By Dr Carl Hallgren
Ultrasound has long been used to diagnose pleural fluid by pulmonary physicians or radiologists, but in the hands of anesthesiologists, this instrument is nowadays readily available and useful for everyday clinical diagnostics of heart and lung function in intensive care patients.
Lung Ultrasound Examination (LUX) allows us to visualize pathological findings within the thorax cavity such as pleural effusion, pneumonia, lung consolidation, pneumothorax etc. in a simple and efficient way.
Several different ultrasound devices are available in different sizes with different capacity, where the technical development is rapidly advancing with improved functions every year. There is no consensus today regarding the choice of the best transducer (ultrasound probe) at LUX, but several types are available with different resolution and different angulation. There are curvillinear transducers, linear transducers and phased array transducers that all can be used. The curvillinear transducer is optimal for examining the pleura, the linear transducer is ideal for the surface of the pleura and the phased arrayed transducer is a reasonable selection for examining deeper structures in the lungs.
When investigating the pleura (for finding or excluding pneumothorax) it is recommended a linear, high frequency transducer that gives high resolution but does not penetrate as deeply as the pleura is a superficial structure.
In a normal, completely airy lung, the only structure you usually see with ultrasound will be the pleural line; a healthy lung parenchyma is not seen. Air in the lungs spreads the ultrasonic beams and usually no echo is reflected from deeper parenchyma.
In healthy tissues, the parietal pleura and visceral pleura adapt to each other and slide well over each other during normal breathing. In the case of pulmonary ultrasound examination (LUX), this gives the typical picture of so-called “lung sliding” (see video-clip).
An artifact that may occur when the lung is airy are so-called “A-lines”. The A-line is a horizontal artifact indicating a normal lung surface. A hyperechogenous repetitive artifact (“horizontal artifacts”) occurs at even intervals deeply related to and parallel to the pleural line (see figure). A-lines arise from the fact that some of the ultrasound beams bounce back and forth between the transducer and the pleura, some of which bounce several times. The latter takes longer time and the ultrasonic machine interpret this as an echo from a deeper depth.
A-lines appear after about 5:30 minutes inside the clip.
The finding of A-lines means that the area under the transducer is air-conducting which is normal. As the air volume in the lung decreases and the density increases, for example, with increased amount of fluid or blood, it will cause an increased number of echo points. This finding has been named “B-line”, which is seen as single or multiple vertical beams going through the thoracic cavity. The B-line is a kind of comet-tail artifact indicating subpleural interstitial edema.
B-lines appear after about 8:13 minutes inside the clip.
A B-line is, by definition, a hyperechogenous reverb artifact. The B line starts at the pleural line and stretches like a beam of light all over the screen without fading. It moves synchronously with the pleural line1. Exactly what the B line consists of is currently not certain. At LUX, a difference is made between a so-called “Diffuse Interstitial Syndrome” (DIS) with B-lines in several zones that indicates a global process in the lung, like pulmonary edema, ARDS, or lung fibrosis while focal B-lines instead indicates a localized process like pneumonia, atelectasis or lung contusion.
The number of B-lines has been shown to correlate with the degree of interstitial fluid (Extra Vascular Lung Water – EVLW), both in patients with spontaneous breathing and in patients with positive pressure ventilation 2. More than three B-lines in the same intersect indicate a pathological finding. Several B-lines mean more fluid and at fulminant pulmonary edema, the B lines will melt together to form a single large, headlight-like artifact. In order to name the finding as a diffuse interstitial syndrome, a pathological number of B lines in two or more zones are required bilaterally. Scientific studies show a correlation between the number of B-lines and biological markers for cardiac failure, such as BNP or Wedge pressure.
B-lines occur and disappear in real-time 3, which allows the use of lung ultrasound to allow dynamic monitoring and continuous evaluation of changes in pulmonary pathology. The finding of B-lines can be, for example, used in acute dyspnea to distinguish COPD exacerbation against increased interstitial fluid as seen in cardiac failure. Several studies have shown that LUX can be used alone but that the combination with pro-BNP increases the predictive value. The number of B-lines can be used as an independent prognostic indicator for negative outcome and 16 months of mortality and rehospitalization of patients with decompensated heart failure 4.
Since pneumothorax involves a separation of the visceral pleura and the parietal pleura due to air leakage, LU is well suited to diagnose and exclude pneumothorax. Studies show that LU has better sensitivity but the same high specificity as chest x-ray to diagnose pneumothorax. Since pneumothorax can be life threatening, LU contributes to high utility and the short examination time a targeted examination takes advantage of LU. Since air accumulates at the highest point, one should look for pneumothorax just at the highest point in the chest. In the lying patient, this means anteriorly but it varies with body constitution. The observation is that certain patient groups, e.g. emphysema patients may have bullied emphysema lesions located at demarcated sites. Likewise, subcutaneous emphysema can be difficult to interpret and even make the examination impossible.
There are several causes of missing lung sliding even when pneumothorax is not present, such as inflammation, previous pleurodesis, main stem intubation or apnéa. In these cases it can help to visualize a so-called “pulmonary pulse”. Pulmonary pulse implies a conception of the heartbeat that causes the pleura to pulsate synchronized with the heartbeat (video).
The presence of lung sliding or pulmonary pulse is speaking against pneumothorax . To further strengthen the diagnostics some users examine in “M mode” (motion mode) at LU. A lung that moves normally gives rise to so-called “Seashore signs“, where a clear difference can be seen between immobile subcutaneous tissue and the moving lung parenchyma (see picture). With a pathological lung that does not move alternatively, it is dislocated as with pneumothorax, the pattern in M mode will look different, with no difference between tissues above or below the pleura (see figure). The strength of LU is thus to exclude pneumothorax and progress in clinical diagnostics.
