Cerebrovascular responses to incremental exercise during hypobaric hypoxia: effect of oxygenation on maximal performance

Abstract

Subudhi AW, Lorenz MC, Fulco CS, Roach RC. Cerebrovascular responses to incremental exercise during hypobaric hypoxia: effect of oxygenation on maximal performance. Am J Physiol Heart Circ Physiol 294: H164-H171, 2008. First published November 21, 2007 doi:10.1152/ajpheart.01104.2007.-We sought to describe cerebrovascular responses to incremental exercise and test the hypothesis that changes in cerebral oxygenation influence maximal performance. Eleven men cycled in three conditions: 1) sea level (SL); 2) acute hypoxia [AH; hypobaric chamber, inspired PO2 (PIO 2 ) 86 Torr]; and 3) chronic hypoxia [CH; 4,300 m, PIO 2 86 Torr]. At maximal work rate (Ẇ max), fraction of inspired oxygen (FIO 2 ) was surreptitiously increased to 0.60, while subjects were encouraged to continue pedaling. Changes in cerebral (frontal lobe) (COX) and muscle (vastus lateralis) oxygenation (MOX) (near infrared spectroscopy), middle cerebral artery blood flow velocity (MCA Vmean; transcranial Doppler), and end-tidal PCO2 (PETCO 2 ) were analyzed across %Ẇ max (significance at P Ͻ 0.05). At SL, PETCO 2 , MCA Vmean, and COX fell as work rate rose from 75 to 100% Ẇ max. During AH, PETCO 2 and MCA Vmean declined from 50 to 100% Ẇ max, while COX fell from rest. With CH, PETCO 2 and COX dropped throughout exercise, while MCA Vmean fell only from 75 to 100% Ẇ max. MOX fell from rest to 75% Ẇ max at SL and AH and throughout exercise in CH. The magnitude of fall in COX, but not MOX, was different between conditions (CH Ͼ AH Ͼ SL). FIO 2 0.60 at Ẇ max did not prolong exercise at SL, yet allowed subjects to continue for 96 Ϯ 61 s in AH and 162 Ϯ 90 s in CH. During FIO 2 0.60, COX rose and MOX remained constant as work rate increased. Thus cerebral hypoxia appeared to impose a limit to maximal exercise during hypobaric hypoxia (PIO 2 86 Torr), since its reversal was associated with improved performance. altitude; near infrared spectroscopy; cerebral blood flow; fatigue; muscle oxygenation CEREBRAL HYPOXIA HAS BEEN proposed to be a critical factor limiting exercise performance (37), particularly in hypoxia (7), yet little evidence exists to directly support this theory. Kayser et al. (31) were the first to show that rapidly increasing the fraction of inspired oxygen (FI O 2 ) at the point of maximal exertion prolonged exercise in hypoxia-acclimatized subjects. They concluded that the effect of increased FI O 2 was too quick to have reversed metabolic factors associated with peripheral (intramuscular) fatigue and suggested that cerebral reoxygenation was a more likely explanation for the improvement in exercise performance. Calbet et al. (11) arrived at similar conclusions after using a comparable model to study factors limiting O 2 uptake (V O 2 ). They suggested that exercise under hypoxic conditions may have presented a significant threat to cerebral oxygenation; thus cardiac and/or motor output was curtailed to maintain favorable tissue oxygenation status.While these studies insinuate the importance of preserving cerebral oxygenation during exer...

Cerebral perturbations during exercise in hypoxia

“…This reduction has been observed even in cases of enhanced CBF (51,101), and no correlation was observed between changes in prefrontal oxygenation and MCAV during exercise (2). An elevated cerebral metabolic rate associated with greater O 2 consumption may explain this reduction in cerebral oxygenation during whole body exercise in hypoxia (101). In hypoxia, whole body exercise and isolated contractions differ regarding cerebral oxygenation since the former accentuates cerebral deoxygenation (13,100,101), whereas the later is associated with some degree of cerebral reoxygenation (40,91) compared with hypoxic resting levels.…”

“…2) and maximal intensities (2, 51, 81, 91, 93, 100 -102). This reduction has been observed even in cases of enhanced CBF (51,101), and no correlation was observed between changes in prefrontal oxygenation and MCAV during exercise (2). An elevated cerebral metabolic rate associated with greater O 2 consumption may explain this reduction in cerebral oxygenation during whole body exercise in hypoxia (101).…”

“…An elevated cerebral metabolic rate associated with greater O 2 consumption may explain this reduction in cerebral oxygenation during whole body exercise in hypoxia (101). In hypoxia, whole body exercise and isolated contractions differ regarding cerebral oxygenation since the former accentuates cerebral deoxygenation (13,100,101), whereas the later is associated with some degree of cerebral reoxygenation (40,91) compared with hypoxic resting levels. While most previous studies have only evaluated prefrontal oxygenation, Subudhi et al (102) showed using multichannel NIRS that cerebral oxygenation during hypoxic exercise is well correlated between the prefrontal, premotor, and motor regions, although at maximal exercise deoxygenation of the prefrontal cortex was greater than in other regions.…”

“…In acute severe hypoxia, CNS alterations may precede the development of peripheral muscle fatigue and underlie the reductions in central motor output and exercise performance (13). This theory is supported by studies showing that hyperoxia at exhaustion quickly restores the ability of subjects to sustain the target workload and increases exercise performance in hypoxia (13,17,56,101). Objective measurements of cerebral perfusion and oxygenation (3,51,81,85,91,100,101,110) and supraspinal neuromuscular alterations (40,86,104,105) have recently provided indications of brain adaptations to exercise in hypoxia.…”

High-Intensity Interval Exercise and Cerebrovascular Health: Curiosity, Cause, and Consequence

“…77 vascular function (as detailed above), exercising above~70% _ VO 2 max induces a hyperventilation-induced hypocapnia and subsequent cerebral vasoconstriction, reducing CBF toward resting values. [32][33][34] This constrictive effect may serve as a neuroprotective response to prevent BBB disruption and hyperperfusion injury, and has been associated with improved CBF regulation during changes in blood pressure (i.e., autoregulation). [92][93][94] However, whether this vasoconstriction is sufficient to counteract the increased cerebral perfusion pressure induced from HIT is not known.…”