DOES REFLEX SYMPATHOEXCITATION EVOKE CORRESPONDING CHANGES IN BLOOD FLOW AND TISSUE OXYGENATION IN HUMAN FOREARM?

Abstract

Using near infrared spectroscopy (NIRS) to measure tissue oxygenation in human forearm, previous studies indicated that the ability of sympathetic activation to decrease tissue oxygenation was attenuated during forearm exercise. Changes in tissue oxygenation were presumed to reflect changes in blood flow (O2 delivery) because the sympathetic stimulus was applied during steady-state metabolic conditions (constant O2 demand). However, that presumption remains to be tested. PURPOSE To determine if reflex sympathetic activation elicits qualitatively similar changes in tissue oxygenation and blood flow in resting and exercising human forearm. METHODS In 7 men and 5 women, we simultaneously measured brachial artery blood flow with Doppler ultrasound and forearm tissue oxygenation with NIRS during reflex sympathetic activation evoked by randomized 2-min trials of lower body negative pressure (LBNP) at −10, −20, −30, and −40 mmHg. LBNP at −20 mmHg was repeated during steady-state rhythmic handgrip at 30% of maximal voluntary contraction. RESULTS With the forearm at rest, LBNP at −10 to −40 mmHg evoked graded decreases in brachial blood flow and tissue oxygenation that were highly correlated (r=0.78, p < 0.0001). In resting forearm, LBNP at −20 mmHg decreased brachial blood flow (−30 ±3%) and tissue oxygenation (−18 ±2%). During handgrip, LBNP evoked significantly attenuated decreases in blood flow (−8 ±2%; p < 0.05) and tissue oxygenation (−3 ±2%; p < 0.05). CONCLUSIONS The close correspondence between NIRS and Doppler responses to LBNP indicates that changes in tissue oxygenation reliably reflect changes in blood flow and can be used to assess sympathetically-mediated vasoconstriction in human forearm. Also, the parallel attenuation of sympathetically-mediated decreases in tissue oxygenation and blood flow during handgrip confirms and extends previous NIRS studies showing that sympathetic vasoconstriction is sensitive to modulation in exercising human skeletal muscle. Supported by NIH grant HL64784.