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Changes in muscle tissue oxygenation assessed by near-infrared spectroscopy (NIRS)

We examined the relation between cytochrome aa3 (cyt) oxidation level, oxygen delivery (QO2) and oxygen uptake (VO2) during progressive hindlimb ischemia in pigs. The right hindlimb of 6 anesthetized- ventilated pigs was subjected to progressive ischemia over 100 min by 10 stepwise decreases in pump-controlled blood flow (Q). Hindlimb QO2 was Q x arterial content and VO2 was Q x arteriovenous difference. Cyt oxidation level was measured by multiple wavelength NIRS (Hamamatsu). Measurements were done very 9 min after each change of Q. O2 extraction (ER)2) was determined as VO2/QO2. Critical values for both VO2 and cyt oxidation level as a function of QO2 were determined in each animal and compared by t-test. As QO2 was progressively lowered, VO2 remained constant (3.5 +/- 0.1 ml/min per kg) until QO2 reached a critical value of 5.4 +/- 0.4 ml/min per kg, not statistically different than the QO2 (5.9 +/- 0.4 ml/min per kg) critical for maintaining cyt oxidation level. Critical ERO2 determined using the QO2-VO2 relation (0.60 +/- 0.09) and by the QO2-cyt oxidation relation (0.57 +/- 0.03) were also not significantly different. At the lowest QO2, VO2 was 1.0 ml/min per kg and cyt oxidation level was -7 (mu) M. NIRS appears capable of accurately and noninvasively describing the O2 supply dependency relationship during local ischemic hypoxia.