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Canadian Journal of Anesthesia, Vol 30, 148-154, Copyright © 1983 by Canadian Anesthesiologists' Society

Rebreathing and End-Tidal CO2 during Spontaneous Breathing with the Bain Circuit

W. E. SPOEREL MD FRCP(C)1

1 Department of Anaesthesia, University of Western Ontario and University Hospital, London, Ontario

Address Correspondence to: Dr. W.E. Spoerel, Department of Anaesthesia, University of Western Ontario, P.O. Box 5339, Postal Station "A," London, Ontario, N6A 5A5.

In order to examine the relationship between end-tidal CO2 (FETCOCO2) and inspired CO2 (FICOCO2) in anaesthetized patients breathing spontaneously with a Bain breathing circuit and a fresh gas volume (VF) of 100 ml.kg-1.min-1, the respiratory rate (f) and minute ventilation (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 a rapid fall in FICOCO2 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 VE (137 ± 29 to 55 ±13 ml.kg-1. min-1), while the FETCOCO2 rose gradually from 5.2 ± 0.9 per cent 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 causeda rise in FICOCO2from 0.5 ± 0.3 per cent to 2.9 ± 0.6 per cent simultaneous with a rise in f (11 ±2 to 25 ± 7 breath.min-1]) and VE (70 ± 25 to 133 ± 34ml.kg-1.min-1), while the FETCOCO2 declined gradually over a ten-minute period from 7.6 ± 0.7 per cent to 6.4 ± 0.4 per cent.

The change in FICOCO2 always occurred exactly at the same time as the drug-induced change in respiration. It was associated with a corresponding change in the degree of mixing of fresh gas and expired gas within the breathing system and appeared to correlate with the change in the ratio VE/VF. There was no indication that the FICOCO2 or the distribution of CO2 within the system had any effect on FETCOCO2 or CO2 elimination. Under these conditions the FICOCO2 and the volume of rebreathed CO2 can not be the cause but must be regarded as a passive change consequent to the altered pattern of breathing.

Key Words: ANAESTHETIC TECHNIQUES, NON REBREATHING: Bain circuit






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Copyright © 1983 by the Canadian Anesthesiologists' Society.