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

Abstract

BACKGROUND: Transcutaneous carbon dioxide (P tcCO 2 ) 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 aCO 2 . 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 tcCO 2 was measured using a digital monitor. Relations between P aCO 2 and P tcCO 2 were assessed by the Pearson correlation coefficient. BlandAltman analysis was used to test data dispersion, and an analysis of variance test was used to compare the differences between P aCO 2 and the corresponding P tcCO 2 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 tcCO 2 correlated well with P aCO 2 (r2 ‫؍‬ 0.93, P < .001). Bland-Altman analysis showed a mean P aCO 2 ؊ P tcCO 2 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 aCO 2 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 tcCO 2 monitoring with arterial blood gas analysis. However, we should consider that P tcCO 2 underestimates P aCO 2 levels, and its accuracy depends on the level of hypercapnia, so this method would not be suitable for acute patients with severe hypercapnia.

Evaluating a novel formula for noninvasive estimation of arterial carbon dioxide during post‐resuscitation care

“…The accuracy of the novel formula is the highest in the normoventilation range. Previous studies of end‐tidal CO 2 and PtcCO 2 and show similar results with high PaCO 2 levels, which can be the result of increased dead space and shunting 22,23 . The method underestimated the highest PaCO 2 values, which may occur with large alveolar dead space.…”

“…In adults, PtcCO 2 has shown conflicting results 29,30 and may be affected by hypotension, peripheral perfusion disturbances and the use of vasoconstrictors 31,32 . Transcutaneous PCO 2 appears to be a more accurate method compared with PETCO 2 , but its accuracy might deteriorate with extreme PaCO 2 values and is also affected by V/Q mismatch 23,33,34 …”

How should we monitor patients with acute respiratory failure treated with noninvasive ventilation?

“…In addition, transcutaneous CO 2 monitoring in the context of ARF has been carefully evaluated [44][45][46][47]. Compared to arterial PaCO 2 measurements, transcutaneous CO 2 monitoring showed high agreement in different timings and disease categories [45,46,48], suggesting that it could be very convenient for estimating the ventilatory response to NIV, facilitating proactive (rather than reactive) ventilator titration.…”

“…Moreover, subjects requiring NIV for hypercapnic ARF preferred transcutaneous monitoring to ABG measurement [46]. One of the major limitations is that transcutaneous CO 2 measurement underestimates PaCO 2 levels, especially for those who are severely hypercapnic [47,49]. It is important to know that there are differences in accuracy between devices [43], and methodological issues may have an influence, for example placing the electrode in the earlobe [45,46,[48][49][50], in the upper chest [47,51,52] or abdomen [44].…”

“…Thus, transcutaneous CO 2 monitoring could replace frequent ABG measurements for monitoring patients with ARF undergoing NIV, since it is patient-friendly, feasible, reliable and safe. A synchronous ABG analysis at baseline is recommended to validate levels and may be advisable in cases of higher transcutaneous CO 2 values (⩾60 mmHg) [47].…”