PŘEHLEDOVÉ ČLÁNKY / REVIEW ARTICLES Imaging of COVID-19 in critical care with a focus on chest ultrasound 66 | ANESTEZIOLOGIE A INTENZIVNÍ MEDICÍNA / Anest intenziv Med. 2023;34(2):61-68 / www.aimjournal.cz indicating acute disease and peripheral ground‑glass opacities on CT and, thus, as a compelling predictor of an impending positive RT‑PCR test [11, 41, 43]. Echocardiography greatly increases the specificity of isolated CUS findings. SARS‑CoV-2 related endothelial inflammation raises pulmonary artery pressure causing RV overload, especially when on mechanical ventilation (Fig. 2). Those patients may, however, present with a negative CT angiography, also due to the limited sensitivity to pick up subsegmental thrombosis which is a nature of later developing acute fibrinous organising pneumonitis (AFOP) with obstructive lung mechanics [44]. It is, however, understood that in patients who have no documented pulmonary or heart disease, it can take about 30-50% of pulmonary bed obstruction to induce pulmonary hypertension. Those with an existing pulmonary or heart disease, minor derangements of the pulmonary circulation due to inflammatory changes and microthrombi are enough to produce pulmonary hypertension [45]. Given the prothrombotic nature of the COVID-19 infection, pulmonary embolism (PE) has a cumulative incidence of as much as 30% according to the literature, and is mostly seen in ICU patients [45–48]. These patients are routinely reffered to CT pulmonary angiography (CTPA) for the diagnosis. Pulmonary angiography diagnostic yield indicated upon positivity of d‑dimers reaches 27%-30%, with the remainder of patients showing no pathology [48–50]. Combining CUS with echocardiography and Duplex ultrasound of the legs (multiorgan ultrasound) can further aid in the early detection of pulmonary embolism (PE) and avoid CTPA, thus reducing the radiation burden in all patients with suspected pulmonary embolism and in those who cannot undergo CTPA because of other factors [49, 50]. On CUS, a pulmonary infarct due to PE may be seen as a triangular subpleural consolidation in addition to suggestive echocardiography/Doppler findings [51]. The use of CUS in COVID-19 offers the detection of complications such as pneumothorax, pleural effusion, empyema, atelectasis, or cardiogenic pulmonary oedema [35, 36, 52, 53]. Pleural effusions are not commonplace in COVID-19 patients with incidences reaching 4%, and may suggest an alternate aetiology such as arrhythmia‑induced heart failure, stress cardiomyopathy, or polyserositis [15]. The incidence of barotrauma in the most severe forms of COVID-19 hovers around 12-15% and reaches 26% in high‑volume ECMO centres admitting patients and also due to their mechanical ventilation associated complications [28]. A prompt diagnosis of pneumothorax with bedside ultrasound is essential in ventilated patients [35] (Fig. 7). CUS assessment can also help to guide positioning and proning seeing that the posterior zones of supine patients are the most affected coupled with physiologic hypostatic‑hypoventilation changes occurring in the dorsal parts of the lungs whilst the anterior and lateral zones may be better aerated at CUS. The lung score can be utilised to monitor the dynamics of lung aeration of the patient [54]. With dysfunction of the diaphragm, there is a reduced diaphragmatic amplitude and thickening of the diaphragm with a concomitant adaptation of the extra-diaphragmatic apparatus whence the extra-diaphragmatic muscles are recruited and this manifests, for example, as a thickening of the parasternal intercostal muscles [22, 23]. By incorporating the lung ultrasound score and echocardiography, mechanical ventilation weaning failure can be anticipated by visualisation of aeration changes of the lung on LUS before and during the weaning. When patients are placed on mechanical ventilation with positive pressures, there is a reduction in venous return, preload, and afterload of the left ventricle (LV). Therefore, when a patient is liberated from mechanical ventilation, the ensuing decrease in the intrathoracic pressure raises the central blood volume, systemic venous return, preload and afterload of the LV. This is seen clinically as a subsequent rise in LV filling pressure and, ultimately, pulmonary oedema on CUS as an increase in the number of B-Lines [12, 38]. Limitations of CUS There are several limitations related to performing and accurately interpreting CUS. CUS is not able to visualise deep lung parenchyma and mediastinal structures. The inability to detect central lesions not abutting the pleura can be particularly limiting in morbidly obese patients with a thick chest wall. Typical limitations in the intensive care setting are also subcutaneous emphysema and extensive wound dressings. A chronic interstitial disease can make interpretation of the CUS findings difficult as can chest wall deformities and technical limitations [1, 5, 7]. Conclusion CUS is an excellent tool in the assessment of various pulmonary pathologies due to the fact that there is virtually no radiation involved and it is easy to perform at the bedside. Especially in ICU patients, there is a reduction in cumbersome transport to the CT suite and in suboptimal bedside CXR limiting adequate diagnosis. An array of pulmonary and extrapulmonary pathologies can be assessed and accurately diagnosed. However, there may be limitations impeding the CUS examination, especially subcutaneous emphysema. In the recent COVID-19 atmosphere, there is a huge potential for CUS in being used as a tool in triage, in indicating invasive measures, in the assessment of response to therapy, and as a guide in mechanical ventilation setting, weaning and patient positioning. Take‑home message: Imaging methods in severe COVID-19 Chest ultrasound (CUS) is the method of first choice in suspected severe COVID-19. CUS becomes far more specific if combined with at least basic echocardiography protocol. Radiographic methods (CT, X-ray) lack sensitivity in the first 48 h of the disease, i.e. 5-6 days after acquisition including the incubation period. Computed tomography (CT) is not indicated as a default diagnostic method in every patient with respiratory insufficiency. Calculation of percentage of diseased lung parenchyma on CT has a limited prognostic value and does not possess any therapeutic implications. Absence of significant alveolo-interstitial syndrome on CUS may exclude lung involvement and severe respiratory COVID-19 with sufficient specificity. A significant alveolo-interstitial syndrome on CUS is specific for COVID-19 pneumonitis and ARDS. CT may help in triage of intermediate severity patients where a lack of CUS specificity, if combined with echocardiography, cannot exclude COVID-19.
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