Altitude illness occurs when a person ascends more rapidly than the body can adjust (acclimatize) to the decreased oxygen at a higher altitude. Some people adjust very easily, while others cannot go above even moderate heights without experiencing symptoms. There is no way to know ahead of time whether one will be a good acclimatizer, except based on past experience. If symptoms occurred before, it is likely they may occur again at the same altitude. The ability to acclimatize may be genetic.
That high altitude can have deleterious effects on the body has long been recognized. Perhaps the first reference to illness associated with altitude was recorded nearly 2000 years ago by Tseen Hanshoo, who described the "Great and Little Headache" mountains on the journey along the Silk Road. With the advent of hot-air ballooning in the 19th century, French and English adventurers described deleterious effects of ascent to high altitude. By the end of the 19th century, Angelo Mosso, an Italian physiologist and climber, had built the advanced Regina Margerita on Monte Rossa in the Italian Alps (4,559) m). In 1960, Charles Houston, an Aspen internist, described non-cardiogenic pulmonary edema i.e. high-altitude pulmonary edema (HAPE), in a healthy 21-year-old cross-country skier. The large number of Indian troops stationed in the Himalayas provided further description of high-altitude pulmonary edema in otherwise healthy young men. In 1991, a group of experts met to define criteria for the diagnosis of illnesses associated with high altitude. These are known as Lake Louis Criteria.
There are three subtypes of acute altitude sickness: acute mountain sickness (AMS), high- altitude pulmonary edema (HAPE) and high- altitude cerebral edema (HACE). Although these conditions occur for different physiological reasons, there are easy steps travellers can take to minimize their risk from these disorders since they all stem from the lack of oxygen at altitude. The chance of developing any of these conditions and the seriousness of the sickness depends on four main variables: the rate of ascent; height above sea level; length of stay at that altitude; and amount of physical exertion undertaken while there. In order to lessen the occurrence of altitude-related disorders, good acclimatization is essential. To allow appropriate time for acclimatization to occur, it is currently advised that the first night sleeping at altitude should be no higher than around 2400 m. Once above 2700 m. subsequent daily ascent should not exceed around 300 m. Professional climbers often use the maxim of 'Climb High, Sleep Low when planning their itinerary, meaning that it is allowable to ascend higher than your daily allowance of 300 m in one day, as long as you descend and sleep at a lower, 'allowed' altitude overnight.
Acclimatization or acclimatization (also called acclimation or acclimatation) is the process in which an individual organism adjusts to a change in its environment (such as a change in altitude, temperature, humidity, photoperiod, or pH), allowing it to maintain performance across a range of environmental conditions. The lack of enough oxygen at altitude stimulates physiological adaptations that improve tolerance of the changed conditions both at rest and during exercise. One of the best known examples of acclimatization in humans can be observed when travelling to high altitude locations – such as tall mountains or hill stations. For instance, if an individual hikes to 3,000 meters above sea level and stays there for 1-3 days, they become acclimatized to 3,000 meters if the same individual hikes to 4000 meters in altitude, then their body have to acclimatize once again.
This process of acclimatization allows the body time to undertake various physiological adjustments at both the molecular and cellular levels. These alterations result in increased systemic blood flow and diuresis, causing a decrease in plasma volume and subsequent increase in red blood cell concentration. This allows increased oxygen carrying capacity per unit of blood. A secondary effect further enhances the capacity of the blood to carry oxygen due to an increased secretion of erythropoietin, which is detectable within two hours of ascent to altitude. This leads to an amplified production of red blood cells in the bone marrow. Alterations in the pulmonary and cerebral circulations also occur to facilitate oxygenation and cerebral oxygen delivery at altitude.
Prevention of all altitude illnesses requires ascent at a gradual rate allowing time for acclimatization. A general guideline is that at altitudes greater than 3000 m, one should not spend subsequent nights 300 m higher than the previous night. A rest day is recommended every 2 to 3 days. Anyone with the symptoms of AMS should not ascend until the symptoms are improved.
The clinician should evaluate patients who are seeking advice on trips to high altitude. A history of high-altitude problems, the altitude profile and the speed of ascent are all important factors. The physician should educate patients about the signs and symptoms of high-altitude illnesses and inform them of the risks assumed by the high-altitude traveler.
Acetazolamide prophylaxis Acetazolamide is the drug of choice for prophylaxis against AMS. A carbonic anhydrase inhibitor, the drug slows the hydration of carbon dioxide. Acetazolamide affects the red blood cells, kidneys, lungs and brain. Being a renal CA inhibitor, acetazolamide decreases bicarbonate reabsorption, causing diuresis. The resultant acidosis effectively stimulates the medullary respiratory center, increasing ventilation and enhancing oxygen delivery to the cells. In addition, acetazolamide inhibits CSF production and CSF pressure and also inhibits nocturnal antidiuretic hormone (ADH) secretion.
Indications for acetazolamide prophylaxis include rapid ascent (in 1 day or less) to altitudes greater than 3000 m; a rapid gain in sleeping altitude, for example, moving camp from 4000 to 5000 m; and a past history of AMS or high-altitude pulmonary edema. Side effects include paresthesias, polyuria; and less commonly, nausea, drowsiness, impotence and myopia. Because it impairs the hydration of carbon dioxide on the tongue, it allows carbon dioxide to be tasted and can ruin the flavor of carbonated beverages, including beer. It should be given with caution in patients with allergies to sulfa drugs; it is contraindicated in the rare case of allergy to acetazolamide itself.
