Portable chest roentgenograms are used regularly to provide useful information about chest pathologic findings in critically ill patients. However, several different types of pathologic conditions are commonly missed on the portable chest roentgenogram. A pneumothorax can be very difficult to identify on supine AP x-ray films because the air may disperse widely throughout the pleural space instead of forming focal collections. Although the use of upright positioning can lessen this problem, complete upright positioning is frequently impossible. As is shown in this and earlier reports, the pneumothorax may be loculated within or behind roentgenographically dense structures, making detection with a portable chest roentgenogram very difficult. Often, the only sign of penumothorax on the supine portable chest roentgenogram is the failure to silhouette a mediastinal structure because of air trapped between the mediastinum and other lung pathologic findings. Pleural effusion also can present a diagnostic dilemma, since it may be difficult to detect the presence of fluid and to differentiate between pleural effusion and posterior-basal atelectasis. It also may be difficult to quantify the amount of fluid present, since it may redistribute in the pleura] cavity and appear merely as a thickened pleural space, a thickened major and/or minor fissure, or simply as a uniform increase in the density of the entire lung field. add comment
The technique used to produce the portable chest roentgenogram profoundly influences resulting information. Patient position, inspiratory effort, and degree of penetration can vary widely and cause significant confusion. False abnormalities can occur if the x-ray beam is not directed tangentially toward a given structure (eg, diaphragm). In this situation, outlines may be obscured, yielding a “pseudosilhouette sign,” and may lead one to falsely suspect the presence of an abnormality The decubitus bedside technique for ICU chest roentgenography can be quite helpful in evaluating the “upside” for the presence and degree of pneumothorax. If the pneumothorax is loculated anteriorly or posteriorly, the decubitus roentgenogram can be obtained with the patient rotated either posteriorly or anteriorly to demonstrate this loculation. Decubitus roentgenography of the upside may also demonstrate better aeration of the lung base as the up diaphragm descends maximally. The decubitus roentgenogram for the downside frequently requires higher technique and is best accomplished with the central ray positioned two to three inches towards the side which is dependent; in this way, free dependent pleural effusions may flow out of the field of the lower lobe uncovering more pathologic findings (ie, a cavity with a fluid level) in the previously opaque lung base. When possible, it is advisable to obtain decubitus views with each side down for their complementary value.