'Cross-adaptation': habituation to short repeated cold-water immersions affects the response to acute hypoxia in humans

Abstract

Adaptation to an environmental stressor is usually studied in isolation, yet these stressors are often encountered in combination in the field, an example being cold and hypoxia at altitude. There has been a paucity of research in this area, although work with rodents indicates that habituation to repeated short cold exposures has a cross-adaptive effect during hypoxia. The present study tested the hypothesis that cross-adaptation is also possible with humans. Thirty-two male volunteers were exposed to 10 min bouts of normoxic and hypoxic (FIO2 0.12) rest and exercise (100 W on a recumbent cycle ergometer). These were repeated after a 96 h interval, during which participants completed six, 5 min immersions in either cold (12°C, CW) or thermoneutral water (35°C, TW). Venous blood samples were taken immediately after each bout, for determination of catecholamine concentrations. A three-lead ECG was recorded throughout and the final 5 min of each bout was analysed for heart rate variability using fast fourier transformations (and displayed as log transformed data (ln)). In comparison with the first hypoxic exercise exposure, the second exposure of the CW group resulted in an increased ln high frequency (ln HF) power (P < 0.001) and reduced adrenaline (P < 0.001) and noradrenaline concentrations (P < 0.001). Adrenaline and noradrenaline concentrations were lower in the CW group during the second hypoxic exercise compared to the TW group (P = 0.042 and P = 0.003), but ln HF was not. When separated into hypoxic sensitive and hypoxic insensitive subgroups, ln HF was higher in the hypoxic sensitive CW group during the second hypoxic exercise than in any of the other subgroups. Cold habituation reduced the sympathetic response (indicated by the reduced catecholamine concentrations) and elevated the parasympathetic activity (increased ln HF power) to hypoxic exercise. These data suggest a generic autonomic cross-adaptive effect between cold habituation and exposure to acute hypoxia in humans.

Figure 1. Adrenaline concentrations during 100 W of hypoxic cycling

Mean (dotted line) and individual data (continuous lines) for plasma volume corrected adrenaline concentrations during 100 W of hypoxic cycling before and after the repeated cold water (n = 4) or thermoneutral water immersions (n = 4). Mean (s.d.) for the normoxic rest (filled diamonds), exercise (filled squares) and hypoxic rest (filled triangles) conditions are also shown. *Difference between hypoxic exposure 1 and 2 (P < 0.05); †difference between groups (P < 0.05).

Figure 2. Noradrenaline concentrations during 100 W of hypoxic cycling

Mean (dotted line) and individual data (continuous lines) for plasma volume corrected noradrenaline concentrations during 100 W of hypoxic cycling before and after the repeated cold water (n = 4) or thermoneutral water immersions (n = 4). See Fig. 1 for further details.

Figure 3. High frequency power between the first and second hypoxic exercise exposures

Mean (s.d.) change in log transformed high frequency power between the first and second hypoxic exercise exposures when the cold water and thermoneutral water groups have been separated into the hypoxic sensitive and hypoxic insensitive subgroups. *Different from other subgroups (P < 0.05).

Figure 4

Change in peak cardiac frequency (beats·min⁻¹) from the first to the final cold-water immersions against the change in peak cardiac frequency to hypoxic exercise separated by those in the hypoxic sensitive group (open circles) and hypoxia insensitive group (filled circles).

Figure 5. Number of symptoms during the hypoxic exposures

Mean (s.d.) number of symptoms during the hypoxic exposures for participants in the thermoneutral water immersion group (black bars) and cold water immersion (white bars). †Different from first hypoxic exposure (P < 0.05); *difference between CW and TW (P < 0.05).

Figure 6. Symptom scores during the hypoxic exposures

Mean (s.d.) symptom scores during the hypoxic exposures for participants in the thermoneutral water immersion group (black bars) and cold water immersion (white bars). †Different from first hypoxic exposure (P < 0.05); *difference between CW and TW (P < 0.05).