Role of CO sub(2) in the cerebral hyperemic response to incremental normoxic and hyperoxic exercise

Cerebral blood flow (CBF) is temporally related to exercise-induced changes in partial pressure of end-tidal carbon dioxide (P sub(ET)CO sub(2)); hyperoxia is known to enhance this relationship. We examined the hypothesis that preventing P sub(ET)CO sub(2) from rising (isocapnia) during submaximal exercise with and without hyperoxia [end-tidal PO sub(2), [P sub(ET)CO sub(2)] = 300 mmHg] would attenuate the increases in CBF. Additionally, we aimed to identify the magnitude that breathing, per se, influences the CBF response to normoxic and hyperoxic exercise. In 14 participants, CBF (intra- and extracranial) measurements were measured during exercise [20, 40, 60, and 80% of maximum workload (W sub(max))] and during rest while ventilation (Ve) was volitionally increased to mimic volumes achieved during exercise (isocapnic hyperpnea). While Ve was uncontrolled during poikilocapnic exercise, during isocapnic exercise and isocapnic hyperpnea, V sub(E) was increased to prevent P sub(ET)CO sub(2) from rising above resting values ( similar to 40 mmHg). Although P sub(ET)CO sub(2) differed by 2 plus or minus 3 mmHg during normoxic poikilocapnic and isocapnic exercise, except for a greater poikilocapnic compared with isocapnic increase in blood velocity in the posterior cerebral artery at 60% W sub(max), the between condition increases in intracranial ( similar to 12-15%) and extracranial (15-20%) blood flow were similar at each workload. The poikilocapnic hyperoxic increases in both intra- and extracranial blood-flow ( similar to 17-29%) were greater compared with poikilocapnic normoxia ( similar to 8-20%) at intensities >40% W sub(max) (P < 0.01). During both normoxic and hyperoxic conditions, isocapnia normalized both the intracranial and extracranial blood-flow differences. Isocapnic hyperpnea did not alter CBF. Our findings demonstrate a differential effect of P sub(ET)CO sub(2) on CBF during exercise influenced by the prevailing P sub(ET)CO sub(2).