Chapter 11 Intensive Care Unit

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CHAPTER

Intensive Care Unit Imaging

Co-written with Andrew Hartigan, MD

• Key Points 1. The value of a portable CXR usually depends upon obtaining an appropriately penetrated, upright expo- sure in full inspiration. Consistency of technique from day to day is essential to optimize the value of films. 2. Parenchymal infiltrates have many common poten- tial etiologies (including atelectasis, embolism, edema, and hemorrhage). Only a small minority of infiltrates represents infection; the diagnosis of nosocomial pneumonia requires clinical correlation and microbiologic confirmation. 3. Although certain signs may be highly suggestive, the CXR does not reliably distinguish the high- permeability edema of ARDS from the hydrostatic pulmonary edema of volume overload or left heart failure. 4. Ultrasound has burgeoned as an imaging modality for applications by the intensivist, as it poses no risk of contrast or radiation exposure, can provide defini- tive, dynamic, and high-value information. With its near-immediate availability, ultrasound can facilitate an expanding variety of percutaneous bedside pro- cedures and answer emergent questions relating to effusion, lung edema, and pneumothorax. 5. Chest CT is quickly completed and often reveals conditions that were not suspected by plain radio- graphs. Reconstructed images sharply and convinc- ingly define pathoanatomy, especially when contrast agents can be safely given. 6. CT is the single best imaging modality for evaluating the abdomen unless the primary working diagnosis is cholelithiasis, ureteral obstruction, or ectopic pregnancy. Ultrasound may be equally informative in such cases and is often the better choice when contrast exposure for CT is contraindicated. 7. Assuming the availability of CT scanning, magnetic resonance imaging (MRI), a modality that provides superb soft tissue imaging without ionizing radiation

exposure is but is time consuming and has relatively few ICU applications that do not relate to neurocriti- cal care. 8. Early interactive consultation with the diagnostic or interventional radiologist usually assures the best selection of procedure, optimal patient prepara- tion, and efficient, bundled sequencing of tests and interventions. OVERVIEW OF RECENT ADVANCES IN ICU IMAGING Conventional and specialized imaging techniques are vital to the care of the critically ill. Diagnostically, computed tomographic (CT) scanning and mag- netic resonance imaging (MRI) are indispensable for neurologic, chest, abdominal, and sinus evalua- tions. Ultrasound (US) facilitates cardiac, vascular, renal, gallbladder, pleural, and lung assessment, and though sparingly used, nuclear medicine techniques sometimes help in confirming embolic disease, gastrointestinal (GI) bleeding, and fistulous com- munications. Bedside availability of US has made thoracentesis and central venous catheter (CVC) placement safer and easier. Interventional radiol- ogy assumes an ever-increasing role in performing repairs that once could only be addressed surgically. This ever-increasing list includes embolization of cerebral aneurysms, percutaneous aortic aneurysm grafting, embolization of life-threatening bleeding vessels, placement of intravascular filters, emergent stroke intervention, and pulmonary embolism (PE) lysis. These and other specialized applications are discussed here and elsewhere in this volume in con- junction with the specific diseases they help define. This chapter concentrates on imaging applications relevant to the critical care setting: the chest X-ray (CXR) and chest CT, the abdominal plain film, ICU ultrasound, and interventional procedures.

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Major advances have occurred in ICU radiology over the last two decades as technological progress has perfected digital filming techniques, accelerated acquisition and processing speeds, deployed ultraso- nography to the bedside, and dramatically enabled improved imaging communications to and from the ICU point of care. Clinical data and background information can be rapidly reviewed by both clini- cian and radiologist, and digital images can now be viewed remotely on almost any computer, por- table X-ray machine, or handheld electronic device. This technological revolution has brought a host of improvements. Among them: 1. “Hard copy” films are no longer lost or out of chronological order. 2. Delays in availability have decreased. 3. It is now possible to manipulate image bright- ness and contrast and to compare new images side-by-side with previous ones. 4. Geographically separated physicians can simul- taneously view a study. 5. Physicians no longer need to leave the ICU to view studies. There are two important disadvantages of the digital revolution. First, although the situation is rap- idly improving, the expensive high-resolution displays necessary to see the smallest details are not widely available; hence, studies are often examined on sub- optimal screens. Second, the frequent meetings of the intensivist with radiologist that nearly always occurred when hard copy X-ray films were used have all but vanished. Although “throughput” efficiency may be enhanced, such isolation is unquestionably detrimental. Failure to connect face-to-face often deprives the radiologist of important clinical infor- mation to aid in effective consultation, may result in clinicians overlooking subtle but important findings, and eliminates a valuable educational function.

