Intensive care unit imaging

Tracheal tube position

ET positioning should be confirmed with a CXR because of the high frequency (25%) of suboptimal ET placements. Ideally the tube should be positioned in the mid-trachea 5 cm above the carina, which is usually located just inferior to the level of the aortic arch. If the carina cannot be visualized on the CXR, the ET tube tip should be positioned overlying the T6 vertebra ( Fig. 59.3 ). Another method to locate an unseen carina uses the intersection of the midline of the trachea with a 45-degree bisecting line, which passes through the middle of the aortic knob. Positioning of the ET tube too deeply usually leads to right mainstem bronchus intubation. This often will result 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. If the tube is positioned too high in the trachea (above the level of the clavicles), there will be an increased risk of unintended extubation.

ET tubes move with flexion, extension, and rotation of the neck. An ET tube will generally move in the same direction that the neck flexes or extends (chin down = tip down or chin up = tip up). Head rotation away from the midline will elevate the ET tube tip. Total tip excursion can range up to 4 cm with extremes of head movement.

An 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 indicative of cuff overinflation. This can result in tracheal wall ischemia and subsequent airway stenosis and should therefore be avoided. Gradual dilation of the trachea may occur during long-term positive-pressure ventilation and should be avoided by minimizing both ventilator cycling pressure and cuff sealing pressures.

In patients who are post-tracheostomy, a CXR may detect subcutaneous air, pneumothorax, pneumomediastinum, or malposition of the tube. However, routine use of a CXR after both surgical and percutaneously inserted tracheostomy placement is controversial. It may best be reserved for patients with either emergent or technically difficult procedures or who have clinical decompensation after them. The T3 vertebral level is considered the ideal position of the tracheostomy stoma. A tracheostomy tube tip should usually lie halfway between the stoma and the carina. Unlike an ET tube, a tracheostomy tube does not change position with neck flexion or extension. Lateral radiographs are necessary for evaluation of AP angulation. Sharp anterior angulation of the tracheal tube is associated with the development of trachea-innominate fistulas, and continued posterior angulation risks erosion and tracheoesophageal fistula. Massive hemoptysis usually signals the former condition, whereas sudden massive gastric distention with air occurs in the latter. Fortunately, both complications are quite rare in modern practice.

Tracheal stenosis

Previous intubation or tracheostomy can be complicated by tracheal stenosis. Narrowing of the trachea can be seen at the level of the tracheal tube tip, at the cuff, or, most commonly, at the tracheostomy tube stoma. Stenosis must be substantial (luminal opening <4 mm) to be symptomatic. The typical hourglass-shaped narrowing can be hard to visualize on a single AP radiograph, so CT is best for establishing a definitive diagnosis.

Central venous catheters

The tip of a 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). Proximal placement will result in an inaccurate measurement of the central venous pressure and is also associated with increased risk of thrombus formation. Distal placement is associated with the risks of cardiac perforation and tamponade. To avoid this, CVCs should be placed with the catheter tip in the middle-superior superior vena cava (SVC), with the tip directed inferiorly. Radiographically, catheter tips positioned above the superior margin of the right mainstem bronchus are unlikely to rest in the atrium. There are some risks to positioning catheters in the middle SVC, including vascular abutment and potential perforation. This is especially notable with left-sided catheters which will enter the SVC at an oblique angle. Caution should be exercised, especially with more rigid hemodialysis lines placed from this approach. Such catheters may benefit from placement in the lower SVC to avoid this complication. Vascular perforation can result in air embolism, fluid infusion into the pericardium or pleural space, hemopneumothorax, and pericardial tamponade.

Postprocedure radiographs reveal complications, including catheter misplacement, in around 10% of CVC placements. For example, catheters 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. This is more commonly seen in patients with proximal stenosis. To minimize this risk, ultrasound should be used to demonstrate collapsibility of the jugular vein before placement. 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 the right subclavian vein. Many clinicians are comfortable leaving CVCs, which terminate in the contralateral subclavian, in place provided there are no clinical effects, but are less at ease with CVCs terminating in the internal jugular vein. In addition to catheter malposition, the complications of CVC insertions include pneumothorax, arterial injury, stroke, and pleural cavity entry. Catheter-associated thrombosis can also occur with CVCs and peripherally inserted central catheter (PICC) lines.

The width of the mediastinal and cardiac shadows should be assessed after placement of CVCs because perforation of the free wall of the right ventricle (RV), although rare, has the potential to result in pericardial tamponade.

Pulmonary artery (swan-ganz) catheter

A properly inserted pulmonary artery catheter (PAC) should follow the same route as a CVC but continues through the right atrium, RV, and into a pulmonary artery. The tip of the PAC ideally overlies the middle third of a well-centered AP CXR 3–5 cm from the midline, within the mediastinal shadow, or at least not more than 2 cm past the hilum. Ideally, the tip should also lie below the level of the left atrium.

