What is NOT normal?

Interstitial syndromes of the lung include a variety of pathologic conditions that involve either localized or diffuse involvement of the lung. The thickened interlobular septa from fibrosis, edema or excess extravascular lung water result in a characteristic appearance on ultrasound.  In these conditions the air-filled alveoli and water-filled interstitium interact to create a reverberation artifact called B-lines. B-lines are characterized as (figure 4):

• Sharp, vertical lines
• Arising from the pleura and extending to the edge of the screen
• Move with respiration
• Erase A-lines

Figure 4:  Sonographic B-line

 

While one or two B-lines can sometimes be seen normally in dependent portions of lung or areas of interlobar fissures, when more than three are seen in one field of view they are considered pathological. In addition, when B-lines are seen diffusely in the chest in multiple fields of view, particularly in non-dependent areas they indicate pathology.

 

Quantifying B-lines

In the acute setting, a qualitative assessment of LUS findings is usually adequate. The number of B-lines on ultrasound correlates well with disease severity as well as response to therapy. In less severe diseases B-lines appear multiple and discrete, as disease progresses, they can become confluent giving a “white out appearance of the lung (figure 5). Studies have shown that B-lines rapidly resolve in response to treatment for heart failure and through the course of dialysis for those with ESRD [11]. In addition, B-lines and other abnormalities found in patients with viral pneumonia, ARDS and bronchiolitis resolve along with their clinical course [1,3].

 

Figure 5:  In severe disease B-lines coalesce and form confluent B-lines resulting in a “white out” appearance of the lung.

 

In non-critical patients a more careful assessment with quantification of B-lines can be useful for assessment and monitoring response to therapy. While beyond the scope of this module there are several techniques described to quantify B-lines.

 

Interpreting B-lines

Sonographic B lines have multiple causes including pulmonary edema, infection, ARDS, contusion, and fibrosis; US interpretation must occur within the clinical context. Focal B-lines with associated pleural line abnormalities or consolidation indicate pneumonia or contusion (video 2).

Video 2: Focal B-lines adjected to pleural irregularity with consolidation in bacterial pneumonia

 

Multifocal but patchy B-lines with spared areas can be seen in bronchiolitis, viral pneumonia, pneumonitis, contusion, ARDS and fibrosis—often along with pleural line abnormalities and/or small subpleural consolidations. Finally diffuse and homogenous B-lines with a regular pleural line is a pattern expected pulmonary edema or fluid overload (video 3) [2]. Generally, the patient’s history and clinical information can help guide the diagnosis improving the sensitivity and specificity of LUS in diagnosing specific etiologies.

 

Video 3: Diffuse and symmetric B-lines with a normal pleural line in a patient with congestive heart failure.

 

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 periosteum
• 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
• Monitoring for volume overload

 

Equipment

• Ultrasound machine
• High frequency linear probe (6-12 MHz) is most used, although a curvilinear may be better especially in older patients. A 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 shallowest depth that gives an appropriate image to maximize resolution.
   2. Orient the probe marker to the patients’ head and identify the pleural line deep to the ribs in the longitudinal orientation. Ensure the probe is perpendicular to the chest wall to give the clearest image of the pleural line possible.
   3. Scan the patient’s chest systematically: this can be done by dividing the chest into anterior and lateral areas and by investigating each zone superiorly and inferiorly (figure 1). Alternatively, you can choose to investigate each rib space by sliding the probe from cranial to caudal until the diaphragm is reached in each zone as you would in pneumonia, including the posterior zone (figure 2).
   4. For any abnormalities detected, the area should be investigated fully by sliding the probe along the rib space or changing the probe to a transverse view to scan in-between the ribs.

 

Figure 1: 8 zone technique for interstitial disease

 

Figure 2: Alternative technique similar to pneumonia

 

Tips:

• It may be easiest to position the child seated in a parents’ arms
• For lateral and posterior views lifting the arm or crossing arms in front of the body can allow greater access to the chest superiorly
• Consider warming the gel
• Consider the size of the patient when selecting a probe
• Consider using the “lung preset” if available on your US machine

Introduction

Point of Care Ultrasound (PoCUS) is the use of portable ultrasonography to answer specific, focused clinical questions at the bedside. It is an extension of both our clinical acumen and physical exam.

The most common initial test for the dyspneic patient is chest x-ray. Recently, growing evidence has shown that PoCUS can reliably detect interstitial edema or thickening with equal, if not better, sensitivity than CXR. While CT scanning provides the best test characteristics, it is impractical and comes with the cost of significant radiation.

Ultrasound images of normal lung show predictable artifacts. These artifacts are disturbed in disease states and can be readily differentiated by ultrasound.

 

Why Ultrasound?

Traditionally, it was the thought was that ultrasound would not be a useful modality for investigating lung pathology because air scatters ultrasound waves. However, lung pathology (i.e. viral pneumonia/pneumonitis, cardiogenic pulmonary edema, contusion, pulmonary fibrosis, ARDS) will often lead to edema and fluid accumulation in the intersitium and alveoli. If this fluid or thickening reaches the pleural line these pathologies can be seen on ultrasound. This is the case in most patients, particularly children who have smaller lungs. This is supported by an ever-growing body of evidence.

