IS DISTAL SAMPLING OF END-TIDAL CO2 NECESSARY IN SMALL SUBJECTS

The authors compared Pa(CO2) with end-tidal CO2 (ET(CO2)) 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 ventilation 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 ET(CO2). A 0.75-mm ID polyethylene tube inserted in the side of the ETT sampled ET(CO2) at the distal tip and at the 6-, 12-, and 15-cm marks on the ETT. ET(CO2) was also measured at the standard proximal connector. The differences (P < 0.05) between Pa(CO2) and ET(CO2) 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 mmHg (x̄ ± SEM), respectively, and did not change with FGF or mode of ventilation. The difference between Pa(CO2) and ET(CO2) 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 mmHg) and at higher FGF (19.4 ± 1.3 versus 16.8 ± 1.6 mmHg). The differences (P < 0.05) between ET(CO2) at the distal tip and ET(CO2) 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 ET(CO2) 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 ET(CO2) 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.