Time and Frequency Domain Representations of the Left Ventricle: Theoretical and Experimental Results

Abstract

A frequency domain and a time domain model of the left ventricle are described in this work. These representations provide insight into the function of the healthy left ventricle and show how ventricular function may be altered by heart disease. In developing the frequency domain representation of the left ventricle, the flow and pressure waveforms generated by the left ventricle were described as superpositions of sinusoidal oscillations at different frequencies. Flow and pressure waveforms were obtained experimentally at two different left ventricular afterloads. The two different afterloads were obtained utilizing an intraaortic balloon. The left ventricle was modeled by an equivalent source pressure and source impedance analogous to a Thevenins equivalent representation. The model parameters (source pressure and source impedance) appear to be sensitive to cardiovascular changes such as myocardial infarction and increased left ventricular end-diastolic pressure. The source pressure expressed in the time domain may be a useful myocardial contractility index. For aortic input impedance much greater than source impedance, a change in left ventricular afterload would result in small change in aortic pressure. In this case the left ventricle would be functioning as a pressure source. By comparing the source resistance and aortic input resistance, the left ventricle appears to be a D.C. pressure source. For analysis in the time domain, the left ventricle was represented by truncated, confocal ellipsoids approximated by a series of cylindrical shells. The properties of the left ventricle were distributed over the cylindrical sections. The timing and sequence of contraction of the cylindrical shells were prescribed to simulate the mechanical action of the left ventricle. Plow and pressure waveforms produced by the model were similar to those obtained experimentally. Results of the simulation indicate that the pressure distribution in the ventricular chamber may be a useful index for determining the status of the left ventricle.