Diastolic‐Systolic Coronary Flow Differences are Caused by Intramyocardial Pump Action in the Anesthetized Dog

The effect of cardiac contraction on coronary arterial flow has been described in terms of an intramyocardiaJ pump, which displaces blood backward and forward during systole and diastole, respectively. Normally, the mean forward flow exceeds, and consequently conceals, this backflow. The main left coronary artery of six anesthetized open-chest dogs was perfused with a Gregg cannula from a constant pressure source via a pcrfusion line containing an adjustable stenosis. At mean left main arterial pressures, Pic, of 65, 90, 125, and 155 mm Hg, the hearts were perfused via different grades of stenosis, while a constant mean perfusion pressure (Pic) distal to the stenosis was maintained. Mean coronary flow was then independent of stenosis grade. However, with increasing stenosis grade, the systolic-diastolic coronary flow difference decreased, whereas the dlastolic-systolic coronary pressure difference increased. By varying the stenosis grade at constant Pic linear relationships between diastolic-systolic pressure difference and flow difference were obtained and were interpreted as being a result of an electrical analog potential-source equivalent. From the potential-source equivalent, the diastolic-systolic pressure changes of the intramyocadial pump, pin, can be determined as well as the coronary resistance, Rc impeding the flow variations originated by pin. We found pin = 53.1 ± 7.02 (SD) nun Hg, and independent of Pic.c was correlated with the resistance to coronary flow, Rc via Ra = 0.63 × Rc – 12.9 mm Hg»s/ml (r = 0.939, n = 25). Rc was defined as (Pic - 14 nun Hg)/mean coronary flow. The waterfall model extended to allow for autoregulation to achieve an equal division of mean flow over the myocardium could not explain these results. From a decay curve of coronary arterial pressure following clamping of the perfusion line, intramyocardial coronary capacitance was estimated to be approximately 0.07 ml/mm rig/100 g LV. This value is in agreement with published volume pressure relationships of the intramyocardial blood compartment. The phasic coronary blood flow component requires intramyocardial arterial volume. We conclude that systolic-diastolic variations in coronary blood flow are not due to varying resistances but are caused by an active intramyocardial pump. Circ Res 49.