The effect of coronary arterial pressure on myocardial distensibility. Absence of a "garden hose" effect during in-vivo conditions

The effect of the coronary perfusion pressure on myocardial distensibility was studied in 11 open-chest dogs. The left anterior descending coronary artery was cannulated, and coronary perfusion pressure and blood flow were measured. Regional myocardial segment length was measured using sonomicrometers. The temporal relationship between the phasic coronary pressure fall and change in myocardial segment length was analyzed during the early phase of coronary occlusion. Diastolic myocardial segment length was completely unaffected by the substantial fall in coronary pressure over a period of 9.3±0.8 s (20±2 heart beats). During this period, coronary pressure fell from 98±7 to 28±2 mm Hg. Subsequently, diastolic segment length increased, presumably due to ischemia rather than to a delayed compliance change. In order to differentiate between a possible long-time constant for coupling of the intravascular pressure to myocardial compliance versus a primary ischemic effect, regional cardiac contraction was abolished by an intracoronary potassium chloride infusion in three dogs. Coronary occlusion during regional cardioplegia produced no further segment length changes for a 1-min period, effectively excluding viscoclastic coupling time constants of up to 1 min. From these results we conclude that the coronary distending pressure does not contribute to passive myocardial properties over the physiological perfusion pressure range, and that the "garden hose" effect is not operative for the in vivo working heart.