Usefulness of Measurement of End-tidal CO(2) 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 CO(2) retention because of excessive oxygen administration. The CapnoEye™ is a novel portable capnometer that can measure end-tidal CO(2) (EtCO(2)) noninvasively. This retrospective study evaluated the usefulness of this device. METHODS: EtCO(2) was measured using the CapnoEye™. The EtCO(2) 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 EtCO(2) 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 EtCO(2) gradient was correlated with only the body mass index in a multivariate analysis (p=0.0235). The EtCO(2) 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 ΔEtCO(2) and ΔPvCO(2) (r=0.4). A receiver-operating characteristic curve analysis revealed the optimal cut-off EtCO(2) value for identifying hypercapnia to be 34 mmHg (p=0.0005). CONCLUSION: The evaluation of EtCO(2) by the CapnoEye™ was useful for predicting PvCO(2). The body mass index was identified as a possible predictor of the PvCO(2) and EtCO(2) gradient. An increase in EtCO(2) may indicate deterioration of the respiratory status in patients with chronic respiratory failure receiving LTOT.