Physiological Risk Factors for Severe High-Altitude Illness

Clinician's corner: What do we know about safe ascent rates at high altitude?

Although pharmacologic strategies are available for decreasing the risk of acute altitude illness, the best means of preventing these problems remains undertaking an adequately slow ascent. Guidelines regarding appropriate ascent rates have been published in various forums, and while these guidelines are generally similar to each other in regards to the recommended ascent rates and use of rest days, there is actually little evidence in the literature supporting the particular recommendations. The purpose of this review is to consider these guidelines and the issue of ascent rates in greater detail. Following a discussion of the evidence regarding ascent rates and acclimatization, the review considers several unanswered questions regarding the current guidelines, including the applicability of the guidelines for all altitude travelers, how best to determine the ascent rate, how to implement rest days, and whether pre-acclimatization strategies can be used to facilitate faster than recommended ascents. Given the current state of evidence, there is no reason to alter the current guidelines, as they likely work for the substantial majority of high altitude travelers. It is individuals traveling to high altitude for the first time for whom they remain most important, while those individuals with substantial prior experience at high altitude may opt for faster or slower ascent rates based on their prior experience. Rest days should remain a part of any ascent profile and should be used following any large gains in elevation rather than simply at specified time intervals. Pre-acclimatization strategies may decrease the risk of acute altitude illness but there is insufficient evidence to suggest they can be used to facilitate faster than recommended ascents. Further research may allow changes in practice in the future but for the time being, adherence to the current recommendations is the prudent approach for the majority of high altitude travelers.

The effect of hypoxemia and exercise on acute mountain sickness symptoms

Performing exercise during the first hours of hypoxic exposure is thought to exacerbate acute mountain sickness (AMS), but whether this is due to increased hypoxemia or other mechanisms associated with exercise remains unclear. In 12 healthy men, AMS symptoms were assessed during three 11-h experimental sessions: 1) in Hypoxia-exercise, inspiratory O(2) fraction (Fi(O(2))) was 0.12, and subjects performed 4-h cycling at 45% Fi(O(2))-specific maximal power output from the 4th to the 8th hour; 2) in Hypoxia-rest, Fi(O(2)) was continuously adjusted to match the same arterial oxygen saturation as in Hypoxia-exercise, and subjects remained at rest; and 3) in Normoxia-exercise, Fi(O(2)) was 0.21, and subjects cycled as in Hypoxia-exercise at 45% Fi(O(2))-specific maximal power output. AMS scores did not differ significantly between Hypoxia-exercise and Hypoxia-rest, while they were significantly lower in Normoxia-exercise (Lake Louise score: 5.5 ± 2.1, 4.4 ± 2.4, and 2.3 ± 1.5, and cerebral Environmental Symptom Questionnaire: 1.2 ± 0.7, 1.0 ± 1.0, and 0.3 ± 0.4, in Hypoxia-exercise, Hypoxia-rest, and Normoxia-exercise, respectively; P < 0.01). Headache scored by visual analog scale was higher in Hypoxia-exercise and Hypoxia-rest compared with Normoxia-exercise (36 ± 22, 35 ± 25, and 5 ± 6, P < 0.001), while the perception of fatigue was higher in Hypoxia-exercise compared with Hypoxia-rest (60 ± 24, 32 ± 22, and 46 ± 23, in Hypoxia-exercise, Hypoxia-rest, and Normoxia-exercise, respectively; P < 0.01). Despite significant physiological stress during hypoxic exercise and some AMS symptoms induced by normoxic cycling at similar relative workload, exercise does not significantly worsen AMS severity during the first hours of hypoxic exposure at a given arterial oxygen desaturation. Hypoxemia per se appears, therefore, to be the main mechanism underlying AMS, whether or not exercise is performed.

Ageing and cardiorespiratory response to hypoxia

The risk of severe altitude-induced diseases is related to ventilatory and cardiac responses to hypoxia and is dependent on sex, age and exercise training status. However, it remains unclear how ageing modifies these physiological adaptations to hypoxia. We assessed the physiological responses to hypoxia with ageing through a cross-sectional 20 year study including 4675 subjects (2789 men, 1886 women; 14-85 years old) and a longitudinal study including 30 subjects explored at a mean 10.4 year interval. The influence of sex, training status and menopause was evaluated. The hypoxia-induced desaturation and the ventilatory and cardiac responses to hypoxia at rest and exercise were measured. In men, ventilatory response to hypoxia increased (P < 0.002), while desaturation was less pronounced (P < 0.001) with ageing. Cardiac response to hypoxia was blunted with ageing in both sexes (P < 0.001). Similar results were found in the longitudinal study, with a decrease in cardiac and an increase in ventilatory response to hypoxia with ageing. These adaptive responses were less pronounced or absent in post-menopausal women (P < 0.01). At exercise, desaturation was greater in trained subjects but cardiac and ventilatory responses to hypoxia were preserved by training, especially in elderly people. In conclusion, respiratory response to hypoxia and blood oxygenation improve with ageing in men while cardiac response is blunted with ageing in both sexes. Training aggravates desaturation at exercise in hypoxia, improves the ventilatory response and limits the ageing-induced blunting of cardiac response to hypoxia. Training limits the negative effects of menopause in cardiorespiratory adaptations to hypoxia.

[Altitude and the cardiovascular system]

A stay at high altitude exposes an individual to various environmental changes (cold, exercise, isolation) but the most stressful for the body is hypoxia. However, the cardiovascular system yields some efficient mechanisms of acclimatization to oxygen lack. Hypoxia activates the adrenergic system and induces a tachycardia that decreases during a prolonged stay at altitude. The desensitization of the adrenergic system leads to a decrease in maximal heart rate and a protection of the myocardium against an energy disequilibrium that could be potentially harmful for the heart. Hypoxia induces a peripheral vasodilation and a pulmonary vasoconstriction, leading to few changes in systemic blood pressure and an increase in pulmonary blood pressure (PHT) that can contribute to a high altitude pulmonary edema. Advice to a cardiac patient who plans to go to high altitude should take into account that all diseases aggravated by increased adrenergic activity or associated with a PHT or a hypoxemia (right-to-left shunt) will be aggravated at high altitude. As altitude increases, a patient with a coronary disease will present an ischemic threshold for a lower power output during an EKG exercise test. The only test allowing predicting the tolerance to high altitude is the hypoxia exercise test realized at 30% of maxVO(2)and at an equivalent altitude of 4,800m.