and exposure technique. One simple measure to improve the ability to interpret CXRs is to reposi- tion overlying devices (e.g., ECG monitoring wires, ventilator and IV tubing, external pacing pads, and nasogastric or orogastric tubes) out of the field of the radiograph. Orientation of the patient with respect to the radiographic beam is of critical importance. Kyphotic, lordotic, and rotated projections impact the apparent dimensions of intrathoracic structures and detection of pathology. The use of “gravity- dependent” radiopaque markers on the corners of portable films helps clarify a patient’s position. The AP technique blurs and magnifies the anterior medi- astinum and great vessels, in some cases by as much as 20%. Obese patients present particular challenges in separating what is normal from what is not, espe- cially when filmed supine (Fig. 11-1). Moreover, apart from the AP requirement itself, radiographs obtained in supine patients exaggerate apparent cardiovascular dimensions because of augmented venous filling, higher diaphragms, and reduced lung volume. For example, the azygous vein distends in the supine normal subject but collapses in the upright position (Fig. 11-2). Conversely, supine films often render imperceptible a small pneumothorax or pleural effusion. Rotation produces artifactual hemi- diaphragm elevations and depressions. In diffuse infiltrative processes, lateral positioning accentuates asymmetry—making the dependent lung appear more affected. Film penetration may emphasize or diminish parenchymal lung markings. Consistency in exposure technique is critical to allow day-to-day comparison of radiographs. A properly exposed CXR should reveal vertebral interspaces in the retrocar- diac region. Films on which these interspaces are not visualized are underpenetrated, exaggerating parenchymal markings and making visualization of any air bronchograms more difficult. Changes in lung volume influence the appear- ance of parenchymal infiltrates, especially in mechanically ventilated patients and in those receiving positive end-expiratory pressure (PEEP). Infiltrates seen on a CXR obtained in full inspira- tion on the ventilator usually appear less dense than when viewed in partial inspiration. Similarly, many patients will have a “less-infiltrated” appear- ing CXR following the application of higher PEEP. Unfortunately, there is no predictable relationship between the level of PEEP applied and its impact on the appearance of the film. To facilitate comparison, therefore, serial films ideally should be exposed with the patient in the same position, during the same

CHEST RADIOGRAPHY Technique

Although the CT has displaced the bedside film from its former diagnostic prominence, the simple portable film suffices to answer many questions that require repeated follow up and do not require CT precision. Bedside radiography, therefore, retains a strong place for many applications. However, the usefulness of the portable anterior–posterior (AP) CXR is largely determined by positioning

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Normal PA CXR

Normal AP Obese

FIGURE 11-1. Left: Normal posterior–anterior (PA) upright chest radiograph. Note the definition and dimensions of the heart and vas- cular structures. Right: Supine AP chest radiograph in massively obese normal subject. Note the widened mediastinum, enlarged heart shadow, and symmetrically elevated hemidiaphragms.

phase of the respiratory cycle, and with comparable tidal volume and end-expiratory pressure. (Clearly, such an ideal for interpretation may not be feasible or clinically advisable, but such influences should be borne in mind.) Infusions of large volumes of flu- ids, the development of oliguria, or superimposed myocardial dysfunction produce a rapidly deterio- rating radiographic picture. Bronchoalveolar lavage may cause the appearance of localized infiltrates because of residual lavage fluid and atelectasis.

Bedside lung US for lung and pleural interrogation by the ICU provider has the potential to obviate the need for repeated radiation exposure to resolve diag- nostic questions or track progress. Film Timing Because of the high likelihood of finding significant abnormalities (e.g., tube malposition, pneumotho- rax), it is worthwhile to obtain a CXR on almost all

Brachiocephalic veins

Superior vena cava

Azygos vein

Right atrium

FIGURE 11-2. Distention of azygos vein, indicating higher than normal pressures in the SVC, is seen on frontal chest film as a circular or lenticular shadow ( arrow ) at its point of anatomic insertion.

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patients upon arrival in the ICU. The frequency with which radiographs are necessary after stabiliza- tion is much more controversial. General agreement exists that CXRs should be obtained promptly after invasive procedures such as endotracheal (ET) intu- bation, feeding tube placement, transvenous pace- maker insertion, thoracentesis, pleural biopsy, and central vascular catheter placement to ensure proper tube or catheter position and exclude complica- tions. Likewise, a film should probably be obtained routinely after transbronchial biopsy, although the need for such a study in the nonintubated patient is debated. In all but emergency situations, a CXR should follow failed attempts at catheterization via the subclavian route before contralateral placement is attempted. Although many ICUs continue to routinely obtain daily or even more frequent radiographs in patients with cardiopulmonary disease or dysfunc- tion, regularly scheduled films are not necessary in all patients. Despite data indicating that a quarter to two thirds of routine ICU CXRs demonstrate an abnormality or minor change, many of these findings are nonacute or inconsequential. Most important developments are signaled by clinically suggestive signs or careful examination of the patient before obtaining the radiograph. Prospective study indi- cates that fewer than 10% of films demonstrate a new significant finding, and only a fraction of these are not anticipated by clinical examination. A reason- able compromise position is to obtain daily “routine”

radiographs on mechanically ventilated patients who have hemodynamic or respiratory instability. The need for additional films should be dictated by changes in the patient’s clinical condition and by the performance of procedures. In the stable, mechani- cally ventilated patient, especially those with a tracheostomy, studies can safely be obtained less frequently. Obviously, deterioration should prompt reevaluation. Because up to 25% of ET tubes are initially posi- tioned suboptimally, radiographic confirmation of tube location is crucial; positioning the ET tube in the right main bronchus often results in right upper lobe or left lung atelectasis. (Left main intubations are uncommon because the left main bronchus is smaller and angulates sharply from the tracheal axis.) Conversely, if the tube tip lies too high in the trachea (above the level of the clavicles), unin- tended extubation is likely. When the head is in a neutral position, the tip of the ET tube should rest in the midtrachea, approximately 5 cm above the carina. In adult patients, the T6 vertebral level is a good estimate of carinal position if it cannot be directly visualized (Fig. 11-3). The carina is usually located just inferior to the level of the aortic arch. (Another method to locate an unseen carina uses the intersection of the midline of the trachea with a Placement of Tubes and Catheters Tracheal Tube Position

7

FIGURE 11-3.  Location of the main carina on the frontal film. The separation between the right and left main bronchi ( arrow ) almost invariably occurs at the level of the 6 and 7 posterior ribs, directionally “southwest” of the aortic knob.

