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 (Pa(CO2), Pet(CO2), Ptc(CO2), respectively) were obtained in 134 children receiving mechanical ventilation (ages, 2 days to 16 years; mean, 2.5 years). The mean +/- SD Pet(CO2) bias (Pa(CO2) - Pet(CO2)) was 3.4 +/- 6.6 mmHg. When the Pet(CO2) bias was plotted against the Pa(O2)/P(AO2) ratio, a change in the scatter was obvious at about 0.3. The Pet(CO2) bias for patients with Pa(O2)/P(AO2) under 0.3 was 7.8 +/- 7.3 mmHg compared to 0 +/- 3.4 in patients with Pa(O2)/P(AO2) above 0.3 (P < 0.001). Pet(CO2) differed significantly from Pa(CO2) (P < 0.001) only for patients with Pa(O2)/P(AO2) under 0.3. The slope (Pa(CO2) versus Pet(CO2)) for these patients was 1.59, while the slope for patients with Pa(O2)/P(AO2) above 0.3 coincided with the line of identity (1.00). The mean +/- SD Ptc(CO2) bias (Pa(CO2) - Ptc(CO2)) 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 Ptc(CO2) 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 Ptc(CO2) against Pa(CO2) 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 Pa(CO2) estimation by both Pet(CO2) and Ptc(CO2) is sufficiently precise and reliable for clinical use in critically ill children. Certain limitations stem from the nature of the techniques. Measurement of alveolar to arterial O2 ratio may improve the precision of Pa(CO2) estimation by capnography; assessment of skin perfusion is important in order to increase the accuracy of the transcutaneous method, especially in critically ill children.