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Bedside Ultrasonography for Pneumothorax

Overview

Background

This article discusses the theoretical and practical aspects of using bedside ultrasonography for pneumothorax. Although the Focused Assessment with Sonography in Trauma (FAST) examination has been part of Advanced Trauma Life Support (ATLS) for over a decade,
the addition of pneumothorax evaluation to this protocol to create the extended FAST (e-FAST) examination is a relatively new development.

Over the past 15 years, the use of bedside ultrasonography in the emergency department (ED) has revolutionized patient care.
 Trauma patients, who are particularly vulnerable to the complications of delayed diagnostic studies, have benefited tremendously from rapid ultrasound-guided evaluation at the bedside.
 In 2011, the Eastern Association for the Surgery of Trauma updated their guidelines, giving a level 2 recommendation for the use of thoracic ultrasound to diagnose pneumothorax.

The traditional initial evaluation of patients with a suspected traumatic pneumothorax was with chest radiography, typically performed with the patient in the supine position so as to preserve cervical spine immobilization. However, this method is grossly inadequate for detecting pneumothoraces, with sensitivities as low as 36–48% in some studies.
 Given the low sensitivity of portable chest x-ray, ultrasound has also been suggested as an alternative to portable chest x-ray in the detection of traumatic pneumothorax.  In a retrospective review performed at a level I trauma center, ultrasound was shown to predict the absence of pneumothorax with 93.8% sensitivity and a negative predictive value of 99.9% when compared to chest x-ray.  One pneumothorax was missed on ultrasound that was visualized on chest x-ray; however, this was a small, low anterior pneumothorax that was not clinically significant. All patients in this study who required tube thoracostomy placement had pneumothoraces that were detected on ultrasound. These findings suggest that for clinically significant pneumothoraces, ultrasound is a feasible alternative or replacement for portable chest x-ray, as it is more readily available and has higher sensitivity.

In a retrospective study, the diagnostic accuracy of the e-FAST was compared to that of multidetector computed tomography (MDCT) and of invasive interventions. With the use of CT scanning, 87 pneumothoraces were detected among the 763 lung fields studies; with the use of ultrasound, 67 of 87 pneumothoraces were detected. Of the 20 missed, 17 were considered mild and not life-threatening. The diagnostic performance of ultrasound in this study was found to have a sensitivity of 77% and a specificity of 99.8%. Although ultrasonography may not be as diagnostically accurate as CT scanning, CT is limited in unstable patients, and this limitation can result in delayed diagnosis and treatment.
  

Although bedside sonography was first implemented by emergency medicine physicians, it is now used by multiple specialties, including radiologists,
surgeons, and adult and pediatric intensivists.
Furthermore, in addition to its traditional use in the ED, bedside ultrasound is starting to be used to confirm central line placement
and to monitor postoperative patients.
Studies have also shown that bedside ultrasonography is a valuable tool in diagnosing iatrogenic pneumothoraces.
In the past few years, there has been an international effort to standardize the application of lung ultrasound, and in 2011, the International Liaison Committee on Lung Ultrasound (ILC-LUS) published a set of evidence-based recommendations for lung ultrasound, including the application of ultrasound for the detection of pneumothorax.

Ultrasound use has also been suggested as a feasible diagnostic tool in the prehospital setting. In a transport vehicle, ambient noise and movement may limit physical exam and delay diagnosis. In a prospective, observational study, nonphysician aeromedical providers were trained to perform and interpret thoracic ultrasound in the detection of pneumothoraces in adult trauma patients and adult medical patients requiring endotracheal intubation. Twenty pneumothoraces and one mainstem intubation were identified, of which 16 were correctly identified and the mainstem intubation was incorrectly diagnosed. Prehospital ultrasound had a sensitivity of 68% (95% confidence interval (CI) 46-85%), a specificity of 96% (95% CI 90-98%), and an overall accuracy of 91% (95% CI 85-95%). In comparison, ED ultrasound had a sensitivity of 84% (95% CI 62-94%), a specificity of 98% (95% CI 93-99%), and an accuracy of 96% (95% CI 90-98%).

Indications

It is critical to maintain a high suspicion for a pneumothorax in all trauma patients with a significant mechanism, as well as in nontrauma patients with a history or physical findings suggestive of the diagnosis.

