During the cardiac cycle, the aortic valve closes when the pressure in the aorta exceeds the pressure in the left ventricle. At what point in the cardiac cycle does this occur?

Explanation

## Aortic Valve Closure Timing **Key Point:** The aortic valve closes when aortic pressure exceeds left ventricular (LV) pressure, which occurs during isovolumetric relaxation (early diastole). ### Mechanism of Closure After the left ventricle finishes ejecting blood (end of systole), the LV pressure begins to fall rapidly. The aortic valve remains open as long as LV pressure > aortic pressure. Once LV pressure drops below aortic pressure, blood begins to flow backward toward the ventricle, pushing the valve leaflets together and sealing the aorta from the ventricle. ### Cardiac Cycle Phases | Phase | LV Pressure | Aortic Pressure | Aortic Valve Status | |-------|-------------|-----------------|---------------------| | Isovolumetric contraction | ↑ rapidly | Unchanged | Closed | | Rapid ejection | LV > Aortic | Aortic > LV | **Open** | | Reduced ejection | LV > Aortic | Aortic > LV | **Open** | | Isovolumetric relaxation | ↓ rapidly | Unchanged | **Closes** | | Ventricular filling | Low | High | Closed | **High-Yield:** The aortic valve closes at the **dicrotic notch** on the aortic pressure tracing — this is the point where aortic pressure exceeds LV pressure and the valve snaps shut. **Clinical Pearl:** The second heart sound (S₂) is produced by the closure of both the aortic and pulmonary valves, with aortic closure occurring slightly earlier and being louder in the right upper sternal border. ### Mnemonic: **"AV closes when A > LV"** Aortic Valve closes when Aortic pressure exceeds Left Ventricular pressure.