Effect of Altitude on Blood Pressure

Abstract

The effect of higher altitude over the blood pressure (BP) is not exactly known, and it appears to vary both between and within individuals. The effects of lower temperature, light, increased physical stress, and duration of living at higher altitude also play a significant role. Most of the studies showed that higher altitude correlated with rising BP. This change in BP is due to many factors, the important one being the effect of hypoxia causing activation of sympathetic nervous system. Activation of sympathetic nervous system causes an increase in cardiac output, heart rate, and peripheral vasoconstriction, which lead to increase in BP. The incidence of BP in high latitudes is controversial ; some research shows greater rates while others show lesser rates of hypertension in these individuals. It may be due to the effects of multiple other confounding factors like genetic, ecological, and lifestyle variables. Hence, increasing popularity of high-altitude travel needs further research to evaluate this problem. Furthermore, the implications of altitude-induced hypertension for cardiovascular risk and end-organ damage require more clarification. INTRODUCTION Studies evaluating the effects of high altitude on arterial BP are not enough. There is a contradiction about the effect of higher altitude on arterial BP. The results of various research generally support the positive relationship between arterial BP and higher altitude. 1-5 Individual predisposition is very important, as the effect of higher altitude appears to vary both between and within individuals. 6 Even there are differences in susceptibility among races. 2 Duration of living at higher altitude also plays a significant role. Persons who have acclimatized over weeks show less effect of higher altitude on BP. Studies have shown that altitude-induced BP elevations can persist for months. 1 The effects of lower temperature, light, and increased physical stress also play an important role in survival in more difficult environments. Most of the studies showed that higher altitude correlated with rising BP. 2,5 Susceptible individuals undergoing short-term ascents above 2,500 m showed an acute effect on BP. This change in BP is due to many factors, the important one being the effect of hypoxia causing activation of sympathetic nervous system. Activation of sympathetic nervous system causes increase in cardiac output, heart rate, and peripheral vasoconstriction. These facts have many implications for patients suffering from disease conditions that can lead to hypoxia. Hence, appropriate measures should be taken in these patients whenever they are going to higher altitudes. Some studies demonstrate an increase in BPs, 7-9 and others describe a decrease in BP 10 in nonhypertensive individuals acutely exposed to hypoxia, while some other studies report no change in BP. 11,12 Similarly, in the hypertensive patient, various studies report elevations in BP upon acute exposure to higher altitudes, 13-16 yet others report no significant change. 9,11,17 A recent study demonstrated that among residents of Tibet, there is a significant correlation between the higher altitude and incidence of raised BP. It was found that prevalence of hypertension increased by 2% for every 100 m increase in altitude. 18 In another study, Italian researchers found that during ascend at higher altitude BP steadily increases. They also found that medicines are not very effective for treatment of high BP once a certain height is reached. Forty-seven individuals traveled to the base camp of Mount Everest at an altitude of approximately 5400 m. All participants checked BP round-the-clock during their ascent. The volunteers were also randomized to take either a placebo or 80 mg of telmisartan. Telmisartan is an angiotensin receptor blocker. Angiotensin receptor blocker blocks angio-tensin II, which causes constriction of blood vessels. The study also showed that at an altitude of 5400 m an increase in BP by 14/10 mm Hg is caused. 19 This study also showed that although telmisartan significantly reduced BP up to 3400 m but no effect was seen at 5400 m. 19 The findings impact on people who ascend at high altitude and also in those living at sea level. Some people may also be prone to consistent elevations in BP while living at high altitudes. On the contrary, in the higher land, the prevalence of hypertension is more mixed. 4,5 Some research show greater rates while others show lesser rates of hypertension in these individuals. It