High-altitude illnesses
High-altitude illnesses have profound consequences on the health of many unsuspecting and otherwise healthy individuals who sojourn to high altitude for recreation. The clinical manifestations of high-altitude illnesses are secondary to the extravasation of fluid from the intravascular to extravascular space, especially in the brain and lungs. The most common of these illnesses, which can present as low as 2,000 m, is acute mountain sickness, which is usually self-limited but can progress to the more severe and potentially fatal entities of high-altitude cerebral edema and high-altitude pulmonary edema. This article will briefly review normal adaptation to high altitude and then more extensive reviews of the clinical presentations, prevention, and treatments of these potentially fatal conditions. Research on the mechanisms of these conditions will also be reviewed. A better understanding of these disorders by practitioners will lead to improved prevention and rational treatment for the increasing number of people visiting high-altitude areas around the globe. There will not be space for writing about high-altitude residents, medical conditions in low-altitude residents going to high altitude, or training for athletes at high altitude. These topics deserve another article.
Safety of air travel in patients with lung diseases
Commercial air travel remains popular despite escalating oil prices, international security and environmental concerns. The number of commercial aircraft passengers with respiratory disease is unknown, but in 1974, 5% were described as "ambulatory patients". As populations age, the proportion of passengers with pre-existing morbidity is likely to rise. Longer flights further increase the risk of in-flight medical emergencies.
There are no established methods for quantifying morbidity associated with air travel. However, major airlines consistently report ∼10% of in-flight medical emergencies resulting from respiratory conditions (West, 2007). Medaire, a USA company providing radio-link emergency medical assistance to major commercial airlines, cites respiratory conditions as the third most common cause of in-flight medical emergencies. Respiratory problems are also the third most common reason for medical diversion, with the most common being cardiac and neurological conditions (including syncope).
Commercial aircraft fly at ∼38,000 ft, but are pressurised to a cabin altitude of 8,000 ft (2,438 m). Variations in cabin altitude, up to 2,717 m, have been reported (Bisgard, 1996). The reduced alveolar oxygen partial pressure (PAO2) at this altitude equates to breathing 15% oxygen and lowers the PAO2 of a healthy passenger to 53–64 mmHg. The effect, limited by the shape of the hemoglobin dissociation curve, usually goes unnoticed. However, this exposure may have a profound effect on those with lung disease, especially if they are hypoxemic at sea level, because the steeper part of the dissociation curve is involved. Other consequences of air travel include immobility, predisposing to venous thrombosis, increased gas volumes, lowered humidity and potential for transmission of infection.
A careful preflight medical evaluation is essential to determine which patients with pulmonary disease can fly safely, which patients require supplemental oxygen, and which patients should not fly at all. All adults with pulmonary disease who have a preflight arterial oxygen tension of less than 70 mmHg or a preflight pulse oximetry saturation of less than 92% should receive supplemental oxygen during air travel. The hypoxia altitude simulation test and the 6-minute walk test are useful when additional evaluation for supplemental in-flight oxygen is needed. Patients with an unstable condition, an acute exacerbation of their pulmonary disease, severe pulmonary hypertension (Class III and Class IV), or an active pneumothorax should not fly.
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