Climate, altitude, and blood pressure

Human Biology, Aug 1999 by Hanna, Joel M

The pattern of acclimatization has been verified on numerous occasions (Young 1996). After acclimatization there are small increases in blood pressure, limited to the period of cold exposure (Muza et al. 1988; Hesslink et al. 1992). It has been suggested that this is cold habituation, accompanied by a reduction in sympathetic response. This may not be the case, however, because measurement of plasma hormones in well-acclimatized individuals showed that catecholamine levels are actually elevated above those observed in the nonacclimatized state (Young et al. 1986; Savourey et al. 1996).

There are sex and age differences for whole body exposure to mild cold. By virtue of subcutaneous fat distribution, women typically show greater tissue insulation and suppression of shivering (Rennie et al. 1962; Kollias et al. 1974). During acclimatization, this would favor lower blood pressures among women. Considering the effects of aging, Collins et al. (1985) compared the responses of 18-24-year-old men to those aged 63-70 years. The clothed men were exposed to several levels of cold, the lowest being 6C. At milder temperatures the blood pressures of the 2 age groups were similar, but with the colder exposures the older group exhibited higher systolic and diastolic blood pressures. Age does not appear to have an important impact on resting blood pressure during mild cold exposure.

Indigenous Groups Naturally Exposed to Total Body Cooling. The Scholander protocol has been applied to Australian Aborigines and Bushmen, 2 populations naturally exposed to whole body cooling. The Australian Aborigine men were nomadic when tested. They regularly slept on the ground without the benefit of protective clothing; they had become naturally acclimatized to cold. The responses of the Aborigines were an absence of shivering, a gradual reduction in body temperature, and a significant decrease in metabolic rate. They also enjoyed a good night's sleep. Again, blood pressure was not measured; however, the reduced metabolic rate and peripheral vasoconstriction suggest that their blood pressure was not substantially elevated. Hicks and Matters (1933) collected blood pressure data on a similar group of nomadic Aborigines and found pressures to be uniformly low. Ages were unknown, but mean blood pressure values were 109/73 for the men and 107/66 for the women. The highest value among the 45 adults was 130/92.

The other cold-exposed group was the Bushmen, who were also nomadic at the time. The type of cold exposure was similar to that of the Aborigines, except the Bushmen slept under skin cloaks for warmth. The Bushmen also showed resistance to shivering and a somewhat blunted metabolic response, probably not adequate to stimulate an elevation in blood pressure. The blood pressures of a similar group of 42 nomadic Bushmen men and 36 women were recorded by Kaminer and Lutz (1960). In the pooled adult sample blood pressures were clearly not elevated. The mean blood pressures were 108/66 for the men and 113/70 for the women-low but normal by Western standards.