Comparison of noninvasive measurements of carbon dioxide tension during withdrawal from mechanical ventilation.

End tidal CO2 tension (PetCO2) and transcutaneous CO2 tension (PtcCO2) were compared with arterial CO2 (PaCO2) before and after withdrawal of mechanical ventilation in 20 patients predisposed to hypercarbia. With stable PaCO2 during mechanical ventilation, the correlation coefficient (r) between PaCO2 and PetCO2 was .9, and between PaCO2 and PtcCO2, .87. PtcCO2 considerably overestimated PaCO2 in three patients who were receiving dopamine. After withdrawal of mechanical ventilation, changes in PaCO2 were closely paralleled by changes in PetCO2 and PtcCO2 (r = .82 and .86, respectively). Nine of 20 patients had an increased PaCO2 of 10 torr or greater. In eight of these, PetCO2 and PtcCO2 rose by at least 5 torr, and in seven, the rise in PetCO2 and PtcCO2 was within 5 torr of the rise in PaCO2. During mechanical ventilation, PetCO2 and PtcCO2 estimated stable PaCO2 with sufficient accuracy for clinical use, except in patients with cutaneous vasoconstriction. After withdrawal of mechanical ventilation, changes in PetCO2 and PtcCO2 were predictive of important PaCO2 increases, warranting continued exploration and evaluation as to their use in monitoring patients predisposed to hypercarbia.