Effects of Patrol Operation on Hydration Status and Autonomic Modulation of Heart Rate of Brazilian Peacekeepers in Haiti

The stress of operational missions may challenge the maintenance of body homeostasis, affecting soldiers' cardiac autonomic control, promoting dehydration, and compromising performance. Therefore, we aimed to determine the effects of peacekeeper patrol operation in Haiti on soldiers' hydration status and cardiac autonomic modulation, and to determine whether fluctuations in autonomic modulation were associated with changes in hydration status, energy expenditure (EE), and aerobic fitness (V[Combining Dot Above]O2max). A group of 20 soldiers (23.5 ± 4.7 years; V[Combining Dot Above]O2max 52.9 ± 4.5 ml·kg⁻¹·min⁻¹) completed an operational patrol mission with a mean duration of 160.6 ± 28.6 minutes. Before (Pre) and after (Post) the operation, the soldiers' body masses (BMs) were measured and 5-minute heart rate interbeat (R-R) intervals were recorded at rest to estimate heart rate variability (low-frequency [LF] and high-frequency [HF] power, and sympathovagal balance [LF/HF]). During the mission, EE was estimated using heart rate (HR) monitors. Changes from Pre to Post in BM (%BM loss) and LF/HF (ΔLF/HF) were used to evaluate the soldiers' dehydration levels and autonomic modulation, respectively. The mean EE was 711.0 ± 208.7 kcal. From pre to post, increases (p < 0.01) were noted in LF normalized units (n.u.) and LF/HF and decreases (p < 0.01) were noted in BM, R-R interval, and HF n.u. The variation in ΔLF/HF correlated with EE (r = 0.49; p = 0.02), V[Combining Dot Above]O2max (r = -0.42; p = 0.05), and %BM loss (r = 0.53; p = 0.02). The results demonstrated that an operational peacekeeper patrol with an approximate duration of 160 minutes promoted both dehydration and an imbalance in the autonomic modulation of soldiers' HR. The reduction in sympathovagal balance correlated with EE, dehydration, and aerobic conditioning.