Effects on pulmonary function of daily exposure to dry or humidified hyperbaric oxygen

Airway reactivity and diving in healthy and atopic subjects

BACKGROUND

The short- and long-term effects of self-contained underwater breathing apparatus (SCUBA) dives on airway responsiveness in nonasthmatic atopic subjects have not been systematically investigated.

PURPOSE

To compare the effect of SCUBA diving at 50-m depth on lung function and airway responsiveness to methacholine (MCh) in atopic nonasthmatics and healthy subjects.

METHODS

We studied 15 atopic nonasthmatic subjects and 15 controls who underwent the visit for the professional SCUBA-diving license at the Navy Medical Center, La Spezia, Italy. All subjects underwent spirometry and skin-prick test for common environmental allergens. MCh challenge was performed 24 h before, and 20 min and 24 h after a standardized SCUBA-dive test and after hyperbaric-chamber test.

RESULTS

At 20 min, the provocative dose of MCh causing 20% fall of the forced expiratory volume at the first second (MCh PD20 - FEV1) was significantly reduced in atopic, asymptomatic subjects from 1712 x 2.6 microg (mean x geometric standard deviation) to 1202 x 2.2 microg (P < 0.0005) after the hyperbaric-chamber test and to 1204 x 2.3 microg (P < 0.005) after SCUBA diving. In healthy subjects, the baseline value of MCh PD20 was 2977 x 1.1 microg, and this value did not change significantly after the hyperbaric-chamber test (2575 x 1.4 microg) and after SCUBA dives (2553 x 1.4 microg, P > 0.1 for both comparisons). In atopic subjects, the MCh PD20 returned near to the baseline value 24 h after the hyperbaric-chamber test (1776 x 2.4 microg) and after the SCUBA test (1500 x 2.67 microg). No significant change in FEV1 was observed after the tests in both groups.

CONCLUSION

SCUBA diving is associated with development of early airway hyperresponsiveness to MCh in atopic subjects.

Humidity does not affect central nervous system oxygen toxicity

Central nervous system (CNS) oxygen toxicity can occur as convulsions and loss of consciousness when hyperbaric oxygen is breathed in diving and hyperbaric medical therapy. Lin and Jamieson (J Appl Physiol 75: 1980-1983, 1993) reported that humidity in the inspired gas enhances CNS oxygen toxicity. Because alveolar gas is fully saturated with water vapor, we could not see a cause and effect and surmised that other factors, such as metabolic rate, might be involved. Rats were exposed to 507- and 608-kPa O(2) in dry (31 or 14%) or humid (99%) atmosphere until the appearance of the first electrical discharge preceding the clinical convulsions. Each rat served as its own control. A thermoneutral temperature (28 +/- 0.4 degrees C) yielded resting CO(2) production of 0.81 +/- 0.06 ml x g(-1) x h(-1). Latency to the first electrical discharge was not affected by humidity. At 507-kPa O(2), latency was 23 +/- 0.4 and 22 +/- 0.7 min in dry and humid conditions, respectively, and, at 608-kPa O(2), latency was 15 +/- 4 and 14 +/- 3 min in dry and humid conditions, respectively. When no effects of CO(2) and metabolic rate are present, humidity does not affect CNS oxygen toxicity. Relevance of the findings to diving and hyperbaric therapy is discussed.