Cardiac Cycle - Isovolumetric Contraction (Phase 2)

All Valves Closed

This phase of the cardiac cycle begins with the appearance of the QRS complex of the ECG, which represents ventricular depolarization. This triggers excitation-contraction coupling, myocyte contraction and a rapid increase in intraventricular pressure. Early in this phase, the rate of pressure development becomes maximal. This is referred to as maximal dP/dt.
The AV valves to close as intraventricular pressure exceeds atrial pressure. Ventricular contraction also triggers contraction of the papillary muscles with their attached chordae tendineae that prevent the AV valve leaflets from bulging back into the atria and becoming incompetent (i.e., “leaky”). Closure of the AV valves results in the first heart sound (S₁). This sound is normally split (~0.04 sec) because mitral valve closure precedes tricuspid closure.

During the time period between the closure of the AV valves and the opening of the aortic and pulmonic valves, ventricular pressure rises rapidly without a change in ventricular volume (i.e., no ejection occurs). Ventricular volume does not change because all valves are closed during this phase. Contraction, therefore, is said to be "isovolumic" or "isovolumetric." Individual myocyte contraction, however, is not necessarily isometric because individual myocyte are undergoing length changes. Individual fibers contract isotonically (i.e., concentric, shortening contraction), while others contract isometrically (i.e., no change in length) or eccentrically (i.e., lengthening contraction). Therefore, ventricular chamber geometry changes considerably as the heart becomes more spheroid in shape; circumference increases and atrial base-to-apex length decreases.
The rate of pressure increase in the ventricles is determined by the rate of contraction of the muscle fibers, which is determine by mechanisms governing excitation-contraction coupling.
The "c-wave" noted in the LAP may be due to bulging of mitral valve leaflets back into left atrium. Just after the peak of the c wave is the x'-descent.

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Revised 04/13/07