During a Valsalva manoeuvre, a healthy subject shows a rise in arterial pressure immediately after release of the strain. In a patient with diabetic autonomic neuropathy, this late pressure response is absent. The normal late pressor overshoot depends primarily on:

• A Transient fall in cardiac output caused by sudden loss of intrathoracic pressure
• B Compression of the aorta against the thoracic spine during strain
• C Rebound increase in venous return meeting sustained sympathetic venous and arteriolar constriction ✓
• D Reflex withdrawal of vagal tone triggered by carotid sinus distension

Explanation

In phase IV, releasing the strain abruptly restores venous return while sympathetic tone built up during phase II still constricts vessels, producing a pressure overshoot with reflex bradycardia. Option B describes phase I, where aortic compression briefly raises pressure. Option A describes the start of phase III. In autonomic failure, impaired sympathetic constriction means venous pooling persists and neither the phase II recovery nor the phase IV overshoot occurs.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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