Estimation of arterial carbon dioxide by end-tidal and transcutaneous PCO2 measurements in ventilated children.

Simultaneous measurements of arterial, end-tidal, and transcutaneous carbon dioxide (PaCO2, PetCO2, PtcCO2, respectively) were obtained in 134 children receiving mechanical ventilation (ages, 2 days to 16 years; mean, 2.5 years). The mean +/- SD PetCO2 bias (PaCO2 - PetCO2) was 3.4 +/- 6.6 mmHg. When the PetCO2 bias was plotted against the PaO2/PAO2 ratio, a change in the scatter was obvious at about 0.3. The PetCO2 bias for patients with PaO2/PAO2 under 0.3 was 7.8 +/- 7.3 mmHg compared to 0 +/- 3.4 in patients with PaO2/PAO2 above 0.3 (P less than 0.001). PetCO2 differed significantly from PaCO2 (P less than 0.001) only for patients with PaO2/PAO2 under 0.3. The slope (PaCO2 versus PetCO2) for these patients was 1.59, while the slope for patients with PaO2/PAO2 above 0.3 coincided with the line of identity (1.00). The mean +/- SD PtcCO2 bias (PaCO2 - PtcCO2) was -1.3 +/- 7.2 mmHg. Skin perfusion was recorded at the area close to the transcutaneous CO2 monitor electrode and was defined as normal when capillary refill was below 3 seconds. The PtcCO2 bias for patients with normal skin perfusion was -0.2 +/- 5.4 mmHg (P = 0.73) compared to -4.1 +/- 9.9 for patients with decreased skin perfusion (P = 0.01). The slope of PtcCO2 against PaCO2 was closer to identity in patients with normal skin perfusion (1.17) than in patients where it was decreased (slope, 1.40). We suggest that PaCO2 estimation by both PetCO2 and PtcCO2 is sufficiently precise and reliable for clinical use in critically ill children. Certain limitations stem from the nature of the techniques.(ABSTRACT TRUNCATED AT 250 WORDS)