The impact of ventilation-perfusion inequality in COVID-19: a computational model

Abstract

COVID-19 infection may lead to acute respiratory distress syndrome (CARDS) where severe gas exchange derangements may be associated, at least in the early stages, only with minor pulmonary infiltrates. This may suggest that the shunt associated to the gasless lung parenchyma is not sufficient to explain CARDS hypoxemia. We designed an algorithm (Vent_riQ_lar), based on the same conceptual grounds described by J.B. West in 1969. We set 498 ventilation-perfusion (V_A/Q) compartments and, after calculating their blood composition (PO₂, PCO₂, and pH), we randomly chose 10⁶ combinations of five parameters controlling a bimodal distribution of blood flow. The solutions were accepted if the predicted PaO₂ and PaCO₂ were within 10% of the patient's values. We assumed that the shunt fraction equaled the fraction of non-aerated lung tissue at the CT quantitative analysis. Five critically-ill patients later deceased were studied. The PaO₂/FiO₂ was 91.1 ± 18.6 mmHg and PaCO₂ 69.0 ± 16.1 mmHg. Cardiac output was 9.58 ± 0.99 L/min. The fraction of non-aerated tissue was 0.33 ± 0.06. The model showed that a large fraction of the blood flow was likely distributed in regions with very low V_A/Q (Q_mean = 0.06 ± 0.02) and a smaller fraction in regions with moderately high V_A/Q. Overall LogSD, Q was 1.66 ± 0.14, suggestive of high V_A/Q inequality. Our data suggest that shunt alone cannot completely account for the observed hypoxemia and a significant V_A/Q inequality must be present in COVID-19. The high cardiac output and the extensive microthrombosis later found in the autopsy further support the hypothesis of a pathological perfusion of non/poorly ventilated lung tissue.NEW & NOTEWORTHY Hypothesizing that the non-aerated lung fraction as evaluated by the quantitative analysis of the lung computed tomography (CT) equals shunt (V_A/Q = 0), we used a computational approach to estimate the magnitude of the ventilation-perfusion inequality in severe COVID-19. The results show that a severe hyperperfusion of poorly ventilated lung region is likely the cause of the observed hypoxemia. The extensive microthrombosis or abnormal vasodilation of the pulmonary circulation may represent the pathophysiological mechanism of such V_A/Q distribution.

Keywords: COVID-19; gas exchange; lung physiology; mechanical ventilation; ventilation-perfusion.