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45-degree bisecting line, which passes through the middle of the aortic knob.) ET tubes move with flex- ion, extension, and rotation of the neck. Contrary to what might be expected, the tube tip moves cau- dally when the neck is flexed (i.e., chin down = tip down). Conversely, head rotation away from the midline and neck extension elevates the ET tube tip. Total tip excursion may be as much as 4 cm. The normal ET or tracheostomy tube should occupy one half to two thirds of the tracheal width and should not cause bulging of the trachea in the region of the tube cuff. Bulging is associated with an increased risk of subsequent airway stenosis, pre- sumably the result of tracheal wall ischemia from cuff overinflation. Gradual dilation of the trachea may occur during long-term positive pressure ven- tilation, but every effort should be made to prevent this complication by minimizing both ventilator cycling pressure and cuff sealing pressures. After tracheostomy, a CXR may detect subcu- taneous air, pneumothorax, pneumomediastinum, or malposition of the tube. The T3 vertebral level defines the ideal position of the tracheostomy site. (This usually places the tip halfway between the stoma and the carina.) Unlike the orally placed ET tube, the tracheostomy tube does not change posi- tion with neck flexion or extension. Lateral radio- graphs are necessary for evaluation of anteroposterior angulation. Sharp anterior angulation of the tracheal tube is associated with the development of tracheo- innominate fistulas, whereas continued posterior angulation risks erosion and tracheoesophageal fis- tula. Massive hemoptysis usually signals the former condition, whereas sudden massive gastric disten- tion with air occurs in the latter. Fortunately, both complications are quite rare in modern practice. In patients with previous intubation or trache- ostomy, the tracheal air column should be exam- ined for evidence of stenosis. Tracheal narrowing is relatively common and can occur at the level of the tracheal tube tip, at the cuff, or at the tracheostomy tube stoma (most common site). The typical hour- glass-shaped narrowing can be hard to visualize on a single AP radiograph, and stenosis must be substan- tial (luminal opening <4 mm) to be symptomatic. CT establishes a definitive diagnosis. Central Venous Catheters For accurate pressure measurement, the tip of the CVC should lie within the thorax, well beyond any venous valves. These valves are typically located in the

subclavian and jugular veins, approximately 2.5 cm from their junctions with the brachiocephalic trunk (at the radiographic level of the anterior first rib). Because CVC catheters in the right atrium or ven- tricle may cause arrhythmias or perforation, the desir- able location for these lines is in the midsuperior vena cava, with the tip directed inferiorly. Radiographically, catheter tips positioned above the superior mar- gin of the right mainstem bronchus are unlikely to rest in the atrium. Catheters should have no sharp bends along their course and should descend lateral and parallel to the spine. Stiff catheters, particularly hemodialysis lines inserted through the left subcla- vian vein may impinge on the lateral wall of superior vena cava, potentially resulting in vascular perfora- tion. Complications resulting from vascular puncture include air embolism, fluid infusion into the pericar- dium or pleural space, hemopneumothorax, and peri- cardial tamponade. Imaging studies reveal that partial thrombosis occurs distressingly often with CVCs and peripherally inserted central catheters (PICC lines). Postprocedure radiographs reveal complications in up to 15% of CVC placements. On occasion, cath- eters inserted via the subclavian route can pass across the midline into the contralateral subclavian vein, or even turn cephalad entering the internal jugular veins. Similarly, catheters inserted in the internal jugular veins may track into the subclavian vein of either side. The phenomenon of a subclavian catheter crossing the midline is most common when a triple-lumen catheter is threaded through a larger bore channel already in placed in the right subclavian vein. Many clinicians are comfortable leaving CVCs, which ter- minate in the contralateral subclavian in place, pro- vided there are no clinical effects but are less at ease with CVCs terminating in the internal jugular vein. As a general rule, it is a good idea to obtain a CXR following failed attempts at CVC placement before attempting insertion on the contralateral side. Doing so reduces the already tiny chance of producing bilateral pneumothoraces. Obviously, this safeguard must be abandoned under truly emergent circumstances where venous access must be obtained immediately. Pulmonary Artery (Swan–Ganz) Catheter Every insertion-related complication of CVCs, including pneumothorax, pleural entry, and arterial injury, can result from the placement of the pulmo- nary artery catheter (PAC) as well. Unique complica- tions of PAC placement include knotting or looping

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Pacing Wires When transvenous pacing wires are inserted emer- gently, they often lie malpositioned in the coronary sinus, right atrium, or pulmonary artery outflow tract. On an AP view of the chest, a properly placed pacing catheter should have a gentle curve with the tip overlying the shadow of the right ventricu- lar apex. However, it is often difficult to assess the position of the pacing wire on a single film. On a lateral view, the tip of the catheter should lie within 4 mm of the epicardial fat stripe and point ante- riorly. (Posterior angulation suggests coronary sinus placement.) In patients with permanent pacemak- ers, leads commonly fracture at the entrance to the pulse generator, a site that should be checked rou- tinely. Pacing wires can also result in cardiac per- foration, so it is important to examine the CXR for signs of tamponade and if suspicion is sufficient, perform bedside cardiac US. Chest Tubes The optimal position for a chest tube depends on the reason for its placement. Posterior positioning is ideal for the drainage of free-flowing pleural fluid, whereas anterosuperior placement is preferred for air removal. On an AP chest film, posteriorly placed tubes are closer to the film than those placed ante- riorly. This proximity of the chest tube to the film results in a “sharp” or focused appearance of the catheter edge and its radiopaque stripe. Conversely, anteriorly placed chest tubes often have fuzzy or blurred margins. Chest tube location may appear appropriate on a single AP film, even though the tube actually lies within subcutaneous tissues or lung parenchyma. Unexpected failure to re-expand the pneumothorax or drain the effusion should be a clue to extrapleural placement. A chest CT may be necessary to confirm appropriate positioning. On plain film, another clue to the extrapleural location of a chest tube is the inability to visualize both sides of the catheter. Larger chest tubes are constructed with a “sentinel eye,” an interruption of the longitu- dinal radiopaque stripe that delineates the opening of the chest tube closest to the drainage appara- tus. This hole must lie within the pleural space to achieve adequate drainage and ensure that no air enters the tube via the subcutaneous tissue. After removal of a larger chest tube, fibrinous thicken- ing may produce a persisting tube track, which mimics the visceral pleural boundary, suggesting pneumothorax.

