Muscle Reflex Effects on the Autonomic Hemodynamic Response to Passive Leg Movement in Older Individuals

Abstract

Utilizing passive leg movement (PLM), it has recently been documented that leg afferent‐mediated muscle reflexes (predominantly group III) play a critical role in central (cardiac output, Q; stroke volume, SV; heart rate, HR) and peripheral (leg blood flow, LBF) hemodynamic responses in young individuals. The purpose of this study was to examine the impact of age on the role of the muscle reflex in PLM‐induced hemodynamics. Eight men (67 ± 9 yrs) underwent 2 min of passive knee‐extension before (CON) and after partial blockade of lower limb muscle afferents via lumbar intrathecal fentanyl (FENT). Q (plethysmography), leg perfusion pressure (PP), and LBF (Doppler ultrasound) were measured continuously. Leg vascular conductance (LVC) was calculated as LBF/PP. All variables, assessed at baseline, were significantly affected by PLM. The changes in peak values were similar between CON and FENT for Q (~20% increase; P = 0.11), HR (~15% increase; P = 0.42), SV (~18% increase; P = 0.15), and PP (~7% decrease; P = 0.68). However, as FENT resulted in a greater increase in LBF (266 ± 51% vs. 139 ± 21%; P < 0.05), LVC was also greater (288 ± 51% vs. 155 ± 18%; P < 0.05). These findings are in contrast to prior observations in young individuals in whom, compared to CON, FENT during PLM caused a significant decrease in Q (34% vs. 22%), LBF (295% vs. 210%), and LVC (322% vs. 231%). These findings imply an age‐related shift in the role of the mechanoreceptor‐mediated muscle reflex in regulating central and peripheral hemodynamics. Specifically, group III afferents transition from a facilitating effect on central and peripheral hemodynamics during PLM in the young to having no influence on Q and evidence of restraining LBF and LVC in the old.