EFFECT OF HYPOXIA AND CARBON-MONOXIDE ON MUSCLE OXYGENATION DURING EXERCISE

We used near-infrared spectroscopy (NIRS) to test the hypothesis that reducing oxygen availability during exercise would affect the rate of muscle oxyhemoglobin (O(2)Hb) desaturation when performing work above the lactic acidosis threshold (LAT), but not below it Seven healthy me n each performed two constant work intensities (60%LAT and the LAT plu s 40% of the difference between the LAT and Vo(2)max [40%Delta]) four times under the following conditions: (1) 10 min air; (2) 5 min 15%O-2 + 5 min air; (3) 5 min air + 5 min 15%O-2; (4) 5 min after carbon mon oxide (CO) loading to increase the carboxyhemoglobin (COHb) saturation to 15%. During each test, cardiorespiratory parameters and muscle oxy genation measured with NIRS were continuously monitored. Forearm venou s blood lactate was measured every 2 to 3 min. Hypoxia and CO accelera ted muscle deoxygenation only for exercise above the LAT; for exercise below the LAT, neither progressive deoxygenation nor lactate accumula tion occurred after initital, rapid muscle deoxygenation. The rate of decrease in muscle oxygenation between 3 to 5 min of exercise correlat ed with the increase in Vo(2) (r = 0.61, p < 0.01) and blood lactate ( r = 0.70, p < 0.01) over the same period. These results support the hy pothesis that progressive muscle deoxygenation occurs above the LAT an d that the rate of deoxygenation is sensitive to oxygen delivery.