and entanglement with other catheters or pacing wires and pulmonary artery rupture and infarction. Knotting or entanglement of PACs with other cath- eters is a frightening prospect but can usually be avoided and need not be dangerous if a few simple steps are followed. Knotting can largely be avoided by not advancing the catheter more than 20 cm before the next chamber’s pressure tracing is observed. For example, a right ventricular tracing should be seen with less than 20 cm of catheter advancement after obtaining a right atrial pressure tracing, and a pulmo- nary artery tracing should be obtained before another 20 cm is advanced after first obtaining the right ven- tricular tracing. Doing so prevents the catheter from forming a large loop in the right atrium or ventricle. If the PAC does become knotted or entangled with another device (e.g., pacing wire or vena caval filter), it is essential to resist the temptation to pull on the catheter harder to extract it; doing so only tightens the knot, making eventual extraction more difficult. Almost always, knotted catheters can be “untied” under fluoroscopic guidance by an interventional radiologist simply by loosening the knot, with aid of a stiff internal guidewire. Unrelieved pulmonary arterial blockage has been a reported complication in 1% to 10% of PAC placements. The most common radiographic find- ing is distal catheter tip migration, with or without pulmonary infarction. With an uninflated balloon, the tip of the PAC ideally overlies the middle third of a well-centered AP CXR (within 5 cm of the midline). Distal migration is common in the first hours after insertion as the catheter softens and is propelled distally by repeated right ventricular con- tractions. If pressure tracings suggest continuous wedging, it is important to look for distal migration, as well as a catheter folded on itself across the pulmonic valve or a persistently inflated balloon (appearing as a 1-cm diameter, rounded lucency at the tip of the catheter). Inflating the balloon of an inappropriately distal PAC can result in immediate catastrophic pul- monary artery rupture or delayed formation of a pul- monary artery pseudoaneurysm. Pseudoaneurysms present as indistinct rounded densities on CXR 1 to 3 weeks after PAC placement. The diagnosis is easily confirmed by MRI or contrasted chest CT. The width of the mediastinal and cardiac shad- ows should be assessed following placement of PACs and CVCs, because perforation of the free wall of the right ventricle (fortunately, rare) has the potential to result in pericardial tamponade.

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Intra-aortic Balloon The intra-aortic balloon (IAB) is an inflatable device placed in the proximal aorta to assist the failing ven- tricle. Diastolic inflation of the balloon produces a distinct, rounded lucency within the aortic shadow, but in systole, the deflated balloon is not visible (whereas the supporting catheter is). Ideal posi- tioning places the catheter tip just distal to the left subclavian artery. Placed too cephalad, the IAB may occlude the carotid or left subclavian artery. Placed too caudally, the IAB may occlude the lumbar or mesenteric arteries and produce less-effective coun- terpulsation. Daily radiographic assessment is pru- dent to detect catheter migration or a change of the aortic contour suggestive of IAB-induced dissection. Gastric Access Tubes Whether inserted through the nose (NG) or mouth (OG), it is usually prudent to obtain a CXR to con- firm gastric tube position before administration of medication, fluid, or feeding, even when clinical evaluation indicates proper positioning. Even in intubated patients, a small number of tubes intended for the stomach do end up in the lung (usually the right mainstem bronchus). Vigorous insertion tech- nique can force the gastric tube through the lung into the pleural space. Inadvertent airway entry is most likely to occur when using a small-bore-stylet- stiffened tube, especially when inserted in comatose or deeply sedated patients. When inserted via the esophagus, the side holes of the enteral tube should be fully advanced past the lower esophageal sphinc- ter to minimize reflux. Following similar safety pre- cautions, an abdominal film should be obtained after placement of a percutaneous endoscopic gastric (PEG) tube to search for common complications, such as extragastric migration or peritoneal leakage. As already mentioned the outset of this discussion, it must be recognized that the chest CT offers far greater diagnostic precision than the bedside radio- graph. Yet, for many purposes, the humble bed- side chest radiograph remains indispensible, being cheaper and quicker to obtain, presenting less expo- sure to ionizing radiation, and sparing the patient the hazards associated with transport from the ICU environment. Specific Applications of Chest Radiography