International evidence-based guidelines have been published which support the use of point-of-care lung ultrasound in investigating various pulmonary pathologies, including interstitial disease [1]. In fact, LUS far outperforms radiography in the detection of interstitial diseases including viral pneumonia/pneumonitis, pulmonary edema, and pulmonary fibrosis [1,2]. In COVID-19 patients, LUS far outperformed conventional radiography, performing nearly as well as CT while decreasing radiation exposure and patient movement within hospital [3].  A recent meta-analysis looking at the test characteristics of LUS for pulmonary edema found a pooled sensitivity of 94% and specificity of 92% [4]. The test characteristics of LUS are superior to that of CXR in the diagnosis of acute pulmonary edema with the sensitivity of CXR ranging from 14-68% and specificity of 53-96% [5-7]. Also appealing, LUS is easier to interpret than CXR and the inter-reader reliability is consistently higher as well [8]. While these are adult studies and the test characteristics need to be studied in children, it is well proven that LUS performs at least as well in the pediatric population.

Logistically, a typical bedside ultrasound looking for pulmonary edema will take less than 5 minutes [9]. While system implications have not been studied for those with interstitial syndromes, a recent RCT studying the use of PoCUS in the diagnosis of pneumonia showed a 30-60% reduction in the use of CXRs, as well as significantly shorted ED length of stay in those patients’ receiving ultrasound vs. CXR [10].  Similar benefits could be expected in those with interstitial syndromes.

References

1. Patel, G et al. Point-of-Care Cardiac Ultrasound (POCCUS) in the Pediatric Emergency Department. Clinical Pediatric Emergency Medicine. 2018. 19: 323-327. DOI: 10.1016/j.cpem.2018.12.009
2. Lanoix et al. A Preliminary Evaluation of Emergency Ultrasound in the Setting of an Emergency Medicine Training Program. Americal Journal of Emergency Medicine. 2000. 18:41-45. DOI: 10.1016/S0735-6757(00)90046-9
3. Mayron, R et al. Echocardiography Performed by Emergency Physicians: Impact on Diagnosis and Therapy. Annals of Emergency Medicine. 1988. 17: 150-154. DOI: 10.1016/S0196-0644(88)80301-9
4. Plummer, D et al. Emergency Department Echocardiography Improves Outcome in Penetrating Cardiac Injury. Annals of Emergency Medicine. 1992. 21: 709-712. DOI: 10.1016/S0196-0644(05)82784-2
5. Mandavia, D et al. Bedside Echocardiography by Emergency Physicians.  Annals of Emergency Medicine. 2001. 38: 377-382. DOI: 10.1067/mem.2001.118224
6. Ma, J et al. Prospective Analysis of a Rapid Trauma Ultrasound Examination Performed by Emergency Physicians. The Journal of Trauma: Injury, Infection, and Critical Care. 1995. 38: 879-885. http://ovidsp.ovid.com/ovidweb.cgi?T=JS&PAGE=reference&D=ovftb&NEWS=N&AN=00005373-199506000-00009
7. Miller AF, Arichai P, Gravel CA, et al. Use of Cardiac Point-of-Care Ultrasound in the Pediatric Emergency Department. Pediatric Emergency Care. 2020 Oct. DOI: 10.1097/pec.0000000000002271. PMID: 33122503.
8. “Pericardial vs Pleural Effusion.” Temple Emergency Ultrasound, Feb 8 2018. https://www.templeemergencyultrasound.com/tips-tricks/2018/2/3/pericardial-vs-pleural-effusion

Summary

• POCUS can easily identify pericardial effusions with good sensitivity and specificity.
• Indications for performing PoCUS looking for pericardial effusion include blunt and penetrating trauma, chest pain, unexplained hemodynamic instability, or dyspnea.
• In the subxiphoid view free fluid is first visualized in the pericardium between the myocardium and liver.
• In the parasternal long axis, a pericardial effusion will be found between the heart and the descending aorta.

Pitfalls

Subxiphoid View

False Positive – Epicardial Fat Pad

• • An epicardial fat pad can be found in young healthy individuals.
  • It will move synchronously with the heart and will not encompass the heart.
  • The epicardial fat pad will also be more prominent anteriorly, while PCE will be more prominent posteriorly.

 

FIGURE 13: Epicardial fat pad on subxiphoid view.

 

Parasternal Long View

False Positive – Pleural Effusion

• • Pleural effusions can be seen on the parasternal long view but will typically be found in the far field, deep to the descending aorta (reminder: a pericardial effusion will be seen near field to the descending aorta).

What is NOT normal?

Subxiphoid View

A pericardial effusion will first be visible in the posterior part of the pericardium given the patient is laying supine. The fluid will be seen between the myocardium and the liver. As fluid accumulates, the effusion will progress to encompass more of the heart. Effusion will be seen as an anechoic collection surround the heart, as if the heart is beating within a sac of fluid.

FIGURE 10: Pericardial Effusion Subxiphoid View

 

Parasternal Long View:

A pericardial effusion will be seen between the left atrium and ventricle and the descending aorta.

It will again appear as an anechoic, black, collection on your screen.

FIGURE 11: Pericardial Effusion Parasternal Long View

 

How to Distinguish Between Pleural Effusion and Pericardial Effusion

Pericardial Effusion

1. Crosses the midline
2. Separates descending aorta from pericardium
3. Beware tamponade physiology

 

Pleural Effusion

1. Accumulates posterolateral to the descending aorta
2. Typically, not present in the subxiphoid view given no pleural reflection between the liver & heart

FIGURE 12: Pericardial effusion vs pleural effusion on parasternal long view [8].

 

Tamponade

Cardiac Tamponade is a clinical diagnosis. Hemodynamic instability within the context of pericardial effusion should be taken seriously and prompt consideration of tamponade. While there are sonographic clues to tamponade these are beyond the scope of this module and will be discussed in a future ”advanced“ module

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.