What is normal?

Our area of interest is the space between the pericardium and the myocardium of the left ventricle, extending to the apex of the heart. It is normal for some patients to have visible trace pericardial fluid, which will act as a lubricant for cardiac motion. This, however, will be minimal.

 

 

FIGURE 8: Subxiphoid. Physiologic free fluid.

 

FIGURE 9: Subxiphoid. Pericardial fat pad.

It is also normal for some patients to have a pericardial fat pad. This fat pad sits between the pericardium and the myocardium and has a similar, black, appearance. Unlike fluid, however, the fat pad will be more obvious anteriorly, and disappear as you move posteriorly. Unlike fluid which will move posteriorly to the most dependent area. Also, the fat pads will have very little variation with contraction, while pericardial effusions will show significant variation with contraction.

References

1. Orso D, Ban A, Guglielmo N. Lung ultrasound in diagnosing pneumonia in childhood: a systematic review and meta-analysis. J Ultrasound 2018;21:183– 95.
2. Balk, DS, Lee, C, Schafer, J, et al. Lung ultrasound compared to chest X-ray for diagnosis of pediatric pneumonia: A meta-analysis. Pediatric Pulmonology. 2018; 53: 1130– 1139. https://doi.org/10.1002/ppul.24020
3. Pereda et al. Lung Ultrasound for the Diagnosis of Pneumonia in Children: A Meta-analysis. Pediatrics (2015) DOI: 10.1542/peds.2014-2833
4. Nazerian et al. Accuracy of lung ultrasound for the diagnosis of consolidations when compared to chest computed tomography (2015) DOI:10.1016/j.ajem.2015.01.035
5. Jones BP, et.al Feasibility and Safety of Substituting Lung Ultrasonography for Chest Radiography When Diagnosing Pneumonia in Children A Randomized Controlled Trial. CHEST 2016; 150(1):131-138
6. Tsou PY et al Diagnostic Accuracy of lung ultrasound performed by novice versus advanced sonographers for pneumonia in children: A systematic review and meta-analysis. Academic Emergency Medicine. 2019; 26(9): 1074. doi: 10.1111/acem.13818
7. Gravel CA, Neuman MI, Monuteaux MC, Neal JT, Miller AF, Bachur RG. Significance of Sonographic Subcentimeter, Subpleural Consolidations in Pediatric Patients Evaluated for Pneumonia. J Pediatr. 2022 Apr;243:193-199.e2. doi: 10.1016/j.jpeds.2021.12.052. Epub 2021 Dec 27. PMID: 34968499.
8. Dietrich CF, et.al. Lung B-line artefacts and their use. Journal of Thoracic Disease, Vol 8, No 6 June 2016. doi: 10.21037/jtd.2016.04.55
9. Cox M, et al. Pedaitric chest ultrasound: A practical approach. Pediatr Radiol (2017) 47:1058–1068. DOI 10.1007/s00247-017-3896-8

Summary 

• PoCUS is sensitive, specific, and reliable for the detection of pulmonary consolidations
• Characteristic findings of bacterial pneumonia include hypoechoic subpleural consolidation with and irregular posterior border and hepatization of lung tissue with air bronchograms
• Consolidation greater than 1cm in size correlates best with the presence of bacterial pneumonia
• LUS for the detection of pneumonia can quickly, easily and accurately be performed at the bedside

Pitfalls

Disease states other than pneumonia can also result in appearance of consolidation on ultrasound—just as in X-ray. Consolidation is not specific to pneumonia and must be clinically correlated. Any disease state that results in loss of aeration of alveoli either through fluid accumulation or collapse, including pulmonary contusions, hemorrhage, and atelectasis, can appear as areas of sonographic consolidation.

While there are some findings that can help differentiate alveolar disease from atelectasis, these are not always reliable. In pneumonia, gas moving within the bronchioles of consolidated lung can sometimes be seen and will appear as bright dots moving in a liner fashion through an area of consolidation. These are known as dynamic air bronchograms and are generally thought to rule out atelectasis. Atelectasis can also be differentiated from pneumonia in that surrounding b-lines are unlikely to be present. Color doppler can be applied to the area of hepatization to assess flow to the area, which will be decreased or absent in cases of atelectasis but normal or increased in pneumonia.

Video 2: Dynamic air bronchogram differentiating consolidation from atelectasis.

 

In children the thymus can sometimes be visualized and confused for hepatized lung. The thymus appears as hypoechoic structure with bright echogenic foci scattered throughout and is often described as a “stary sky” appearance. Apart from its anatomic location on the chest, the thymus also has clearly demarcated margins due to a fibrous capsule and no surrounding b-lines or shred sign. Additionally, if color doppler is applied there is hypo vascularity compared to an area of consolidated lung.

