Thoracic Imaging 2003 - Society of Thoracic Radiology

WEDNESDAY
232
Critical Care Unit Imaging
Joel E. Fishman, M.D., Ph.D.
Assistant Professor of Radiology
University of Miami School of Medicine
Objectives
1. Review published data regarding efficacy of ICU chest radiology
2. Discuss specific pleural & parenchymal diseases affecting
ICU patients, including atelectasis, pneumonia, aspiration,
edema, ARDS, and effusion
3. Identify abnormal air collections and barotrauma
4. Evaluate life support devices and their complications
5. Discuss the role of chest CT in the ICU
Introduction
Physical examination is often difficult in the ICU setting,
and for many years has been complemented by the portable
chest radiograph (CXR). The most common specific indications
for obtaining such a radiograph, whether film-screen or digital,
have included
1. rapidly evolving changes in cardiopulmonary status
2. evaluation of the immediate postprocedure and subsequent
position of life support devices, as well as their complications
3. ventilator-associated complications including barotrauma
4. specific pleuroparenchymal diseases such as pleural effusion,
atelectasis, aspiration, pneumonia, edema, ARDS, and
pulmonary embolism (1).
The interpretation of portable ICU radiographs is often difficult,
due to the limitations of applying optimal radiographic
technique in the ICU setting, as well as patient condition and
the presence of monitoring and other devices (either in or on the
patient) obscuring portions of the chest. As might be expected,
studies have shown a wide range of measures of effectiveness of
these radiographs, and there has been to date only limited study
claiming to demonstrate cost-effectiveness, but this is not universally
accepted. According to some studies, 43-65% of all
ICU CXR had “unexpected or abnormal findings”, many affecting
management. However, considering routine, daily chest
radiographs lowers the efficacy to 15-18% in an MICU/respiratory
ICU setting and even lower (5%) in the cardiothoracic ICU.
The American College of Radiology has weighed in to the discussion
with its Appropriateness Criteria (2), stating that a daily
CXR is indicated for patients with acute cardiopulmonary problems
and for patients on mechanical ventilation. In cases of
CVP, feeding tube, and chest tube placement, only post-procedure
radiographs are indicated. Postprocedure radiography is
possibly indicated for nasogastric (nonfeeding) tubes.
According to the ACR, Swan-Ganz catheters only require postprocedure
films, but this is more controversial.
Atelectasis
Atelectasis may occur in any patient, especially those having
undergone general anesthesia, or whom have preexisting lung
disease, smoking history, obesity, or elderly age. Approximately
50% of patients with atelectasis but without pneumonia have
fever. Lobar atelectasis is usually secondary to poor inspiratory
effort and is often seen postoperatively and after extubation. It
has a propensity to affect the left lower lobe more than others;
one evaluation revealed left lower lobe atelectasis in 66%, right
lower lobe in 22%, and right upper lobe in 11%. Lobar atelectasis
is generally a result of small airway collapse, not mucus
plugs, so it may not respond to bronchoscopy. Pneumonia is
more likely to occur if atelectasis persists past 3 or 4 postoperative
days. Radiographically, lobar atelectasis is indistinguishable
from lobar pneumonia. Atelectasis without air bronchograms
suggests mucus plugging as the etiology. The CXR
has approximately 74% sensitivity and 100% specificity for
detecting atelectasis (or consolidation) vs. CT scanning (3). The
mildest and most common radiographically visible form is subsegmental
atelectasis, which often appears platelike or discoid,
and can cross fissures.
Pneumonia
ICU patients are at high risk for nosocomial pneumonia, a
disease that has an incidence of 6/1000 discharges and accounts
for 18% of all infections. It occurs in 8-12% of MICU/SICU
patients, and in up to 60% of patients with ARDS. The presence
of an ET tube and feeding tube promotes aspiration with
possible subsequent infection, and immunocompromised
patients are naturally predisposed to infection. The mortality
rate is 13-55% (4). Organisms include aerobic gram-negative
rods, especially pseudomonas, enterobacter, and klebsiella. Of
gram-positive organisms, staph aureus is most common (15%),
and is increasingly methicillin-resistant (MRSA). The most
common non-bacterial agent is candida (4%). Nosocomial
pneumonia can be difficult to diagnose. Bronchoalveolar lavage
has become an important tool in helping to diagnose pneumonia
in the ICU.
A study of ventilated patients showed a CXR accuracy of
50% for diagnosing pneumonia (5); interestingly, clinical input
and the use of prior films did not improve accuracy. Other studies
have shown a CXR specificity for nosocomial pneumonia of
only approximately 30% (4). The CXR is especially difficult in
patients with ARDS. Radiographic opacities may be due to
atelectasis (see above), edema, ARDS, aspiration, hemorrhage,
or infarct (see below). Bronchopneumonia typically displays
alveolar opacities. Helpful signs include airspace opacity abutting
a fissure, air bronchogram (especially if solitary), and cavitation.
The “silhouette sign” and bilateral airspace disease is
more nonspecific. Rapid change in minutes or hours suggests
atelectasis, aspiration, or even patchy edema rather than pneumonia.
CT shows that the CXR underestimates basal consolidations
in post-op patients by 26% (3). Nondependent opacities
are more suspicious for pneumonia than dependent ones.
Antibiotics do not alter the CXR appearance for the first two
days, but lack of radiographic improvement after 14 days
strongly suggests treatment failure.
Edema
Pulmonary edema is broadly grouped into two categories:
(1) Hydrostatic edema: heart failure, overhydration, and renal
failure; (2) Permeability edema: aspiration, sepsis, drug reaction
or allergy, near drowning, smoke or toxic fume inhalation, neu-