The most proximal and accurate site for sampling end-tidal CO2 in infants

Abstract

The most proximal site to sample end-tidal CO2 with reasonable accuracy in infants during pulmonary ventilation using a Mapleson D circuit remains controversial. The utilisation of high fresh gas flow near the site of gas sampling dilutes the expired gas and causes an underestimation of end-tidal CO2. In this study a laboratory model was used to identify, qualitatively and quantitatively, the most proximal site in the Mapleson D circuit where the measurement of end-tidal CO2 is not influenced by mixing with fresh gas. A fresh gas flow rate of between 2 and 15 L · min-1 with a respiratory rate of 20-30 · min-1 and a tidal volume of 30-100 ml · min-1 was evaluated. This experiment was divided into two parts. Firstly, an infant lung model was used to visualize the site of mixing between fresh gas and smoke-labelled exhaled gas. Secondly, fresh gas flow and expired gas flow were controlled and the end-tidal CO2 concentration was measured along the length of the anaesthetic circuit to identify the site of mixing of fresh gas and expired gas during steady-state conditions. Three expired gas flows were studied at six fresh gas flows. In all our studies, the rate of fresh gas flow and expired gas flow influenced the site of mixing and degree of dilution but no mixing was observed distal to the point at which the endotracheal tube connector narrows to the diameter of the endotracheal tube (P < 0.05). This laboratory study allows us to suggest that the most proximal and acceptably accurate site to sample end-tidal Co2 in infants during ventilation with the Mapleson D circuit is at the point of narrowing of the endotracheal tube connector with the endotracheal tube. © 1994 Canadian Anesthesiologists.