This study examined how the autonomic nervous modulation of healthy subjects was influenced by altitude in low-altitude mountain tourism. Our research was different from previous studies performed for trekkers at higher altitudes in that the confounding factors, such as oxygen consumption
, physical exertion, water and food intake, etc., were excluded or minimized. Therefore, the major changes in HRV measures observed in this study might be caused predominantly by the change in altitude during tourism. It was demonstrated that travelling in the low-altitude mountain area could result in the change in the autonomic nervous modulation of healthy sea-level residents. As altitude was increased from 30 MASL to 520 MASL and 1080 MASL, the HR and BP were decreased, while the overall HRV (SDRR
and TP), the Vagal Modulation (HFP) and sympatho-vagal modulation (LFP) were increased. The increase in overall HRV reflected the enhancement of mixed sympathetic and vagal modulations [22
]. The overall HRV has strong prognostic value in adverse cardiovascular events
]. The finding of increase in overall HRV in the low-altitude area in this study is in accordance with the general notion of feeling relaxed and comfortable during low-altitude tourism, because the sensation of comfort and relaxation is often correlated with increased vagal modulation [7
]. Thus, mountain tourism in low-altitude area within 1080 MASL may have good effect on human physiological fitness of the sea-level residents in terms of autonomic nervous modulation and blood pressure regulation.
In previous studies about the HRV in the mountains higher than 1500 MASL, the HR was increased and the HRV was decreased when the altitude was increased [24
]. The major contributing factor for the decrease in HRV was shown to be the hypoxia induced by low partial pressure oxygen at altitudes above 1500 meters [25
]. In contrast, this study in low-altitude tourism showed that many HRV measures were increased as altitude was increased. The main reason for our favourable results in low-altitude tourism as compared with higher altitude tourism might be that there is less hypoxia
effect in low-altitude area because the partial pressure of oxygen is relatively preserved when the attitude is below 1500 MASL.
Comparison of the changes in BP and HRV measures among 3 altitude changes revealed that the main changes in HRV measures occurred in the ascent from 30 to 520 MASL, rather than the ascent from 520 to 1080 MASL (Figure 1). Furthermore, the SBP, MABP, mRRI, SDRR
, TP, VLFP, and HFP reached their peak values at around 520 MASL, rather than at 1080 MASL (Table 1). It seems that the greatest change in BP and HRV measures occurred at around 520 MASL which meets the altitudes categories between “near sea-level” (0–500 MASL) and “low-altitude” (500–2000 MASL) [4
]. Further studies may be necessary in the future to find out the best altitude that is easily accessible to the public while giving rise to the greatest decrease in BP and the greatest increase in HRV measures.
Many studies have reported that HRV correlated significantly and negatively with age [26
]. In adult, both time and frequency domain HRV measures including SDRR
, Root Mean Squared Successive Difference (rMSSD), pNN50, VLF, LF and HF decline with age [27
]. In this study, we found similarly that age correlated significantly and negatively with SDRR, CVRR, TP, LFP and HFP whatever the altitude was. In addition, though the age has positive correlation with nVFP, it has no correlation with nLFP, nHFP and LHR. This finding suggested that the renin-angiotensin-aldosterone
modulation and thermoregulation of the subject increased with increasing age, but the vagal and sympathetic modulation of the subject was not affected by age. In this study we found that the SBP, MABP and PP were all significantly decreased when the subjects ascended from 30 MASL to a higher altitude in the old group, but not in the young group. This finding suggested that the blood pressures of the old subjects can be decreased to more extent than the young subjects by ascending to a higher altitude in the low-altitude mountain area. Thus, travel in the low-altitude mountain area might have some anti-hypertensive effects in the older subjects, but not in the young subjects.
Gender-related difference in HRV has been reported by many researches. Huikuri et al. [28
] reported that the nLFP and LHR of male subjects were larger than those of female subjects. Antelmi et al. [27
] reported that women had higher HFP, rMSSD, and pNN50, while men had higher VLFP, LFP, and SDRR
. In this study, no difference in HRV was observed between different genders at 30 and 1080 MASL. However, at altitude 520 MASL the SDRR
, TP and LFP in male subjects were significantly higher than those of the female subjects. Our results suggested that different genders reacted somewhat differently to altitude in terms of autonomic nerve modulation. The male subjects had higher overall HRV at 520 MASL, especially the low-frequency components, as compared with those of the female ones.
There were no significant differences in CVRR
and VLFP between genders at 30 MASL in this study. When the subjects were ascended to a higher altitude, the CVRR
and VLFP of the female subjects were increased gradually and significantly, while the CVRR
and VLFP of the male subjects remained unchanged. This difference in the response of CVRR
and VLFP to altitude change between genders might be related to the differences in the body composition and the structure of cardiovascular trees between male and female subjects, because the VLFP is an indicator of renin-angiotensin-aldosterone modulation and vagal withdrawal, and the CVRR is an index of overall vagal modulation, and because the blood pressures of the female subjects were lower than those of the male subjects at 30 MASL.
In addition to altitude, environmental variables might also be the factors that could affect the autonomic nerve modulation of the subjects in real wilderness. For instance, the ambient temperature might influence the VLF component which was known to be related to thermoregulation [21
]. The VLFP and DBP were increased in cold room, and that the HR and nLFP were increased while the nHFP and SBP were decreased in hot room [29
]. In this study, though the humidity at 1080 MASL was significantly lower than that at 30 and 520 MASL, no relationship was found between the relative humidity and HRV measures at altitude 1080 MASL. However, we found that the nVLFP in the old group was significantly increased at 520 and 1080 MASL, though at 30 MASL it was not significantly different between the young and old groups. In addition, though the nHFP was not significantly different between the young and old groups at 30 MASL, it was significantly increased at 520 MASL in the old group. Since the nVLFP is often used as the index of renin-angiotensin-aldosterone modulation and vagal withdrawal [19
], or the index of thermoregulation [21
], and the nHFP is often used as the index of vagal modulation, the increase in nVLFP and the decrease in nHFP at higher altitudes in the old group suggested that the renin-angiotensin-aldosterone modulation, vagal withdrawal and thermoregulation in the old subjects were more likely to be affected by the altitude than the young subjects. Environmental factors related to altitude changes might have played some role in this phenomenon.