Muscle sympathetic nerve activity response to heat stress is attenuated in chronic heart failure patients

Abstract

Heat stress evokes significant increases in muscle sympathetic nerve activity (MSNA) in healthy individuals. The MSNA response to heat stress in chronic heart failure (CHF) is unknown. We hypothesized that the MSNA response to heat stress is attenuated in CHF. Passive whole body heating was applied with water-perfused suits in 13 patients (61 ± 2 yr) with stable class II-III CHF, 12 age-matched (62 ± 2 yr) healthy subjects, and 14 young (24 ± 1 yr) healthy subjects. Mild heating (i.e., increases in skin temperature ΔTsk ~2-4°C, internal temperature ΔTcore <0.3°C) significantly decreased MSNA in CHF patients; however, it did not significantly alter the MSNA in the age-matched and young healthy subjects. Heat stress (i.e., ΔTsk ~4°C and ΔTcore ~0.6°C) raised MSNA in the age-matched (32.9 ± 3.2 to 45.6 ± 4.2 bursts/min; P < 0.001) and young (14.3 ± 1.7 to 26.3 ± 2.4 bursts/min; P < 0.001) controls, but not in CHF (46.2 ± 5.3 to 50.5 ± 5.3 bursts/min; P = 0.06). The MSNA increase by the heat stress in CHF (Δ4.2 ± 2.0 bursts/min) was significantly less than those seen in the age-matched (Δ12.8 ± 1.7 bursts/min, P < 0.05) and young (Δ12.0 ± 2.7 bursts/min, P < 0.05) control groups. These data suggest that the MSNA response to heat stress is attenuated in CHF patients. We speculate that the attenuated MSNA response to heat stress may contribute to impaired cardiovascular adjustments in CHF in a hot environment.

Keywords: heart failure; hemodynamics; nervous system; stress; sympathetic.

Fig. 1.

Representative tracings of mean skin temperature (Tsk), internal temperature (Tcore), heart rate (HR), mean arterial blood pressure (MAP) and muscle sympathetic nerve activity (MSNA) during whole body heating in a CHF patient (top) and a young healthy control subject (bottom). Mild heating: Tsk > ~36°C, while ΔTcore <0.3°C. Heat stress: ΔTcore >0.5°C when Tsk was ~38°C.

Fig. 2.

Systolic blood pressure (SBP), diastolic blood pressure (DBP), HR, and MSNA during baseline, mild heating, and heat stress. *P < 0.05 vs. baseline. †P < 0.05 vs. heat stress within the subject group. ‡P < 0.05 vs. young control subjects. §P < 0.05 vs. age-matched control subjects under the respective thermal condition.

Fig. 3.

Slopes of the relationship between MSNA burst incidence (BI) and DBP during normothermia in a chronic heart failure (CHF) patient, an aged-matched control subject, and a young control subject.

Fig. 4.

Representative tracings of MSNA under normothermia and heat stress conditions (A), the MSNA response to a fatiguing handgrip (B) during a separate visit in one CHF patient. The patients were not heated during the visit for the handgrip exercise. Fatiguing handgrip exercise evoked significant MSNA increase in the CHF patients (C; n = 5). HG denotes last minute of the handgrip exercise.