Cross-adaptation: the effect cold habituation has on the physiological responses to acute hypoxia in humans
Student thesis: Doctoral Thesis
Physiological adaptation to environmental stressors is often studied in isolation, but these stressors are frequently combined outside of laboratory settings, for example cold and hypoxia at altitude. There is also limited information about the effect that adaptation to one environment has on exposure to another. The five studies in this thesis were conducted in humans to assess the effect cold habituation has on the response to a simulated hypoxic exposure, and also to investigate a possible mechanism through which any change may occur. A possible site for the 'cross-adaptation' between cold habituation and hypoxia is the autonomic nervous system. Heart rate variability (HRV) is an non-invasive measurement technique which has been used to quantify autonomic activity. The two main frequency bands of interest when using HRV are referred to as the Low-Frequency (LF) band (the power found between 0.04 and 0.15 Hz) and the High-Frequency (HF) band (the power found between 0.15 and 0.4 Hz). Study One assessed the reliability of heart rate variability as a technique to indicate autonomic activity during both paced (breathing in time to a standard audible signal) and spontaneous breathing conditions, and at different cycling exercise intensities in a thermoneutral environment. It was hypothesised that within each condition HRV indices would be reliable between repeated recordings, which were separated by 96 hours. Eight participants performed each condition on the two occasions. Analysis of the data (coefficients of variation [CV] and intraclass correlation coefficients [ICC]) showed that the paced breathing condition was the most reliable condition, and time domain HRV indices were reliable, whilst not all frequency domain HRV indices were. Normalising and log transforming the raw data did improve reliability and log transformed total and high frequency (Ln HF) power and low:high frequency ratio (Ln LF:HF) met the a priori criteria (CV <10 % and ICC > r=0.8). It was concluded that most log transformed HRV indices were reliable at rest, during paced breathing and during moderate intensity exercise. Thus, the hypothesis was accepted, but caution was advised as several of the indices were close to exceeding the reliability criteria (Ln total power, Ln HF and Ln LF:HF) and a second autonomic measurement technique may be considered to substantiate its use. The previous study identified that Ln HF power increased when breathing frequency was reduced at rest. Study Two investigated the effect that alterations in breathing patterns had on HRV indices during rest and unloaded seated cycle ergometery (0 Watts) in 16 male participants. It was hypothesised that breathing which was externally paced would increase HF power compared to spontaneous breathing conditions. HF power was elevated during the paced breathing conditions in comparison to spontaneous breathing at rest and during unloaded exercise. Consequently, the hypothesis was accepted. Thus, ventilatory variables should be recorded in following studies as there may be links between ventilation and HRV indices. The previous studies used participants' freely chosen cadence when cycling, this may have influenced the HRV. The third study tested the hypothesis that cycling cadence affected HRV indices. HRV indices from 16 male participants were analysed when cycling at 40, 60, 80 and 100 revs.min-1 on an unloaded (0 Watts) and loaded (100 Watts) seated cycle ergometer. HRV indices declined as cadence was increased. Thus, the hypothesis was accepted. If HRV indices were to be calculated during subsequent experiments, both cadence and power output would have to be standardised. The first three studies provided information on the conditions which must be present to produce reliable HRV data during moderate intensity exercise. These studies also indicated that an additional means of measuring autonomic activity should be included. Study Four was designed to establish if one hypoxic exposure would influence a second exposure, if there was no effect the model could be adopted for the final experiment. This study also examined the effect of hypoxia on HRV indices at rest and during exercise. It was hypothesised that exercise and hypoxia would exert separate and additive effects on HRV indices and catecholamine concentrations. Twelve male participants rested and exercised on a loaded cycle ergometer (100 Watts) in normoxic (faction of inspired Oxygen, FIo2 0.2093) and hypoxic conditions (FIo2 0.15) on two occasions, separated by 96 hours. HRV and catecholamine concentrations were similar between the normoxic and hypoxic resting conditions. During exercise in normoxia catecholamine concentrations increased and Ln HF power was reduced, further increases in catecholamine concentrations and a reduction in Ln HF power were found during exercise in hypoxic conditions. The hypothesis was rejected for resting conditions, and accepted for the exercise conditions. It was also found that the first hypoxic exposure did not influence the HRV indices and catecholamine concentrations of the second hypoxic exposure and this model could therefore be used for the final experiment. The final study (Study Five) tested for the presence of a 'cross-adaptation' response in cold habituated humans to hypoxic exposures during rest and moderate intensity exercise. This study was designed on the basis of the information obtained from the previous four experiments and tested the hypothesis that cold habituation by repeated cold-water immersions would reduce the sympathetic activity and cardio-respiratory responses during loaded cycling (100 W) in hypoxic conditions (FIo2 0.12). Thirty-two male participants underwent six, five minute immersions in either cold (12 °C) or thermoneutral (35 °C) water over a three day period. The normoxic and hypoxic exposures were performed before and after the water immersions. It was established that cold habituation attenuated the sympathetic response to loaded exercise during an acute hypoxic exposure and reduced the number and severity of acute mountain sickness (AMS) symptoms. The study provides the first evidence of a cross-adaptation between cold habituation and hypoxic exposure in humans. This was not found in participants who performed thermoneutral water immersions. Therefore, the hypothesis was accepted. In conclusion, in four of four participants whose catecholamine concentrations were analysed and eight from 16 volunteers whose HRV was analysed, showed that cold habituation reduces the sympathetic response to an acute hypoxic stimulus during loaded cycling. However, it is not known if this cross-adaptation provides an adaptive or maladaptive response to prolonged exposure to hypoxia or altitude. Additionally, the permanence of the cross-adaptation also requires further investigation.
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