Circulatory responses to breath holding in mad

Human circulatory responses to voluntary breath holding were studied following preparation with prolonged hyperventilation or I o min of IOO y0 oxygen breathing. Heart rate (HR), mean arterial blood pressure (MBP), chest circumference, end-tidal Pco~, and bilateral forearm blood flow (FBF) were recorded continuously before and during voluntary breath holding in the resting end-tidal chest position. One forearm received IO mg intra-arterial phenoxybenzamine. Arterial blood samples analyzed with electrodes demonstrated that hypoxia (Pao, 19-50 mm Hg) occurred during breath holding after hyper-ventilation; no hypoxia (Pao, above 250 mm Hg) occurred during breath holding after oxygen breathing. HR, MBP, and FBF were not significantly altered by the breath holding either with or without development of hypoxia, nor was there a sig-nificant difference between the response of the intact an