Blunted Exercise Pressor Response to Isometric Knee Extension and Post-Exercise Ischemia in Individuals with Down Syndrome.

Introduction: Individuals with Down syndrome (DS) exhibit autonomic dysfunction, which contributes to reduced work capacity. The metaboreflex produces exercise-induced sympathoexcitation and can be assessed via post-exercise muscle ischemia (PEMI). Blunted sympathoexcitation is common in individuals with DS and contributes to the physiological basis for reduced work capacity observed this population, but the influence of the metaboreflex is unknown. Using unilateral isometric knee extension exercise with PEMI, we hypothesized that individuals with DS would demonstrate a reduced metaboreflex compared with individuals without DS.

Methods: Twenty-four individuals with DS (M/F: 13/11; 24 ± 5 yr; 30.3 ± 6.2 kg·m -2 ) and without DS (M/F: 13/11; 25 ± 4 yr; 26.5 ± 4.5 kg·m -2 ) performed a unilateral isometric knee extension at 30% of their maximal voluntary contraction on a leg dynamometer. Following 2 min of contraction, a thigh cuff was rapidly inflated to 220 mm Hg on the exercised leg for 3 min to isolate the activation of the muscle metaboreflex via PEMI. Beat-to-beat mean arterial pressure (MAP) and systolic blood pressure (SBP) were assessed using finger photoplethysmography. Heart rate (HR) was collected via three-lead electrocardiogram.

Results: Despite similar baseline values of all variables in both groups, individuals with DS demonstrated a blunted pressor response to unilateral isometric knee extension compared with individuals without DS (MAP; DS: 103 ± 14 vs non-DS: 125 ± 19 mm Hg), and the blunted MAP response was maintained with PEMI (MAP; DS: 95 ± 13 vs non-DS: 106 ± 18 mm Hg; group-time interaction, P < 0.001). Individuals with DS also exhibited reduced HR 2 min into contraction compared with individuals without DS (HR; DS: 90 ± 16 vs non-DS: 114 ± 22 bpm; group-time interaction, P < 0.001).

Conclusions: Individuals with DS demonstrated a diminished metaboreflex response compared with their peers without DS, during a task known to induce sympathoexcitation. Our findings suggest that reduced influence of the metaboreflex contributes to the reduced exercise pressor response in individuals with DS. Such exercise-specific peripheral autonomic alterations extend beyond our previous cardiac autonomic findings demonstrating blunted sympathoexcitatory perturbations in individuals with DS, which may contribute to reduced work capacity observed in this population.