Rebreathing and end-tidal CO2 during spontaneous breathing with the Bain circuit

Abstract

In order to examine the relationship between end-tidal CO2 (Fetco2) and inspired CO2 (Fico2) in anaesthetized patients breathing spontaneously with a Bain breathing circuit and afresh gasvolume (\(\dot VF\)) of 100 ml · kg-1 · min-1, the respiratory rate (f) and minute ventilation (\(\dot VE\)) was changed in two groups of six patients each by the induction or reversal of narcotic respiratory depression. During light nitrous oxide-halothane anaesthesia (Group I), the intravenous injection of 0.1 mg · kg-1 of alphaprodine caused arapid fall in Fco2 from 2.3 ± 0.5 per cent to 0.7 ± 0.1 per cent concomitant with the reduction inf(37 ± 5 to 16 ± 4) breath -min-1 and\(\dot VE\) (137 ± 29 to 55 ± 13 ml · kg-1 · min-1), while the Fetco2 rose gradually from 5.2 ± 0.9 percent to 6.4 ± 0.9 per cent over a ten-minute period. During light nitrous oxide-halothane anaesthesia supplemented by alphaprodine (Group II), 0.2 mg of naloxone intravenously caused a rise in Fico2 from 0.5 ± 0.3 per cent to 2.9 ± 0.6 per cent simultaneous with arise in f (11 ± 2 to 25 ± 7 breath · min-1) and\(\dot VE\) (70 ± 25 to 133 ± 34 ml · kg-1 · min-1), while the FETco2 declined gradually over a ten-minute period from 7.6 ± 0.7 per cent to 6.4 ± 0.4 per cent.