Left ventricular systolic dynamics in terms of its chamber mechanical properties.

To determine the mechanical properties of the left ventricle (LV) as a pump, a mathematical model of its systolic dynamics was developed. Initially the model consisted of three elements, i.e., elastance, resistance, and inertance. Results from three experiments, however, indicated that the inertial component was negligible compared with the other two components. The functional forms of elastance and resistance were determined by applying the flow-pulse response technique to an isovolumetrically beating, isolated canine heart. Results from three experiments indicated that the systolic elastance and resistance can be represented by a third-order polynomial in time and a linear function of instantaneous ventricular pressure (LVP), respectively. The simplified model was then tested by calculating the systolic elastance and resistance from LVP, volume, and flow data of an ejecting LV obtained over a single cardiac cycle. A total of 225 combinations (10 expts) of end-diastolic volume (EDV), ejection pressure (EP), heart rate (HR), and contractile state (CS) were evaluated. The results indicated that 1) the elastance function was insensitive to variations in EDV and EP but was a function of CS and HR; 2) the linear resistance-pressure relationship was insensitive to variations in EDV, EP, HR, and CS; and 3) the model could "prospectively" predict the LV isovolumetric pressure from the data of an ejecting beat. Thus a model of LV systolic dynamics has been established that can be used to calculate the intrinsic chamber mechanical properties, i.e., elastance and resistance, of an ejecting LV.