Heart rate variability during high ambient heat exposure

Introduction: This study observed heart rate variability (HRV) measured in order to detect the changes in sympathovagal activity during high ambient (74 degrees C) temperature. HRV is a measure of heart rate (HR) fluctuations around the mean HR and has been used to indicate the balance between parasympathetic and sympathetic function of the autonomic nervous system in medicine and exercise, but not in extreme environments. Methods: There were 10 healthy male subjects who wore Polar HR monitors (PE 5000) during a 15-min exposure (74.3 +/- 5.9 degrees C). Results; HR increased significantly (p < 0.05) from 66.5 bpm preexposure to 106.0 bpm during exposure to the dry heat. A significant decrease in parasympathetic drive was indicated by: 1) a reduction of 89.1 +/- 31.3% in the proportion of interval differences between two normal R-R intervals that were in excess of 50 ms in length (pNN50); 2) an 83.4 +/- 7.0% decrease in the root mean square of successive R-R differences (RMSSD); and 3) an 82.7 +/- 11.4% decrease in the normalized high-frequency (HF norm) values. An increase in a sympathetic drive was indicated by a significant increase in: 1) the normalized low-frequency (LF norm) values (of 84.5 +/- 19.4%); and 2) the LF:HF ratio (10.9 +/- 10.8%). Discussion: The results from this study show that exposure to high ambient temperature produces a significant increase in HR through an increase in sympathetic and decrease in parasympathetic drive.