Review article
Ultrasound Lung Comets: A Clinically Useful Sign of Extravascular Lung Water

https://doi.org/10.1016/j.echo.2005.05.019Get rights and content

Assessment of extravascular lung water is a challenging task for the clinical cardiologist and an elusive target for the echocardiographer. Today chest x-ray is considered the best way to assess extravascular lung water objectively, but this requires radiology facilities and specific reading expertise, uses ionizing energy, and poses a significant logistic burden. Recently, a new method was developed using echocardiography (with cardiac probes) of the lung. An increase in extravascular lung water—as assessed independently by chest computed tomography, chest x-ray, and thermodilution techniques—is mirrored by appearance of ultrasound lung comets (ULCs). ULCs consist of multiple comet tails originating from water-thickened interlobular septa and fanning out from the lung surface. The technique requires ultrasound scanning of the anterior right and left chest, from the second to the fifth intercostal space. It is simple (with a learning curve of < 10 examinations) and fast to perform (requiring < 3 minutes). ULC assessment is independent of the cardiac acoustic window, because the lung on the anterior chest is scanned. It requires very basic 2-D technology imaging, even without a second harmonic or Doppler. ULCs probably represent an ultrasonic equivalent of radiologic Kerley B-lines. On still-frame assessment, cardiogenic watery comets can be difficult to distinguish from pneumogenic fibrotic comets, although the latter are usually more localized and are not dissolved by an acute diuretic challenge. Functionally, ULCs are a sign of distress of the alveolar-capillary membrane, often associated with reduced ejection fraction and increased pulmonary wedge pressure. The ULC sign is quantitative, reproducible, and ideally suited to complement conventional echocardiography in the evaluation of heart failure patients in the emergency department (for the differential diagnosis of dyspnea), in-hospital evaluation (for tailoring diuretic therapy), home care (with portable ultrasound), and stress echocardiography lab (as a sign of acute pulmonary congestion during stress). In conclusion, ULCs represent a useful, practical, and appealingly simple way to image directly extravascular lung water.

Section snippets

Historical background

The “comet-tail” sign was first described in 1982 concerning an intrahepatic shotgun pellet,2 giving rise to a roughly vertical narrow-based artifact spreading up to the edge of the screen. Subsequently, it was noted at the lung surface in normal or pathological conditions, including lung sarcoidosis.3 Following these initial ultrasound anecdotes, Lichtenstein et al first described the potential usefulness of comet-tail artifact as a diagnostic marker of alveolar-interstitial syndrome.1, 4, 5, 6

Physical basis and differential diagnosis

All diagnostic ultrasound methods are based on the principle that ultrasound is reflected by an interface between media with different acoustic impedance. In normal conditions, with the transducer positioned on the chest wall, the ultrasound beam finds the lung air (ie, high impedance and no acoustic mismatch on its pathway through the chest).1 In the presence of extravascular lung water, the ultrasound beam finds subpleural interlobular septa thickened by edema (ie, a low-impedance structure

Methodology

The echocardiographic examination is performed using any commercially available 2-D scanner, also portable, with any transducer frequency (from 1.6 to 5 MHz). There is no need for a second harmonic or Doppler imaging mode. The echocardiographic examinations are performed with patients in a near-supine or supine position (Figure 5). Ultrasound scanning of the anterior and lateral chest is obtained on the right and left hemithorax, from the second to the fourth (on the right side to the fifth)

Extravascular lung water: Clinical validation

There are 2 other main methods of assessing extravascular lung water in the clinical setting: chest x-rays (used extensively in the clinical arena) and a catheter-based thermodilution technique (sometimes used in intensive care).8, 9 The ULC sign was compared with each of these 2 gold standards, and results were reassuring.

Nevertheless, each of the 2 standards suffers important limitations. Usually, chest x-ray allows adequate recognition of pulmonary edema, with signs evolving as a function of

Echocardiographic description

At present, we have incorporated in our laboratory ULC assessment for all patients with dyspnea and with known or suspected heart failure. For clinical and research purposes, the best approach is to describe the “comet score,” summing up all scores of all scanned spaces. This requires < 3 minutes and is added to our echo data bank. For clinical purposes, the response in the written report forwarded to the patient and the referring physician can be coded in a semiquantitative fashion (from

Potential clinical impact

At this point, ULCs represent a useful, practical, appealingly simple way to assess extravascular lung water. This sign should be systematically assessed for in the echocardiographic evaluation of patients with dyspnea and/or known or suspected heart failure. Although cardiologic research with ULC has just started, we can already identify some clinically relevant correlates of this sign. It is directly related to radiologically assessed extravascular lung water7 and is associated with a rise in

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