Arterial oxygen tension of patients with abnormal lungs treated with hyperbaric oxygen is greater than predicted

The arterial oxygen (O2) tension (PaO2) of patients with normal gas exchange treated with hyperbaric oxygen (HBO2) can be predicted from their pre-HBO2 arterial to alveolar O2 tension ratio (a/A) which remains constant up to a PaO2 of 2,000 mm Hg. We observed that the a/A could not be used to predict the PaO2 of patients with impaired gas exchange (reduced pre-HBO2 a/As) treated with HBO2. Our study provides information about the PaO2 of patients with abnormal lungs treated with HBO2. For clinical reasons, we measured the PaO2 of 24 patients treated with HBO2. We obtained arterial blood gas values from patients with lung dysfunction (a/A < 0.75) prior to, during, and after HBO2. The pre-HBO2 a/A = 0.45 +/- 0.17 (mean +/- 1 SD). During HBO2 the a/A ranged from 0.7 to 0.8 depending on chamber pressure and returned to the pre-HBO2 baseline after HBO2. We conclude the following: (1) The hyperbaric PaO2s of patients with a/A < 0.75 is greater than expected. (2) However, the PaO2 is lower than in patients with normal lung function (a/A > 0.75). Possible explanations include improvement in ventilation/perfusion matching, reduction of venous admixture, and/or extra-alveolar uptake of O2. (3) Exposures to HBO2 treatment pressures greater than recommended by existing protocols may be required in patients with impaired transfer of O2 across the lung to achieve PaO2s similar to patients with normal lung function treated with HBO2.