Correlation of PaCO2 and ETCO2 in COPD Patients with Exacerbation on Mechanical Ventilation

Abstract

Introduction: Chronic obstructive pulmonary disease (COPD) patients in hypercapnic respiratory failure require multiple arterial blood gas (ABG) analysis for monitoring. It is a painful, invasive, and expensive investigation. This study was aimed at finding an agreement between end-tidal carbon dioxide (ETCO 2 , a noninvasive modality) and arterial carbon dioxide (PaCO 2 ) in COPD patients with acute exacerbation on mechanical ventilation. Materials and methods: A prospective observational study was conducted in COPD patients who required mechanical ventilation. ETCO 2 was recorded by mainstream capnography along with ABG analysis. An agreement between PaCO 2 and ETCO 2 was assessed. The effect of various factors on correlation was also studied. Results: A total of 100 patients with COPD in hypercapnic respiratory failure were included. Seventy-three percentage of patients were managed on invasive mechanical ventilation (IMV). The mean ETCO 2 and PaCO 2 were 48.66 15.57 mm Hg and 75.52 21.9 mm Hg, respectively. There was a significant correlation between PaCO 2 and ETCO 2 values (r = 0.82, 95% confidence interval of r = 0.78-0.86, p <0.0001). The Bland-Altman analysis shows the mean bias as -19.4 (95% limits of agreement = -40.0 to 1.1). Pearson's correlation coefficient was 0.84 in intubated patients and 0.58 in patients on noninvasive ventilation (NIV). Pearson's correlation coefficient between PaCO 2 and ETCO 2 in subjects with consolidation, cardiomegaly, hypotension, and raised pulmonary artery pressures was 0.78, 0.86, 0.85, and 0.86, respectively. Conclusion: Mainstream ETCO 2 measurement accurately predicts the PaCO 2 in COPD patients on IMV. However, for patients on NIV, ETCO 2 is insufficient in monitoring PaCO 2 levels due to weak correlation. Clinical significance: ETCO 2 can be used as a noninvasive modality in intensive care unit for monitoring the PaCO 2 in COPD patients on IMV. This can reduce the requirement of arterial blood sampling to a minimum number, in turn reducing the cost of the treatment and discomfort to the patients.