## Isovolumetric Contraction Phase ### Definition and Characteristics Isovolumetric contraction is the phase of the cardiac cycle during which: - **Volume remains constant** (both AV and semilunar valves are closed) - **Pressure increases rapidly** (ventricular myocardium contracts) ### Why Volume is Constant During isovolumetric contraction, the mitral (AV) valve closes at the end of diastole, and the aortic valve does not yet open because ventricular pressure has not exceeded aortic pressure. Therefore, no blood can enter or leave the ventricle—the volume is "locked in." ### Why Pressure Increases Despite constant volume, the contracting ventricular wall exerts increasing force on the blood within the chamber, causing a rapid rise in intraventricular pressure. This phase accounts for the steep portion of the ventricular pressure curve at the beginning of systole. ### Pressure-Volume Loop Representation **Key Point:** On a ventricular pressure-volume loop, isovolumetric contraction appears as a **vertical line** (constant volume, increasing pressure) at the left edge of the loop. **High-Yield:** The duration of isovolumetric contraction is approximately 0.05 seconds and is clinically important—prolongation of this phase can indicate ventricular dysfunction or outflow obstruction (e.g., aortic stenosis). **Mnemonic:** **ISOVOLUMETRIC = ISO (same) + VOLUMETRIC (volume)** — the defining feature is that volume does not change, even though pressure does. **Clinical Pearl:** In aortic stenosis, isovolumetric contraction is prolonged because the ventricle must generate higher pressure to overcome the stenotic valve resistance before ejection can begin. 
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