For decades, chest x-ray has been the standard of care for detecting or ruling out pneumothorax in the bronchoscopy suite. Although CT scans are considered the gold standard, use of ultrasonography is rapidly gaining acceptance for the detection of pneumothorax. Historically, the evaluation of the lung by ultrasonography was impeded by artifacts generated from the reflection and by limited penetration of sound waves. However, astute interpretation of these artifacts has paved the way for using ultrasonography in the evaluation of normal pulmonary parenchyma as well as pulmonary pathology including hydrostatic pulmonary edema, noncardiogenic pulmonary edema, and pneumothorax.1
In chest ultrasonography, presence of lung sliding is sufficient to rule out pneumothorax and has a 100% negative predictive value.2 Specifically, lung sliding involves a to and fro movement of the visceral and parietal pleura. In contrast, confirmed detection of pneumothorax by ultrasonography requires a combination of loss of lung sliding, absence of B-lines, absence of lung pulse, and presence of lung point. Lung point is the pathognomic sign of pneumothorax,3 which is the intermittent visualization of lung sliding from a mobile partially collapsed lung. In addition to these criteria, loss of lung sliding can also be demonstrated using M-mode (stratosphere sign). Interestingly, the loss of lung sliding as a sign of pneumothorax was first described in a horse in 1986 and a year later in humans.4,5 Since then a multitude of studies have assessed the use of ultrasonography in pneumothorax. Notably, 3 meta-analyses comparing the diagnostic accuracy of ultrasound with chest x-ray in the identification of pneumothorax indicate increased sensitivity, improved accuracy, and at least similar specificity.6–8 Overall, an increasing body of evidence has made a compelling case for the routine use of ultrasonography at the bedside in the diagnosis of pneumothorax in trauma patients, in patients on mechanical ventilation, and after procedures of lung biopsy.2,9
In this issue of the Journal of Bronchology and Interventional Pulmonology, Kumar and colleagues used a real-time 2D ultrasound with a high-frequency linear transducer to compare the diagnostic accuracy with chest x-ray in the detection of pneumothorax after transbronchial lung biopsy in a tertiary care hospital in India. Pneumothorax by ultrasonography was defined as the absence of lung sliding, lung pulse, B-lines, and the presence of lung point and stratosphere sign in consecutive adult patients undergoing elective flexible bronchoscopy in a single center. A majority of the studied patients had interstitial lung disease including sarcoidosis, nonresolving pneumonia, or tuberculosis. Pneumothorax was reported in 7% (8 of 113) of those undergoing transbronchial lung biopsies by ultrasonography as opposed to 6.1% (7 of 113) by chest x-ray. The authors thus reported a sensitivity, specificity, and overall accuracy of 100% for ultrasound compared with a standard posteroanterior chest x-ray in standing position.
Although encouraging, this 100% sensitivity for ultrasonography has not been described before. In the seminal article by Lichtenstein and colleagues, lung point was observed in 44 of 66 cases of pneumothorax, thereby yielding an overall sensitivity of 66%. In patients with occult pneumothorax, the sensitivity increased to 75%. In another retrospective analysis by Lichtenstein and colleagues, lung point was present in 34 of 43 cases of occult pneumothorax, resulting in a sensitivity of 79% and a specificity of 100%. The high sensitivity observed in the present study therefore needs to be interpreted with caution, and may likely be explained by several factors. First, as compared with other studies, this study only had 8 patients with pneumothorax. Second, the mean BMI (24±3) of the studied patients was on the lower side. This is known to enable easier visualization of the pleural line and the lung point by ultrasonography, and hence reproducing the same sensitivity in patients with a higher BMI may be challenging. In addition, this study included a unique population of patients with underlying chronic lung disease as opposed to previous studies. Indeed, the presence of underlying chronic lung disease with pneumothorax rarely results in complete collapse of the lung, thereby ensuring identification of lung point. In contrast, pneumothorax in patients with no underlying lung disease can result in complete lung collapse and interference with lung point identification, which may in part explain the lower sensitivity in previous studies. Furthermore, the sensitivity and specificity of ultrasound were compared with chest x-ray, and not with a CT scan, which is the current gold standard for the detection of pneumothorax. This may have falsely improved the diagnostic accuracy of ultrasonography in the study. More importantly, all of the ultrasounds in this study were taken by a single experienced operator, and hence extrapolation of these results with novice users may not be feasible.
Nevertheless, this study has elegantly demonstrated the role of ultrasonography in monitoring the progression of lung point as a surrogate for improving or worsening of the pneumothorax and in aiding the bedside clinician in intervention for the pneumothorax. However, the use of an ultrasound has not shown superiority over chest x-ray in this study, and future studies aimed at determining relevant clinical outcomes are warranted. Given its noninvasive nature, absence of radiation, dynamic real-time images rather than static images, and easier as-needed repetition per changes in the clinical situation of the patient, ultrasound represents a valid and useful tool in the bronchoscopy suite in the hands of a trained operator. Importantly, basic ultrasound machines with a linear array transducer are commonly used for vascular access and are widely available in most US hospitals. As more evidence accumulates for the utility of ultrasonography and more trained physicians come out of fellowships, it will not be long before the chest x-ray slides out10 and ultrasound slides in.
The authors thank Nirupama Mulherkar, PhD, for writing support.
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