Transcutaneous arterial carbon dioxide pressure monitoring in critically ill adult patients

Objective To evaluate the accuracy of transcutaneous PCO₂ (PtcCO₂) as a surrogate for arterial PCO₂ (PaCO₂) 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 PCO₂. 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 PCO₂ measures were obtained. Twenty-one were excluded from analysis, because profound skin vasoconstriction was present (PCO₂ 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°C for 27 measurements. Mean duration of monitoring was 17 ± 17 h. The mean difference between PaCO₂ and PtcCO₂ was −0.2 ± 4.6  mmHg with a tight correlation (R ² = 0.92, p > 0.01). PCO₂ bias did not significantly change among three successive measurements. Changes in PaCO₂ and in PtcCO₂ between two blood samples were well correlated (R ² = 0.78, p > 0.01). Variations of more than 8 mmHg in PtcCO₂ had 86% sensitivity and 80% specificity to correctly predict similar changes in PaCO₂ in the same direction. Catecholamine dose or respiratory support did not affect PtcCO₂ accuracy. Hypothermia has only a small effect on accuracy. No complication related to a prolonged use of the sensor was observed Conclusion Transcutaneous PCO₂ provides a safe and reliable trend-monitoring tool, provided there is no major vasoconstriction. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.