Laboratory Studies
No laboratory studies assist in the diagnosis of barotrauma. Arterial blood gas evaluations allow assessment of acid-base status, oxygenation, and ventilation and, therefore, the consequences of barotrauma. However, arterial blood gas values do not help establish the diagnosis.
Chest Radiography
The portable chest radiograph often provides the first indication of barotrauma, especially in an otherwise asymptomatic patient. Initial findings can be subtle because patients often have other pulmonary opacities that may obscure the appearance of extra-alveolar air (see the image below).
Image shows subtle manifestations of barotrauma, pulmonary interstitial emphysema, and subcutaneous emphysema. This patient was being treated with noninvasive ventilation. Importantly, recognize that barotrauma can be associated with noninvasive ventilation.
Findings such as nonbranching, fixed-caliber radiolucencies radiating from the hilum to the periphery or small collections of air in interlobular septa suggest pulmonary interstitial emphysema (PIE) or subpleural air cysts (see the image below). Pneumomediastinum causes outlining of the great vessels (the superior vena cava and the left subclavian, common carotid, and innominate arteries) with extension into the neck, whereas pneumopericardium causes outlining of the pericardium and contiguous diaphragm.
This patient was undergoing treatment for acute respiratory distress syndrome when a new lucency was found on a routine portable chest radiograph. The lucency over the right midlung zone represents a subpleural air cyst. Such cysts can increase in size and eventually rupture, creating a pneumothorax.
Pneumothoraces, especially small ones, may be difficult to detect on portable chest radiographs in mechanically ventilated patients. In most of these patients, studies are performed while they are supine, a position that changes the highest point in the hemithorax from an apical-lateral location to an anteromedial location, where air rises and accumulates. Air can also be subpulmonic and may be seen as a hyperlucent upper quadrant or a deep lucency in the lateral costophrenic angle; this is often referred to as the deep sulcus sign.
The obvious concern is that pneumothoraces may progress to a life-threatening tension pneumothorax. Although this is usually clinically evident, radiographic signs of structures under tension include displacement of mediastinal structures, collapsed lung, and flattening and inversion of the diaphragm (see the images below).
This patient developed a left tension pneumothorax during treatment of a severe pneumonia. Note the marked shift of the mediastinal structures to the right, the partial collapse of the left lung, and the inversion and downward displacement of the left hemidiaphragm.
This patient had a left pneumothorax with placement of a left thoracostomy tube. However, this portable chest radiograph shows a persistent retrocardiac lucency, which raised questions about a persistent pneumothorax.
Computed Tomography
In patients with small collections of air, the diagnosis may be difficult with portable chest radiography. Alternative imaging studies in these situations include decubitus studies of the side in question and computed tomography (CT) of the chest. The logistics of decubitus imaging in a critically ill, mechanically ventilated patient can be daunting, and the quality of the study results may make interpretation difficult. Chest CT is desirable, but patients must be in a sufficiently stable condition to tolerate transport to the CT scanner.
Chest CT is rarely indicated to establish the diagnosis of barotrauma, but it may be helpful in determining the size of a pneumothorax in mechanically ventilated patients (see the image below). Because the plain portable chest radiograph provides only a two-dimensional view of the thorax, the size of pneumothoraces that span the hemithorax may be underestimated. Likewise, it may not be easy to appreciate pneumothoraces that are primarily anterior or basilar.
This chest CT scan was obtained on the same day as the chest radiograph of the patient in Media File 4. The image shows a loculated pneumothorax in the mid left lung. This image illustrates the information a chest CT scan can add and the difficulty in diagnosing a pneumothorax with the limited views provided by a portable chest radiograph.
Chest CT may help with the placement of thoracostomy tubes in patients in whom the pneumothorax may be confined or loculated. In some patients with large air leaks, more than one thoracostomy tube may be required, and CT can assist with their placement.
The additional information a chest CT scan provides about the lung parenchyma, the pleural surfaces, and the vascular structures may also be useful in patient care.
Evaluation of Ventilator Parameters
Many ventilators currently in use also have a respiratory mechanics graphics package. This, along with the traditionally monitored airway pressures, may provide some insight into the care of patients at risk for barotrauma and into the diagnosis of barotrauma. However, these graphics packages should never be used in isolation to diagnose barotrauma. They provide complementary information and can lead the clinician to the diagnosis.
