In this prospective observational study, we found poor agreement between auscultation and LUS for the diagnosis of pulmonary edema in acutely admitted critically ill patients.
Several previous studies focused on the diagnostic accuracy of LUS compared to other imaging modalities, such as chest X-ray and CT scan [4, 10, 20]. However, few studies have compared the diagnostic accuracy of LUS with the stethoscope, one of the most frequently used instruments at the bedside. Lichtenstein et al. prospectively compared the diagnostic performance of auscultation, LUS, and chest X-ray for detecting alveolar consolidation and alveolar-pulmonary edema with CT scan in 32 patients with acute respiratory distress syndrome and in 10 healthy volunteers . The authors found that auscultation had a diagnostic accuracy of 55% for alveolar-pulmonary edema, which corresponds fairly to the 67% accuracy in our study . In that study, LUS had a diagnostic accuracy of 97% for alveolar consolidation and 95% for alveolar-pulmonary edemas, and chest X-ray had a diagnostic accuracy of 75% for alveolar consolidation and 72% for alveolar-pulmonary edema . In a sensitivity analysis, we observed that the agreement and diagnostic accuracy of LUS for pulmonary edema were limited when compared to chest X-ray, which is in line with other studies .
Another study by Torino et al. prospectively investigated the agreement between auscultation and LUS in non-admitted patients before and after undergoing hemodialysis . The authors similarly found a very poor agreement (κ statistic 0.16, in this study κ statistic 0.25) between the presence of crepitations on auscultation and the presence of B lines on LUS in a total of 1106 measurements in 79 patients . Although their population seems different to ours, patients receiving dialysis may also suffer from pulmonary edema as a consequence of fluid overload. Their results and conclusions are similar to ours, and therefore, these observations may be generalizable to populations beyond the critically ill.
We found that the diagnostic accuracy of auscultation improved if patients were not mechanically ventilated; no previous study has reported this finding. Acoustic disturbances caused by the ventilators might explain the complicated appreciation of subtle auscultation findings.
Implications and generalizability
Improved diagnostic accuracy for detecting pulmonary edema could lead to improved treatment leading to increased benefits and decreased harms for the patient. In critically ill patients, typically multiple pathophysiological processes are co-occurring at the same time, which hampers the extrapolation of the test characteristics for diagnosing abnormalities in these patients, such as pulmonary edema. As some physicians still use auscultation to detect pulmonary edema, we think our study clarifies that auscultation may not be as reliable for detecting pulmonary edema as classically perceived, especially in the ICU. Ultrasonography becomes increasingly available, and our data add nuance to the discussion surrounding how this technology might be properly integrated into clinical practice in the care of the critically ill. These observations encourage further research of LUS; the need for external validation remains to increase the generalizability of this diagnostic modality.
Several limitations of this study must be acknowledged. First, the clinical examination and ultrasonography were conducted as early as possible after ICU admission which limits the applicability of use in patients with prolonged admission. Further studies should explicate how auscultation and LUS compare in other departments and more specifically other pathologies such as a pneumothorax. Second, we were not able to validate all our LUS assessments by experts, also because there are no reference standards for the interpretation of LUS. Chest X-ray and CT are other diagnostic methods that are frequently used for the assessment of pulmonary edema. However, previous studies have suggested that LUS is superior to chest X-ray and comparable to chest CT scan for diagnosing pulmonary edema [3, 8]. Therefore, we decided not to use these modalities as a reference standard and only included a sensitivity analysis of chest X-ray. We limited LUS reporting to the number of B lines per field and did not use further qualitative commentary. Third, the auscultation was not standardized. During clinical examination, researchers performed both auscultation and LUS; however, in contrast to LUS, we did not describe in detail the location of auscultation. In practice, these were similar to the LUS scan sites. Therefore, we think the influence on our results is minimal. Also, the researchers only specified whether they heard significant crepitation or rhonchi on auscultation. Other abnormal breathing sounds were not recorded and we only documented their overall presence or absence; we are unable to compare auscultation with LUS for each specific scan site. In addition, ideally, we ask the patient to cough to distinguish between rhonchi and/or crepitations. Unfortunately, the large majority of the patients in the ICU are not cooperative with this request. Fourth, even though the researchers who performed the measurements were not involved in patient care, they were not blinded for patient information, such as admission diagnoses, other clinical variables and the results of auscultation when performing the CCUS. However, as ultrasonography was always performed after auscultation, we believe it is proper to discuss this potential source of bias but do not believe that it substantially influenced our results due to the objective nature of B line appearance. Fifth, since researchers were senior medical students and junior residents, auscultation by more experienced medical doctors could potentially improve the diagnostic accuracy. Last, 83 (8%) patients were excluded from the analyses due to the absence of LUS or auscultation data. However, the relatively small proportion of this excluded patient group makes it unlikely that excluded patients would have altered the conclusions. Despite the potential biases and limitations, we showed that the agreement between auscultation and lung ultrasound was poor. This is important as current data is scarce on the diagnostic value of new non-invasive bed tools such as CCUS, especially in comparison with clinical examination in critically ill patients.