Fit for high altitude: are hypoxic challenge tests useful?

Abstract

Altitude travel results in acute variations of barometric pres- sure, which induce different degrees of hypoxia, changing the gas contents in body tissues and cavities. Non ventilated air containing cavities may induce barotraumas of the lung (pneumothorax), sinuses and middle ear, with pain, vertigo and hearing loss. Commercial air planes keep their cabin pres- sure at an equivalent altitude of about 2,500 m. This leads to an increased respiratory drive which may also result in symp- toms of emotional hyperventilation. In patients with pre- existing respiratory pathology due to lung, cardiovascular, pleural, thoracic neuromuscular or obesity-related diseases (i.e. obstructive sleep apnea) an additional hypoxic stress may induce respiratory pump and/or heart failure. Clinical pre-altitude assessment must be disease-specific and it includes spirometry, pulsoximetry, ECG, pulmonary and sys- temic hypertension assessment. In patients with abnormal values we need, in addition, measurements of hemoglobin, pH, base excess, PaO2, and PaCO2 to evaluate whether O2- and CO2-transport is sufficient.

Instead of the hypoxia altitude simulation test (HAST), which is not without danger for patients with respiratory insuffi- ciency, we prefer primarily a hyperoxic challenge. The supple- mentation of normobaric O2 gives us information on the acute reversibility of the arterial hypoxemia and the reduc- tion of ventilation and pulmonary hypertension, as well as about the efficiency of the additional O2-flow needed during altitude exposure. For difficult judgements the performance of the test in a hypobaric chamber with and without supple- mental O2-breathing remains the gold standard. The increas- ing numbers of drugs to treat acute pulmonary hypertension due to altitude exposure (acetazolamide, dexamethasone, nifedipine, sildenafil) or to other etiologies (anticoagulants, prostanoids, phosphodiesterase-5-inhibitors, endothelin receptor antagonists) including mechanical aids to reduce periodical or insufficient ventilation during altitude exposure (added dead space, continuous or bilevel positive airway pressure, non-invasive ventilation) call for further randomized controlled trials of combined applications.