Sympathetic nervous withdrawal in the vasodepressor (vasovagal) reaction.

When 40 subjects had 134 ml of blood drawn, 8 of them developed hypotension. These 8 subjects were younger, and when they stood they decreased their systemic arterial pressure, pulse rate and plasma norepinephrine. In contrast, normal subjects increased all these parameters after standing. The fall in plasma norepinephrine suggests a generalized withdrawal of sympathetic nervous tone during vasodepressor reactions. Three subjects maintained a heart rate above 60, but still developed hypotension. Withdrawal of sympathetic tone to the vasculature appears more important than increased vagal tone to the heart in causing hypotension in this reaction.