Page 4, Cardiac Cycle, Dr. D. Penney

The electrical events, i.e. the ECG, are coordinated with, but lead the mechanical events. For example, the QRS complex always begins some msec before ventricular systole, and the P wave begins prior to atrial systole (Figure 1.02).

The phonocardiogram is normally made up of the two heart sounds, S1 and S2. S1 is caused by closure of the tricuspid and mitral valves, while S2 results from pulmonic and aortic valve closure. The third and fourth heart sounds, S3 and S4, occur during rapid filling and atrial systole, respectively. They are not normally heard.

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