Summary

• PoCUS is both sensitive and specific for the diagnosis of pneumothorax.
• Use the linear probe to scan the anterior chest in a supine patient.
• Absence of shimmering, comet tails and lung pulse indicate pneumothorax.
• Don’t forget to document your findings in the chart and save images fro QA.

References

1. Alrajhi K, Woo MY, Vaillancourt C. Test characteristics of ultrasonography for the detection of pneumothorax: a systematic review and meta-analysis. Chest 2012;141:703-8. doi:10.1378/chest.11-0131.

2. Seow et al. Comparison of Upright Inspiratory and Expieratory Chest Radiographs for Detecting Pneumothoraces. AJR 1996; 166:313-316. doi: 10.2214/ajr.166.2.8553937.

3. Murphy et al. CT and Chest Radiography are Equally Sensitive in the Detection of Pneumothorax After CTGuided Pulmonary Interventional Procedures. AJR 1990;154:45-46. doi: 10.2214/ajr.154.1.2104723.

4. Raimondi et al. Lung Ultrasound for Diagnosing Pneumothorax in the Critically Ill Neonate. J Pediatr 2016;175:74-8. doi:10.1016/j.jpeds.201.04.018

5. Cattarossi et al. Lung Ultrasound Diagnostic Accuracy in Neonatal Pneumothorax. Canad Resp J. 2016. doi 10.1155/2016/6515069

6. Liu et al. Lung ultrasonography to diagnose pneumothorax of the newborn. AJEM 2017;35:1298-1302. doi: 10.1016/j.ajem.2017.04.001.

7. Volpicelli et al. Semi-quantification of pneumothorax volume by lung ultrasound. Int Care Med 2016;40:14607. doi:10.1007/s00134-014-3402-9.

Quantifying Pneumothorax

In the unstable trauma patient, the absence of lung sliding is indication enough for chest tube placement due to pneumothorax. In the stable trauma or medical patient with absent lung sliding identification of the lung point confirms the presence of the pneumothorax and can help estimate its size. The lung point is the abrupt change from normal lung sliding to absence of lung sliding. It represents the edge of the pneumothorax (figure 6, video 6).

The lung point can be identified by rotating the probe to be in line with a rib space and following the pleural line laterally until the junction is identified.

A more posterolateral lung point corresponds to a larger pneumothorax. A lung point posterior to the mid-axillary line has an 82% sensitivity and 83% specificity for greater than 15% lung collapse on CT which is generally considered the recommended size for chest tube placement or drainage in a stable patient but again clinical judgement is warranted [7].

Note: The lung point may move subtly with respiration.

Figure 6: Lung point – CT illustration

Video 6: Lung point

Pitfalls – Cardiac Lung Point and Diaphragmatic Lung Point

There are two important mimics of the lung point, which should be recognized. The visceral and parietal pleura separate around the heart and at the edge of the diaphragm. These create a normal lung point at the left chest and inferiorly at the diaphragm (video 7&8). These are normal findings.

Care must be taken to not confuse the normal pleura at the heart or diaphragms as a lung point. In both cases, be sure to recognize the anatomy that the normal lung interfaces with. If normal lung touches pulsatile tissue in the left chest, it is likely the heart. If solid organ is visible consider that the diaphragmatic lung point and not a pathologic lung point is being identified.

Video 7: Cardiac lung point

Video 8: Diaphragmatic lung point

 

Pitfalls:

In the absence of lung sliding one must still consider the clinical context as other conditions can cause decreased or absent sliding, including:

• Large blebs
• Pleural adhesions
• Apnea
• Right main stem intubation (decreased movement on left chest)
• Dense consolidation: atelectasis, pneumonia, contusion
• Pleural effusion

In the case of apnea, main stem intubation and atelectasis often cause an artefact called the lung pulse that can be seen, differentiating these conditions from true pneumothoraces. A lung pulse is the slight motion of the pleura caused by the movement of the heart transmitted to the pleura and can only appear when the parietal and visceral pleura are opposed (video 4).

