Blood pressure trend of lowlanders during first 6 days of acclimatisation to high altitude

INTRODUCTION

Low barometric pressure at high altitude (HA) lowers the alveolar oxygen in human sojourns in these areas. This hypobaric hypoxia stimulates oxygen sensitive chemoreceptors in the carotid body and the brainstem resulting in an increased efferent sympathetic outflow.^[1,2] The sustained activation of sympathetic nervous system ensures oxygen supply to cells and tissues by increasing heart rate and cardiac output.^[3] Subsequently, the acclimatisation process partially restores arterial oxygen content thereby reducing sympathoexcitatory chemoreceptor drive. Despite this normalisation, some healthy humans exhibit increased systemic arterial pressure irrespective of the hypoxia exposure duration.^[4]

It is observed that the normotensive individuals at sea level frequently end up with hypertension when exposed to HA stress. This hypoxic stress aggravates the 24-h ambulatory systolic, diastolic, day time and night time blood pressure of individuals, in both normotensive as well as hypertensive.^[5] This augmentation of BP in hypertensive individuals is more pronounced; hence, poorly controlled hypertensives are advised to refrain from travel to HA. Various investigations have demonstrated differing impacts of BP in response to HA in normotensive subjects. Studies in healthy individuals without hypertension have either shown increase, decrease, or no change in BP at HA.^[5-7] It is important to note that effect of HA on BP is inadequately characterised. Some studies have associated modified sympathoadrenal activity for systemic hypertension at HA.^[8,9]

One of the intriguing findings is that normotensives who develop hypertension at HA revert to normal values on returning to sea level. Such type of hypertension has been arbitrarily labelled as HA systemic hypertension. This is of significance in case of people who spend a longer tenure at HA like soldiers and mine workers. Hypertension in such individuals regularly faces the repercussion of increased BP and is frequently put on antihypertensive medication.

It has been seen that hypoxia of HA increases BP and also an appreciable number of lowlanders who stay at HA for longer time develop systemic hypertension. The present study was hence planned to delineate the BP response of the lowlanders during initial days of acclimatisation to HA.

MATERIALS AND METHODS

RESULTS

Data were collected from 398 participants out of 500 participants eligible during the study period. The physical characteristics of the participants are shown in [Table 1].

At HA out of 398 participants on 6^th day, 51 (12.8% [95% CI: 9.6–16.5]) had BP more than 140/90 mmHg and 347 had BP <140/90 mmHg. The pattern of change of BP over time is shown in [Figure 1].

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Figure 1:

Pattern of change in blood pressure at different days at high altitude.

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On HA day 1, both the groups exhibited significantly increased BP from sea level (SL) and on HA day 4, HBP group showed significantly higher BP than NBP group SBP (144.5 ± 8.7 mmHg vs. 135.0 ± 7.0 mmHg, P < 0.05) and DBP (88.9 ± 6.0 mmHg vs. 85.3 ± 5.1 mmHg, P < 0.05). Similarly, on HA day 6, HBP group showed significantly higher BP than NBP group SBP (148.1 ± 7.9 mmHg vs. 131.7 ± 5.3 mmHg, P < 0.05) and DBP (93.8 ± 6.3 mmHg vs. 83.7 ± 3.8 mmHg, P < 0.05) [Table 2].

There was a significant difference in BP values through the extent of time as well as between HBP and NBP group for SBP, DBP and MBP (Repeated Measurement ANOVA: P < 0.001 for all three measurements). Repeated measurement model using restricted maximum likelihood method was done for adjustment of the age and BMI. However, the difference among and within group remains significant [Table 3] as regard to systematic BP, similar findings have been shown for diastolic and MBP (Not Shown). In the two groups, the difference in blood pressure became statistically significant from fourth day onwards [Table 3].

Mean MBP increased on 1^st day of HA. The mean MBP decreased gradually for both HBP and NBP groups; however, for HBP, it started increasing on 4^th day onwards at HA [Figure 2].

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Figure 2:

Change in mean blood pressure on different days at high altitude.

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DISCUSSION

The main finding of the present study is that the majority of the subjects had a continuous fall of BP during the days of acclimatisation making them normotensive whereas in others BP continuously increased 4^th day onwards. The immediate effect of HA ascent is a decrease in BP by endothelium-dependent and endothelium-independent systemic vasodilation^[12] but curiously our subjects did not show such instantaneous hypotensive impact of HA. This physiological response is counteracted by sympathetic vasoconstriction essentially by arterial hypoxemia after few hours of stay at HA. Peripheral chemoreceptors located in the carotid bodies and cardiovascular control regions of the mid-brain mediate hypoxia-induced vasoconstriction resulting in critical and steady increase in arterial BP.^[13] In our study, there was increased BP on the 1^st day at HA and this change was sustained over the first 3 days after which NBP group starts to come back to normal as acclimatisation sets in. In the HBP group, the increase was sustained and there was further increase. This was distinctly different from a study by Roach et al.,^[14] in which the MBP of 64 normotensive and 33 older hypertensives was higher at 2500 m than recorded home measurements, but gradually diminished, stabilising on day 3. The present study observed this pattern in both the groups initially for 3 days but the HBP participants, started having higher BP from the 4^th day onwards at HA and in NBP subjects BP continues to decline. Hence, on the 6^th day at HA, BP reverted to normal indices (i.e., ≤140/90 mmHg) in the NBP group but persisted above 140/90 mmHg in the HBP group. The underlying mechanisms through which BP returns to near-normal or even lower values have been generally thought to be a part of the acclimatisation process. Despite the extant of the literature with regard to BP and acclimatisation timing, the period when continually exposed lowlanders achieves that acclimatisation is not well established,^[3] and there are no such epidemiological studies.

The sustained high DBP and borderline SBP values observed in many subjects of our study group for a protracted period of time requires further investigation. The phenomenon could be a steady portrayal of sympathetic stimulation, though it is not yet known when it ceases.^[3] Another factor playing a role in the elevation of BP in chronic hypoxia could also be an overstated reaction to enhanced chemoreceptor reflex described for hypertension prone individuals.^[15] An increased blood viscosity^[16] and alteration of the endothelial function^[17] have likewise been implicated. Hence, a single factor will be insufficient to explain the phenomenon.

The increase of sympathetic tone might be a characteristic reaction to balance the impacts of hypoxia in case of nonacclimatised subjects. Hypoxia directly causes vasodilation and if acting through peripheral chemoreceptors leads to vasoconstriction through sympathetic stimulation.^[18,19] Interactions occur between the hypoxic vasodilatation of systemic arterioles and the chemoreceptor-mediated responses in the systemic circulation.^[20-22] This, thus prompts baroreceptor-mediated sympathetic excitation. Other varied reasons could be alterations in baroreflex function, an increase in the ‘set point’ and possibly a decrease in gain. These autonomic adjustments may have a role in an escalation in BP during sustained hypoxia.^[22,23] Moreover, few data exist regarding the relative importance of other neurohormones such as angiotensin, aldosterone or vasopressin, all of which may be involved in some degree with raising BP during sustained high-altitude exposure.

CONCLUSION

There was a contrasting BP response of individuals during acclimatisation on continual recording. The majority of the subjects could revert the BP back to normal levels after the initial rise while in few, BP remained elevated throughout. Hence, it can be concluded that few percentage of lowlanders remain with high BP after acclimatisation to HA. Such individuals should be monitored regularly after acclimatisation as these individuals can have a delayed acclimatisation normalising BP later on or can become hypertensive subsequently.