[abstract] INFLUENCE OF ELEVATED OXYGEN (O2) PARTIAL PRESSURE ON CARBON DIOXIDE (CO2) NARCOSIS

Abstract

Background: Divers risk CO2 retention from inadequate ventilation or inspired CO2. CO2 is narcotic and affects consciousness. Previous studies suggested that hyperoxia reduced hypercapnic symptoms (Donald. AEDU Report XVI, 1945). We investigated O2 and CO2 effects on cognitive performance and discomfort. Methods: Twelve males (37±11 [28-59] years [mean±SD, range]) breathed four gases: A (0.21atm O2/0.0atm CO2), B (0.21atm O2/high CO2), C (1.3atm O2/0.0atm CO2), and D (1.3atm O2/high CO2) for 4 min at 1.45 ata. ÒHigh CO2Ó was defined for each subject as the maximum inspired CO2 tolerable for 4 min among gases presented in order 0.055, 0.065, 0.075, or 0.085atm CO2. Gas A was paired with gases B, C, and D in a Latin Square design of nine experimental pairs. Experiments were conducted at rest and with 75watts dry, cycling exercise. Cognitive function was assessed with an n-back auditory test. Subjects reported a rating of perceived discomfort (RPD; 0-10). Minute ventilation (VE) and end-tidal CO2 (PETCO2) were recorded. Measurements were assessed by multiple linear regression with repeated measures. Results: Tolerable resting CO2 ranged from 0.065 to 0.085atm. Exercise measurements were comparable, but exercise often lowered CO2 tolerance by 0.01atm. Four subjects who breathed gas C had tunnel vision, dizziness, panic, exhaustion, or near-syncope (2 rest, PETCO2=57.4, 61.3 mmHg; 2 exercise, PETCO2=51.3, 74.8 mmHg). A-Normoxia/ B-Hyperoxia/ C-Normoxia/ D-Hyperoxia/ Gas Normocapnia Normocapnia Hypercapnia Hypercapnia n-back (%correct) 84.6±7.0 84.4±6.5 70.2±10.7* 73.5±9.9@ RPD 0.4±0.8 0.6±1.0 4.4±1.6* 3.9±1.3@ PETCO2 (mmHg) 36.3±3.4 31.8±3.6# 60.4±5.1* 57.4±5.2@ VE (L/min BTPS) 11.9±5.4 14.3±6.1# 60.5±17.1* 59.5±18.9 p<0.05 for #-A vs. B, @-C vs. D, *-A vs. C Conclusions: Hyperoxia reduced perceived discomfort slightly in subjects breathing hypercapnic gas for 4 min and slightly improved n-back scores. PETCO2 decreased but hyperventilation was unaffected. Severe symptoms were most commonly associated with normoxic/hypercapnic gas. (Supported by Divers Alert Network.)