The second heart sound (S2) consists of aortic and pulmonary components. Physiological findings disclose that the split between the two components is modulated by respiration. It is assumed that the waveforms of the aortic and pulmonary components remain constant in adjective cardiac cycles. Then, the S2s are aligned with respect to the aortic component in time domain. So, the average of the aligned S2s produces the aortic component, and the pulmonary component tends to disappear due to the average operation. The average is taken as the template of the aortic component. The pulmonary component of a cardiac cycle is separated by subtracting the template from the S2 of this cycle. The timing of a component is defined as the weighted energy peak locations. Therefore, the split between the two components is measured by the timing difference. Data collected from 12 healthy human subjects are used to evaluate the method. The experiment results show that the minimum split occurs at the end of expiration and its value is about 20 ms. Meanwhile, the maximum split is about 50 ms at the end of inspiration. Both the trend of split varying with respect to respiratory phase and the numerical range of split varying are comparable to the previous results. The timing difference measures successfully the split modulated by respiration. This method has potential applications in monitoring hemodynamic response to respiration.
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