The Effect of Adding CO2 to Hypoxic Inspired Gas on Cerebral Blood Flow Velocity and Breathing during Incremental Exercise

作者：Fan Jui Lin; Kayser Bengt^*

来源：PLos One, 2013, 8(11): e81130.

DOI：10.1371/journal.pone.0081130

Hypoxia increases the ventilatory response to exercise, which leads to hyperventilation-induced hypocapnia and subsequent reduction in cerebral blood flow (CBF). We studied the effects of adding CO2 to a hypoxic inspired gas on CBF during heavy exercise in an altitude naive population. We hypothesized that augmented inspired CO2 and hypoxia would exert synergistic effects on increasing CBF during exercise, which would improve exercise capacity compared to hypocapnic hypoxia. We also examined the responsiveness of CO2 and O-2 chemoreception on the regulation ventilation ((V) over dotE) during incremental exercise. We measured middle cerebral artery velocity (MCAv; index of CBF), (V) over dotE, end-tidal PCO2, respiratory compensation threshold (RC) and ventilatory response to exercise ((V) over dotE slope) in ten healthy men during incremental cycling to exhaustion in normoxia and hypoxia (FIO2 = 0.10) with and without augmenting the fraction of inspired CO2 (FICO2). During exercise in normoxia, augmenting FICO2 elevated MCAv throughout exercise and lowered both RC onset and (V) over dotE slope below RC (P%26lt;0.05). In hypoxia, MCAv and (V) over dotE slope below RC during exercise were elevated, while the onset of RC occurred at lower exercise intensity (P%26lt;0.05). Augmenting FICO2 in hypoxia increased (V) over dotE at RC (P%26lt;0.05) but no difference was observed in RC onset, MCAv, or (V) over dot slope below RC (P%26gt;0.05). The (V) over dotE slope above RC was unchanged with either hypoxia or augmented FICO2 (P%26gt;0.05). We found augmenting FICO2 increased CBF during sub-maximal exercise in normoxia, but not in hypoxia, indicating that the %26apos;normal%26apos; cerebrovascular response to hypercapnia is blunted during exercise in hypoxia, possibly due to an exhaustion of cerebral vasodilatory reserve. This finding may explain the lack of improvement of exercise capacity in hypoxia with augmented CO2. Our data further indicate that, during exercise below RC, chemoreception is responsive, while above RC the ventilatory response to CO2 is blunted.

• 出版日期2013-11-21