Skeletal muscle StO2 kinetics are slowed during low work rate calf exercise in peripheral arterial disease

The time course of muscle oxygen desaturation (StO2 kinetics) following exercise onset reflects the dynamic interaction between muscle blood flow and muscle oxygen consumption. In patients with peripheral arterial disease (PAD), muscle StO2 kinetics are slowed during walking exercise; potentially reflecting altered muscle oxygen consumption relative to blood flow. This study evaluated whether StO2 kinetics measured using near infrared spectroscopy (NIRS) would be slowed in PAD during low work rate calf exercise compared with healthy subjects under conditions in which blood flow did not differ. Eight subjects with PAD and eight controls performed 3 min of calf exercise at 5, 10, 30, and 50% of maximal voluntary contraction (MVC). Calf blood flow responses were measured by plethysmography. Power outputs were similar between groups for all work rates. In PAD, the time constants of StO2 kinetics were significantly slower than controls during 5% MVC (13.5 +/- 1.7 vs. 6.9 +/- 1.2 s, P < 0.05) and 10% MVC work rates (14.5 +/- 2.7 vs. 6.8 +/- 1.1 s, P < 0.05). Blood flow assessed when exercise was interrupted after 30 s did not differ between PAD and control subjects at these work rates. In contrast, the StO2 time constants were not different between groups during 30 and 50% MVC work rates, where blood flow responses in PAD subjects were lower as compared with controls. Thus in PAD, the slowed StO2 kinetic responses under conditions of unimpaired calf blood flow reflect slowed muscle oxygen consumption in PAD skeletal muscle during low work rate plantar flexion exercise as compared with healthy skeletal muscle.