Continuous and simultaneous on-line measurements of VO2 and VCO2 during endotracheal anesthesia

A single method was developed for the continuous measurement of oxygen consumption (VO2) and carbon dioxide production (VCO2) during endotracheal anesthesia. The total vented flow from a Bain circuit was measured during anesthesia in 28 adult patients and was found to attain at least 97% of a 6-L/min fresh gas inflow (FGI). Therefore, the circuit overflow was assumed to approximate the FGI. Total volume of vented oxygen was calculated by multiplying FGI by oxygen concentration in the vented (mixed expired) gas. VO2 was assessed as the difference between the oxygen volumes entering and leaving the circuit. VCO2 was derived from the product of FGI and mixed expired CO2 concentration. VO2 measured on the Bain circuit correlated with measurements made on a non-breathing circuit (n = 60 pairs, r = 0.99). A good correlation was also found between VCO2 obtained on both circuits (n = 60 pairs, r = 0.99).