKNESS DURING ISOVOLUMIC CONTRACTION: AN ISOLATED PIG HEART STUDY

Abstract-To investigate the independent effect of myocardial wall thickness and myocardial perfusion pressure on integrated backscatter, experiments were designed in which integrated backscatter of normally perfused myocardial tissue was measured while changes in wall thickness during the cardiac cycle were reduced to a minimum. In nine blood-perfused isolated pig hearts, perfusion pressure was uncoupled from left ventricular pressure generation (Langendorff method) and isovolumic contraction and relaxation were realized by inserting a noncompressible water-filled balloon into the left ventricle. In a first experiment, at constant perfusion pressure (85 mmHg), the integrated backscatter (3-7 MHz), the myocardial wall thickness and the left ventricular pressure were determined simultaneously at various balloon volumes (5-25 mL). A quasistatic increase of balloon volume by 50 % resulted in an average decrease of wall thickness of 6.5 % @ < 0.01) and a mean increase in the integrated backscatter level of 1.1 dB @ < 0.01). Integrated backscatter levels increased statistically significant by 0.14 f 0.014 dB per percent decrease of wall thickness. Measurements of percentage end-systolic myocardial waU thickening ranged from-10 % to +lO%, mean 0.15 r 4.5 % (NS from zero) ; whereas cyclic variation of integrated backscatter ranged from-3.9 to +3.9 dB, mean 0.19-C 1.5 dB (NS from zero). In a second experiment, at a constant midrange balloon volume, the same parameters were determined simultaneously at various perfusion pressures (20-120 mmI-Ig). A