Atelectasis Atelectasis is a frequent cause of infiltration on ICU CXRs. The wide spectrum of findings ranges from invisible microatelectasis, through plate, segmental, and lobar atelectasis, to collapse of an entire lung. Differentiating between segmental atelectasis and segmental pneumonia is often difficult, because these conditions often coexist. However, marked volume loss, rapid onset, and quick reversal are more characteristic of acute collapse. Atelectasis tends to develop in dependent regions and, more commonly, in the left rather than the right lower lobe by a 2:1 margin. Radiographic findings of atelectasis include hemidiaphragm elevation, parenchymal density, vascular crowding (especially in the retrocardiac area), deviation of hilar vessels, ipsilateral mediastinal shift, and loss of the lateral border of the descending aorta or heart. Each lobe has a characteristic pattern of atelecta- sis. With right upper lobe collapse, apical density increases as the minor fissure rotates superior medi- ally producing an easily recognizable curvilinear arch extending to the mediastinum. Because the left lung does not have a middle lobe or minor fissure, upper lobe collapse occurs anteriorly, producing a diffuse haziness of the hemithorax and loss of the upper left cardiac border. In both cases, the main pulmonary artery shadow moves cephalad. On lat- eral CXR, right middle lobe atelectasis appears as a prominent wedge with its apex directed toward the hilum, as the minor fissure and major fissure move toward each other. Unfortunately, on frontal films, the findings are typically more subtle, often mani- fest only as obscuration of the right heart border. A lordotic film increases the density and sharpens the definition of the airless but thin right middle lobe. Partial collapse of either the right or left lower lobe produce similar patterns of diaphragmatic “silhou- etting.” When lower lobe volume loss is extensive, a triangular posteromedial density can be seen with its base resting on the diaphragm. Contrary to popu- lar belief, the “silhouette sign” is not always reliable on portable films, particularly in the presence of an enlarged heart or when the film was obtained in a lordotic or rotated projection. Air bronchograms extending into an atelectatic area suggest that col- lapse continues without total occlusion of the cen- tral airway and that attempts at airway clearance by bronchoscopy or aggressive suctioning, therefore, are likely to fail.

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FIGURE 11-4. Appearance of a mobile pleural effusion in three positions. In the supine position, a “ground-glass” lateralized diffuse density (with preservation of vascular markings) may be the only sign of layered pleural fluid. A changing appearance with position confirms the diagnosis.

Pleural Effusion and Hemothorax Pleural effusions occur very commonly among ICU patients; however, their appearances vary with body positioning (Fig. 11-4). On the supine AP CXR, large effusions redistribute—potentially causing a hazy density to overlie the entire hemithorax without loss of vascular definition (Fig. 11-5). Apical pleural capping is another radiographic sign of large collec- tions of pleural fluid in the supine patient. Upright or lateral decubitus radiographs may help confirm the presence of an effusion (Fig. 11-6). If a large collec- tion of pleural fluid obscures the lung parenchyma, a contralateral decubitus film often helps visualize the ipsilateral lung. Pleural fluid is not ordinarily visible until several hundred milliliters have accumulated. On lateral decubitus films, 1 cm of layering fluid indicates a volume that can usually be tapped safely. If there is any question about the quantity or mobility of fluid, bedside ultrasonography is usually helpful.

Subpulmonic or loculated fluid may be diffi- cult to recognize. Hemidiaphragm elevation, lateral displacement of the diaphragmatic apex, abrupt transitions from lucency to solid tissue density, and increased distance from the upper left hemi- diaphragmatic margin to the gastric bubble (on an upright film) are all signs of a subpulmonic effusion (Fig. 11-7). US and chest CT are useful adjuncts in detecting the presence of such collections of pleural fluid and in guiding drainage. US has the obvious advantages of portability, repeatability, cost effi- ciency, safety, and real-time imaging for drainage. Extra-alveolar Gas/Barotrauma Extra-alveolar gas can manifest as interstitial emphysema, cyst formation, pneumothorax, pneu- momediastinum, pneumoperitoneum, or subcuta- neous emphysema (see Chapter 8). Pulmonary Interstitial Emphysema Radiographic signs of gas in the pulmonary intersti- tium include lucent streaks that do not conform to air bronchograms and new cysts at the lung periph- ery, usually at the bases. Interstitial emphysema may also generate small “target lesions” as air surrounds small peripheral pulmonary arterioles viewed en face. These signs, best seen when the parenchyma is densely infiltrated, portend the imminent (but not invariable) development of pneumothorax. Subpleural Air Cysts Subpleural air cysts, a potential sign of impending pneumothorax in mechanically ventilated patients, are small (3- to 5-cm wide) basilar rounded lucen- cies. The cysts often appear abruptly and tend to rapidly increase in size (sometimes to as large as 9 cm). Subpleural air cysts frequently progress to tension pneumothorax in the presence of contin- ued high-pressure mechanical ventilation. The role of prophylactic tube thoracostomy remains undefined; however, when subpleural air cysts are

FIGURE 11-5. Mobile right pleural effusion supine. Diffuse haziness with well-preserved outlines of the ipsilateral pulmonary arteries are characteristic.

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Bilateral Pleural Effusion

Pleural Effusion (decubitus)

FIGURE 11-6. Left: Bilateral pleural effusions with characteristic crescentic blunting on the upright PA film. Right: Mobile effusion in left lateral decubitus orientation. Arrows demarcate the fluid separating left lung from ribs.

Pneumothorax Pneumothorax is often difficult to detect on porta- ble CXRs. Relatively few ICU patients exhibit the typical patterns seen on upright CXRs performed in noncritically ill patients. Proper positioning assumes great importance in detection. On supine films or

noted, the clinician should reduce airway pressures to the extent possible and maintain a high level of vigilance and be prepared to emergently insert chest tubes. Fortunately, such catastrophic developments have become much less likely in the present era of lung-protective ventilation.

A B FIGURE 11-7. A: Radiographic signs of a subpulmonic effusion ( 1 ) hemidiaphragm elevation with separation of lung from gastric bubble, ( 2 ) lateralization of the diaphragmatic dome, and ( 3 ) abrupt transition from lucency to soft tissue density. B: Left subpulmonic effusion in upright position. Note abrupt transition of density at the lung base and lateral displacement of what appears to be the hemi- diaphragmatic dome.

