Transcutaneous Carbon Dioxide Monitoring in Subjects With Acute Respiratory Failure and Severe Hypercapnia

BACKGROUND: Transcutaneous carbon dioxide (P_tcCO2) monitoring is being used increasingly to assess acute respiratory failure. However, there are conflicting findings concerning its reliability when evaluating patients with high levels of P_aCO2. Our study evaluates the accuracy of this method in subjects with respiratory failure according to the severity of hypercapnia.

METHODS: We included subjects with respiratory failure, admitted to a respiratory intermediate care unit, who required arterial blood gas analysis. Simultaneously, P_tcCO2 was measured using a digital monitor. Relations between P_aCO2 and P_tcCO2 were assessed by the Pearson correlation coefficient. Bland-Altman analysis was used to test data dispersion, and an analysis of variance test was used to compare the differences between P_aCO2 and the corresponding P_tcCO2 at different levels (level 1, <50 mm Hg; level 2, 50–60 mm Hg; level 3, >60 mm Hg).

RESULTS: Eighty-one subjects were analyzed. The main diagnosis was COPD exacerbation (45%). P_tcCO2 correlated well with P_aCO2 (r2 = 0.93, P < .001). Bland-Altman analysis showed a mean P_aCO2 − P_tcCO2 difference of 4.9 ± 4.4 with 95% limits of agreement ranging from −3.6 to 13.4. The difference between variables increased in line with P_aCO2 severity: level 1, 1.7 ± 3.2 mm Hg; level 2, 3.7 ± 2.8; level 3, 6.8 ± 4.7 (analysis of variance, P < .001).

CONCLUSIONS: Our study showed an acceptable agreement of P_tcCO2 monitoring with arterial blood gas analysis. However, we should consider that P_tcCO2 underestimates P_aCO2 levels, and its accuracy depends on the level of hypercapnia, so this method would not be suitable for acute patients with severe hypercapnia.