Establishing a Gradient between Partial Pressure of Arterial Carbon Dioxide and End-Tidal Carbon Dioxide in Patients with Acute Respiratory Distress Syndrome

Abstract

End-tidal carbon dioxide (ETCO₂) monitoring is useful in many situations. However, ETCO₂ monitoring is unreliable in patients with acute respiratory distress syndrome (ARDS) due to widespread lung inflammation. In our study, we attempt to establish the gradient between the arterial pressure of carbon dioxide (PaCO₂) and ETCO₂ in patients with ARDS, which we defined as the PaETCO₂ gradient. The main objective of the study was to establish a PaETCO₂ gradient in each severity of ARDS. We analyzed 35 patients with ARDS and a total of 88 arterial blood gases were included. PaCO₂, PaO₂/FiO₂ and ETCO₂ were measured. Patients were stratified into mild, moderate and severe ARDS as classified by the Berlin ARDS criteria. PaCO₂ and ETCO₂ were compared at each severity stratification. The mean PaCO₂ was 50.0, the mean ETCO₂ was 26.6 and the gradient among all samples was 23.24 (±12.02). The mean gradient for each severity is as follows: mild: 19.3 (±9.9), moderate: 27.9 (±13.2) and severe: 23.9 (±7.8). The difference between the PaETCO₂ gradient of the mild to moderate (p=0.001) and mild to severe groups (p=0.01) reached statistical significance. However, the difference between the moderate to severe groups did not reach statistical significance (p=0.48). We found the gradient between PaCO₂ and ETCO₂ in patients with ARDS is vast and tends to worsen with increasing severity of ARDS. This indicates that the gradient between the 2 may be used as an indicator of increasing severity of ARDS.