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Near infrared spectroscopy (NIRS) has been used to evaluate changes in blood volume and oxygen levels in the brain while people exercise. The technique demonstrates that even if you are healthy, there could be limitations to just how hard you can exercise without affecting brain activity and potentially doing more harm than good for your health. Yagesh Bhambhani and Rohit Malik of the Faculty of Rehabilitation Medicine, at the University of Alberta, Edmonton, Canada and Swapan Mookerjee of Bloomsburg University, Bloomsburg, Pennsylvania, USA explain how during an incremental exercise test, in which exercise intensity is gradually stepped up to exhaustion, the arterial carbon dioxide pressure (PaCO2) and end tidal carbon dioxide (PETCO2) levels breathed out begin to decline at the respiratory compensation threshold (RCT-GEX). Such a fall off in carbon dioxide levels in the arterial system results in a decrease in blood flow to the brain and alters exercise capacity. The researchers used NIRS as a potentially invaluable, non-invasive technique for monitoring cerebral blood oxygenation. In the brain, the two light absorbing compounds of interest are haemoglobin (Hb) and cytochrome oxidase. At 760 nm, haemoglobin is in the deoxygenated state, whereas at 850 nm it occurs in the oxygenated form. The difference in absorbency between these two wavelengths indicates the relative change in haemoglobin oxygen saturation in the arterioles, capillaries, and venules, while the sum of absorbencies indicates a relative change in localized blood volume, explain the researchers. Seventeen healthy male volunteers of varying everyday exercise and activity levels undertook an incremental exercise test on a cycle ergometer. During the test, NIRS measurements were recorded from the left forehead along with the respiratory gas exchange responses. The NIRS results demonstrated that cerebral oxygenation and cerebral blood volume decreased just beyond the RCT; the intensity at which you begin to breathe very rapidly and exhale large amounts of carbon dioxide. This Bhambhani and colleagues point out correlates, albeit with a timelag, with the significant decline in PETCO2 which is an indirect estimate of PaCO2. They explain that the decline in oxygenation and blood volume in the brain suggests reduced neuronal activation, thereby limiting the maximum exercise capacity of even healthy subjects.
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