Causes of Hypoxemia in COVID-19 Acute Respiratory Distress Syndrome: A Combined Multiple Inert Gas Elimination Technique and Dual-energy Computed Tomography Study

Background: Despite the fervent scientific effort, a state-of-the art assessment of the different causes of hypoxemia (shunt, ventilation-perfusion mismatch, and diffusion limitation) in COVID-19 acute respiratory distress syndrome (ARDS) is currently lacking. In this study, the authors hypothesized a multifactorial genesis of hypoxemia and aimed to measure the relative contribution of each of the different mechanism and their relationship with the distribution of tissue and blood within the lung. Methods: In this cross-sectional study, the authors prospectively enrolled 10 patients with COVID-19 ARDS who had been intubated for less than 7 days. The multiple inert gas elimination technique (MIGET) and a dual-energy computed tomography (DECT) were performed and quantitatively analyzed for both tissue and blood volume. Variables related to the respiratory mechanics and invasive hemodynamics (PiCCO [Getinge, Sweden]) were also recorded. Results: The sample (51 +/- 15 yr; Pao(2)/Fio(2), 172 +/- 86 mmHg) had a mortality of 50%. The MIGET showed a shunt of 25 +/- 16% and a dead space of 53 +/- 11%. Ventilation and perfusion were mismatched (LogSD, Q, 0.86 +/- 0.33). Unexpectedly, evidence of diffusion limitation or postpulmonary shunting was also found. In the well aerated regions, the blood volume was in excess compared to the tissue, while the opposite happened in the atelectasis. Shunt was proportional to the blood volume of the atelectasis (R-2 = 0.70, P = 0.003). V(center dot)A/Q(center dot)T mismatch was correlated with the blood volume of the poorly aerated tissue (R-2 = 0.54, P = 0.016). The overperfusion coefficient was related to Pao(2)/Fio(2) (R-2 = 0.66, P = 0.002), excess tissue mass (R-2 = 0.84, P < 0.001), and Etco(2)/Paco(2) (R-2 = 0.63, P = 0.004). Conclusions: These data support the hypothesis of a highly multifactorial genesis of hypoxemia. Moreover, recent evidence from post-mortem studies (i.e., opening of intrapulmonary bronchopulmonary anastomosis) may explain the findings regarding the postpulmonary shunting. The hyperperfusion might be related to the disease severity.