Page 5, Cardiac Cycle, Dr. D. Penney
The length of 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 (Table 1.01). 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.
Ventricular systole shortens because of:
- increased speed of conduction (dromotropicity)
- increased speed of contraction and relaxation (Ca++ release / uptake).
- increased role of atrial kick.
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 or Staircase).
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