Video 4: Lung Pulse

Another cause of confusion is subcutaneous emphysema. Subcutaneous emphysema is generally associated with pneumothorax but the air in the subcutaneous tissues scatters the ultrasound beams creating a grey haze similar to lung. In this case it is often difficult to visualize the ribs or pleural line but pneumothorax can be assumed in the right clinical context (video 5).

Video 5: Subcutaneous emphysema

Rarely absent lung slide can be caused by pathology other than pneumothorax. Looking deep to the pleural line can also help distinguish absent lung sliding due to pneumothorax from absent slide due to other pathologies. On ultrasound, air appears as homogenous grey and regularly-spaced, echogenic, horizontal A-lines appear deep to the pleural line (figure 5). This pattern would be seen deep to a motionless pleural line in pneumothorax, large blebs and adhesions so the clinical context must be considered.

Figure 5: A-lines

Pleural effusions can be differentiated as a cause of absent lung sliding by anechoic fluid deep to the relatively hyperechoic pleural line. In dense consolidation caused by pneumonia, contusions and atelectasis not only is a lung pulse likely present but the lung deep to the pleural line will have absent a-lines and take on the appearance of a hypoechoic or organ-like structure.

What is NOT normal?

When air enters the pleural space separating the parietal and visceral pleura the air cannot be seen directly. The absence of shimmering and comet tails indicates the two pleural layers are not in contact as is the case with pneumothorax (video 3).

Video 3: Absent pleural motion: pneumothorax

If no shimmering or comet tails are visualized, pneumothorax is likely but the clinical context should be considered. Some conditions can mimic pneumothorax with absent or minimal pleural motion.

A pneumothorax at the level of the probe is indicated by the ABSENCE OF BOTH:

• Lung sliding
• Comet tail artifacts

Confirm with M-mode

When pleural motion is minimal it can be difficult to appreciate. If there is uncertainty M-mode can be used to help identify the presence or absence of a pneumothorax. M-mode, or motion mode, plots structures visualized on a thin line of the ultrasound image across time on the x-axis and depth on the y-axis. To use M-mode, press the “M” button on the machine and make sure the vertical marker is over a section where the pleural line is not shadowed by a rib. Identify the pleura in the M-mode image identify the brightest white line on the, M-mode view at the corresponding depth.

In normal lung, the motionless chest wall appears as horizontal lines and the and movement of pleural and lung beneath the pleura creates a grainy, coarse pattern (figure 3). When obtaining an M-mode image it is imperative to hold the probe very still on the chest wall. The movement of an agitated or very dyspneic patient can make the image difficult to interpret.

Figure 3: Seashore sign (normal)

In pneumothorax motion is absent in the chest wall, pleural line and below. This lack of motion creates smooth, horizontal lines throughout when viewed in M-mode (figure 4).

Figure 4: Barcode sign (pneumothorax)

What is normal?

Our thorax is lined by a continuous serous membrane called the pleura. The pleura is further divided into the visceral pleura which is attached to the lung and the parieta pleural which is attached to the chest wall. The pleural space is a potential space between the visceral and parietal pleura. It is normally filled with small amounts of physiologic fluid. On ultrasound, this interface is seen as a hyperechoic line running deep to the ribs. As the patient breathes the fluid moves between the pleura and is called lung sliding or “shimmering” (video 2).

Video 2: Normal Lung Sliding

In addition to shimmering, normal lung sliding leads to a reverberation artefact called a comet tail. Comet tails are formed by ultrasound pulses bouncing between the reflective surfaces of the visceral and parietal pleura and appear as vertical echogenic lines originating from the pleura which move with respiration (figure 2). Because the surfaces are not parallel many of the bouncing beams are scattered so the artefact fades as it gets further from the pleural line.

Figure 2: Comet tail artifact

A normal pleural interface at the level of the probe is indicated by the PRESENCE OF EITHER:

• Lung sliding
• Comet tail artifacts

What am I looking at?

