Lung distribution of gas and blood volume in critically ill COVID-19 patients: a quantitative dual-energy computed tomography study

Abstract

Background Critically ill COVID-19 patients have pathophysiological lung features characterized by perfusion abnormalities. However, to date no study has evaluated whether the changes in the distribution of pulmonary gas and blood volume are associated with the severity of gas-exchange impairment and the type of respiratory support (non-invasive versus invasive) in patients with severe COVID-19 pneumonia. Methods This was a single-center, retrospective cohort study conducted in a tertiary care hospital in Northern Italy during the first pandemic wave. Pulmonary gas and blood distribution was assessed using a technique for quantitative analysis of dual-energy computed tomography. Lung aeration loss (reflected by percentage of normally aerated lung tissue) and the extent of gas:blood volume mismatch (percentage of non-aerated, perfused lung tissue—shunt; aerated, non-perfused dead space; and non-aerated/non-perfused regions) were evaluated in critically ill COVID-19 patients with different clinical severity as reflected by the need for non-invasive or invasive respiratory support. Results Thirty-five patients admitted to the intensive care unit between February 29th and May 30th, 2020 were included. Patients requiring invasive versus non-invasive mechanical ventilation had both a lower percentage of normally aerated lung tissue (median [interquartile range] 33% [24–49%] vs. 63% [44–68%], p < 0.001); and a larger extent of gas:blood volume mismatch (43% [30–49%] vs. 25% [14–28%], p = 0.001), due to higher shunt (23% [15–32%] vs. 5% [2–16%], p = 0.001) and non-aerated/non perfused regions (5% [3–10%] vs. 1% [0–2%], p = 0.001). The PaO2/FiO2 ratio correlated positively with normally aerated tissue (ρ = 0.730, p < 0.001) and negatively with the extent of gas-blood volume mismatch (ρ = − 0.633, p < 0.001). Conclusions In critically ill patients with severe COVID-19 pneumonia, the need for invasive mechanical ventilation and oxygenation impairment were associated with loss of aeration and the extent of gas:blood volume mismatch. Graphic abstract

The evolution of the ventilatory ratio is a prognostic factor in mechanically ventilated COVID-19 ARDS patients

“…Nevertheless, in our study ventilatory ratio exceeds PaO 2 /FiO 2 in predicting in-hospital mortality. In fact, for its association to the extent of poorly aerated lung tissue [ 29 ], the physiologic dead space has been suggested as a stronger predictor than oxygenation of the non-COVID ARDS outcomes [ 30 ]. Unlike typical ARDS, the profound hypoxemia observed in COVID-19 might be found in patients with relatively preserved lung volumes and respiratory compliances [ 1 ], thus probably not reflecting the burden of parenchymal disease [ 31 , 32 ].…”

Inhaled pulmonary vasodilators are not associated with improved gas exchange in mechanically ventilated patients with COVID-19: A retrospective cohort study

“…Heterogeneity of lung perfusion has been described in contrast enhanced CT scans of patients with COVID-19 with and without pulmonary embolism [ 10 , 13 ]. Maldistribution of aerated lung tissue and lung perfusion correlated with impairment in P A O 2 :F I O 2 in spontaneously breathing and mechanically ventilated patients with COVID-19 [ 35 ]. Pulmonary vascular involvement including neutrophil recruitment, neutrophil extracellular trap formation, and vascular occlusion has been described in COVID-19 ARDS and found to be more severe than in influenza [ 14 ].…”

Personalized mechanical ventilation in acute respiratory distress syndrome

“…Dual-energy CT scan has been recently used to investigate ventilation perfusion relationships at different ventilation settings in COVID-19 patients [ 66 ]. LUS has been proposed as an alternative tool to monitor lung morphology at bedside [ 67 ], for several reasons: (1) LUS and chest CT findings are well correlated with regard to aeration [ 68 ] and identifying patients with focal or non-focal ARDS morphology [ 69 ], (2) LUS may influence clinical decision making related to the individualized mechanical ventilation management [ 70 , 71 ], and (3) worsening aeration in several LUS regions, associated with deterioration of respiratory mechanics and blood gases, requires prompt reevaluation of mechanical ventilation settings.…”