Combined methazolamide and theophylline improves oxygen saturation but not exercise performance or altitude illness in acute hypobaric hypoxia

Abstract

New findings: What is the central question of this study? Does the combination of methazolamide and theophylline reduce symptoms of acute mountain sickness (AMS) and improve aerobic performance in acute hypobaric hypoxia? What is the main finding and its importance? The oral combination of methazolamide (100 BID) and theophylline (300 BID) improved arterial oxygen saturation but did not reduce symptoms of AMS and impaired aerobic performance. We do not recommend this combination of drugs for prophylaxis against the acute negative effects of hypobaric hypoxia.

Abstract: A limited number of small studies have suggested that methazolamide and theophylline can independently reduce symptoms of acute mountain sickness (AMS) and, if taken together, can improve aerobic exercise performance in normobaric hypoxia. We performed a randomized, double-blind, placebo-controlled, cross-over study to determine if the combination of oral methazolamide and theophylline could provide prophylaxis against AMS and improve aerobic performance in hypobaric hypoxia (∼4875 m). Volunteers with histories of AMS were screened at low altitude (1650 m) and started combined methazolamide (100 mg BID) and theophylline (300 mg BID) treatment, or placebo, 72 h prior to decompression. Baseline AMS (Lake Louise Questionnaire), blood (haemoglobin, haematocrit), cognitive function, ventilatory and pulse oximetry ( $S_{p{O}_2}$ ) measures were assessed at low altitude and repeated between 4 and 10 h of exposure to hypobaric hypoxia (P_B = 425 mmHg). Aerobic exercise performance was assessed during a 12.5 km cycling time trial (TT) after 4 h of hypobaric hypoxia. Subjects repeated all experimental procedures after a 3-week washout period. Differences between drug and placebo trials were evaluated using repeated measures ANOVA (α = 0.05). The drugs improved resting $S_{p{O}_2}$ by ∼4% (P < 0.01), but did not affect the incidence or severity of AMS or cognitive function scores relative to placebo. Subjects' performance on the 12.5 km TT was ∼3% worse when taking the drugs (P < 0.01). The combination of methazolamide and theophylline in the prescribed dosages is not recommended for use at high altitude as it appears to have no measurable effect on AMS and can impair aerobic performance.

Keywords: acute mountain sickness; carbonic anhydrase; cognitive function; cycling time trial; methylxanthine.