Venomotor responses during central and local hypoxia

Venomotor responses, measured as the pressure rise in occluded forearm veins, were used in a two-part experiment to test presence or absence of sympathetic neuroeffector mechanisms in 10 men made moderately to severely hypoxemic. In part I, forearm venoconstriction was induced by ice water on the contralateral forearm (a spinal reflex) in eight supine, resting men who breathed air, 10.3% oxygen or 7.7% oxygen. Large reflex venoconstrictions persisted during hypoxia. In part II (seven men), venoconstriction was centrally induced by exercise while subjects were 1) normoxic; 2) arm hypoxic, body normoxic; 3) arm hyperoxic (or normoxic), body hypoxic; or 4) both arm and body hypoxic. Arm vs. body oxygen tensions were separated by occluding the arm as one gas mixture was breathed, then switching the subject to another mixture as the arm remained occluded. Strong venoconstrictor responses to moderate exercise (100-150 W) persisted during both local and central hypoxemia. We conclude that moderate to severe hypoxemia does not block, pre- or postjunctionally, sympathetic venoconstriction that originates from spinal reflexes (cold). Venoconstriction in exercise (presumably originating in higher centers) was not blocked by moderate hypoxemia; severe hypoxemia was not studied.