Non-invasive ventilation (NIV) is the treatment of choice for persistent hypercapnic ventilatory failure during acute exacerbations of chronic obstructive pulmonary disease (AECOPD), despite optimal medical treatment.^1,2 Assessment of the partial arterial pressure of carbon dioxide (Paco₂) is the “gold standard” for the evaluation of the adequacy of alveolar ventilation in this setting. However, repeated intermittent invasive arterial puncture carries inherent risks, including pain.³ Transcutaneous measurement of carbon dioxide (TcPco₂) theoretically appears more appropriate for monitoring Paco₂. This measurement is based on the observation that CO₂ has a high tissue solubility and diffuses through the skin. While available data as to the precision of TcPco₂ measurements have given conflicting results,^4–6 no study has attempted to assess simultaneous recordings of TcPco₂ and Paco₂ in patients requiring NIV for AECOPD.

We prospectively studied the agreement between TcPco_{2 …}