The PAC balloon is used both for placement and obtaining postcapillary wedge pressure (PCWP). Care should be taken to ensure that the balloon is left deflated except during these two procedures, as it can result in pulmonary infarct or pulmonary artery rupture. Unrelieved pulmonary arterial blockage has been a reported complication in 1%–10% of PAC placements. The most common radiographic finding is distal catheter tip migration, with or without pulmonary infarction. Distal migration is common in the first hours after insertion as the catheter softens, adding slack to the line. This slack results in the tip being propelled distally by repeated right ventricular contractions. If pressure tracings suggest continuous wedging, it is important to look for distal migration, 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 pulmonary artery rupture or delayed formation of a pulmonary artery pseudoaneurysm. Pseudoaneurysms present as indistinct rounded densities on CXR 1–3 weeks after PAC placement. The diagnosis is easily confirmed by magnetic resonance imaging (MRI) or contrasted chest CT.

In addition to these pulmonary infarct and pulmonary artery rupture issues, PAC insertion can result in knotting or looping and entanglement with other catheters or pacing wires. The risk of knotting or entanglement of PACs can largely be avoided by following two simple steps. First, do not advance the catheter more than 20 cm before observing the next chamber’s pressure tracing. For example, a right ventricular tracing should be seen within 20 cm of catheter advancement after obtaining a right atrial pressure tracing. This prevents the catheter from having enough length to form a large loop in the right atrium or ventricle. Second, if the PAC does become knotted or entangled with another device such as a pacing wire, it is essential to resist the temptation to pull on the catheter harder to extract it. Forceful pulling will only tighten the knot, making eventual extraction more difficult. Rather, knotted catheters can be “untied” under fluoroscopic guidance by an interventional radiologist by simply loosening the knot using a stiff internal guidewire.

Pacing wires

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 ventricular apex. To accurately assess the wire’s position in three-dimensional space, a lateral film is required. On a lateral view, the tip of the catheter should lie within 4 mm of the epicardial fat stripe and point anteriorly. Posterior angulation suggests coronary sinus placement. Other common areas for malposition include the right atrium or pulmonary artery outflow tract.

In patients with permanent pacemakers, leads can fracture at the entrance to the pulse generator, a site that should be checked routinely. Pacing wires can also result in cardiac perforation, so it is important to examine the CXR for signs of tamponade and perform bedside cardiac US if it is suspected.

Chest tubes

The optimal position for a chest tube depends on the reason for its placement. Inferior and posterior positioning is ideal for the drainage of free-flowing pleural fluid, whereas anterosuperior placement is preferred for pneumothorax. On an AP chest film, posteriorly placed tubes are closer to the film than those placed anteriorly. 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. In a complicated pleural effusion, a CT can visualize loculations that a plain film can miss. On plain film, another clue to the extrapleural location of a chest tube is the inability to visualize both sides of the catheter. The radiopaque stripe on many tubes includes a break or “sentinel eye,” which denotes the proximate port of entry for air or fluid. This should be included within the pleural cavity to achieve drainage and ensure no subcutaneous entry of air into the tube. After removal of a larger chest tube, fibrinous thickening may produce a persisting tube track, which mimics the visceral pleural boundary, suggesting pneumothorax.

Intraaortic balloon

The intraaortic balloon (IAB) is an inflatable device placed in the proximal aorta to assist a failing ventricle. Diastolic inflation of the balloon produces a distinct, rounded lucency within the aortic shadow, but in systole, the deflated balloon is not visible (unlike its supporting catheter). Ideal positioning places the catheter tip just distal to the left subclavian artery ( Fig. 59.4 ). 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 counterpulsation. Daily radiographic assessment is prudent to detect catheter migration or a change of the aortic contour suggestive of IAB-induced dissection.

Temporary ventricular assist devices

Temporary ventricular assist devices (VADs), such as Impella catheters, are also available to assist a failing left ventricle (LV). They are placed via a patient’s femoral artery with the catheter extending through the aorta and aortic valve, with the catheter tip being visualized in the LV. Repositioning of the catheter requires visualization beyond a standard CXR, with bedside US or formal echocardiography being required at the time of placement and positioning. Like PACs, slack can develop in a temporary VAD catheter because of a patient’s body heat, resulting in an inappropriate position (often too deep in the LV). Clinical signs of catheter malposition include hemolysis (potentially massive) with change in urinary color and blood counts, hypotension, and reduced variability in the motor current monitor tracing.

Gastric access tubes

Whether inserted through the nose (nasogastric [NG]) or mouth (orogastric [OG]), it is usually prudent to obtain a CXR or plain abdominal film to confirm appropriate gastric tube position before administration of medication, fluid, or feeding, even when clinical evaluation indicates proper positioning. Tube feeding or medication administration into the airway or pleura can result in severe complications. 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 technique can force the gastric tube through the lung into the pleural space ( Fig 59.5 ). 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.

