V•O2 On-Kinetics in Skeletal Muscle in Normoxia and Hypoxia with Matched Convective O2 Delivery

Abstract

The aim of this investigation was to evaluate O2 uptake (V•O2) on-kinetics under hypoxic conditions while holding convective O2 delivery constant in isolated canine muscles in situ. Canine gastrocnemius muscles (n=7) were surgically isolated and then electrically stimulated via the sciatic nerve (8 V, 0.2-ms duration, 50Hz, 200-ms trains) eliciting contractions at a rate of 1 contraction every 2 seconds (about 70% of peak V•O2). The dogs inspired 3 different O2 fractions: 21% (normoxia), 12% (hypoxia), and 8% (severe hypoxia). Convective O2 delivery [blood flow (Qm) x arterial O2 content (CaO2)] to the muscle was held constant by elevating muscle blood flow via a perfusion pump. Blood flow was recorded with an ultrasonic flowmeter while venous O2 saturation was monitored continuously in order to allow measurement of contraction-by-contraction muscle VO2. Relative muscle deoxygenation (ΔHHbMb) was assessed with continuous near-infrared spectroscopy (NIRS). Arterial blood PO2 (torr) was measured during each stimulation period and the mean per condition was 111 ± 11 for 21% inspired O2, 41 ± 3 for 12% inspired O2, and 30 ± 3 for 8% inspired O2. Venous blood PO2 (torr) measured prior to the end of contractions was significantly different (P< 0.05) among the three conditions (21% = 20 ± 5, 12% = 16 ± 5, 8% = 13 ± 5). The arterial O2 content (CaO2) was significantly lower in the hypoxic conditions and pump perfusion was significantly higher in the hypoxic conditions resulting in matched convective O2 delivery (ml/Kg-1/min-1) that were not different (P > 0.05) among the three different O2 fractions, (21% = 202 ± 57, 12% = 206 ± 57 and 8% = 205 ± 59). There was also no difference in tau, the primary time constant for the VO2 on-kinetics among treatments (21% = 10.9 ± 2.4s, 12% = 10.1 ± 1.8s, 8% = 9.2 ± 1.9s ). Additionally, the NIRS data showed no significant time difference in the Δ HHb time course, mean response time (MRT) (21% = 15.3 ± 2.7s, 12 % = 14.7 ± 4.4s, 8% = 13.9 ± 3.0s) for each treatment. The primary component of both VO2 and ΔHHbMb on-kinetics were characterized by a monoexponential function. Thus in the presence of unchanged (or "matched") convective O2 delivery with arterial hypoxemia, skeletal muscle V•O2 on-kinetics were not significantly impaired indicating there is not a peripheral diffusion limitation due to lower mean capillary PO2.