In order to recognize a pneumothorax, 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 anatomy

Indications

• All trauma patients in whom a chest x-ray is indicated or a FAST exam is performed
• Clinical suspicion of pneumothorax
• During systematic assessment of lungs with PoCUS in dyspneic patient

Equipment

• Ultrasound machine
• Linear* or curvilinear transducer
  Gel
  *Linear probe is preferred for investigating pleural pathology

Technique

• Position the patient supine
• Place the probe longitudinally on the anterior chest between the parasternal and mid-clavicular lines below the clavicle (Figure 1).
• Identify chest wall, rib and pleural line.
• Assess for markers of lung sliding.
• Slide one rib space down and repeat.
• Slide down another rib space and repeat.
• Repeat on the opposite side.

Introduction

The most common initial test for respiratory distress is chest x-ray. Recently, growing evidence has shown that PoCUS can reliably detect thoracic pathology with equal if not better sensitivity than chest x-ray (CXR). In the case of pneumothorax PoCUS is significantly more sensitive than CXR. It also has the advantage of being performed at the bedside more quickly than CXR. Computed tomography offers little advantage over PoCUS, is impractical and comes with the cost of significant radiation.

Why Ultrasound?

Traditionally it was thought that ultrasound was not a useful modality for investigating lung pathology because air scatters ultrasound waves. There has been a growing body of evidence in recent years contradicting this practice. In fact, ultrasound images of normal lung show predictable artifacts that are disturbed in disease states and can be readily differentiated by ultrasound.

A recent meta-analysis comparing PoCUS to computed tomography in adults with pneumothorax found that in the hands of trained users PoCUS had a sensitivity of 91% and specificity of 98%. The same study found supine CXR had a sensitivity of only 50% [1]. While upright CXR may be more sensitive, it has never been formally compared to CT and the few studies available have found a sensitivity of 83-84% [2,3]. While these are mainly adult studies, PoCUS maintains its superior test characteristics in the neonatal population with novice sonographers who received one hour of training with sensitivities and specificities nearing 100% suggesting it is a useful tool throughout the lifespan [4,5,6].

Ultrasound has the additional benefits of being free from ionizing radiation and quicker to perform at the bedside. In a study of PoCUS versus CXR for neonatal pneumothorax ultrasound took on average 5 minutes versus 19 minutes with portable x-ray [4].

PoCUS is safe, timely and effective for the diagnosis of pneumothorax.

Summary

• PoCUS is sensitive, and specific for the detection of pleural effusions.
• Position the probe in the posterior axillary line with the marker towards the head
• Identify the liver/spleen, diaphragm, vertebral bodies and lung
• The characteristic (positive) finding is a hypoechoic collection above the diaphragm with distinct borders and dynamic movement.
• Don’t forget to document your findings in the chart and save images for QA.

References

1. Grimberg et al. Diagnostic accuracy of sonography for pleural effusion: systematic review. Sao Paulo Med J (2010) 128;2: 90-95.

2. Asthon-Cleary, DT. Is thoracic ultrasound a viable alternative to conventional imaging in the critical care setting? BJA (2013) doi:10.1093/bja/aet076

3. Kurian et al. Comparison of Ultrasound and CT in the Evaluation of Pneumonia Complicated by Parapneumonic Effusion in Children. AJR (2009) DOI:10.2214/AJR.09.2791

4. Calder and Owens. Imaging of parapneumonic pleural effusions and empyema in children. Ped Rad (2009) DOI: 10.1007/s00247-008-1133-1

5. Volpicelli et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med (2012) DOI:10.1007/s00134-012-2513-4

6. Islam et al. The diagnosis and management of empyema in children: a comprehensive review from the APSA Outcomes and Clinical Trials Committee. J Ped Surg (2012) DOI: 10.1016/j.jpedsurg.2012.07.047

7. Peris et al. The Use of Point-of-Care Bedside Lung Ultrasound Significantly Reduces the Number of Radiographs and Computed Tomography Scans in Critically Ill Patients. Anesth&Analg (2010) DOI: 10.1213/ANE.0b013e3181e7cc42

8. Jones BP, Tay ET, Elikashvili I, Sanders JE, Paul AZ, Nelson BP, Spina LA, Tsung JW, Feasibility and Safety of Substituting Lung Ultrasound for Chest X-ray When Diagnosing Pneumonia in Children: A Randomized Controlled Trial, CHEST (2016), doi: 10.1016/j.chest.2016.02.643