Continuous end-tidal CO2 sampling within the proximal endotracheal tube estimates arterial CO2 tension in infants

End-tidal CO2 (ETCO2) sampled using a 22-gauge needle inserted through the wall of the proximal endotracheal tube was compared with ETCO2 obtained from the standard proximal connector to determine which was the more accurate sampling site for estimation of arterial CO2 tension (PaCO2). Fourteen infants were anaesthetized and their lungs ventilated using a Drager ventilator and a paediatric circle system. Blood gas determination of PaCO2 was obtained from an arterial catheter and compared with continuous sampling of ETCO2 analyzed by raman spectroscopy. The PaCO2 (35.3 +/- 4.9 mmHg, x +/- SD) was not different from the ETCO2 sampled within the proximal endotracheal tube (34.7 +/- 3.8 mmHg), but was greater (P less than 0.05) than the ETCO2 at the proximal connector (31.6 +/- 4.0 mmHg). We conclude that in infants during ventilation with a circle system, the PaCO2 can be accurately assessed by continuous sampling of ETCO2 from the proximal endotracheal tube.