During the Valsalva manoeuvre in a healthy adult, in which phase does arterial pressure FALL below baseline while heart rate rises, and what is the direct cause?

• A Phase I, due to aortic compression raising afterload
• B Phase II, due to sustained intrathoracic pressure impeding venous return and reducing stroke volume ✓
• C Phase III, due to sudden release of intrathoracic pressure flooding the circulation
• D Phase IV, due to baroreceptor-mediated vagal withdrawal

Explanation

During Phase II strain, raised intrathoracic pressure collapses the great veins, cutting venous return and stroke volume, so arterial pressure dips and reflex tachycardia follows. Phase I shows a brief pressure rise from aortic compression. Phases III and IV occur after release: III shows a transient dip as pooling begins, and IV shows overshoot hypertension with reflex bradycardia. Autonomic failure abolishes the Phase IV overshoot, a useful clinical correlate.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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