Training improves divers' ability to detect increased CO₂

Background: Elevated arterial PCO₂ (hypercapnia) is a known risk in diving with closed circuit breathing apparatus. In a retrospective study, we determined CO₂ retention and the ability to detect CO₂ in novice divers who were either CO₂-recognition-trained subjects (TS) or untrained subjects (UTS). Methods: Ventilatory and perceptual responses to variations in inspired CO₂ (range 0-5.6 kPa, 0-42 mm Hg) during moderate exercise were assessed in novice Israeli Navy divers on active duty. Tests were carried out on 231 TS and 213 UTS. Results: The minimal mean inspired PCO₂ that could be detected was 4.8 ± 1.6 kPa (36 ± 12 mm Hg) in UTS and 2.9 ± 0.7 kPa (22 ± 5 mm Hg) in TS (p < 0.0001). No significant changes were found in PETCO₂ between the two groups during exposure to a PICO₂ of 5.6 kPa (42 mm Hg). There were 46 TS who were found to be CO₂ retainers (more than +1 SD above the mean) and 19 were classified as poor detectors (more than +1 SD above the mean). Seven subjects exhibited both traits. During actual oxygen diving performed later by this group, the only four cases of CNS-oxygen toxicity were among those seven subjects (p < 0.01). Conclusions: We conclude that CO₂ recognition training improves the diver's capability to detect CO₂. We suggest that a diver who is both a poor CO₂ detector and a CO₂ retainer will be prone to CNS-oxygen toxicity.