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T ension P neumothorax Action on a presumptive diagnosis of tension pneu- mothorax occasionally must be initiated solely on clinical grounds without imaging confirmation so as to address an advancing threat to hemodynamic stability. Radiographically, tension pneumothorax often shifts the mediastinum and flattens or inverts the hemi- diaphragm ipsilateral to the pneumothorax. Yet, ten- sion is usually difficult to diagnose with confidence on a single film; infiltrated or obstructed lungs fail to collapse completely, and an unyielding medias- tinum may not shift noticeably, despite a marked pressure gradient. Comparison of past films and clinical correlation may be required. When doubt exists and the patient is hemodynamically unstable, emergent decompression is indicated. The experience of past decades showed that pneumothorax occurs in up to 50% of patients receiving mechanical ventilation with peak infla- tion pressures exceeding 60 cm H 2 O, and a large fraction of those were under tension. The adoption of lower tidal volume ventilation has decreased the incidence of pneumothorax dramatically. When it does occur, pneumothorax commonly complicates the course of patients with necrotizing pneumo- nias, acute respiratory distress syndrome (ARDS), secretion retention, or expanding cavitary or bullous lesions. Tension pneumothorax may be very difficult to distinguish from bullous disease under tension by plain radiograph alone. Although a chest CT can be revealing, patients in extremis cannot wait for a diag- nostic CT scan. In such emergent settings, erring on the side of chest tube insertion is probably the best course of action, even though rupturing a large bulla can create a bronchopleural fistula. Pneumomediastinum After gaining access to the mediastinum, gas nor- mally decompresses into adjacent soft tissues. Apart from discomfort or pain, pneumomediastinum itself rarely produces important physiologic effects in adults. Mediastinal gas may arise from neck injuries, from rupture of the trachea or esophagus, or (most commonly) from alveolar rupture and retrograde dissection of air along bronchovascular bundles. Pneumomediastinum appears radiographically as a lucent band around the heart and great vessels as gas within the space separates the parietal pleura from the mediastinal contents. On the heart’s inferior bor- der, this lucency can extend across the mediastinum, linking the two sides of the chest with a “complete

in patients with pleural adhesions, gas may collect exclusively in the basilar (anterior) regions of the thorax. Thus, gas may outline the minor fissure or may move anteriorly over the heart, mimicking pneu- momediastinum or pneumopericardium. Loculated pneumothoraces can be very difficult to detect with- out CT, and it is surprising how many times residual localized air collections are found by CT among patients with one or more chest tubes. Radiographic signs of pneumothorax on the supine CXR include a “deep sulcus sign” and lucency over the upper portions of the spleen or liver (see Chapter 8). At the bedside, an upright expiratory CXR is often the best film for detecting a pneumothorax. This view confines a fixed amount of intrapleural air within a smaller volume, accentuating the proportion of tho- racic volume it occupies and the separation of the lung from chest wall. Provider-implemented bedside US has facilitated such diagnoses and should be con- sidered when doubt persists after CXR examination. The visceral pleura provides a specific marker: a radiodense (white) thin stripe of appropriate curva- ture with lucency visible on both sides and absent lung markings beyond. Skin folds often mimic the pleural edge but can be distinguished by certain fea- tures: lucency present only on one margin, poorly defined limits, and extension beyond the confines of the rib cage. Because pneumothorax reduces blood flow to the collapsed lung, its density may be surprisingly normal, even with an extensive gas collection. Here again, failure to detect dynamic lung sliding and the presence of a lung point on US nicely complement or even supplant the radio- graphic evidence (see ICU Ultrasound, following). Pneumothoraces are often characterized by the percentage of the hemithorax they occupy. This practice is highly imprecise, both because the frontal CXR is only two-dimensional and because apparent percentage changes occur with variations in breath- ing depth and position. As with pleural fluid, pre- cise determination of the size of a pneumothorax is neither possible nor necessary. A tension pneumo- thorax (of any size) and a “large” pneumothorax both require drainage—the former because of its immedi- ate physiologic effects, the latter because it creates a pleural pocket that is unlikely to reabsorb spontane- ously over an acceptable time. The reabsorption rate of a pneumothorax has been estimated to be “1% to 2% per day,” a crude rule of thumb that emphasizes the slowness of this process. Thus, a 15% pneumo- thorax would typically take 2 weeks to reabsorb.

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diaphragm sign.” An unnaturally sharp heart border is the first indicator of pneumomediastinum, a sign that must be distinguished from the “kinetic halo” seen at the heart or diaphragm border of an edema- tous lung. The mediastinal pleura, outlined by gas on both sides of a thin radiodense line, can often be detected. On a lateral film, pneumomediastinum usually appears as a thin crescent of gas outlining the ascending aorta. Not uncommonly, extrapleural gas extends from the mediastinum, lifting the pari- etal pleura off the diaphragm, or outlining the infe- rior pulmonary ligament. Pneumomediastinum is an important harbinger of pneumothorax, which follows in up to 30% of mechanically ventilated patients. In doubtful cases where progression is feared, defini- tive diagnosis can be established by CT. Subcutaneous Gas In the adult, subcutaneous gas, also known as subcu- taneous emphysema, usually has important diagnos- tic but little physiologic significance. Subcutaneous gas produces lucent streaks or bubbles in the soft tissues that contrast with the normal densities of the chest and neck. However, there is almost no limit to the path the gas may take, as it may track into the retroperitoneum, the peritoneal cavity, and even the scrotum. During mechanical ventilation, bilateral subcutaneous gas usually results from alveolar rup- ture and medial gas dissection, indicating both a via- ble decompression pathway and an increased risk of pneumothorax. Once pneumothorax has occurred, progressive accumulation of gas in the subcutaneous tissue suggests the presence of a bronchopleural fis- tula or a malfunctioning chest tube, especially if the gas is bilateral. Ipsilateral subcutaneous gas detected shortly after chest tube placement generally entered

