Design
This multicentre observational study was conducted in four university hospitals. We performed a retrospective analysis comparing the result of LUS examinations and chest CT, performed as part of routine care, in a convenience sample of patients in either the Emergency Department (ED) or the Intensive care unit (ICU). The ED and ICUs were located in different institutions.
Ethical considerations
The study was approved by the Committee for Research Ethics of the French Society of Anesthesia and Intensive Care Medicine (CERAR IRB000102542020-062). In accordance with French law, patients were informed regarding the use of their data for publication [16].
Population
We included adult patients admitted to the ED or ICU with confirmed SARS-CoV-2 infection, diagnosed by acute dyspnoea (SpO2 < 94% and/or breathlessness [17]) together with a positive polymerase chain reaction (PCR) test in a nasopharyngeal or bronchoalveolar sample, who had a LUS exam at admission as well as a chest CT within the 24 h following the LUS.
Clinical features
At inclusion, each patient had a standard medical history and examination, monitoring of heart and respiratory rate, blood pressure and SpO2, and arterial blood gas analysis.
For patients who were breathing spontaneously, FiO2 was calculated as follows: FiO2 = (21 + 3* oxygen flow (L/min)/100) [18].
For the purposes of analysis, we defined a low SpO2/FiO2 ratio as < 357 and high SpO2/FiO2 ratio as ≥ 357. This cut-off was chosen as it is equivalent to a PaO2/FiO2 ratio of 300 mmHg [19].
LUS and chest CT examination
LUS was performed within the first 2 h after admission. LUS was performed by imaging 12 lung regions, modified for critical illness. Thus, we imaged the posterior areas behind the posterior axillary line rather than in the paravertebral areas to avoid turning completely the patient [20]. LUS examinations were performed by emergency physicians or intensivists in charge of the patient [21, 22]. The skill of the operators was rated as follows [21]:
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Level 3 operator; LUS academic teacher with several publications in the field
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Level 2 operator; more than 25 supervised procedures and 200 non-supervised procedures
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Level 1 operator; at least 25 supervised procedures and less than 200 non-supervised procedures
Chest CT was performed at an appropriate time during the clinical course using a 128-slice CT (OPTIMA CT660, GE Healthcare, Chicago, Illinois, US) in the supine position, with the patient instructed to hold their breath after a deep inspiration. Most CT scans were non-contrast, low-dose chest CT.
Further details of the diagnostic procedures are available in the Electronic Supplemental Material (ESM 1).
Sample size considerations
The sample size calculation was performed to determine whether an area under the curve (AUC) of ≥ 0.80 was achieved for a receiver operator characteristic (ROC) plot of LUS versus chest CT scan. Based on the unpublished data, with a precision of 10% and an expected proportion of severe pneumonia on chest CT scan of 40%, the sample size required was 87. Taking into account the potential for incomplete data from LUS or chest CT scans, we included 100 patients.
Statistical analysis
The characteristics of the patients are summarized as medians and interquartile ranges for continuous variables, and as numbers and percentages for qualitative variables. Comparisons of patients’ characteristics, CT and ultrasound parameters were performed between patients managed in the ED versus the ICU. The LUS score was compared between the three severity grades (mild, moderate, and severe pneumonia) on CT scan using an ANOVA test. The receiver operator characteristic (ROC) curve and AUC estimates were determined for the relationship of LUS score and CT scan to diagnose severity of pneumonia. The optimal threshold for best discrimination between non-severe and severe pneumonia was calculated using the Youden index. Sensitivity (Se), specificity (Sp), negative predictive value (NPV), positive predictive value (PPV) and DA are provided with their 95% confidence intervals (CIs). A grey zone represents a predictive test of low accuracy, that is, the Se and Sp are both < 90% [23]. Se and Sp curves were constructed to calculate the grey zone for an LUS score that was inconclusive for predicting severe pneumonia [24]. ROC and AUC, the optimal thresholds, Se, Sp, NPV, PPV and DA were determined for LUS score to diagnose consolidation, interstitial syndrome, pleural effusion and pleural irregularity according to the chest CT findings and use of mechanical ventilation. Mean LUS scores were compared between low and high SpO2/FiO2 ratios. For all calculations, R software (R Development Core Team) and SPSS Statistics for Windows, Version 20.0 (IBM, Armonk, NY) were used. The significance level was set at p < 0.05.