Effect of extracorporeal carbon dioxide removal on respiratory quotient measured by indirect calorimetry: Unraveling the mystery

New Findings: What is the main observation in this case? Several studies have reported progressive hypoxaemia once extracorporeal carbon dioxide removal is started in patients with hypercapnic respiratory failure, possibly attributable to an altered respiratory quotient. What insights does it reveal? In this quality control report, we show that the respiratory quotient exhibits only minimal alteration when extracorporeal carbon dioxide removal is started and assume that the progressive hypoxaemia is attributable to an increase in intrapulmonary shunt. Abstract: The use of extracorporeal carbon dioxide removal (ECCO ₂R) has been proposed in patients with acute respiratory distress syndrome to achieve lung-protective ventilation and in patients with selective hypercapnic respiratory failure. However, several studies have reported progressive hypoxaemia, as expressed by a need to increase the inspired oxygen fraction (F _iO ₂) to maintain adequate oxygenation or by a decrease in the ratio of arterial oxygen tension (P _aO ₂) to F _iO ₂ once ECCO ₂R is started. We present the case of a patient who was admitted to the intensive care unit for a coronavirus disease 2019 pneumonia and who was intubated because of hypercapnic respiratory insufficiency. Extracorporeal carbon dioxide removal was started, and the patient subsequently developed progressive hypoxaemia. To test whether the hypoxaemia was attributable to the ECCO ₂R, blood samples were taken in different settings: (1) ‘no ECCO ₂R’, blood flow 150 ml/min with a ECCO ₂R gas flow of 0 L/min; and (2) ‘with ECCO ₂R’, blood flow 400 ml/min with gas flow 12 L/min. We measured P _aO ₂, alveolar oxygen tension, P _aO ₂/F _iO ₂, alveolar–arterial oxygen tension difference, arterial carbon dioxide tension and the respiratory quotient (RQ) by indirect calorimetry in each setting. The RQ was 0.60 without ECCO ₂R and 0.57 with ECCO ₂R. The alveolar oxygen tension was 220.4 mmHg without ECCO ₂R and increased to 240.3 mmHg with ECCO ₂R, whereas P _aO ₂/F _iO ₂ decreased from 177 to 171. Our study showed only a minimal change in RQ when ECCO ₂R was started. We were the first to measure the RQ directly, before and after the initiation of ECCO ₂R, in a patient with hypercapnic respiratory failure.