via the tube track itself. Subcutaneous gas detected immediately after blunt chest trauma should raise the possibility of tracheobronchial or esophageal dis- ruption (see Chapter 36). Pulmonary Edema Without invasivemonitoring, distinguishing between normal permeability (fluid overload and congestive heart failure [CHF]) and high-permeability pulmo- nary edema, ARDS can be difficult. Considerable overlap exists in the radiographic findings of these entities, but certain CXR findings may be helpful in determining the etiology of excess lung water. These forms of edema are often distinguished by three fea- tures: size of the heart and great vessels, distribution of vascular markings, and the pattern of infiltration (Table 11-1). CHF and volume overload are char- acterized by a widened vascular pedicle, an even or inverted pattern of vascular markings, and a ten- dency toward a gravitational distribution of edema (“bat wing” or basilar). Pleural effusions, particularly those of substantial size, are also more common with CHF than ARDS. The vascular pedicle is measured at the point the superior vena cava crosses the right main bronchus to a perpendicular dropped from the point of takeoff of the left subclavian artery from the aorta. Kerley B lines, because of perilymphatic interstitial fluid, are common in established CHF (usually of several days’ to weeks’ duration), whereas crisp air bronchograms are unusual. Conversely, the less mobile infiltrates of ARDS are widely scat- tered, patchy, and often interrupted by distinct air bronchograms. These criteria are better for correctly classifying CHF and volume overload edema and less accurate for identifying ARDS. Although useful

Table 11-1.  Radiographic Features of Pulmonary Edema Characteristics Cardiogenic or Volume Overload Edema

High-Permeability Edema

Heart size

Enlarged

Normal

Vascular pedicle Flow distribution

Normal/enlarged Balanced/cephalad Normal/increased

Normal/small Basal/balanced

Blood volume Septal lines

Normal Absent

Common

Peribronchial cuffing Air bronchograms Edema distribution

Very common Uncommon

Uncommon Very common

Even/central/gravitational Very common/moderate–large

Patchy/peripheral/nongravitational

Pleural effusion

Infrequent/small

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when applied with appropriate clinical correlation, widespread application of these criteria to evaluate the etiology of pulmonary edema has shown them to be less reliable than stated in the original investiga- tions. Because most of the radiographic deteriora- tion seen in ARDS occurs within the first 5 days of illness, a worsening CXR appearance after this time suggests superimposition of pneumonia, fluid over- load, CHF, or ventilator-induced lung injury (VILI). Although pulmonary edema is usually bilateral and symmetric, it may collect asymmetrically when medi- astinal tumor, bronchial cyst, or massive thromboem- bolism diverts flow preferentially to one lung. The recently transplanted lung is also prone to developing unilateral pulmonary edema. Asymmetry may also be observed following unilateral aspiration, reexpansion pulmonary edema, or in the presence of extensive bul- lous disease. Gravity may redistribute edema fluid and atelectasis to newly dependent lung regions over rela- tively brief periods after patient repositioning. Mediastinal Widening Mediastinal widening on a well-centered film (par- ticularly following chest trauma or an invasive pro- cedure) should raise suspicion of aortic disruption. (A rotated or lordotic film may be misleading.) A contrast-enhanced chest CT provides the definitive diagnosis. Obtaining a high-quality upright PA CXR, though desirable, is frequently not possible because of injuries or hypotension. Radiographic clues to aor- tic disruption include a widened superior mediasti- num (the most sensitive sign), a blurred aortic knob, rightward deviation of a nasogastric tube or aortic shadow, and tracheal deviation to the right and anteri- orly. Inferior displacement of the left main bronchus, left-sided pleural effusion (with or without apical capping), and displacement of intimal calcifications

of the aorta provide other signs suggestive of aortic disruption (see Chapter 35). Mediastinal widening with vascular injury is frequently associated with traumatic fractures of the sternum, first two ribs, or clavicle. Widening of the cardiac shadow should prompt careful review of the aortic contour because blood may dissect from the aorta into the pericar- dium. If aortic disruption is suspected, angiography by catheter (the diagnostic “gold standard” for many years) is seldom needed. Contrast-enhanced CT scanning, MRI, and echocardiography almost invari- ably provide definitive evidence. Pericardial Effusion Pericardial effusion is recognized radiographically by enlargement of the cardiac shadow. The classic “water bottle configuration” of the cardiac silhouette, although highly characteristic, is unusual. An epicardial fat pad visible on the lateral CXR should raise suspicion of a pericardial effusion, as should splaying of the tracheal bifurcation. Echocardiography is the procedure of choice for the detection and evaluation of pericardial effusions, and it simultaneously affords the opportu- nity to assess heart chamber size, contractile function, and vena caval diameter. When a transthoracic echo- cardiogram cannot obtain images of adequate qual- ity because of patient weight or chest hyperinflation, transesophageal echocardiogram is usually diagnostic.

Air–Fluid Levels (Lung Abscess vs. Empyema)

Several radiographic features help to distinguish whether an air–fluid level lies within the pleural space or within the lung parenchyma (Fig. 11-8). On an AP film, pleural fluid collections generate wide, moderately dense air–fluid levels, whereas

FIGURE 11-8.  Intraparenchymal versus intrapleural fluid collections. Fluid collections within the pleural space usually have a greater horizontal than vertical dimension, do not cross fissure lines, and may have sloping attachments to the pleural surface on one or more views. Furthermore, pleural collections typically have different dimensions on AP and lateral views. By contrast, intrapa- renchymal collections tend to be more spherical, with equal dimensions on AP and lateral views.

