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

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 myo cardial tissue was measured while changes in wall thickness during the cardiac cycle were reduced to a minimum. In nine blood-perfused isola ted pig hearts, perfusion pressure was uncoupled from left ventricular pressure generation (Langendorff method) and isovolumic contraction a nd relaxation were realized by inserting a noncompressible water-fille d balloon into the left ventricle. In a first experiment, at constant perfusion pressure (85 mmHg), the integrated backscatter (3-7 MHz), th e myocardial wall thickness and the left ventricular pressure were det ermined simultaneously at various balloon volumes (5-25 mL). A quasist atic increase of balloon volume by 50% resulted in an average decrease of wall thickness of 6.5% (p < 0.01) and a mean increase in the integ rated backscatter level of 1.1 dB (p < 0.01). Integrated backscatter l evels increased statistically significant by 0.14 +/- 0.014 dB per per cent decrease of wall thickness. Measurements of percentage end-systol ic myocardial wall thickening ranged from -10% to +10%, mean 0.15 +/- 4.5% (NS from zero); whereas cyclic variation of integrated backscatte r ranged from -3.9 to +3.9 dB, mean 0.19 +/- 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 pressur es (20-120 mmHg). An increase in perfusion pressure by 50% resulted in a small but statistically significant increase of 1.5% in myocardial wall thickness, which could be explained by an increase of intravascul ar volume. The integrated backscatter levels did not change statistica lly significantly. Measurements of percentage end-systolic myocardial wall thickening ranged from -8.9 to +7.8%, mean 0.13 +/- 4.0% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -1.8 to +4.2 dB, mean 0.37 +/- 1.3 dB (NS from zero). The magnitude o f cyclic variation of integrated backscatter of myocardial tissue in a contractile state is reduced if myocardial muscle is prevented from n ormal thickening. In addition, changes in intravascular volume during the cardiac cycle have a negligible influence on the absolute backscat ter level or its cyclic variation. We conclude, if only wall thickness and perfusion pressure are involved, that integrated backscatter is m ainly determined by myocardial wall thickness.