Elevated arterial Pco(2) (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 CO(2) and to determine the CO(2) retention trait after 1 year of active oxygen diving with closed-circuit apparatus. Ventilatory and perceptual responses to variations in inspired CO(2) (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 Pco(2) that could be detected. The mean (with SD in parentheses) end-tidal Pco(2) during exposure to an inspired Pco(2) 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 < or = 0.001]. One year of shallow oxygen diving activity with a closed-circuit apparatus does not affect the ability to detect CO(2) nor does it lead to increased CO(2) 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 CO(2) retention.
5 Figures and Tables
Table 1. Age, height, weight, vital capacity, and FEV1 in the ND and ED phases
Fig. 1. Distribution of CO2 detection threshold in the novice (ND) and experienced (ED) diving phases (n 40 subjects). N.S., no significant difference between groups; PICO2, inspired PCO2.
Fig. 2. Distribution of end-tidal PCO2 (PETCO2) at the detection point in the ND and ED phases (n 40 subjects).
Fig. 3. Frequency of the 5 CO2-related symptoms [hyperpnea (A), air hunger (B), warm sensation (C), dizziness (D), and headache (E)] and their perceived intensity (1 very slight to 5 extreme), in the ND and ED phases as a function of PICO2.
Fig. 4. Distribution of CO2 retention (PETCO2 at an inspired CO2 of 5.6 kPa, 42 Torr) in the ND and ED phases (n 40 subjects; P 0.01).
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