Since “lung sliding” occurs when the pleural blades slide against each other, this ultrasonic finding eliminates pneumothorax in the area under investigation. A B-line, which is defined by the visceral pleura, also excludes pneumothorax. In order to be able to determine pneumothorax with LU, it is necessary to find the so-called “pulmonary point.” It is the point that defines the pneumothorax shaft against the area of the lung where the pleural blades still slide towards each other (video). This finding is diagnostic for pneumothorax. By finding the pulmonary point, one can also quantify the extent of the pneumothorax cavity. LU allows following the course of a pneumothorax that does not require emergency drainage, which may avoid unnecessary drainage. For the diagnosis of pneumothorax, one should find the following findings:
- Lung point
- Absence of sliding
- Absence of B-lines
- Absence of pulmonary pulse
LUX may in some cases help us to distinguish between inflammation and atelectasis of the lung tissue (resorption vs compression). When a disease process results in the consolidation of lung tissue with reduced air content and increased fluid content, it can be detected by LUX.
To be detected, the process is required to be pleural. The cause may be e.g. pneumonia, atelectasis, pulmonary embolism, primary or metastatic cancer or lung contusion. Consolidation’s appearance in LUX helps in differential diagnosis. Consolidation can be a subpleural hypoechogenic dark area, but with increasing consolidation, the lung will assume a more tissue-like appearance similar to the liver, a.k.a. “hepatization” (image).
An inflammatory process (like bacterial pneumonia) can give rise to fluid effusion, which at LUX may be seen as a focal interstitial syndrome, with B-lines at the site of inflammation. The inflammation also affects the pleura and gives a thickened and irregular appearance, in contrast to pulmonary edema where the pleural line normally is narrow and sharp (image). An inflammatory process can also cause reduced or missing lung sliding. A consolidation may contain air, so-called air bronchogram. At LUX, this provides punctual or linear hyperecogenous (white) areas within the consolidation.
One may distinguish between dynamic and static air bronchogram where you in the dynamic air bronchogram can see gas bubbles move in the bronchi (video + image). This excludes atelectasis and strengthens additionally the pneumonia diagnosis 5.
With the help of ultrasound, pleural fluid can be visualized relatively easily. The fluid is usually seen as an anechogenous (dark) gap between the parietal and visceral blades of pleura 5. Frequently, also compression atelectasis of the lung bases are seen. Although the echogenicity of the fluid may give rise to suspicion of its contents, it requires the need for precision diagnosis and analysis for accurate diagnostics. 6 The puncture for pleural drainage is then made ultrasonic.
In the M mode you can see a so-called “Sinusoid sign“, which is a variation of the interpleural distance during inhalation and exhalation, as the visceral blade of the pleura during the breathing cycle exhibits a movement similar to a sinusoidal curve.
Pulmonary ultrasound is better at detecting pleural fluid and on differentiating between fluid and consolidation compared to conventional chest X-ray. Studies show that pulmonary ultrasound detects pleural fluid as good as a CT-scan. 7-8. Pleural fluid is common in intensive care patients and is a common cause of atelectasis development that can cause pathophysiological effects such as reduced compliance, impaired oxygenation and increased pulmonary vascular resistance. 1
Go forward until 23:30.
In a person in a sitting position, free pleural fluid is collected in the lower regions of the pleural cavity. The fluid is therefore readily seen in the rear axillary line at the height of diaphragm.3 In a conventional chest X-ray, rounding of the lateral costo-phrenic angle is a common sign of the presence of pleural fluid, but sometimes as much as 500 ml of pleural fluid can be found without this rounding being seen on X-ray. A rounding of the posterior costo-phrenic angle 6 can sometimes detect as little as 75 ml of fluid. The patient’s habitus, extent of lung tissue consolidation and other causes may complicate the assessment with conventional x-ray.
Since the pleura has a complex structure, it is difficult to find a definite mathematical formula to calculate the amount of pleural fluid using LU. Several formulas have been proposed that take into account body surface and multiplane analysis of the pleural fluid. Balik et al. has published a simplified formula where pleural fluid was measured as maximum separation (“sep”) between the parietal and visceral blades of the pleura in end-expiration. The simplified formula provides a quick estimate of the amount of fluid and can help the clinician in decision on pleurocentesis. Note that the patients in the study were in a lying position with a 15 degree angle of the head end. 9
Thus the volume of fluid (V) is calculated as; V (ml) = 20 x sep (mm)
In summary, it can be noted that ultrasound examination of the lungs provides excellent opportunities for visualization of pathological processes in the lungs that allow quick and easy diagnostics well suited to intensive care patients.
- Volpicelli G, Elbarbary M, Blaivas M, et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Medicine. 2012;38:577-91.
- Enghard P, Rademacher S, Nee J, et al. Simplified lung ultrasound protocol shows excellentprediction of extravascular lung water in ventilated intensive care patients. Critical Care. 2015;19:36.
- Noble VE, Murray AF, Capp R, et al. Ultrasound assessment for extravascular lung water in
patients undergoing hemodialysis. Time course for resolution. Chest. 2009;135:1433-9.
- Frassi F, Gargani L, Tesorio P, et al. Prognostic value of extravascular lung water assessed withultrasound lung comets by chest sonography in patients with dyspnea and/or chest pain. Journal ofCardiac Failure. 2007;13:830-5.
- Lichtenstein D, Meziere G,Seitz J. The dynamic air bronchogram. A lung ultrasound sign of
alveolar consolidation ruling out atelectasis. Chest. 2009;135:1421-5.
- Blaivas M, Lyon M,Duggal S. A prospective comparison of supine chest radiography and bedsideultrasound for the diagnosis of traumatic pneumothorax. Academic emergency medicine : Officialjournal of the Society for Academic Emergency Medicine. 2005;12:844-9.