A Model-based Study of Relationship between Timing of Second Heart Sound and Systolic Blood Pressure

� Abstract—The onset of second heart sound is triggered by the closure of aortic valve due to the interaction of left ventricle and arterial system. Noninvasive experiments found that RS2 defined by the time delay from the peak of ECG R wave to the onset of the second heart sound had a close inverse correlation with arterial systolic blood pressure. However, no theoretical study has been carried out to investigate the underline connections between them. A modified model of heart-arterial system is proposed in the present study. In this model the heart is described as a pressure source depending on time, ventricular volume, outflow, and heart rate, and the arterial system as a nonlinear system incorporating a pressure-dependent compliance. Simulation results show that the modified model is able to reflect the cardiovascular function qualitatively. The results also demonstrate that RS2 is inversely correlated with aortic blood pressure under the effect of changing peripheral resistance, heart rate and contractility. The present study gives insight into the significant functional relations between the parameters characterizing the cardiovascular system and hemodynamics characteristics and provides an interpretation of the experimental observation on the relationship between RS2 and aortic blood pressure. I. INTRODUCTION The heart sounds result from the interplay of the dynamic events associated with the contraction and relaxation of the artia and ventricles, the valve movements and the blood flow (1). The first heart sound (S1) occurs at the onset of ventricular contraction during the closure of the mitral and the tricuspid valves. The second heart sound (S2) marks the end of ventricular systole and the beginning of the ventricular relaxation, following the closure of the aortic and pulmonary valves. The diagnostic value of heart sound has been recognized and used for primary screening of various heart diseases for long time. With the development of phonocardiogram, the timing of heart sound has been utilized in cooperation with electrocardiogram (ECG) and carotid pulsation to measure systolic time intervals (STI) which was established as a noninvasive test of cardiac function (2, 3).