Effects of forearm bier block with bretylium on the hemodynamic and metabolic responses to handgrip

Abstract

We tested the hypothesis that a reduction in sympathetic tone to exercising forearm muscle would increase blood flow, reduce muscle acidosis, and attenuate reflex responses. Subjects performed a progressive, four-stage rhythmic handgrip protocol before and after forearm bier block with bretylium as forearm blood flow (Doppler) and metabolic (venous effluent metabolite concentration and 31P-NMR indexes) and autonomic reflex responses (heart rate, blood pressure, and sympathetic nerve traffic) were measured. Bretylium inhibits the release of norepinephrine at the neurovascular junction. Bier block increased blood flow as well as oxygen consumption in the exercising forearm ( P < 0.03 and P < 0.02, respectively). However, despite this increase in flow, venous K+ release and H+release were both increased during exercise ( P < 0.002 for both indexes). Additionally, minimal muscle pH measured during the first minute of recovery with NMR was lower after bier block (6.41 ± 0.08 vs. 6.20 ± 0.06; P < 0.036, simple effects). Meanwhile, reflex effects were unaffected by the bretylium bier block. The results support the conclusion that sympathetic stimulation to muscle during exercise not only limits muscle blood flow but also appears to limit anaerobiosis and H+ release, presumably through a preferential recruitment of oxidative fibers.