Anatomical, physiological and clinical similarities and differences in ARDS physiological subtypes

Excerpt

This letter is in response to an article by Chiumello et al. recently published in Intensive Care Medicine [1]. This paper compared respiratory mechanics in acute respiratory distress syndrome (ARDS) phenotypes and observed several interesting clinico-physiologic contradictions. However, their physiological justification needs some more introspection. ARDS due to coronavirus disease 2019 (COVID-19) had a lower P/F (PaO₂/FiO₂) ratio than compliance-matched-ARDS despite having a lower non-aerated lung volume. Furthermore, venous admixture was also noted in COVID-19-ARDS patients with low non-aerated lung fraction. Ventilation perfusion mismatch due to low-ventilation/perfusion (V/Q) ratio leads to venous admixture. In COVID-19-ARDS, we expect a higher V/Q ratio due to pulmonary microthrombosis and the higher aerated lung fraction [2]. Even then, segments of the lung may develop low V/Q areas due to diversion of blood from thrombotic vessels to normal vessels. However, this diversion may further increase segments with high V/Q ratio and subsequent increase in alveolar dead space and that was surprisingly not different in COVID-19-ARDS compared to compliance-matched ARDS. Lung with a higher fraction of high V/Q ratio rarely leads to hypoxemia and low P/F ratio because it may get compensated by high minute ventilation during spontaneous or intermittent mandatory ventilation. However, in this study, the authors used matched cohorts of ARDS patients with controlled ventilation. It is likely that controlled ventilation in COVID-19-ARDS with compliant lung and a high V/Q ratio could have produced hypoxemia and low P/F. Notably, COVID-19-ARDS is known to induce ventilatory drive more than the ARDS due to other causes and minute ventilation is generally much higher when patients are allowed to breathe spontaneously during mechanical ventilation. Additionally, ventilatory ratio could have been significantly higher in COVID-19-ARDS than compliance-matched-ARDS in non-paralyzed patients. …