Respiratory and cerebral circulatory control during exercise at .21 and 2.0 atmospheres inspired pO₂

Abstract

Respiratory and cerebral hemodynamic responses to leg exercise during respiration of air at 1.0 atm. and O2 at 2.0 atm. were studied in relation to changes in arterial and internal jugular venous blood oxygen composition, pH, pCO2 and bicarbonate concentration. The hyperpnea of exercise at 1.0 atm. was accompanied by arterial and venous acidemia and hypocapnia. Oxygen administration during exercise at 2.0 atm. lowered ventilation, restored arterial pH and pCO2 toward resting levels and caused venous pCO2 to rise above the resting level; cerebral venous cH remained elevated in spite of reduction of blood fixed acid concentration. The ventilatory response to exercise showed positive correlations with work load, oxygen consumption, and with changes in arterial and internal jugular venous cH and fixed acid. The observed negative correlations of changes in respiratory minute volume with changes in arterial and internal jugular venous pCO2 and bicarbonate concentration suggest that these factors are functions, rather than primary determinants, of ventilation in exercise. Cerebral hemodynamics and oxygen consumption were not significantly altered by exercise at 1.0 atm. The data suggest either a slight elevation of cerebral blood flow or reduction in the rate of cerebral oxygen consumption during exercise breathing O2 at 2.0 atm., without gross elevation of cerebral venous pO2. Observed changes in cerebral vascular resistance during exercise at .21 and 2.0 atm. inspired pO2 appear related to concomitant alterations in arterial pCO2, with no detectable relationship either to brain oxygen requirement of cerebral venous acid-base composition. Submitted on November 25, 1958

Step Hypocapnia to Separate Arterial from Tissue Pco 2 in the Regulation of Cerebral Blood Flow

“…They concluded that arterial blood rather than tissue Pco 2 controlled flow, while noting that a vasoconstriction produced by O 2 could have masked a dilation associated with tissue hypercapnia. They reaffirmed this in a subsequent study during exercise (8). Ledingham,Harper and McDowall (9) have shown that if Paco 2 is held constant, the administration of O2 at 3 atm to dogs resulted in no significant change in cerebral cortical blood flow determined by krypton clearance.…”

Cerebral Circulation and Metabolism During Thiopental Anesthesia and Hyperventilation in Man*

“…In the first four studies CBF was determined over a 10-minute period, by the continuous sampling modification of the nitrous oxide method (6). The "syringe integration" sampling procedure was first suggested by Schienberg and Stead (7).…”

“…Determinations of whole blood metabolic gas composition and N20 content were performed manometrically in duplicate as described elsewhere (6). Percentage oxygen saturation of hemoglobin was estimated from the manometrically measured values after corrections for physically dissolved oxygen and, in addition, for hemoconcentration in the determination of the oxygen capacity (12,13).…”

Pulmonary gas exchange in diving

“…Measurements have included ventilation, oxygen consumption, carbon dioxide elimination, and both end-tidal and arterial gas tensions (51,53). Ensuing experiments demonstrated hypercapnia at rest (91), but to a greater extent during exercise (11, 25, 40 -42, 52, 74, 89 -91, 107, 118).…”

“…Another study failed to find an effect of PO 2 on arterial PCO 2 between 0.7 and 1.3 ATA during immersed prone exercise at 4.7 ATA (8). Factors besides respiratory drive attenuation that may contribute to the reduction in exercise ventilation include peripheral chemoreceptor inhibition and attenuation of the acidemia that occurs during heavy exercise (51).…”