Transcutaneous Carbon Dioxide in the Management of Noninvasive Ventilation

To the Editor:

Respiratory monitoring during noninvasive mechanical ventilation (NIV) allows evaluation of its efficiency and prevention of any delay in the initiation of invasive mechanical ventilation and the occurrence of related complications. The measurement of breathing frequency, pulse oximetry, arterial blood gas analysis, and capnography are commonly employed methods for this purpose. Direct monitoring of P_CO2 is particularly important in the management of patients with hypercapnic respiratory failure. Direct measurement of arterial P_aCO2 using arterial blood gas analysis is the accepted standard method; however, the inability of this method to provide continuous monitoring and its invasive nature have spurred research for an alternative method.¹ The studies have focused on end-tidal P_CO2 and transcutaneous measurement of P_CO2 (P_tcCO2) due to their ability to provide continuous monitoring and their noninvasive nature. P_tcCO2 is especially used to evaluate alveolar ventilation in patients with nocturnal hypoventilation receiving NIV in the home setting. The studies demonstrated the correlation between P_tcCO2 and P_aCO2 values.²

In the study titled “What is the potential role of transcutaneous carbon dioxide in guiding acute noninvasive ventilation?” Van Oppen et al³ evaluated the correlations between P_tcCO2 − P_aCO2 and arterial pH-calculated transcutaneous pH and the relationship of the 2 methods with the pain scores in 9 subjects undergoing NIV due to hypercapnic respiratory failure. The study measurements were performed with 4-h intervals in the first 12 h after the initiation of NIV.

According to Bland-Altman analysis, transcutaneous pH was, in general, consistent with arterial pH; however, this relationship was weaker in the case of severe acidosis, as evidenced by the pH value measured, <7.30. A similar relationship was found between P_tcCO2 and P_aCO2. This relationship was weaker in the case of severe acidosis when P_aCO2 was >65 mm Hg. They suggested that NIV could be guided using bicarbonate and pH values predicted using the algorithms, and this approach would also reduce the number of blood samplings for arterial blood gas analysis. The utility of this method only when pH is not <7.30 and in the presence of pure respiratory acidosis, which is not accompanied by metabolic acidosis, may limit its use in clinical practice. However, the predicted values when baseline bicarbonate is measured >34.0 mmol/L do not reflect the actual values, and this necessitates the use of further algorithms.

As the secondary outcome measure of the study, pain score was significantly lower compared with arterial blood sampling during transcutaneous monitoring. There are also studies suggesting that the correlation between P_tcCO2 and P_aCO2 in subjects undergoing NIV is suboptimal and that P_tcCO2 cannot substitute for P_aCO2.⁴ Although P_tcCO2 monitoring is a noninvasive method and seems to be superior in showing the efficiency of treatment, more studies with a larger number of cases are required to establish its place in cases with acute respiratory failure.

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