Ultrasound detection of pneumothorax

Pathophysiology

In the normal state, the closed pleural system of the lung is made up of physiologic pleural fluid contained between the visceral and parietal pleura making up the pleural sac. Air introduced into this closed system leads to a pneumothorax by separating the normally apposed visceral and parietal pleura. This separation can potentially alter the mechanisms of respiration.

In the supine trauma patient without previous pleural disease, pathologic air within the pleural space tends to rise to the anterior chest wall at the paracardiac regions and anterior costo-diaphragmatic sulci. It is at these anterior sites that bedside ultrasound is ideal (see image 1).

 

Probe Selection

The low-frequency, curvilinear probe (2-5 MHz) has been used by most authors for the detection of anterior pneumothoraces (see image 2, item A). This probe is found on most bedside ultrasound machines and is recommended for use during FAST examinations.

 

The curvilinear probe allows for the continuation of the FAST exam to the thoracic cavity, negating the need for a probe change during the exam. In our experience, using the high-frequency, linear probe (5-10 MHz) allows the novice physician to obtain rapid proficiency in the chest ultrasound exam for detection of pneumothorax (see image 2, item B).

After sufficient experience with pattern recognition of positive and negative ultrasonographic signs of a pneumothorax with the linear, high-frequency probe, the clinician may then utilize the curvilinear, low-frequency probe for the complete evaluation of the abdomen and thorax--thus adding the lung component to the standard FAST exam, commonly termed the Extended FAST or EFAST.²

Procedure

The bedside chest ultrasound exam for the detection of anterior pneumothoraces is performed by placing the probe perpendicular to two ribs spaces in the anterior chest region (see image 2, item C). The hyperechoic (white) ribs, with posterior shadows, act as fixed anatomical landmarks (see image 2, item D). This defined interspace, just deep to the intercostal muscles and soft tissue, is the location of the pleural line.

Sliding of the pleural line indicates the lack of air between the visceral and parietal pleural, and "rules out" an anterior pneumothorax. When pathologic air accumulates in between the parietal and visceral pleura, ultrasound waves are able to image the superficial parietal layer, but cannot visualize the visceral layer. This is because of the small collection of air in the pleural space that causes ultrasound waves to scatter. This is represented as "lack of lung sliding" and can be interpreted as a pneumothorax.

Once the pleural space is interrogated adequately, we recommend utilizing M-mode as an adjunctive imaging modality to confirm the presence or absence of a pneumothorax. Motion mode, or "M-mode," allows the clinician sonographically to evaluate a single line and determine the presence of motion along that singular line.

Again, by leaving the probe in same perpendicular orientation over the rib interspaces, the operator can determine, in a static method, the presence of lung sliding. Lack of motion in the soft tissue region is displayed as horizontal lines, while motion is displayed as unorganized "static" (see image 3).

 

Resuscitative ultrasound image illustrating a lung point in time motion mode. The normal seashore sign (arrows) can be seen alternating with the stratosphere sign in time with respiration.

Split field image demonstrating static 2-D mode depiction of normal pleura (arrowhead) on the left of the image, with M-mode depiction of the sea-shore sign on the right side of the image. 

 

 

Stratosphere sign of a pneumothorax; 2-D image above indicates line of interrogation of pleural interface. The corresponding time motion mode image fails to reveal any underlying pleural movement consistent with a pneumothorax.

 Currently, no consensus exists regarding the number of interspaces that need to be interrogated to exclude an anterior pneumothorax. In a supine patient, we recommend scanning 2-3 interspaces in the midclavicular line in the most anterior region of the chest (usually around the nipple line/4th-5th rib space). Also, we recommend evaluating both hemithoraces to differentiate normal from abnormal lung tissue if a pneumothorax is present.

Secondary Signs of an Anterior Pneumothorax

In "B-mode," or "brightness mode," another sign termed "comet tail" artifact can aid the physician in the ultrasound examination of the chest for the detection of a pneumothorax.

Comet tail artifacts are "ray-like" projections off the pleural line thought to be created when ultrasound waves hit the interface between the apposing pleural and visceral layers of the lung (see image 4). These artifacts are not seen when pathologic air is present between the normally apposed pleural layers. Lack of comet tail artifacts can indicate the presence of a pneumothorax.

 

The point where normal lung interfaces with air of a pneumothorax in the pleural space is defined as the "lung point." In "B-mode," the physician can scan the anterior chest in search of the dynamic transition between normal pleural sliding and absence of pleural sliding (pneumothorax). This dynamic point moves during the respiratory cycle, and can be found at various locations in the chest. Small pneumothoraces tend to have transitions in the anterior chest, while larger pneumothoraces have areas of transition on the lateral aspect of the chest.

Visualization of the lung point is pathognomonic for the presence of a pneumothorax. Experience is necessary to recognize this specific sign, and it may be necessary to interrogate numerous interspaces to locate the lung point. This limits the recognition of lung point during a rapid EFAST examination.

Limitations

Lack of lung sliding and comet tail artifacts may not always indicate a pneumothorax. Recently intubated patients may have a mainstem bronchus intubation preventing adequate aeration of one lung and not demonstrate either lung sliding or comet tail artifacts, giving the operator a false impression of pneumothorax.

Also, when evaluating the paracardiac regions on the left chest, care must be taken to identify the pleural line. The heart rises and falls with the movement of the diaphragm, and this motion may be misinterpreted as a "lung point," especially if the probe marker is pointed caudad instead of cephalad (as recommended).

In stable trauma patients with ultrasonographic signs of a pneumothorax and a negative supine chest radiograph, we recommend a repeat upright plain film after clearance of cervical immobilization for confirmation of an occult pneumothorax. In patients where cervical immobilization cannot be removed, we recommend computed tomography of the chest to delineate the pneumothorax early in trauma care (before transportation or intubation).

Summary

Continuation of the FAST exam to include the chest for the rapid determination of a pneumothorax can be useful for the emergency physician.

The provider must interrogate the most anterior aspect of the chest wall. The operator must identify pertinent landmarks (rib with resultant shadows) and determine the location of the pleural line. Lack of pleural movement can indicate air between the parietal and visceral pleura, allowing for rapid identification of a pneumothorax. This technique has been shown to be more sensitive for the recognition of anterior pneumothoraces in trauma patients, compared with supine chest radiographs.