Altitude illnessBMJ 2003; 326 doi: https://doi.org/10.1136/bmj.326.7395.915 (Published 26 April 2003) Cite this as: BMJ 2003;326:915
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The risks of developing altitude related illness can be minimised by slowly acclimatising to altitude. Exposed to hypobaric hypoxia, the body makes a series of adjustments that serve to increase oxygen delivery to cells. An early change, which may be detected at an altitude of around 1500 metres, is an increase in the rate and depth of ventilation, preserving alveolar partial pressure of oxygen. A consequence of this increased ventilation is hypocapnia and respiratory alkalosis, which limits further increases in ventilation. One effect of acclimatisation is to increase the hypercapnic ventilatory response, allowing an increase in minute ventilation for a given arterial carbon dioxide level. With time the kidneys will start excreting bicarbonate, and arterial pH returns to near normal, allowing additional hyperventilation. The overall effect of this in acclimatised individuals is to increase the ventilatory response to hypoxia and thus to increase the amount of oxygen delivered to the alveoli over a given time.
The heart rate also increases early on exposure to hypobaric hypoxia, increasing cardiac output. Oxygen delivery to the tissues is enhanced by a rise in the packed cell volume, initially as a consequence of haemoconcentration and latterly due to a rise in red cell mass.
Interest in methods that allow a degree of acclimatisation to be achieved before travelling to altitude is increasing. Intermittent exposure to hypoxia at sea level—for example, by breathing hypoxic gas mixes or sleeping in a hypoxic environment—lead to several physiological changes, but there are no studies to show a beneficial effect on subsequent acclimatisation in the mountains. Spending some time in a hypobaric chamber before travelling will lead to acclimatisation, although possibly less than that achieved by an equivalent period in the mountains.1 If acclimatisation is attempted before the trip, the benefits fall off quickly with time spent at sea level, with most benefits lost after two to three weeks.
Chronic mountain sickness
This is an illness of altitude residents that is characterised by excessive polycythaemia, pulmonary hypertension and right heart failure. It is more common in south American than central Asian populations, although it occurs in high altitude regions throughout the world.2 The incidence in parts of south America may be as high as 33% of men aged 60-69 years. It is much lower in younger men, in women, and in other high altitude areas of the world. Symptoms may be vague and include dizziness, headache, somnolence, and poor concentration. Exercise tolerance may be reduced. Sufferers appear markedly cyanosed compared with their peers, and oxygen saturation is reduced more than would be expected at a given altitude. The haemoglobin concentration and packed cell volume are markedly raised; values as high as 280 g/l and 83% have been recorded. This causes a rise in blood viscosity, which compromises cerebral blood flow. Along with altitude related hypoxia, this compromises cerebral oxygenation, explaining some of the symptoms of chronic mountain sickness.
The cause of chronic mountain sickness is not clear, but the additive stimuli of altitude related hypoxia and hypoxia secondary to chronic lung disease, obstructive sleep apnoea, or poor ventilatory drive on the haematopoietic system may be important. The syndrome is reversed by descent to lower altitudes, but in the case of high altitude residents, this may not be possible. Venesection and respiratory stimulants have also been suggested for the treatment of chronic mountain sickness.
Pre-existing medical conditions and altitude illness
The risk of ischaemic heart disease in previously well trekkers is not increased. A recent myocardial infarction is a contraindication to altitude travel, but if the infarct is small, there have been no symptoms for some months, and if modest exercise at sea level is tolerated, additional risk in going to moderate altitude is probably small. The same advice applies to patients who have had successful coronary artery bypass surgery (without myocardial infarction) and have good exercise tolerance. However, angina of effort at sea level is likely to worsen at altitude and ascent to moderate altitude may precipitate angina in patients with previously stable coronary artery disease.
Acute hypoxia leads to an increase in systemic blood pressure at rest and at exercise. However, well controlled hypertension is not a contraindication to altitude travel.
There is no benefit in undertaking tests such as an electrocardiogram to predict potential problems at altitude. Echocardiography while the patient is breathing a hypoxic gas mixture will identify those with a brisk brisk hypoxic pulmonary vascular response, but this test is not very discriminatory for the development of altitude illness.
Asthma is generally unaffected by altitude travel, possibly because of the reduction in inhaled allergens or because of the improved flow characteristics of hypobaric gases. There is no evidence that people with asthma are at greater risk of altitude illness than people without asthma.
Sea level symptoms of chronic obstructive airways disease will be worse at altitude, and performance will deteriorate. Infectious exacerbations are a risk at altitude, so appropriate antibiotics should be carried and started early
Patients with interstitial lung disease such as cystic fibrosis are at high risk of deterioration on travelling to altitude. The measurement of oxygen saturations while breathing hypoxic gas mixtures at sea level has been suggested as a test to detect those who will be intolerant of hypobaric hypoxia experienced in a commercial aircraft flight.3
In itself, exposure to altitude does not worsen diabetes, although the increased physical activity often associated with altitude trips and the variations from normal diet may upset glycaemic control. Symptoms of hypoglycaemia may be confused with high altitude cerebral oedema. Diabetic patients should have ready access to glucose supplements, and their companions should be aware of the symptoms and management of hypoglycaemia.
Altitude per se does not increase the risks of seizures in someone whose epilepsy is well controlled, but clearly the consequences of an epileptic seizure may be more severe in a remote mountain area, and more so if the individual is engaged in a hazardous pursuit such as rock or ice climbing.
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