In the case of pulmonary edema this mismatch occurs between the differing impedances of air and water, however it can also occur anytime there is an area of differing impedances at the surface of the lungs such as pleuritis, fibrosis, or even chronic obstructive pulmonary disease. However, in conditions known as alveolar-interstitial syndromes an area of high acoustic mismatch occurs at the subpleural space were interlobular septa are in contact with the pleural lining. Under normal (non-edematous or fibrotic) conditions comet tails are absent because no acoustic mismatch occurs as the beam passes through the subpleural space. Instead of showing up as a distinct structure, in the right circumstances the areas of highly different acoustic impedance show up as a comet tail. These septa are below the resolution of the ultrasound beam, which can only detect objects larger than 1 mm. Interlobular septa are structures within the lung containing lymphatic vessels. In the case of comet tails this impedance occurs between fluid filled interlobular septa, with the acoustic impedance of water being 1.48x10 5gp/cm 2 and that of an air filled lung with the acoustic impedance of air being of 0.0004x10 5gp/cm 2. All ultrasound images are formed when a reflection occurs at the interface of two regions with differing acoustic impedance. They have a narrow base and form a ray spreading away from the transducer towards the bottom of the screen and synchronously move with lung respiration. With careful attention paid by the examiner at the bedside to the patient’s history and monitoring the response to treatment, the ultrasonographic finding of comet tails can be extremely useful in narrowing the differential diagnosis.Ĭomet-tails or “B-lines” are defined as hyperechoic reflections which originate only from and travel roughly perpendicular to the pleural line of the lung. Chronic conditions include pulmonary fibrosis, whereas acute entities are acute respiratory distress syndrome (ARDS), interstitial pneumonia, and acute pulmonary edema. These syndromes include conditions with diffuse involvement of the pulmonary interstitium which lead to respiratory distress through impairment of alveolar-capillary exchange. The lung ultrasound finding of “Comet Tails” has been well studied in how it relates to alveolar-interstitial syndromes. In the case of acute pulmonary edema, the practitioner using techniques of lung ultrasound, can actually visualize the edema, classify it semi-quantitatively, and prescribe interventions before other traditional diagnostic techniques such as chest radiograph can even occur. This may be due in part to poor radiographic windows of the patient or the intraobserver variability and skills of those interpreting the x-ray. Even when it is obtained, chest radiograph can have a low sensitivity for common causes of dyspnea such as pulmonary edema. The more common standard of care for quantifying pulmonary edema has been a chest radiograph which, depending on the institution, may require more time to perform and a more formal interpretation than a portable ultrasound. Additionally the exam can be repeated as often as necessary to monitor response to treatment without fear of subjecting the patient to ionizing radiation associated with conventional chest radiography.Īdvancing technology has allowed for increasingly miniaturized and portable ultrasound systems to the point where exams can be performed quickly at the bedside, often by the rounding physician. The results can help guide clinicians towards the cause of a patient’s dyspnea and in the case of pulmonary edema even semi-quantitatively asses EVLW. While often overlooked by traditional echocardiography, the lungs and evaluation of extravascular lung water (EVLW) can be assessed by direct visualization with relatively simple ultrasonographic techniques.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |