Objective: To evaluate the accuracy of transcutaneous PCO(2) (PtcCO(2)) as a surrogate for arterial PCO(2) (PaCO(2)) in a cohort of adult critically ill patients in a medical intensive care unit (ICU).
Design: Prospective observational study comparing paired measures of transcutaneous and arterial PCO(2).
Setting: A 26-bed medical ICU.
Patients: Fifty ICU patients monitored with a SenTec Digital Monitor placed at the ear lobe over prolonged periods.
Results: A total of 189 paired PCO(2) measures were obtained. Twenty-one were excluded from analysis, because profound skin vasoconstriction was present (PCO(2) bias =-10.8+/-21.8 mmHg). Finally, 168 were analysed, including 137 obtained during mechanical ventilation and 82 under catecholamine treatment. Body temperature was below 36 degrees C for 27 measurements. Mean duration of monitoring was 17+/-17 h. The mean difference between PaCO(2) and PtcCO(2) was -0.2+/-4.6 mmHg with a tight correlation (R(2)=0.92, p>0.01). PCO(2) bias did not significantly change among three successive measurements. Changes in PaCO(2) and in PtcCO(2) between two blood samples were well correlated (R(2)=0.78, p>0.01). Variations of more than 8 mmHg in PtcCO(2) had 86% sensitivity and 80% specificity to correctly predict similar changes in PaCO(2) in the same direction. Catecholamine dose or respiratory support did not affect PtcCO(2) accuracy. Hypothermia has only a small effect on accuracy. No complication related to a prolonged use of the sensor was observed
Conclusion: Transcutaneous PCO(2) provides a safe and reliable trend-monitoring tool, provided there is no major vasoconstriction.