It is important to confirm that the tip of the tube not only overlies the stomach but also that there is no significant deviation of the tube from the midline esophagus until it has passed the level of the diaphragm. This will help avoid the rare but avoidable complications that can result from tubes that traverse the left mainstem bronchi and into the pleural space. When inserted via the esophagus, the side holes of the enteral tube should be fully advanced past the lower esophageal sphincter to minimize reflux. After similar safety precautions, 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.

Atelectasis

Atelectasis is a frequent cause of densities on ICU CXRs. Findings range 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, and these conditions often coexist. Atelectasis can be differentiated by identifying its commonly found features of volume loss, rapid onset, and quick reversal.

Atelectasis tends to develop in dependent regions and by a 2:1 ratio more commonly in the left rather than the right lower lobe. CXR findings will 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 atelectasis that an experienced ICU provider should be able to recognize, and these are summarized in Figs. 59.6 through 59.10 . Air bronchograms extending into an atelectatic area suggest that collapse continues without total occlusion of the central airway and that attempts at airway clearance by bronchoscopy or aggressive suctioning are therefore likely to fail to re-expand the involved lobe.

Pleural effusion and hemothorax

Pleural effusions occur very commonly among ICU patients. Their appearance varies based on body positioning ( Fig. 59.11 ).

Large effusions can redistribute on a supine AP CXR, causing a hazy density to overlie the entire hemithorax without loss of vascular definition ( Fig. 59.12 ). Apical pleural capping is another radiographic sign of large collections of pleural fluid in the supine patient. Upright or lateral decubitus radiographs may help confirm the presence of an effusion ( Fig. 59.13 ). If a large collection 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 suggests a volume that can usually be tapped safely. However, in the era of bedside US, the performance of decubitus films is becoming less common as point-of-care US (POCUS) offers a faster and cheaper alternative. US will also be able to provide specific localization and guidance for thoracentesis.

Bilateral pleural effusion (decubitus)

Subpulmonic or loculated fluid may be difficult to recognize on a portable CXR. Hemidiaphragm elevation, lateral displacement of the diaphragmatic apex, abrupt transitions from lucency to solid tissue density, and increased distance from the upper left hemidiaphragmatic margin to the gastric bubble (on an upright film) are all signs of a subpulmonic effusion ( Fig. 59.14 ). 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-efficiency, safety, and real-time imaging for drainage.

Extraalveolar gas/barotrauma

Extraalveolar gas can manifest as interstitial emphysema, cyst formation, pneumothorax, pneumomediastinum, pneumoperitoneum, or subcutaneous emphysema.

Pneumothorax.

The identification of pneumothorax in ventilated patients remains of critical importance, despite the decreased incidence of this condition since the introduction of low tidal volume ventilation. When it does occur, pneumothorax commonly complicates the course of patients with necrotizing pneumonias, acute respiratory distress syndrome (ARDS), secretion retention, or expanding cavitary or bullous lesions.

Pneumothorax is often difficult to detect on portable CXRs. Fewer ICU patients exhibit the typical patterns seen on upright CXRs performed in noncritically ill patients. Proper upright positioning is very important in detection. 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 thoracic volume it occupies and the separation of the lung from the chest wall. On supine films or in patients with pleural adhesions, gas may collect exclusively in the basilar (anterior) regions of the thorax. In this case a pneumothorax will be positioned anterior to the heart or may mimic pneumomediastinum or pneumopericardium. Loculated pneumothoraces can be very difficult to detect without CT, and residual localized air collections are often 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. POCUS can greatly facilitate a diagnosis of pneumothorax and should be considered when doubt persists after CXR examination.

When observing a pneumothorax, the visceral pleura provides a specific marker: a radiodense (white) thin stripe of appropriate curvature, with lucency visible on both sides and absent lung markings beyond ( Fig. 59.15 ). Skin folds often mimic the pleural edge but can be distinguished by the following features: (1) lucency present only on one margin, (2) poorly defined limits, and (3) 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 especially the presence of a “lung point” on US nicely complement or even supplant the radiographic evidence (see the section “Ultrasound” later).

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 breathing depth and position. As with pleural fluid, precise determination of the size of a pneumothorax is neither possible nor necessary. A tension pneumothorax is not defined by a specific size of gas collection in the pleural space. Both a tension pneumothorax, regardless of size, and a “large” pneumothorax require drainage—the former because of its immediate physiologic effects, the latter because it creates a pleural pocket that is unlikely to reabsorb spontaneously over an acceptable time. The reabsorption rate of a pneumothorax has been estimated to be 1%–2% per day, a crude rule of thumb that emphasizes the slowness of this process. A 15% pneumothorax would typically take 2 weeks to reabsorb.