A model-based study of relationship between timing of second heart sound and systolic blood pressure

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.