Muscle Sympathetic Nerve Activity Responses to Exercise in Patients with Peripheral Artery Disease

Abstract

Prior reports suggest that exercise pressor reflex is heightened in peripheral artery disease (PAD). It is unclear if PAD alters the response in muscle sympathetic nerve activity (MSNA). We hypothesized that the MSNA responses to leg exercise would be greater than that in healthy control subjects. We studied a total of 21 PAD patients (67 ± 1 yrs) and 18 healthy control subjects (66 ± 2 yrs), while MSNA was recorded in 6 of 21 PAD patients and 5 of 18 healthy control subjects. The ankle-brachial pressure index in the most affected leg in the patients was 0.57 ± 0.02. ECG and beat-by-beat blood pressure (BP, Finometer) and MSNA from the peroneal nerve in the non-exercising leg were recorded for 10 min under resting conditions. Thereafter, the subjects performed rhythmic plantar flexion with incremental loading (2-12 kg, + 1 kg/min) until the subject reached fatigue or exercised for a maximum of 14 min. The PAD patients performed the exercise with the most affected leg. The cardiac baroreflex (BRS) and the HR variability indices were also calculated. In PAD patients the resting MSNA (36.4 ± 4.0 vs. 31.2 ± 3.7 bursts/min) tended to be greater and cardiac BRS (8.0 ± 1.1 vs. 14.0 ± 2.2 ms/mmHg) and vagal tone were lower than those in the control subjects. BP responses to plantar flexion exercise were greater in PAD patients than those in healthy control subjects. MSNA activation during exercise occurred earlier than that seen in healthy controls. Importantly, the MSNA responses (i.e. changes) at the last min of exercise in PAD patients (Δ16.8 ± 4.8 bursts/min) were greater than that in control subjects at the same exercising time (Δ5.8 ± 4.2 bursts/min). We speculate that augmented MSNA responses contribute to the augmented pressor responses to exercise in PAD. Further studies are needed to examine if any (e.g. physical and/or pharmacological) intervention can reduce the augmented MSNA responses to exercise in PAD.