Physiology and Medicine at High Altitude: The Exposure and the Stress

Abstract

, it was sometimes in the mid-nineteenth century that, Bert first showed the association between hypobaria triggered lowering of PiO 2 and high altitude illness 2 . This relationship is best captured by the gas-law equation:PV = nRT where P = pressure, V = volume, n = number of gas molecules, R = universal gas constant, and T = absolute temperature. Since, the partial pressure of a gas, P = Fi x B, at sea level given a B of 760 mmHg (P B ), the partial pressure of inspired O 2 becomes = 20.9 x 760 =159 mm Hg. This fall is most dramatic at 5,486 meters, which is the upper level of functional acclimatisation where a PB of = 395 mm Hg yields a Pi O 2 of 82 mmHg (20.9 x 395), which is nearly half that at sea level. Even at 3000 m, the height of Leh, the barometric pressure (P B ) and inspired PO 2 (PiO 2 ) are only 70 per cent of the sea level values. On the summit of Mt. Everest, at 8848 m, the PiO 2 is less than 30 per cent of its sea level value. These values signify the stress and challenge to the physiology at high altitude. The ADApTATIONTaken suddenly to the top of Mt. Everest (8848 m), a person would lose consciousness in less than 3 min 3 . Taken slowly the same person remains alive and functional. In fact the lowest recorded P A O 2 in a healthy individual at 8400 m on Everest by the Caudwell Xtreme Everest expedition was 19 mmHg (2.55 KPa). The stark contrast between the two scenarios is accounted for by the adaptive process called acclimatisation that tends to increase the oxygen delivery (DO 2 ) to the tissues. The ClAssIC MODelThe classic model holds that this increase in DO 2 is affected by an optimisation between: (i) Pulmonary (ii) haematological and, (iii) cardiovascular processes. Pulmonary process optimisation is the first response to hypobaric hypoxia: hyperventilation is triggered by a combination of stimulation of peripheral chemoreceptor and inhibition of the central chemoreceptors that leads to an increased respiratory rate and depth. This hypoxic ventilator response (HVR) triggers in within a few hours of exposure to hypobaric hypoxia. This HVR tends to restore the falling oxygen gradient across the physiology and Medicine at high Altitude: The exposure and the stress Anil GurtooLady Hardinge Medical College, ABsTRACT Increase in altitude causes decrease in atmospheric barometric pressure that results in decrease of inspired partial pressure of oxygen, a source for stress and pose a challenge to climbers/trekkers or persons posted on high altitude areas. This review discusses about the high altitude sickness, their incidence rates, pathophysiology and the classic model of acclimatisation, which explains about how oxygen requirement in extreme environment is achieved by complex interplay among pulmonary, hematological and cardiovascular processes. The acute high altitude illness (AHAI) is basically composed of two syndromes: cerebral and pulmonary that can afflict un-acclimatised climbers/trekkers. The cerebral syndrome includes acute mountain sickness (AMS) and high altitude cerebral oedem...