Response to CO2 in novice closed-circuit apparatus divers and after 1 year of active oxygen diving at shallow depths

Abstract

Elevated arterial Pco2 (hypercapnia) has a major effect on central nervous system oxygen toxicity in diving with a closed-circuit breathing apparatus. The purpose of the present study was to follow up the ability of divers to detect CO2 and to determine the CO2 retention trait after 1 year of active oxygen diving with closed-circuit apparatus. Ventilatory and perceptual responses to variations in inspired CO2 (range: 0–5.6 kPa, 0–42 Torr) during moderate exercise were assessed in Israeli Navy combat divers on active duty. Tests were carried out on 40 divers during the novice oxygen diving phase (ND) and the experienced oxygen diving phase. No significant changes were found between the two phases for the minimal mean inspired Pco2 that could be detected. The mean (with SD in parentheses) end-tidal Pco2 during exposure to an inspired Pco2 of 5.6 kPa (42 Torr) was significantly higher in the novice diving phase than in the experienced diving phase [8.1 kPa (SD 0.7), 62 Torr (SD 5) and 7.8 kPa (SD 0.6), 59 Torr (SD 4), respectively; P ≤ 0.001]. One year of shallow oxygen diving activity with a closed-circuit apparatus does not affect the ability to detect CO2 nor does it lead to increased CO2 retention; rather, it may even bring about a decrease in this trait. This finding suggests that acquiring experience in oxygen diving with a closed-circuit apparatus at shallow depths does not place the diver at a greater risk of central nervous system oxygen toxicity due to CO2 retention.