Video 3: Thymus, well demarcated hypodense area with a “starry sky” appearance.

If the probe is not directly over the lung but is over the liver or spleen, the diaphragm can at times act as a reflective surface giving the mirror image of the liver or spleen appearing above the diaphragm. This should not be mistaken with hepatized lung—hepatized lung can be imaged directly through the pleura and will not disappear with fanning of the probe. A mirror image will disappear as the transducer is fanned or moved cranially above the diaphragm.

Ultimately clinical context is most useful in determining the etiology of positive ultrasound findings.

Also remember:

• Some areas of lung are difficult to access and image (supraclavicular, scapular, axillary)
• Consolidation that does not reach the pleural line cannot be seen with LUS
• Early pneumonia without consolidation will mimic other aetiologies

What is NOT normal?

As pneumonia develops edema begins to collect in the lung, allowing for visualization of lung on ultrasound. B-lines represent thickening of the lung parenchyma—most often secondary to edema and appear as sharp, vertical lines arising from the pleura and extending to the base of the image on the screen. B-lines obscure A-lines and move in concert with lung sliding (figure 4). 1-2 B-lines per field of view can be normal in dependent regions or areas of interlobar fissures and should be clinically correlated. Localized B-lines may be the only finding early in disease course and they can often be found adjacent to areas of consolidation. While focal B-lines may indicate a bacterial pneumonia they are non-specific and can also be seen in viral etiologies. Additionally, when B-lines are found diffusely they are more likely to indicate other disease states such as cardiogenic pulmonary edema or viral pneumonia (8). B-lines and interstitial syndromes are discussed in detail in a dedicated KidSONO module.

Figure 4: B-lines, indicating interstitial thickening

 

As pneumonia progresses and fluid starts to leak into alveoli, dark, irregular subpleural consolidations can be seen.  They are backed by an irregular posterior border indicating transition from non-aerated to aerated alveoli—this is called the shred sign (figure 5). Subpleural consolidations that are greater than 1cm in diameter are generally considered to be indicative of bacterial pneumonia—smaller consolidations can also be found in viral processes such and bronchiolitis and viral pneumonias so clinical judgement is needed.

Figure 5: Sonographic consolidation with hypoechoic collection abutting the pleural line with an irregular posterior border– the “shred sign”

 

In more severe disease a dense area of consolidated lung begins to look like liver parenchyma on ultrasound. This is described as “hepatization” (figure 6).

Figure 6: Advanced disease with dense consolidation known as “hepatization” of the lung

 

Keep in mind LUS is also more sensitive than CXR for detecting pleural effusions, with just 10 mL detected on LUS, in comparison to 200mL required to be detected on CXR (9). If a parapneumonic effusion is present, LUS will easily find the anechoic area, typically at the base or more dependent areas. PoCUS for the detection of pleural effusions is covered in a dedicated module.

What is normal?

In normal lung, the pleura is easily visualized as ultrasound waves are deflected by aerated lung. Reverberation artifacts from the pleural line (secondary to the significant change in acoustic impedance at the pleural-lung interface) generate horizontally arranged artifacts called A-lines (figure 2). Other normal structures visualized are the rib and corresponding rib shadowing below, and at the base of the lung the double line of the diaphragm muscle (figure 3).

 

Figure 2: Normal lung with A-line reverberation artefact

Figure 3:  Normal lung base with the beginning of the diaphgram noted on screen right

 

What am I looking at?

It is first important to note the following normal anatomic structures and their sonographic appearance (video 1).

Chest wall:

•  Most superficial structure
•  Hypoechoic with irregular fascial lines

Ribs:

• Oval, hyperechoic periosteumo
• Dark shadow behind

Pleural line:

• Hyperechoic horizontal line
• Runs between and deep to the ribs

Lung:

• Deep to the pleural line
• Uniform grey haze

Video 1: Normal lung anatomy

Indications

• Dyspnea
• Cough
• Fever without a source

 

Equipment

• Ultrasound machine
• High frequency linear probe (6-12 MHz) is most used, although a curvilinear or phased array probe may also be used
• Gel

 

Technique 

1. Position the patient: It is easiest to completely expose the thorax. LUS can be conducted with the patient in any number of positions; in the parent’s arms (helpful for posterior exam), seated on or lying in a stretcher (lateral decubitus can be used to examine the posterior chest).
2. Warm the gel if possible: Younger or sleeping patients may respond better if the gel is warmed. Consider warming the gel between your gloved hands and applying a layer of warmer gel to the chest.
3. Scan the patient:
   1. Set the depth to between 5 – 10 cm, choosing the smallest depth that gives an appropriate image to maximize resolution.
   2. Hold the probe with the marker pointed towards the patient’s head. Starting most cephalad with the probe in a sagittal orientation scan from the lung apex to the diaphragm.
   3. Complete this process in 6 planes: posterior between the spine and the scapula left and right chest, at the mid-axillary line left and right chest, and anteriorly at the midclavicular line left and right chest (figure 1)
   4. For any abnormalities detected, the area should be investigated more by sliding the probe along the rib space or changing the probe to a transverse view to scan in-between the ribs.

