Carbon dioxide output in anaesthesia

Abstract

In a Mapleson D circuit the carbon dioxide content of gases, sampled at the breathing bag or near the bellows of the ventilator, is virtually constant throughout the phases of respiration. Assuming that after induction of anaesthesia the fresh gas inflow, if kept constant, is essentially equal in volume to the gas vented at the expiratory valve, CO2 output can be calculated by multiplying the fresh gas inflow by the CO2 content of the vented gas measured with a suitable CO2 analyzer. Anaesthesia with nitrous oxide-oxygen, supplemented with low doses of alphaprodine or halothane was compared in two groups of young patients who underwent dental surgery and who were breathing spontaneously. While the CO2 output in the group supplemented with alphaprodine increased from about 100 to 130 ml/m2/min, the halothane group showed a constant CO2 output of about 90 ml/m2/min followed by a significant rise within 5 minutes after halothane was discontinued. In 42 patients on controlled ventilation, no significant difference was found in the CO2 output estimated one hour after induction of anaesthesia in nitrous oxide-oxygen anaesthesia supplemented by halothane, ethrane or alphaprodine. The values obtained were 87 +/- 11 ml/m2/min for halothane (11 patients), 98 +/- 19 ml/m2/min for ethrane (14) and 93 +/- 13 ml/m2/min for the narcotic supplemented anaesthesia (17). The mean CO2 output for all 42 patients was 93 +/- 14 ml/m2/min. Six markedly obese patients under the same anaesthetic technique had a CO2 output of 114 +/- 17 ml/m2/min; however, their CO2 output was similar to normal patients when calculated on the basis of body weight. A marked increase in CO2 output to a mean of 160 +/- 25 ml/m2/min was found in eight patients undergoing operation while on hyperalimentation. The technique described appears suitable to monitor CO2 output under anaesthesia. In order to avoid hypercarbia when using a partial rebreathing system, the fresh gas inflow must be increased above recommended values in cases with increased metabolic activity (e.g. patients receiving hyperalimentation). In obese patients the fresh gas inflow should be calculated on the basis of body weight.