The interstitial pulmonary oedema is a key parameter in the management of patients with chronic heart failure and an early warning sign of impending acute heart failure 1. The objective diagnosis is traditionally based on chest radiographic findings which, when performed at the bedside, may be difficult to interpret, and may have weak correlations with extravascular lung water 23. The lung is considered poorly accessible using ultrasound since air prevents the progression of the ultrasound beam with production of reverberation artefacts under the lung surface 4. The "comet-tail image" is an echographic image detectable at bedside with ultrasound probes positioned over the chest 5. This image consists of multiple comet tails fanning out from the lung surface originating from water-thickened interlobular septa. Functionally, they are a sign of distress of the alveolar-capillary membrane, often associated with reduced ejection fraction and increased pulmonary wedge pressure and they probably represent an ultrasonic equivalent of radiologic Kerley B-lines 6. These features make ultrasound lung comets (ULCs) an appealing simple clinically useful sign for detecting and quantifying extravascular lung water 7 in patients with known or suspected heart failure.

ULCs require very limited technology (2 D-echo, without Doppler functions or second harmonic option) and a short learning curve. Recently, hand-held echocardiography with compact, low cost, miniaturized, battery-driven ultrasound imaging system has became available 8.

The aim of our study is to compare the number of ULCs obtained by an experienced echocardiologist using a full feature echocardiographic platform and inexperienced sonographer using hand-held echocardiography and to evaluate the learning curve, the feasibility and the time needed for the echo lung examination.

A total of 20 paired observations have been obtained for all the enrolled patients. The feasibility of the chest ultrasound examination for the diagnosis of ULCs was 100%. The time needed for the echo lung examination was <3 minutes in all patients. A significant (<5) number ULCs have been observed in 17 patients, while 3 patients showed a number of comets <5. The mean value of ULCs was 35.7 ± 25.30; and 34.2 ± 26.8 for the low tech beginner. Two examples of two representative patients with and without ULCs, are shown in Figures 2 and 3, respectively. The appearance and numbers was similar with the hand-held echocardiography (Fig. 2 and 3, lower panels) and with the full feature echocardiographic platform (Figure 2 and 3, upper panels). There was a tight and highly significant correlation between the 2 observations in the same patient by the "high-tech veteran" and the "low-tech beginner" (Figure 4).

ULCs are based on the principle that ultrasound is reflected by an interface between different acoustic impedance. In normal conditions, the ultrasound beam finds the lung air (ie, high impedance and no acoustic mismatch on its pathway through the chest) 6. In the presence of extravascular lung water, the ultrasound beam finds sub-pleural interlobular septa thickened by oedema (ie, a low impedance structure surrounded by air and with a high acoustic mismatch). The reflection of the beam creates a phenomenon of reverberation. The result is a wedge – shaped signal with a narrow origin in the near field of the image 9. We have found that ULCs are equally reliable in the hands of highly experienced echocardiologist using expensive and complex technology and in the hands of absolute beginners with hand-held echocardiography. ULCs are easy both to obtain and to measure (learning curve of <10 examinations, 30 minutes) and fast to perform (<3 minutes), require very limited technology, even without a second harmonic or Doppler and are not restricted by cardiac acoustic window limitations or patient decubitus. The feasibility was 100%.

Since the qualitative nature of the interpretation is the recognized Achilles hell of echocardiographic techniques 1516, it is especially reassuring that both the presence and extent of ULCs can be obtained with optimal precision, with very limited training, and with very basic and unsophisticated technology. From the technological and expertise viewpoint, ULCs are the "kindergarten" of echocardiography, ideally suited for bedside evaluation of patients with both known or suspected heart failure.

ULCs = Ultrasound lung comets

The author(s) declare that they have no competing interests.

LG, GB and EP performed all of the ULCs assessments. AC and FF performed all the statistical analysis of the study. GM participated in the study design and coordination an helped to draft the manuscript. All authors read and approved the final manuscript.