Figure 1: Scanning technique for pneumonia

Introduction

Pneumonia is a leading cause of death in children around the world. The most common initial test for pneumonia is chest radiograph/x-ray (CXR) despite it having limited sensitivity and exposing children to radiation. Growing evidence shows that point-of-care ultrasound (PoCUS) can reliably detect lung consolidation with equal, if not better, sensitivity than CXR. Computed tomography (CT) provides the best test characteristics but is impractical and carries the cost of significant radiation.

Lung ultrasound (LUS) has excellent test characteristics, is noninvasive, delivers no ionizing radiation, and does not require a patient to be moved to a radiology department. For these reasons there is great potential for its use at the bedside in the diagnosis of pneumonia in children.

 

Why Ultrasound?

Given that air scatters ultrasound waves, it was traditionally thought that lung ultrasound would not be useful to detect pathology. However, over the last two decades there is a growing body of evidence supporting the use of ultrasound in various lung pathologies. Since lung pathology, such as pneumonia, leads to edema and fluid accumulation within the alveoli, areas of consolidation can be seen on ultrasound as long as this fluid reaches the pleural line. Fortunately, this is the case in most patients, particularly in children who have small lungs.

Several meta-analyses examining test characteristics of LUS for pediatric pneumonia have been published (1,2,3). The initial meta-analysis published in 2015 found a pooled sensitivity of 96% and specificity of 93% and an area under the ROC of 0.98 (3). All studies revealed LUS as equal if not superior to detecting consolidation compared to CXR (1,2,3). In the adult literature, a study comparing the test characteristics of LUS and CXR to the gold standard CT revealed a significantly better sensitivity of lung ultrasound: 86% vs 64%, with similar specificities. (4)

Jones et al. published a randomized-control trial examining the feasibility of replacing CXR with LUS for the diagnosis of pneumonia in children. They showed a 30-60% reduction in the use of CXR, depending on the level of experience of the ultrasonographer, as well as decreased ED length of stay. The inter-rater reliability for LUS in this practical study was 0.81, showing excellent agreement (5). A meta-analysis examining accuracy of LUS in novice vs advanced sonographers did find that novice sonographers had decreased accuracy, but the sensitivity and specificity remained 80% and 96% respectively, with an area under the ROC of 0.97 (6). Though, the question does remain as to what makes someone novice vs expert in LUS.

Additional questions about LUS remain to be answered. Specifically, what is the value of each abnormal finding on LUS and which findings are indicative of bacterial pneumonia necessitating antibiotic treatment? Studies agree that findings such as hepatization, air bronchograms and large subpleural consolidations are consistent with bacterial pneumonia, but focal B-lines, small subpleural consolidation and irregularities of the pleural lining can also be found in viral etiologies. A recent study looking at the significance of sub-centimeter, subpleural consolidations concluded that as an isolated finding, these are not indicative of bacterial pneumonia (7). Further study in this area continues to emerge. Jones et al. raised caution that LUS could unintentionally increase antibiotic usage rates if there is no clear definition of bacterial pneumonia (5).

 

Modality	Sensitivity	Specificity	Area under the ROC
LUS	95.5% (93.6 – 97.1)	95.3% (91.1 – 98.3%)	0.98
CXR	86.8% (83.3 – 90.0)	98.2% (95.7 – 99.6)

 

What am I looking at?

Subxiphoid View

FIGURE 5: Subxiphoid view labeled

The liver can be found in the near field, with the heart in the far field. In standard convention, the right side of the heart will be on the left and in the near field of your screen, with the left side of the heart or the right and in the far field of the screen. The ventricles are found in the near field, with the atria in far field.

The border of the right ventricle and the intra-ventricular septum form a “7” in this view.

 

FIGURE 6: Subxiphoid view illustrating the area of interest (pericardial space).

 

Parasternal Long View:

FIGURE 7: Parasternal long view labeled

This view allows you to see the long axis of the heart. You will see three main sections of the heart moving parallel across the screen. In the near field, you will see the right ventricle. The aortic valve and left ventricular outflow track are found on the left side of the screen in mid field. The third parallel section is the left atrium and mitral valve flowing into the left ventricle. The descending aorta is found in the far field.