Bedside Electrical Impedance Tomography Unveils Respiratory “Chimera” in COVID-19

In 1981, Dr. Reginald Greene and colleagues illustrated extensive pulmonary artery filling defects in patients with different severity of acute respiratory failure by bedside angiographic studies (1). More recently, perfusion distribution and regional lung ventilation can be assessed at bedside by noninvasive, radiation-free electrical impedance tomography (EIT) (2, 3). In this report, we present three patients intubated for acute hypoxic respiratory failure owing to coronavirus disease (COVID-19). The three patients had similar levels of oxygenation but different respiratory system compliance (Figure 1). EIT was used to determine regional ventilation and perfusion distribution. Cases 1, 2, and 3 were assessed after 2, 17, and 19 days of mechanical ventilation, respectively. All patients were assessed by computed tomography (CT) without contrast (Figure 1). Case 1 CT imaging shows peripheral and basilar ground-glass opacities, compatible with known COVID-19 pneumonia. Case 2 and case 3 CT images describe diffuse bilateral ground-glass opacities. Case 1 had threefold higher respiratory system compliance than case 3 (Figure 1). In Case 1, EIT showed severe right-lung perfusion anomalies, homogenous ventilation, and a moderate decrease in respiratory compliance (40 ml/cm H₂O). Clinical diagnosis of pulmonary embolism was suggested by a high D-dimer (∼5,000 ng/ml) and lower extremity dopplers showing deep venous thrombosis. Case 2 and case 3 showed a progressive decrease of respiratory compliance (as low as 11 ml/cm H₂O in case 3), without major perfusion disturbances. This report shows that clinical information of the patient coupled with real-time noninvasive bedside EIT might be helpful to characterize the etiology of hypoxemia of patients with respiratory failure with COVID-19.

Figure 1.

Computed tomography (lung window) images of three cases with severe coronavirus disease (COVID-19) and corresponding electrical impedance tomography ventilation and perfusion images. All patients were assessed while in supine position and receiving protective Vt (5–6 ml/kg of predicted body weight). Positive end-expiratory pressure (PEEP) was set to target the best respiratory system compliance after a decremental PEEP trial (4). No signal of inspiratory effort was identified by esophageal manometry during all assessments. The intubation occurs, on average, 2 days after the onset of respiratory symptoms, and none of the patients required noninvasive ventilation. Images were generated by Enlight 1800 (Timpel SA), with color scale adjusted by linear normalization. Ventilation and perfusion distribution maps were divided in four regions of interest. Electrical impedance tomography perfusion was estimated by the first-pass kinetics method (5). A = anterior; C_RS = respiratory system compliance; CT = computed tomography; L = left; P = posterior; R = right.