1.9.4 Isovolumetric Contraction Definition
Isovolumetric contraction is a phase of the cardiac cycle where the ventricles contract without changing volume, increasing pressure to open the semilunar valves.
Isovolumetric Contraction Definition is the phase of the cardiac cycle occurring at the very beginning of ventricular systole, during which the ventricular myocardium generates tension and pressure rises sharply while the volume of blood contained within the ventricle remains constant. This phase begins immediately after closure of the atrioventricular (AV) valves and ends at the moment the semilunar valves open, meaning that during this brief interval all four cardiac valves are closed and no blood enters or leaves the ventricle.
Mechanical Basis
Isovolumetric contraction reflects the fundamental principle that a fluid-filled chamber sealed on all sides cannot change volume even as the muscle surrounding it contracts.
Valve Closure
At the end of diastole, rising ventricular pressure exceeds atrial pressure, forcing the mitral and tricuspid valves shut. This closure produces the first heart sound (S1) and marks the start of isovolumetric contraction.
Constant Volume
Because the AV valves are closed and ventricular pressure has not yet exceeded aortic or pulmonary arterial pressure, the semilunar valves remain shut as well. With every valve closed, the ventricle is a sealed chamber, so despite active myocardial shortening and rising internal pressure, the enclosed blood volume cannot change.
Rapid Pressure Rise
Continued contraction of the ventricular walls against a fixed volume produces a steep, near-vertical rise in intraventricular pressure, one of the fastest pressure changes observed anywhere in the cardiac cycle.
Termination of the Phase
Isovolumetric contraction ends once ventricular pressure surpasses the pressure in the great vessels it feeds.
Aortic Valve Opening
In the left ventricle, the phase ends when ventricular pressure exceeds aortic diastolic pressure, causing the aortic valve to open and initiating ventricular ejection.
Pulmonary Valve Opening
In the right ventricle, the phase ends when ventricular pressure exceeds pulmonary arterial pressure, causing the pulmonary valve to open.
Pressure–Volume Correlation
On a ventricular pressure–volume loop, isovolumetric contraction appears as a vertical line, since pressure rises steeply while volume remains fixed at the end-diastolic volume.
Clinical and Physiological Relevance
The rate of pressure development during isovolumetric contraction, often expressed as the maximum rate of pressure rise, is used as an index of ventricular contractility, since it reflects the intrinsic force-generating capacity of the myocardium independent of afterload. Conditions that impair myocardial contractility, such as heart failure, characteristically prolong isovolumetric contraction and reduce the rate of pressure development during this phase.