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

• Published in: Journal of applied physiology (1985) 2016-04, Vol.120 (8), p.843-854
• Main Authors: Smith, K. J., Wildfong, K. W., Hoiland, R. L., Harper, M., Lewis, N. C., Pool, A., Smith, S. L., Kuca, T., Foster, G. E., Ainslie, P. N.
• Format: Article
• Language: English
• ISSN: 8750-7587; 1522-1601
• DOI: 10.1152/japplphysiol.00490.2015

Cerebral blood flow (CBF) is temporally related to exercise-induced changes in partial pressure of end-tidal carbon dioxide (Pet CO 2 ); hyperoxia is known to enhance this relationship. We examined the hypothesis that preventing Pet CO 2 from rising (isocapnia) during submaximal exercise with and without hyperoxia [end-tidal Po 2 (Pet O 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 max )] and during rest while ventilation (V̇e) was volitionally increased to mimic volumes achieved during exercise (isocapnic hyperpnea). While V̇e was uncontrolled during poikilocapnic exercise, during isocapnic exercise and isocapnic hyperpnea, V̇e was increased to prevent Pet CO 2 from rising above resting values (∼40 mmHg). Although Pet CO 2 differed by 2 ± 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 max , the between condition increases in intracranial (∼12-15%) and extracranial (15–20%) blood flow were similar at each workload. The poikilocapnic hyperoxic increases in both intra- and extracranial blood-flow (∼17–29%) were greater compared with poikilocapnic normoxia (∼8–20%) at intensities >40% W 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 Pet CO 2 on CBF during exercise influenced by the prevailing Pet O 2 .