Imaging Pulmonary Blood Vessels and Ventilation-Perfusion Mismatch in COVID-19

Abstract

COVID-19 hypoxemic patients although sharing a same etiology (SARS-CoV-2 infection) present themselves quite differently from one another. Patients also respond differently to prescribed medicine and to prone Vs supine bed positions. A severe pulmonary ventilation-perfusion mismatch usually triggers moderate to severe COVID-19 cases. Imaging can aid the physician in assessing severity of COVID-19. Although useful for their portability X-ray and ultrasound serving on the frontline to evaluate lung parenchymal abnormalities are unable to provide information about pulmonary vasculature and blood flow redistribution which is a consequence of hypoxemia in COVID-19. Advanced imaging modalities such as computed tomography, single-photon emission tomography, and electrical impedance tomography use a sharp algorithm visualizing pulmonary ventilation-perfusion mismatch in the abnormal and in the apparently normal parenchyma. Imaging helps to access the severity of infection, lung performance, ventilation-perfusion mismatch, and informs strategies for medical treatment. This review summarizes the capacity of these imaging modalities to assess ventilation-perfusion mismatch in COVID-19. Despite having limitations, these modalities provide vital information on blood volume distribution, pulmonary embolism, pulmonary vasculature and are useful to assess severity of lung disease and effectiveness of treatment in COVID-19 patients.

Keywords: COVID-19; CT angiography; Electrical impedance tomography; HRCT; SPECT-CT.

Fig. 1

Normal ventilation and perfusion are responsible for gas exchange in alveoli (a). Alveolar sac filled with edema and exudates produces hypoxemia by decreasing the alveolar and arterial oxygen level result in shunt (b). Pulmonary embolism develops high ventilation in proportion to perfusion produce a dead space (c). Hypoperfusion to the apparently normal parenchyma due to vasoconstriction of pulmonary blood vessels results in poor gas exchange (d) and hyperperfusion through dilated capillaries to alveolar sac filled with edema and exudates result in shunt (e)

Fig. 2

Normal lung (left) showing smooth blood flow and the effective gas exchange is recognized as normal ventilation and perfusion. COVID-19 infection causes an intense inflammatory reaction (right) results in shunt or dead space or both. Uncontrolled activation of lymphocytes, neutrophil, and pulmonary production of platelets cause lung tissue damages. The virulence in COVID-19 triggers pulmonary microthrombi, endothelial damage, and vascular leakage. The host intends to control the thrombi formation by vigorous fibrinolysis, and degraded fibrin (D-dimer) are released in blood stream. Image Source—https://doi.org/10.1111/jth.14975

Fig. 3

Functional respiratory imaging on HRCT scans found that the COVID-19 patients had significantly reduced blood volume in the smaller caliber blood vessels (BV5, cross-sectional area < 5 mm2) compared with the long hauler COVID-19 subjects and healthy controls. In the long haulers, the blood vessels remain anomalous for a long time. The mid-size vessels, indicated in yellow (BV5_10, cross-sectional area of 5–10 mm²), seem to remain dilated which could indicate sustained microvascular obstruction and endothelial damage. In acute COVID-19, they had a significantly higher proportion of blood volume within large-caliber vessels (BV10, cross-sectional area of > 10 mm²) and mainly projected towards the posterior part of the lungs. Impaired gas exchange in the lungs seen in COVID-19 may be partially a result of redistribution of blood away from the small-caliber pulmonary vessels. Image Source—Fluidda Inc

Fig. 4

Panels display perfusion CT scans with dual energy CT (4A, 4B, and 4C) and conventional CT (4D) in COVID-19 patients. In panel 4A, (a) Peripheral GGO and consolidation within the right upper lobe and smaller GGO in the posterior left upper lobe (green arrowheads) are accompanied by dilated subsegmental vessels proximal to, and within, the opacities (green arrows). (b) The accompanying pulmonary blood volume image shows higher perfusion (green arrows) in GGO and consolidation. Image source—10.1016/S1473-3099(20)30367-4. In panel 4B, (a, c) axial CT image shows wedge-shaped bilateral opacities with surrounding GGO; (b) iodine map image shows a triangular peripheral area of decreased perfusion (yellow arrow) in the right lower, distal to pulmonary embolism (red arrow) lobe compatible with pulmonary infarction; (d) iodine map images showing a peripheral, triangular, and hypoperfused area in the left lower lobe (yellow arrow) suggestive of pulmonary infarction. Image source—https://doi.org/10.1016/j.rec.2020.04.013. In panel 4C, (a) axial CT image shows central GGO and peripheral consolidation a right inferior lobe. (b) Axial iodine color map shows high iodine concentrations in consolidations, and hypoperfusion in left medio-basal segment (dotted line), secondary to the thrombosis of corresponding segmental pulmonary artery (white arrow). (c) Axial CT image shows consolidation in a right inferior lobe and GGO in both inferior lobes. (d) Axial iodine color map shows hypoperfused area in the middle lobe (dotted line). Right inferior lobe consolidation shows high and heterogeneous iodine levels. Image source—https://doi.org/10.21037/qims-20-708. In panel 4D, (a) Axial chest CT of the right upper lobe with subpleural pneumonia (red arrows), surrounded by small GGOs. (b) CTPA shows multiple small subpleural perfusion defects (red arrows) and a larger perfusion defect dorsal in the normal ventilated right upper lobe (Δa), due to microvascular obstruction (Δb). Pulmonary emboli in the right pulmonary upper lobe were not observed. Image source—10.1259/bjr.20200718

Fig. 5

In panel 5A, the axial images showing decreased radiotracer uptake in both perfusion and ventilation scan (white arrow) in fused SPECT-CT (a, b) indicating low probability of pulmonary embolism. SPECT (c, d) observed defect matches with the posterior segment of the right upper lobe alveolar filling (black arrow) on the CT scan (e). In panel 5B, on coronal images, decreased dual radiotracer uptake (green arrows) in ventilation (a) and perfusion (b) SPECT scans indicates low possibility of pulmonary embolism. Increased tracheobronchial tract uptake of (blue arrows), with marked intensity on the proximal bronchi suggesting tracheobronchitis or chronic obstructive pulmonary disease. In panel 5C, the axial SPECT images showing normal perfusion scan (a) and defects in ventilation scan (b) in the posterior segment of the right upper lobe (white arrow) that matches with the GGO on CT scan (c, black arrow). Image source—https://doi.org/10.1007/s00259-020-04920-w, https://doi.org/10.1007/s00259-020-04834-7

Fig. 6

In this study, EIT showed real-time noninvasive bedside ventilation and perfusion in hypoxemic COVID-19 patients with respiratory failure. All the three COVID-19 patients intubated for acute hypoxic respiratory failure (PaO₂/FiO₂ < 300) had different respiratory system compliance. EIT was used to determine regional ventilation and perfusion distribution. On CT scans, case 1 shows peripheral and basilar GGO, while case 2 and 3 describe diffuse bilateral GGO. In case 1, EIT showed severe right-lung perfusion anomalies, homogenous ventilation, and a moderate decrease in respiratory compliance (40 ml/cm H₂O). Case 2 and case 3 showed a progressive decrease of respiratory compliance, without major perfusion disturbances. PaO₂/FiO₂ means ratio of arterial oxygen partial pressure in mmHg to fractional inspired oxygen expressed as fraction. Image source—https://doi.org/10.1164/rccm.202005-1801IM