EFFECT OF [HB] ON BLOOD-FLOW DISTRIBUTION AND O-2 TRANSPORT IN MAXIMALLY WORKING SKELETAL-MUSCLE

We investigated whether the reduction in calculated muscle diffusion capacity for O-2 (Dmo(2)) previously shown to occur with lowered hemoglobin concentration ([Hb]) perfusion of maximally working muscle is related to changes in the blood flow distribution. If blood flow distribution is altered during low [Hb] conditions, the reduction in the calculated Dmo(2) may in fact be due to increasing heterogeneity and not to some other hemoglobin-related factor. Color-stained (15-mu m-diam) microspheres were injected into the artery supplying maximally working isolated in situ dog gastrocnemius muscle (n = 6) while it was being perfused (flow controlled by pump perfusion) with whole blood at three different levels of [Hb] [14.1 +/- 0.5, 8.9 +/- 0.4, and 5.7 +/- 0.4 (SE) g/100 ml] in a blocked-order design. Muscle blood flow and arterial Po-2 were not changed as [Hb] was altered. Maximal O-2 uptake (11.8 +/- 1.3, 8.2 +/- 0.8, and 6.0 +/- 0.9 ml . 100 g(-1). min(-1) for those [Hb] values, respectively) and the associated estimate of Dmo(2) (0.25 +/- 0.03, 0.18 +/- 0.03, and 0.15 +/- 0.03 ml . 100 g(-1). min(-1) - Torr(-1)) declined significantly (P < 0.05) with [Hb]. However, the dispersion of the blood flow distribution did not change significantly and, if anything, indicated less heterogeneity at lower [Hb] (coefficient of variation = 0.52 +/- 0.06, 0.46 +/- 0.05, and 0.43 +/- 0.03). These results suggest that in maximally working canine muscle in situ, when O-2 delivery is reduced by lowering [Hb] (at constant blood flow), changes in blood flow distribution play no significant role in the reduction of maximal O-2 uptake and calculated Dmo(2). The apparent increase in the resistance to O-2 diffusion (i.e., reduction in the Dmo(2)) during anemia may therefore be a result of increased red blood cell spacing in the capillary, slow chemical off-loading kinetics of O-2 from Hb, or some other effect that remains to be determined.