Circulatory responses of supine subjects to the exposure of parts of the body below the xiphisternum to subatmospheric pressure.

1. Observations are reported on the effects of exposure of parts of the body below the level of the xiphisternum of supine subjects to a pressure 70 mm Hg below atmospheric for 1 min.2. The stress on the circulation was greater than when parts below the iliac crests were similarly exposed. Heart rate increased by 15-20 beats/min, there was a sustained fall in arterial blood pressure, and forearm blood flow fell profoundly and in some subjects was reduced to below 0.1 ml./100 ml. min.3. In arms that were sympathectomized, or had received an intraarterial infusion of an adrenergic blocking drug, the fall in forearm blood flow was much less and could be related to the fall in arterial pressure.4. When the suction was released there was a brief overshoot of arterial blood pressure and brief cardiac slowing. Forearm blood flow rose to reach a peak some 15 sec after the release.5. In the sympathectomized forearm on release of suction there was an immediate rise in blood flow which was proportionately much greater than the rise in arterial blood pressure.6. This rise was not due to circulating vasodilator substances or to the activity of cholinergic vasodilator nerves. The possibility that it was the result of a change in the tone of the resistance vessels occurring in response to the sudden change in transmural pressure is discussed.