The effects of head-down tilt on carotid blood flow and pulmonary gas exchange.

Common carotid artery blood flow (CCF), pulmonary gas exchange and ventilation were measured in six subjects in the supine posture (SUP I), serially during 20 min of head-down tilt at -30 degrees (HDT) and after returning to the supine posture (SUP II). CCF was approximately 6% lower during HDT, with a transient increase during the second minute, and was about 7% higher during SUP II than during SUP I. The transition from SUP I to HDT caused increases in O2 uptake (VO2), CO2 output, respiratory exchange ratio and tidal volume in the first minute. Similar responses were apparent following the HDT to SUP II transition, except for VO2, which changed little. Correction of VO2 for changes in estimated lung O2 stores indicated that about 200 ml of blood were shifted within the circulation by the tilt transitions which provided a ventilatory stimulus. HDT can cause a loss in blood and tissue O2 stores and gain in CO2 stores by shifting blood volume toward and blood flow away from the dependent headward vascular compartment and perhaps by producing ischemia in the elevated lower extremities. Cerebral venous congestion during HDT appears to cause periodic breathing and reduce CCF, the latter being partially offset by reduced flow resistance in the carotid artery.