Usefulness of Measurement of End-tidal CO2 Using a Portable Capnometer in Patients with Chronic Respiratory Failure Receiving Long-term Oxygen Therapy

Objective Patients with chronic respiratory failure requiring long-term oxygen therapy (LTOT) are at a risk of CO2 retention because of excessive oxygen administration. The CapnoEye (TM) is a novel portable capnometer that can measure end-tidal CO2 (EtCO2) noninvasively. This retrospective study evaluated the usefulness of this device. Methods EtCO2 was measured using the CapnoEye (TM). The EtCO2 and partial pressure of venous carbon dioxide (PvCO(2)) were analyzed, and other clinical data were assessed. Patients Sixty-one consecutive patients with chronic respiratory failure receiving LTOT in the outpatient department at the Japanese Red Cross Medical Center between July 2017 and March 2018 were retrospectively reviewed. Results There was a significant correlation between EtCO2 and PvCO(2) (r=0.63) in the total study population as well as in the COPD group (r=0.65) and ILD group (r=0.67). The PvCO(2) and EtCO2 gradient was correlated with only the body mass index in a multivariate analysis (p=0.0235). The EtCO2 levels on the day of admission were significantly higher than those in the same patients when they were in a stable condition (p= 0.0049). There was a significant correlation between Delta EtCO2 and Delta PvCO(2) (r=0.4). A receiver-operating characteristic curve analysis revealed the optimal cut-off EtCO2 value for identifying hypercapnia to be 34 mmHg (p=0.0005). Conclusion The evaluation of EtCO2 by the CapnoEye (TM) was useful for predicting PvCO(2). The body mass index was identified as a possible predictor of the PvCO(2) and EtCO2 gradient. An increase in EtCO2 may indicate deterioration of the respiratory status in patients with chronic respiratory failure receiving LTOT.