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Lung Abscess

Empyema

FIGURE 11-9. Top: Large right upper lobe lung abscess with thick cavity wall and air–fluid level indicating communication with the airway. Bottom: Loculated empyema of the right posterior pleural space. The lateral view demonstrates that the opacity is not rounded, abuts the ribs, and is not entirely encapsulated by aerated lung.

intrapulmonary collections are usually smaller, more dense, and rounded. Lung abscesses and liquid-filled bullae tend to project similar diameters on both AP and lateral films (Fig. 11-9 Top panels). The air–fluid level of pleural fluid collections must abut the chest wall on either AP or lateral film (Fig. 11-9 Bottom panels). Fluid collections that cross a fissure line on upright films are located within the pleural space. Lung abscesses generally have distinct, thick, shaggy walls with irregular contours, unlike most liquid- filled bullae and pleural fluid collections. As body

position is altered, pleural fluid collections frequently undergo marked changes in shape or contour. CT scanning reliably differentiates the two conditions. Postthoracotomy Changes After pneumonectomy, fluid accumulates in the vacant hemithorax over days to months. Whereas the absolute fluid level is of little significance, changes in the level of fluid are important. A rapid decline in the fluid level should prompt concern for a

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bronchopleural fistula, a complication that most com- monly develops within 8 to 12 days of surgery. If a fistula develops earlier, failure of the bronchial clo- sure should be suspected, prompting consideration of reoperation. Bronchopleural fistulas tend to displace the mediastinum to the contralateral side, an unusual occurrence during uneventful postoperative recovery. Small residual air spaces may remain for up to a year following pneumonectomy and do not necessarily imply the presence of a persistent fistula. Very rapid postoperative filling of the hemithorax suggests infec- tion, hemorrhage, or malignant effusion. Fistulous Tracts Fistulas between the trachea and innominate artery develop most frequently when a tracheal tube angu- lates anteriorly and to the right in a patients with a low tracheostomy stoma, persistent hyperextension of the neck, or asthenic habitus. Because of this association, anteriorly directed tracheal tubes should be repositioned. Fistulas also may form between the trachea and esophagus during prolonged ET intu- bation. These usually occur at the level of the ET cuff, directly behind the manubrium. Predisposing factors include cuff overdistention, simultaneous presence of a nasogastric tube, and posterior angu- lation of the tracheal tube tip. The sudden occur- rence of massive gastric dilation in a mechanically ventilated patient provides an important clue. A radiographic contrast agent may be introduced into the esophagus after cuff deflation or tube removal in an attempt to confirm the presence of the fistula. Pulmonary Embolism Although the plain CXR rarely if ever diagnoses PE, it is quite useful to detect other conditions in the differential diagnosis including CHF, pneumo- thorax, and aspiration. Despite limited diagnostic utility, large emboli may give rise to suggestive find- ings: ipsilateral hypovascularity, pulmonary artery enlargement, and (rarely) abrupt vascular cutoff. Local oligemia (the Westermark sign) may be seen early in the course of PE, usually within the first 36 hours. “Hampton’s hump,” a pleural-based tri- angular density caused by pulmonary infarction, is seldom seen. About 50% of patients with PE have an associated pleural effusion. For critically ill ICU patients with suspected thromboembolism, it often makes sense to begin

the evaluation with a Doppler examination of the limbs. If the US exam reveals what appears to be fresh clot in any deep vein, the diagnosis of “throm- boembolism” is established, other tests are unneces- sary as anticoagulation is indicated. It has become clear that for ICU patients not only are the legs a potential source of clot, but the neck and arms are as well. Roughly half of all CVCs in place for a week or more are associated with at least a partially occlusive thrombus, and approximately 15% of these patients have concurrent PEs. The initial use of limb US has several advantages, including avoidance of con- trast exposure and travel from the ICU, as well as lower cost and limited interpretive turnaround time. If the US is negative but the clinical suspicion of PE remains high, ventilation/perfusion ( V / Q ) scan- ning or contrasted chest CT may be performed. The rarity of a normal CXR diminishes the value of V / Q scanning in the critically ill. Nonetheless, normal perfusion scans are very helpful, and abnormal scans help guide the angiographic search for emboli if the systemic contrast needed for CT is contraindicated by renal dysfunction. The sensitivity and specificity of chest CT for the diagnosis of PE are now well established. In the right clinical context, it is safe to assume that a large filling defect seen in the pulmo- nary circuit of a technically adequate study repre- sents clot (i.e., high specificity). Primary tumors of the pulmonary artery, primary lung tumors, cancers metastatic to the mediastinum, nonneoplastic medi- astinal adenopathy, hydatid disease, and mediastinal fibrosis rarely can mimic PE. By contrast, because the sensitivity of CT varies among institutions, and even in the best centers is not 100% for subsegmen- tal clots, a negative CT should not be regarded as definitive data excluding PE. Sensitivity is optimized by a scanner with many rows of detectors, quick acquisition time, optimal contrast injection tech- nique and gating, adequate breath-holding by the patient, and experienced interpretation of optimally reconstructed images including three-dimensional views. Although there is controversy about the impor- tance of subsegmental clots in healthy patients, in critically ill patients with impaired cardiopulmonary reserve, it is probably inadvisable to overlook such emboli. Echocardiogram may reveal right ventricular and pulmonary arterial dilation. If the Doppler US is negative, V / Q not practical, echocardiography unre- vealing, and CT nondiagnostic while clinical suspi- cion remains high, angiography is the next, seldom taken step. Frequently, CT and catheter angiography