Skin sympathetic nerve activity as a biomarker of fitness

Background: Exercise stress testing is frequently used to expose cardiac arrhythmias. Aerobic exercise conditioning has been used as a nonpharmacologic antiarrhythmic intervention. Objective: The purpose of this study was to test the hypothesis that noninvasively recorded skin sympathetic nerve activity (SKNA) is increased during exercise and that SKNA response varies according to fitness levels. Methods: Oxygen consumption (VO 2 ) and SKNA were recorded in 39 patients undergoing an incremental exercise test. Patients were grouped by 5 levels of fitness based on age, sex, and VO 2 max. Results: With exercise, all patients had a significant increase in average SKNA (aSKNA) (1.58 ± 1.12 μV to 4.50 ± 3.06 μV, P = .000) and heart rate (HR) (87.40 ± 20.42 bpm to 154.13 ± 16.82 bpm, P = .000). A mixed linear model of aSKNA was used with fixed effects of fitness, exercise time, and recovery time, and random effects of subject level intercept and slopes for exercise time and recovery times. The poor fitness group had significantly higher aSKNA than the other groups (P = .0273). For all subjects studied, aSKNA increased by 5% per minute with progression of exercise and decreased by 15% per minute with progression of recovery. The fitness variable encodes information on both comorbidities and body mass index (BMI). Once fitness level is known, comorbidities and BMI are not significantly associated with aSKNA. In all groups, aSKNA positively correlated with HR (R 2 = 0.47 ± 0.23) and VO 2 (R 2 = 0.68 ± 0.25). Conclusion: Fitness level determines the magnitude and time course of SKNA increase during exercise. SKNA may be a useful fitness biomarker in exercise stress testing.