Pneumothorax should be considered in the following clinical situations:

Penetrating trauma to the thorax and abdomen

Blunt trauma to the thorax or abdomen

Blunt trauma with a significant mechanism (eg, a high-speed motor vehicle accident or a major fall)

Multiple-trauma patients

Post-procedures involving the thorax (eg, thoracentesis, internal jugular or subclavian central line placement)

The following signs are suggestive of pneumothorax:

Respiratory distress

Decreased or absent lung sounds, especially if unilateral

Resistance to ventilation (eg, patients who are “hard to bag”)

Hypoxia

Tachycardia (a nonspecific but common finding)

Jugular venous distention

Tracheal deviation (a late finding in tension pneumothorax)

The following symptoms are suggestive of pneumothorax:

Dyspnea

Chest pain

Anxiety

Contraindications

There are no specific contraindications for bedside ultrasonography in the ED; clinical judgment should be used to determine its appropriate application. Patients with significant pulmonary disease and previous pneumothoraces may have altered findings on ultrasound examinations. In these special circumstances, patients should undergo definitive imaging with thoracic CT scanning when stable.

Technical considerations

Successful application of ultrasonography to the detection of pneumothorax requires a solid understanding of the basic anatomy and pathophysiology of the pulmonary system. Both lungs are divided into lobes. The gross functional subunits of each lung are called segments. The right lung comprises 10 segments: 3 in the right upper lobe (apical, anterior and medial), 2 in the right middle lobe (medial and lateral), and 5 in the right lower lobe (superior, medial, anterior, lateral, and posterior). The left lung comprises 8 segments: 4 in the left upper lobe (apicoposterior, anterior, superior lingula, and inferior lingula) and 4 in the left lower lobe (superior, anteromedial, lateral, and posterior). For more information about the relevant anatomy, see Lung Anatomy.

Normally, the lungs are covered by a continuous serous membrane that folds back on itself to create 2 layers: the visceral pleura, which covers the lungs and adjoining structures, and the parietal pleura, which is attached to the chest wall. The pleural cavity is the “potential” space between these closely apposed layers and is normally filled with a small amount of pleural fluid, which allows for normal lung sliding during respiration. In normal circumstances, negative pressure prevents air from entering the pleural space.

A pneumothorax occurs when air infiltrates the pleural cavity, either as a result of traumatic injury to the chest wall or spontaneously (spontaneous pneumothorax is historically believed to be caused by the rupture of pleural blebs). Although there are many risk factors that predispose patients to the development of a pneumothorax, most patients presenting to the ED have pneumothorax secondary to trauma.

A particularly dangerous complication, the dreaded tension pneumothorax, can develop when an injury to the lung parenchyma or bronchus acts as a 1-way valve, allowing air to enter the pleural cavity but preventing it from escaping. A tension pneumothorax can develop rapidly and is greatly exacerbated by positive-pressure ventilation, posing a great danger to intubated patients. For all of these reasons, rapid detection of pneumothoraces in trauma patients is critical, and bedside ultrasonography is a fast, reliable means of accomplishing this task.

The following considerations should be kept in mind in the performance of the procedure (see Technique):

Small pneumothoraces tend to show the lung point in the anterior chest, whereas larger pneumothoraces have their transition areas on the lateral chest

Subcutaneous emphysema and pleural effusions may obscure visualization, and patients with these conditions should be examined with thoracic CT whenever possible

In intubated patients, the lack of pleural sliding and the absence of comet-tail artifacts do not always indicate a pneumothorax; mainstem intubation and poor ventilation should be considered in the differential (with mainstem intubation, most frequently on the right side, the other lung collapses completely, so that no lung sliding will be seen on the collapsed side)

It is often helpful to compare the 2 sides of the chest to each other; however, pneumothoraces may also occur bilaterally

Expected outcomes

The use of ultrasonography to detect pneumothoraces was first described in 1987.
Since then, numerous studies have been published describing this application of ultrasonography and documenting its significantly better sensitivity and specificity in comparison with chest radiography.
According to the review by the ILC-LUS, lung ultrasound may be more effective at ruling out pneumothorax than ruling in the diagnosis, as compared to supine anterior chest radiography.
Although ultrasonography is widely used in emergency and critical care settings for rapid diagnosis of potentially life-threatening conditions, its utility and reliability remain controversial among some specialists.

In a 2009 analysis of all published papers on the topic, the overall sensitivity of transthoracic ultrasonography for the diagnosis of pneumothorax ranged from 58.9% to 100%, and the specificity ranged from 94% to 100%.
A standardized technique for performing the procedure and specific sonographic signs indicating a pneumothorax have been described and validated in the literature, and these have been incorporated into the core curriculum in emergency medicine.

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