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.
Résumé
Chez l’enfant ventilé, la mesure du CO2 télé-expiratoire avec un degré raisonnable de précision par le site le plus proximal du circuit Mapleson D demeure controversée. L’utilisation d’un grand débit de gaz frais près du site de l’échantillonnage dilue le gaz expiré et sous-estime le CO2 télé-expiratoire. Pour l’étude, on utilise en laboratoire un montage permettant d’identifier qualitativement et quantitativement l’endroit le plus proximal du circuit Mapleson D où la mesure du CO2 télé-expiratoire n’est pas influencée par la dilution avec les gaz frais. Un débit de gaz frais variant entre 2 et 15 L · min−1 associé à une fréquence ventilatoire de 20 à 30 · min−1 et à un volume courant de 30 à 100 ml · min−1 est évalué. L’expérience est divisée en deux parties. D’abord, le montage est utilisé pour visualiser le site du mélange entre gaz frais et le gaz expiré marqué avec de la fumée. Ensuite, le débit de gaz frais et le débit de gaz expiré sont contrôlés et la concentration télé-expiratoire de CO2 est mesurée sur la longueur du circuit anesthésique pour identifier le site de mélange du gaz frais et du gaz expiré. Trois débits de gaz expiré sont étudiés avec six débits de gaz frais. Pendant toutes ces études, la vitesse du débit des gaz frais et du gaz expiré influence le site du mélange et le degré de dilution mais on n’observe pas de mélange à un site plus distal qu’au point où la canule endotrachéale se rétrécit au même diamètre que son raccord (P < 0,05). Cette étude en laboratoire permet de suggérer que le site le plus proximal et le plus précis pour évaluer le CO2 télé-expiratoire chez l’enfant pendant la ventilation avec un circuit Mapleson D est le point où le raccord du tube endotrachéal se rétrécit pour sa jonction avec le tube endotrachéal.
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Halpern, L., Bissonnette, B. The most proximal and accurate site for sampling end-tidal CO2 in infants. Can J Anaesth 41, 984–990 (1994). https://doi.org/10.1007/BF03010942
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DOI: https://doi.org/10.1007/BF03010942