Blunted exercise hyperemic response to lower limb exercise in individuals with Down syndrome.

Background: Autonomic dysfunction is common in individuals with Down syndrome (DS). Coordination of autonomic function is essential for appropriate peripheral blood flow delivery and blood pressure (BP) regulation during lower limb dynamic exercise. Our purpose was to examine the effects of lower limb dynamic exercise on femoral blood flow (FaBF) and BP to both active and inactive limbs at both relative and absolute intensities in individuals with and without DS. Methods: Twenty-four individuals with DS (24±5yrs; 30.3±6.2kg/m²) and without DS (25±4yrs; 26.5±4.5kg/m²) performed five knee extensions at 10%, 20%, 30% of maximal dynamic power (MDP; relative intensity), 11 Nm and 16 Nm (absolute intensity). FaBF was measured using Doppler ultrasound of both limbs simultaneously. Beat-to-beat mean arterial pressure (MAP) was assessed using finger photoplethysmography. Femoral vascular conductance (FaVC) was calculated: FaBF/MAP. Results: Individuals with DS demonstrated blunted FaBF (~55% lower) and FaVC (~53% lower) in their active limb compared to individuals without DS at all (10%, 20%, 30% of MDP) relative intensity sets (groupXtime, p<0.001). Similarly dampened FaBF (~50% lower) and FaVC (~46% lower) were observed during absolute intensities for individuals with DS (groupXtime, p<0.001), without group differences in MAP. FaBF and FaVC to the inactive limb were higher during 30% MDP intensity in individuals with DS (groupXtime, p=0.006). Conclusions: These findings suggest individuals with DS are unable to direct BF to working muscles and show an unfavorable increase in inactive limb BF during lower limb exercise. Inadequate muscle perfusion may limit their ability to meet exercise demands and contribute to reduced work capacity.