Optimal power generation by the left ventricle. A study in the anesthetized open thorax cat.

Abstract

We studied the interaction of the left ventricle and the systemic arterial bed in the open thorax cat. In the steady state, the ventricle can be characterized by the pump function graph (i.e., the relationship between mean left ventricular pressure and mean outflow). From this pump function graph, the apparent source resistance of the heart is found. Apparent source resistance is defined as the ratio of the difference between maximal and actual mean left ventricular pressure, and mean outflow. The arterial system can be characterized by the ratio of mean aortic pressure and mean flow (peripheral resistance). The pressure and flow at which the heart operates is defined as the working point. We have investigated whether the ventricle in the intact cat is working optimally, i.e., that it cannot increase work output further at the end-diastolic volume, contractile state, and prevailing heart rate. This condition is considered as "matching" of ventricle and load. It could be shown that optimal power is transferred when the ratio of peripheral and apparent source resistance equals twice the ratio of mean aortic and mean left ventricular pressure (the matching principle). In four cats, we observed that mean aortic and mean left ventricular pressures are proportionally related. Mean external power (the time integral of the product of pressure and flow divided by cycle length) and steady power (the product of mean pressure and mean flow) were found to be proportional as well. These proportionalities allow for the calculation of peripheral resistance and mean external power from the pump function graph. Pump function graphs were determined in three groups: control (n = 9), atrial pacing (n = 8), and halothane (n = 5). We compared the ratio of peripheral and source resistance at the working point and at the point of optimal work output (expressed in steady ventricular power). It could be shown that, in all investigated groups, the power optimum and the working point coincide. It was concluded that circulatory control in the intact anesthetized cat keeps the ventricle at optimal work output under the conditions studied.