Chest radiography and the physical examination are essential to confirm suspected barotrauma, especially in a life-threatening situation in which tube thoracostomy is contemplated.
Airway pressures have traditionally been used to identify patients at risk for barotrauma. Because the vast majority of patients are ventilated with volume-cycle ventilators, airway pressures required to deliver set tidal volumes can provide insight into the state of the underlying lung.
High peak inspiratory pressure, plateau pressure, and positive end-expiratory pressure (PEEP) have all been implicated as risk factors for barotrauma. These, in turn, are proxy measures of the transalveolar pressure that may define the risk for barotrauma. Plateau pressure provides the best estimate of transalveolar pressure and has been used as both a threshold target and a monitoring tool to adjust ventilator settings to reduce the risk of barotrauma and to identify ventilated patients at risk.
The elliptical pressure-volume curve can provide information about the nature of the underlying lung. A line connecting the origin of the curve to the end of inspiration reflects lung compliance; the expected angle for a normal compliant lung is 45°.
Patients with acute respiratory distress syndrome (ARDS) can be expected to have decreased lung compliance, with a shift downward (to the right). In addition, patients with stiff lungs may reach a point at which increased airway pressure does not notably increase the delivered tidal volume. In these instances, the upper portion of the curve at the end of inspiration may become flattened and narrowed, simulating a bird’s beak or the appearance of a penguin.
Adjustments in ventilator settings, especially reduction of the tidal volume, may eliminate the terminal flattening, which should reduce the peak inspiratory pressures and plateau pressures transmitted to the lung. However, the pressure-volume curve is only a graphic display and a guide. It should never be used in lieu of plateau pressure measurements in the care of ventilated patients.
Some ventilator packages permit calculation of the static compliance of the lung. This compliance is calculated by dividing the tidal volume by the difference between the plateau pressure and PEEP values. It is decreased in patients with ARDS, but a sudden decrease in static compliance might herald the development of a pneumothorax. However, this finding is nonspecific, and any condition that decreases lung compliance is expected to have the same effect. Pulmonary edema is common and is encountered more frequently than pneumothorax.
Other changes in the ventilator parameters may suggest a pneumothorax. Patients at risk for barotrauma may have high peak inspiratory pressures. If a pneumothorax develops, peak pressures may initially decrease in association with a decrease in exhaled tidal volume as air escapes into the pleural space. However, if tension develops, inspiratory pressures may increase as the same tidal volume is being delivered to a shrinking anatomic surface area.
Histologic Findings
The diagnosis of barotrauma does not rely on histologic findings. If barotrauma is diagnosed only on the basis of histologic specimens, it usually represents an incidental finding or a finding noted during postmortem examination. As might be expected, histologic findings may demonstrate alveolar disruption with hemorrhage, edema, and inflammation. [17]
More important, the histologic findings of the lung surrounding the area of barotrauma provide insight into the severity of the lung disease and the patient’s risk for barotrauma. For example, a patient with ARDS is expected to have diffuse alveolar damage with hyaline membrane formation; exudative, proteinaceous alveolar fluid; neutrophils; macrophages; and disrupted alveolar epithelium.
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Image shows subtle manifestations of barotrauma, pulmonary interstitial emphysema, and subcutaneous emphysema. This patient was being treated with noninvasive ventilation. Importantly, recognize that barotrauma can be associated with noninvasive ventilation.
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This patient was undergoing treatment for acute respiratory distress syndrome when a new lucency was found on a routine portable chest radiograph. The lucency over the right midlung zone represents a subpleural air cyst. Such cysts can increase in size and eventually rupture, creating a pneumothorax.
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This patient developed a left tension pneumothorax during treatment of a severe pneumonia. Note the marked shift of the mediastinal structures to the right, the partial collapse of the left lung, and the inversion and downward displacement of the left hemidiaphragm.
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This patient had a left pneumothorax with placement of a left thoracostomy tube. However, this portable chest radiograph shows a persistent retrocardiac lucency, which raised questions about a persistent pneumothorax.
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This chest CT scan was obtained on the same day as the chest radiograph of the patient in Media File 4. The image shows a loculated pneumothorax in the mid left lung. This image illustrates the information a chest CT scan can add and the difficulty in diagnosing a pneumothorax with the limited views provided by a portable chest radiograph.
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Acute Respiratory Distress Syndrome Network reference summarizing the mechanical ventilation protocol.
