Exercise-induced hyperemia unmasks regional blood flow deficit in experimental hindlimb ischemia

Many experimental models of hindlimb ischemia are characterized by spontane ous and rapid normalization of resting muscle blood flow (BF) rates which c omplicates the long-term evaluation of angiogenic therapies to reverse limb ischemia. We tested the hypothesis that peroneal nerve stimulation in an i schemic hindlimb would increase the oxygen (O-2) demand and BF rate, thereb y unmasking a severe blood flow deficit that is not apparent at rest. Methods. Ischemia was induced in adult rats by ligation of the left common iliac, femoral arteries, and their branches. Peroneal nerves were stimulate d to allow measurement of exercise-induced regional BF rates with fluoresce nt microspheres. Hemodynamics were monitored, Fluorescent microspheres were injected before and after 5 min of nerve stimulation 3, 10, and 24 days po stischemia. The tibialis anterior (TA) and gastrocnemius (GC) muscles and s kin were harvested and weighed, and fluorescence was measured. BF rate was calculated as milliters per minute per gram of tissue and compared to norma l muscle and skin of unoperated control rats. In order to determine the acc uracy of BF rate measurements in ischemic muscle when < 400 microspheres wa s delivered per specimen, 3 rats were studied by simultaneous injection of 4 x 10(5) blue and 1 x 10(5) yellow-green fluorescent microspheres. The cor relation coefficient between the number of different colored microspheres d elivered was measured. Results. The ischemia caused atrophy of the TA and G;C muscles. The mean mu scle mass of the ischemic TA and GC as a percentage of total body weight de creased over time vs control [TA 0.13 +/- 0.05% vs 0.25 +/- 0.03%, P < 0.05 ; GC 0.51 +/- 0.27% vs 0.70 +/- 0.07%, P = 0.07 at 24 days (24D)]. Despite clinical evidence of severe hindlimb ischemia in experimental groups, i.e., pressure sores, muscle atrophy, and weakness, resting BF rates were not si gnificantly different from those of control. The BF rate of the TA was of 0 .11 ml/min/g after 3D of ischemia, 0.14ml/min/g after 10D, and 0.13 ml/min/ g after 24D. The mean BF rate in normal muscle of unoperated controls was 0 .16 ml/min/g (P > 0.05). However, the exercise-induced hyperemia in the ske letal muscle was significantly blunted in all of the ischemic groups. The u noperated control TA had a greater than 10-fold increase in BF to 1.95 ml/m in/g in response to exercise while the ischemic TA had no increase in BF at 3D, S-fold increase at 10D, and a 5-fold increase at 24D. Parallel finding s were noted in the GC muscles. There was no significant difference in the BF rate in the skin. The accuracy of this microsphere technique in measurin g very low BF rates found in ischemic muscle was supported by the significa nt correlation coefficient (r = 0.99) comparing two quantities of microsphe res injected simultaneously. Conclusion. Despite clinical signs of severe hindlimb ischemia, resting BF rates in the ischemic groups were not significantly decreased. Peroneal ner ve stimulation resulted in up to 10-fold increase in BF rate and unmasked a severe deficit in vascular reserve in the ischemic groups. Resting BF rate is not always an accurate reflection of the flow deficit in models of crit ical limb ischemia, and this model of exercise-induced hindlimb hyperemia m ay allow better long-term evaluation of angiogenic therapies designed to re verse critical limb ischemia. (C) 2001 Academic Press.