The electrical events, i.e. the ECG, are coordinated with but lead the mechanical events (Figure 1.03). For example, the QRS complex always begins some msec before ventricular systole, and the P wave begins prior to atrial systole. 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.
The phases of the cardiac cycle change with heart rate, but not all in same proportion. With increasing heart rate, ventricular systole shortens a little, while ventricular diastole shortens greatly. At a sufficiently elevated heart rate, diastasis disappears completely, and encroachment on rapid filling may occur. It is in this condition that atrial contraction assumes major importance in ventricular filling. Thus, as in an internal combustion engine operating at high r.p.m., ability to refill the pump between power strokes becomes a limiting factor. Shortened ventricular (& atrial) systole at high heart rate results from an increased inotropic state, partly a function of the Bowditch effect (Treppe).
Animated cardiac cycle cartoons are also available.
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