Is Distal Sampling of End-tidal CO2Necessary in Small Subjects?

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Abstract
The authors compared PaCO2 with end-tidal CO2 (ETCO2) sampled at multiple sites along the endotracheal tube (ETT) in seven anesthetized rabbits (weight, 2.7-3.6 kg) to determine the most convenient, yet accurate, sampling location. Comparisons were made during spontaneous and controlled ventiliation with fresh gas flows (FGF) of two and ten times the minute ventilation using a Mapleson D circuit. An Engstrom Eliza analyzer with a continuous sampling rate of 100 ml/min was used to measure ETCO2. A 0.75-mm ID polyethylene tube inserted in the side of the ETT sampled ETCO2 at the distal tip and at the 6-, 12-, and 15-cm marks on the ETT. ETCO2 was also measured at the standard proximal connector. The differences (P < 0.05) between PaCO2 and ETCO2 at the distal, 6-, 12-, and 15-cm marks were 2.9 .+-. 0.4, 3.1 .+-. 0.4, 3.6 .+-. 0.4, and 4.6 .+-. 0.5 mm Hg .hivin.x .+-. SEM), respectively, and did not change with FGF or mode of ventilation. The difference between PaCO2 and ETCO2 measured at the proximal connector was always large but significantly (P < 0.05) greater during spontaneous than controlled ventilation (24.2 .+-. 1.5 versus 15.0 .+-. 1.4 mm Hg) and at higher FGF (19.4 .+-. 1.3 versus 16.8 .+-. 1.6 mm Hg). The differences (P < 0.05) between ETCO2 at the distal tip and ETCO2 at the 6-, 12-, and 15-cm marks were 0.24 .+-. 0.07, 0.73 .+-. 0.11, and 1.77 .+-. 0.20 mmHg, respectively. This demonstrates that the change in ETCO2 between the distal tip and the 12-cm mark on the ETT is less than 1 mmHg, and that this clinically insignificant difference is independent of FGF and mode of ventilation. The 12 cm-mark is outside of the mouth on a newborn, and sampling ETCO2 at that point, which may be accomplished simply by inserting a small needle in the side of the ETT, may be the most appropriate sampling location.