Accuracy of Transcutaneous CO₂ Values Compared With Arterial and Capillary Blood Gases

Transcutaneous monitors are utilized to monitor a patient's respiratory status. Some patients have similar values when comparing transcutaneous carbon dioxide (P_tcCO ) values with blood gas analysis, whereas others show extreme variability. A retrospective review of data was performed to determine how accurately P_tcCO correlated with CO₂ values obtained by arterial blood gas (ABG) or capillary blood gas.

To determine whether P_tcCO values correlated with ABG or capillary blood gas values, subjects' records were retrospectively reviewed. Data collected included the P_tcCO value at the time of blood gas procurement and the ABG or capillary blood gas P_CO value. Agreement of pairs of methods (ABG vs P_tcCO and capillary blood gas vs P_tcCO ) was assessed with the Bland-Altman approach with limits of agreement estimated with a mixed model to account for serial measurements per subject.

A total of 912 pairs of ABG/P_tcCO values on 54 subjects and 307 pairs of capillary blood gas/P_tcCO values on 34 subjects were analyzed. The P_CO range for ABG was 24-106 mm Hg, and P_tcCO values were 27-133 mm Hg. The P_CO range for capillary blood gas was 29-108 mm Hg, and P_tcCO values were 30-103 mm Hg. For ABG/P_tcCO comparisons, the Pearson correlation coefficient was 0.82, 95% CI was 0.80-0.84, and P was <.001. For capillary blood gas/P_tcCO comparisons, the Pearson correlation coefficient was 0.77, 95% CI was 0.72-0.81, and P was <.001. For ABG/P_tcCO , the estimated difference ± SD was -6.79 ± 7.62 mm Hg, and limits of agreement were -22.03 to 8.45. For capillary blood gas/P_tcCO , the estimated difference ± SD was -1.61 ± 7.64 mm Hg, and limits of agreement were -16.88 to 13.66. The repeatability coefficient was about 30 mm Hg.

Based on these data, capillary blood gas comparisons showed less variation and a slightly lower correlation with P_tcCO than did ABG comparisons. After accounting for serial measurements per patient, due to the wide limits of agreement and poor repeatability, the utility of relying on P_tcCO readings for this purpose is questionable.