Introduction: Cold habituation could affect sympatho-vagal balance, which modulates cold stress responses. The study examined cardiovascular autonomic function at the sinus node level during controlled breathing and while undertaking isometric exercise during whole-body cold exposure before and after cold acclimation.
Methods: There were 10 male subjects who were exposed to control (25 degrees C) and cold (10 degrees C) environments for 2 h on 10 successive days in a laboratory. Time and frequency domain heart rate variability (HRV) in terms of root mean square of successive differences in RR intervals, total, high, and low frequency power were determined from controlled breathing at the beginning and end of cold acclimation. Heart rate and blood pressure during an isometric handgrip test (30% MVC for 3 min) were recorded at the beginning and end of cold acclimation. Catecholamines (NE and E), mean skin (Tsk), and rectal temperatures (Trect) were measured.
Results: Acute cold exposure increased total (36%), low (16%), and high frequency power (25%) and RMSSD (34%). Cold acclimation resulted in higher Tsk (0.6 degrees C) and lower NE (24%) response in cold. The cold-induced elevation in high frequency power became significant after cold acclimation, while other HRV parameters remained unchanged. A smaller increase in heart rate and blood pressure occurred at 10 degrees C during the handgrip test after cold acclimation.
Discussion: Cold exposure increased sympathetic activity, which was blunted after cold acclimation. Parasympathetic activity showed a minor increase in cold, which was enhanced after cold acclimation. In conclusion, cold habituation lowers sympathetic activation and causes a shift toward increased parasympathetic activity.