title: Gravitational stress and autonomic cardiac blockade.
abstract: Arterial pressure, heart rate and cardiac output were recorded in eight healthy, male volunteers in the sitting position at normal gravity (1 G) and during a 4 min exposure in a human centrifuge to 3 G, the G vector in both conditions acting in the head-foot direction. The responses of the observed variables to the change from 1 G to 3 G were compared before and after combined beta-adrenergic and parasympathetic blockade of the heart induced by the i.v. administration of propranolol, 0.25 mg/kg bwt, and atropine, 0.03 mg/kg bwt. After blockade the heart-rate response to increased G averaged 22% of that observed without blockade. Mean arterial pressure at 3 G, and thus G tolerance, was significantly lowered after blockade, resulting in symptoms of retinal hypoxia ("greyout") towards the end of the 3 G runs in some of the subjects. G-induced reductions in cardiac output and stroke volume were significantly larger after blockade. Associated increases in total peripheral resistance were 56% before and 79% after blockade, the stronger vasoconstrictor response in the latter case thus being insufficient for maintenance of arterial pressure homeostasis.
mesh_terms: D000328:Adult; D001285:Atropine / Q000494:pharmacology; D001794:Blood Pressure / Q000187:drug effects; D002302:Cardiac Output / Q000187:drug effects; D006112:Gravitation; D006321:Heart / Q000294:innervation / Q000503:physiopathology*; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D008297:Male; D011187:Posture; D011433:Propranolol / Q000494:pharmacology; D013312:Stress, Physiological / Q000503:physiopathology*; D014655:Vascular Resistance / Q000187:drug effects
chemical_list: D001285:Atropine; D011433:Propranolol