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Cardiac Cycle - Isovolumetric Contraction
(Phase 2)
All Valves Closed
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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.
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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.
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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.
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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.
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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.
Jump to other phases:
Phase 1 - Atrial Contraction
Phase 3 - Rapid Ejection
Phase 4 - Reduced Ejection
Phase 5 - Isovolumetric Relaxation
Phase 6 - Rapid Filling
Phase 7 - Reduced Filling
RK Revised
04/13/07
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Cardiovascular Physiology Concepts
