Lung Ultrasound: Better Than a Chest Radiograph?

Ultrasound use in the intensive care unit (ICU) has become increasingly prevalent, especially as more intensivists gain valuable training and experience in this cost-effective imaging modality. In this month’s issue of Intensive Care Medicine, Xirouchaki et al compared the diagnostic performance of lung ultrasound and bedside chest radiography (CXR) for the detection of four pathologic entities: consolidation, interstitial edema, pneumothorax, and pleural effusion.

Forty-two mechanically ventilated patients in a mixed medical-surgical ICU were prospectively enrolled in this blinded, non-randomized trial. Enrollment in the trial was triggered by the need for thoracic computed tomography (CT), which was used as the gold standard for all patients. All patients had a CXR, CT, and ultrasound examination. The primary author performed all ultrasound exams and was blinded to the CT findings; the exams were not reviewed by a blinded radiologist.

Ultrasound had superior sensitivity and specificity for the detection of consolidation, pleural effusion, and interstitial edema when compared to CXR. A positive likelihood ratio of 14.29 was reported for detecting consolidation with ultrasound (100% sensitivity, 78% specificity); a likelihood ratio of 13.4 was reported for diagnosing interstitial edema (94% sensitivity, 93% specificity). Alternatively, CXR had a sensitivity of only 38% for consolidation and a sensitivity of 46% for interstitial edema. Ultrasound identified six of eight pneumothoraces with a sensitivity of 75%. None of the pneumothoraces were clinically significant. The authors concluded that lung ultrasound demonstrated superior diagnostic performance compared to CXR, and may be considered as an alternative to computed tomography (CT) in some instances.

Since patient selection was based on a predefined need for a CT, the study was subject to considerable verification bias. Moreover, all exams were performed by one investigator, and none of the exams were confirmed by a blinded radiologist or second ultrasonographer. All patients were positioned laterally for the exam, and this positioning might have changed the localization for some abnormalities, and may not always be feasible or safe for many ICU patients in other settings. The case mix, plagued by the limitation of a small sample size, was heavily skewed towards trauma patients (n=11) and patients with sepsis (n=18), further limiting the generalizability across diverse ICU patient populations.

Notwithstanding the significant limitations, this work stands as yet another example of how ultrasound might be used in the ICU as a safer and cheaper alternative to other diagnostic modalities. Training, equipment acquisition and quality control remain significant concerns that must be addressed before ultrasound can be reliably used in place of an established “gold standard” such as chest CT.

Concise Critical Appraisal is a regular feature authored by SCCM member Samuel M. Galvagno Jr., DO. Each installment highlights journal articles most relevant to the critical care practitioner.

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