Understanding hypoxemia on ECCO₂R: back to the alveolar gas equation

Extracorporeal CO₂ removal (ECCO₂R) is a promising technique for ARDS and for severe acute exacerbations of COPD [1]. However, ECCO₂R carries its own risk of complications and side effects. Beyond hemorrhagic and thrombotic complications and hemolysis, the occurrence of progressive hypoxemia has been reported in COPD patients treated by ECCO₂R, leading to a tracheal intubation rate of 28% in the prospective series from Braune et al. [2]. Obviously, progressive hypoxemia can be explained by pulmonary complications such as evolving infiltrates, even if other factors such as modification of the respiratory quotient have been proposed [2, 3]. Accordingly, we illustrate such a mechanism, intrinsically linked to the ECCO₂R technique and not involving any worsening of lung function by itself.

A 76-year-old man was admitted because of a very severe hypercapnic acute exacerbation of a chronic respiratory failure due to non-cystic fibrosis bronchiectasis. Invasive mechanical ventilation (Carescape R860 GE Healthcare) was initiated because of non-invasive ventilation failure. ECCO₂R was started 24 h later with the goals of limiting hypercapnia and dynamic hyperinflation and promoting a rapid weaning process [4]. The iLA-Activve system (Xenios-Novalung, Heilbronn) was used with a 22-Fr right jugular veno-venous catheter. Since weaning was a very difficult process, the sweep gas flow was progressively increased during the next 7 days from 1 to 9 L/min, while the extracorporeal blood flow varied between 0.8 and 1.2 L/min. During the same period, the PaO₂/FiO₂ ratio progressively decreased from 251 to 145, with no obvious pulmonary complication. Table 1 indicates the corresponding ABG and PaO₂/FiO₂ values as well as the Da-aO₂ values calculated either using the classical simplified alveolar air equation, i.e., PaO₂ = PiO₂ − PaCO₂/0.8, or the exact simplified alveolar air equation using the 0.3 value of the respiratory quotient displayed by the ventilator. Despite the apparent changes in PaO₂/FiO₂ ratio, the correct Da-aO₂ and PaO₂ were compatible with clinically negligible changes in intrapulmonary shunt, oscillating around 15%, even if we cannot totally exclude confounding factors inferring with the shunt calculation such as a higher mixed venous PO₂ (even if it is generally believed that ECCO₂R exerts only minimal oxygenation effects), a release of hypoxic pulmonary vasoconstriction due to a higher FiO₂, or a shunt decrease in relation to higher FiO₂ as described in moderate ARDS. The observed changes in PaO₂/FiO₂ were therefore mainly justified by changes in PaO₂ due to changes in the VCO₂/VO₂ ratio of the patient’s own lung, rather than to changes in its oxygenation function. Accordingly, no specific pulmonary complication was diagnosed during the following days.

Table 1 Oxygenation values, ABG values, and invasive mechanical ventilation parameters recorded immediately before initiation of ECCO₂R and under ECCO₂R after raising the sweep gas flow to 9 L/min

ECCO₂R exerts predominantly an effective extracorporeal CO₂ removal, without significant effect on oxygenation which accordingly occurs very predominantly in the native lungs, resulting in a decreased native lung respiratory quotient. It is therefore very important to use during ECCO₂R the exact calculations of PaO₂ and Da-aO₂ when a suitable monitoring system is available, or at least to interpret with great caution any PaO₂/FiO₂ worsening, which could, at least in part, reflect an ECCO₂R-induced modification of the alveolar gas content [5].