Assessment of gradient between partial pressure of arterial carbon dioxide and end-tidal carbon dioxide in acute respiratory distress syndrome

Abstract

Context End-tidal carbon dioxide (EtCO2) is used as a noninvasive bedside test to assess the adequacy of ventilation and physiologic dead space in mechanically ventilated patients. The gradient difference between EtCO2 and partial pressure of arterial carbon dioxide (PaCO2) is directly related to an increase in the physiologic dead space. Aim The aim of this study was to evaluate the role of measuring the gradient between EtCO2 and PaCO2 in adults with acute respiratory distress (ARDS). Settings and design This was a prospective consecutive enrollment study. Patients and methods Overall, 51 cases were recruited after the diagnosis of ARDS was made according to the Berlin definition. Patients were mechanically ventilated as per the lung-protective protocol. Daily arterial blood gases were collected and for every sample, the EtCO2 value was collected electronically by capnography using an endotracheal tube for the first 5 days. Results Cases were classified into survivors and nonsurvivors: 26 cases were because of extrapulmonary causes and 25 cases were because of pulmonary causes. The mean value of the APACHE II score for all cases on admission was 21.6. The mean length of ICU stay was 12.7 days. For all study cases, PaO2/FiO2 was the lowest at day 1 and the highest at day 5. We found a significant negative correlation between PaO2/FiO2 and the gradient at days 2, 4, and day 5, and a significant positive correlation between the gradient on admission and the APACHE II score (r=0.4, P≤0.05). Nonsurvivors had a significantly higher gradient and lower EtCO2 and PaO2/FiO2 levels at all time intervals, whereas PaCO2 alone was found to be nonsignificant. Conclusion In ARDS, EtCO2 and gradient are reliable indicators of severity.