[Monitoring airway gas in pediatric anesthesia: an experimental model for endotracheal gas measurement].

Background: We designed an endotracheal probe for measuring inspired and expired gas fractions during pediatric general anesthesia.

Objective: To compare the gas fractions measured by means of intratracheal and extratracheal monitoring.

Material And Methods: The study included ASA 1 patients between the ages of 7 and 12 years under inhaled anesthesia with mechanical ventilation. The following parameters were recorded inside and outside the trachea: inspired and expired oxygen, nitric oxide (N2O) and sevoflurane fractions; the expired and inspired fraction gradients; PaCO2; and end-tidal carbon dioxide (ETCO2). Measurements were taken by an airflow sensor (Pedi-Lite) in the circuit before the point of connection to the endotracheal tube and by an intratracheal probe placed between the tube and the carina. Both sensors were connected to the same monitor. Measurements were taken on intubation and 5, 10, 15, 20, 30, 40, 50, and 60 minutes thereafter. PaCO2 was recorded at the same time. The recorded values were analyzed using the t test and the Pearson product moment correlation coefficient (r), and regression models were constructed using analysis of variance.

Results: Seventy-one patients were enrolled in the study. The mean difference (SD) ETCO2 was 5 (3) mm Hg higher according to endotracheal measurement (P < .005), and that measurement was almost identical (+/-13 mm Hg) to the PaCO2 (P < or = .5). The inspired/expired gradients of endotracheal measurement of oxygen and N2O were 3 (2) points higher (P < .05) than the gradients of extratracheal measurements. In the case of sevoflurane gradients, however, the extratracheal values were higher (mean difference, 0.6 [0.2] points, P < .05). The inspired/expired oxygen and N2O gradients became equal after 18 (3) minutes; the sevoflurane gradients became equal after 8 (2) minutes.

Conclusions: Intratracheal and extratracheal measurements of the inspired and expired fractions of mixed gases provide different results.