Chapter 11 Intensive Care Unit

243

CHAPTER 11 • Intensive Care Unit Imaging

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.