Activation of cardiac parasympathetic and sympathetic activity occur at different skin temperatures during face cooling.

Sufficiently cold-water temperatures (<7o C) are needed to elicit the sympathetic response to the cold pressor test using the hand. However, it is not known if stimulating the trigeminal nerve via face cooling, which increases both sympathetic and cardiac parasympathetic activity, also has a threshold temperature. We tested the hypothesis that peak autonomic activation during a progressive face cooling challenge would be achieved when the stimulus temperature is £ 7o C. Twelve healthy participants (age: 25 ± 3 years, 4 women) completed our study. Six pliable bags, each containing water or an ice slurry (34o C, 28o C, 21o C, 14o C, 7o C, and 0o C) were applied sequentially to participants' forehead, eyes, and cheeks for 5 minutes each. Mean arterial pressure (photoplethysmography; index of sympathetic activity) and heart rhythm (3-lead ECG) were averaged in 1-minute increments at the end of baseline and throughout each temperature condition. Heart rate variability in the time (root mean square of successive differences; RMSSD) and frequency (high frequency (HF) power) domains were used to estimate cardiac parasympathetic activity. Data are presented as the increase from baseline ± SD. Mean arterial pressure only increased from baseline in the 7o C (13.1 ± 10.3 mmHg; p =0.018) and 0o C (25.2 ± 7.8 mmHg; p <0.001) conditions. Only the 0o C condition increased RMSSD (160.6 ± 208.9 ms; p =0.009) and HF power (11450 ± 14555 ms2 ; p =0.014) from baseline. Our data indicate that peak increases in sympathetic activity during face cooling are initiated at a higher forehead skin temperature than peak increases in cardiac parasympathetic activity.