Does Cerebral Blood Flow Limit Maximal Aerobic Power Output?

Abstract

Hyperventilation-induced hypocapnia reduces cerebral blood flow (CBF) during intense exercise. Previous studies have suggested that a reduction in CBF may limit central motor drive, particularly at high altitude where oxygen delivery is impaired. PURPOSE: We hypothesized that clamping end-tidal CO2 (PETCO2) during incremental exercise would increase cerebral blood flow and improve maximal aerobic power output (Pmax) at low and high altitude. METHODS: Six competitive male cyclists (180.0 ± 6.5 cm, 80.0 ± 11.3 kg, 34 ± 10 years) were familiarized with incremental exercise tests (25 W/min) at low (1,650 m, Pb = 630 mmHg) and high (4,875 m, Pb = 425 mmHg) altitude in a hypobaric chamber. Subsequent experimental trials were conducted in a randomized, counter balanced order. At each altitude, two incremental tests (randomized order) separated by 30 min of rest were used to compare the normal, poikilocapnic response against a clamped condition, in which PETCO2 was held at 50 mmHg by manually adding CO2 to an inspiratory reservoir throughout exercise. Cyclists were naïve to the hypothesis and blinded to the intervention. Metabolic gases, ventilation, middle cerebral artery CBF velocity (CBFv, TCD Doppler), forehead pulse oximetry, and cerebral (prefrontal) and muscle (vastus lateralis) oxygenation (NIRS) were monitored continuously across trials. RESULTS: At low altitude, the increase in PETCO2 at maximal exertion (51 ± 3 vs. 33 ± 2 mmHg: mean ± SD, P < 0.01) elevated CBFv (82 ± 9 vs. 56 ± 8 cm/s, P < 0.01) and improved relative cerebral oxygenation (15 ± 7% > poikilocapnic response, P < 0.01), but decreased Pmax (362 ± 31 vs. 385 ± 30 Watts, P < 0.05) and VO2max (54 ± 8 vs. 58 ± 8 ml/kg/min, P < 0.01). Similar results were found at high altitude. Increasing PETCO2 (53 ± 5 vs. 26 ± 3 mmHg, P< 0.01), elevated CBFv (87 ± 15 vs. 61 ± 11 cm/s, P < 0.01) and improved cerebral oxygenation (17 ± 11% > poikilocapnic response, P < 0.05), but decreased Pmax (227 ± 35 vs. 297 ± 30 Watts, P < 0.05) and VO2max (36 ± 6 vs. 40 ± 7 ml/kg/min, P < 0.05). CONCLUSIONS: Hypercapnia increases cerebral blood flow and oxygenation during exercise, but impairs maximal aerobic performance. These results raise the possibility that hypercapnia limits performance via secondary factors, such as cerebral overperfusion, altered acid-base balance, and dyspnea.