Climate, altitude, and blood pressure

Human Biology, Aug 1999 by Hanna, Joel M

Observations of indigenous groups that were carried out before the changes in lifestyle have also yielded general agreement. Murrill (1955) has critiqued early research, and Poulter and Sever (1994) and James and Baker (1995) have discussed more recent studies. Although it is not possible to summarize all studies, their conclusions are similar. Resting systolic and diastolic blood pressures were low and the prevalence of hypertension was also low by modern standards. There was also little or no change in blood pressure with increasing age. Reports have included most regions of the world and many heat-stressed groups, such as nomadic Aborigines (Hicks and Matters 1933), nomadic Bushmen (Kaminer and Lutz 1960), nomadic Pygmys (Kesteloot et al. 1996), southern Chinese (Ling 1936), New Guinea coastal villagers (Hodge et al. 1995), Yanomamo Indians (Oliver et al. 1975), and American Samoan men (Miller and Benedict 1937). Although all groups were native to tropical climates, they were also lean and fit. Lifestyles were nomadic or rural and largely unacculturated; thus it is difficult to link their lower blood pressures exclusively to a tropical climate.

Although blood pressures have been historically low in tropical residents and indigenous peoples, that seems to be changing. For present-day indigenous residents who have been exposed to a modernized lifestyle the results are also consistent. In the Pacific Islands (McGarvey and Schendel 1986), tropical Australia (Smith et al. 1992), the Arabian desert (Ibraheim et al. 1995), sub-Saharan Africa (Bunker et al. 1995), the Kalahari Desert (Kaminer and Lutz 1960), and elsewhere, obesity and lack of physical activity have resulted in elevations in blood pressure and increasing rates of hypertension. In each case there is now a relationship of increasing blood pressure and rates of hypertension with age. If a tropical climate is mildly hypotensive, it is not apparent in these contemporary populations.

In summary, hot dry and hot humid climates do not appear to have a major influence on resting blood pressure. The climatic stress falls most heavily on the heart during exercise, when demands for increased heat transfer and muscular activity come into conflict. The results of this stress are most evident early in exposure when transient reductions in blood pressure may occur. There is a return to normal as acclimatization proceeds. The response seems to be increased stroke volume and elevated plasma volume, which sustain a higher cardiac output. Long-term residents and rural and indigenous peoples show no clear influence of climate on their resting blood pressure, although mild hypotension seems possible. Recent and widespread changes in lifestyle have favored elevations in blood pressure, probably as a result of decreased activity and increased obesity.

As a final note, some caution is in order. The experimental work reports no clearly defined acclimatization-related change in blood pressure; however, experimental studies typically include exercise as part of the acclimation procedure. This effort tends to magnify responses (Rowell 1974). In particular, the increased stroke volume of the acclimated subjects may reflect improved physical fitness rather than heat acclimation. Some of the migrant studies and studies of unacculturated tropical natives suggest low normal blood pressures. This may be characteristic of heat acclimatization that does not include vigorous exercise. Without the major changes in stroke volume and plasma volume that result from exercise, reduced cardiac output and hemodilution might occasion small reductions in blood pressure. Further investigation of this possibility is in order.