Limitations of a Lung Ultrasound in Critical Care

Dear Editor, Extensive use of bedside ultrasound has hastened the real-time diagnosis and greatly improved the patient management in critical care units. This has also led to overdependence, with clinicians and intensivists refraining from ordering additional investigations due to radiation hazards. However, in spite of being an excellent bedside diagnostic tool, the ultrasound has some common limitations, i.e., operator dependence, only a small section of the entire organ can be scanned at a time, resolution decreases with the depth, and inability to penetrate air (e.g., subcutaneous emphysema), bone, etc. Further, we have presented a few clinical conditions in which the diagnostic role of a bedside ultrasound can be limited. Tracheobronchitis: the clinical presentation may include fever, leukocytosis, and changes in the consistency and color of tracheal secretions. A lung ultrasound in these cases will usually show an A-profile with normal lung sliding, due to air around the infected areas. If treatment is not started promptly, it may progress to pneumonia. Hence, reliance solely on bedside ultrasound might delay the diagnosis. In such cases, a chest X-ray or computed tomography (CT) scan can be more helpful for the diagnosis and may show peribronchial infiltrates and bronchial wall thickening.1 Ventilator-associated pneumonia, aspiration pneumonia, or pleural effusion: micro-aspiration of oropharyngeal and gastric secretions is a risk factor for ventilator-associated pneumonia.2 Pleural effusion results from the collection of fluid in the pleural cavity, which usually accumulates in the dependent portions of the lung. If the aspirated material is located, a routine six-point ultrasound examination following the BLUE protocol posteriorly might miss the involved site. Further air in the intervening normal lung tissue might make imaging of the diseased segment difficult Figure 1, arrows in the CT showing the probable position of the ultrasound probe. A CT or chest X-ray will usually show a patch, which helps with early diagnosis.Figure 1: Chest computed tomography of a patient with pleural effusion, with arrows showing the possible location of the ultrasound probe in the supine position. (a and b) Heart, lung, pleural effusionInvasive fungal infections: the fungal lung infections are frequently caused by Candida, Aspergillus, Mucor, and Fusarium. The affected patients are usually immunocompromised, and fungal pneumonia is the most common presentation.3 The diagnostic pathway usually involves serum galactomannan assay, viral and fungal polymerase chain reaction, and imaging. Since, in most cases, pulmonary involvement is localized and nodular, with well-defined lesions surrounded by the normal lung tissue; ultrasound is not a good modality for diagnosis. A bedside ultrasound will show a normal A-profile or a few b-lines, especially in early cases. The lung CT has a high sensitivity and a high negative predictive value in cases of invasive fungal infections, and typical nodules, halo signs, reverse halo sign, and air crescent signs are only apparent on a CT. Intermediate-low to low-risk pulmonary embolism (PE): patients usually present with worsening oxygenation in spite of normal or no change in lung ultrasound or chest X-ray findings. Transthoracic echocardiography is the bedside modality of choice for an acute life-threatening PE. However, in intermediate-low to low-risk PE without the right ventricular dysfunction, the portable bedside ultrasound is of limited value. The signs of deep-vein thrombosis are present in only 47% of cases and tachycardia in 24%.4 Hence, a high index of suspicion and an early CT pulmonary angiography are warranted. In summary, any patient with worsening oxygenation, fever, or respiratory symptoms, and a normal routine bedside ultrasound, other imaging modalities should be considered early, as a delay in diagnosis based on normal ultrasound findings can be catastrophic, especially in critical patients. Authors’ contributions DS, PB: Concept and design of study. PB, JS, SS, VSA: Acquisition of data. PB: Analysis and interpretation of data. DS: Drafting or revising the article, Final approval of the version to be published. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.