Cardiovascular autonomic control during short-term thermoneutral and cool head-out immersion

Background: Moderately cold head-out water immersion stimulates both baro- and cold-receptors, and triggers complex and contradictory effects on the cardiovascular system and its autonomic nervous control. Objectives: To assess the effects of water immersion and cold on cardiovascular status and related autonomic nervous activity. Methods: Hemodynamic variables and indexes of autonomic nervous activity (analysis of heart rate and blood pressure variability) were evaluated in 12 healthy subjects during 3 exposures of 20 min each in the upright position, i.e., in air (AIR, 24-25 degrees C), and during head-out water immersion at 35-36 degrees C (Win) and 26-27 degrees C (Wlc). Results: Plasma noradrenaline, systolic and diastolic blood pressure, and total peripheral resistances were reduced during Win compared to AIR (263.9 +/- 39.4 vs. 492.5 +/- 35.7 pg center dot ml(-1), 116.5 +/- 3.7 and 65.4 +/- 1.7 mmHg vs. 140.8 +/- 4.7 and 89.8 +/- 2.8 mmHg, 14.1 +/- 1.0 vs. 163 +/- 0.9 mmHg center dot L-1 - min, respectively) while they were increased during Wlc (530.8 +/- 84.7 pg center dot ml(-1), 148.0 +/- 7.0 mmHg, 80.8 +/- 3.0 mmHg, and 25.8 +/- 1.9 mmHg center dot L-1 center dot min, respectively). The blood pressure variability was reduced to the same extent during Wlc and Win compared to AIR. Heart rate decreased during Win (67.8 +/- 2.7 vs. 81.2 +/- 2.7 bpm during AIR), in parallel with an increased cardiac parasympathetic activity. This pattern was strengthened during Wlc (55.3 +/- 2.2 bpm). Conclusions: Thermoneutral Wl lowered sympathetic activity and arterial tone, while moderate whole-body skin cooling triggered vascular sympathetic activation. Conversely, both Wl and cold triggered cardiac parasympathetic activation, highlighting a complex autonomic control of the cardiovascular system.