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..... Imaging

A chest x-ray (OCR) is useful in evaluating other causes of

the symptoms. In PE, CXR is nonspecific and nondiagnostic, with a normal radiograph reported in up to 24% of

patients. Common abnormalities seen in patients with PE

include atelectasis, parenchymal abnormalities, elevated

hemidiaphragm, or pleural effusions. Hampton's hump is

a triangular pleural-based infiltrate, representing a pulmonary infarct (sensitivity 22% and specificity 82%).

Westermarck's sign is dilatation of pulmonary vessels

proximal to the PE with collapse of distal vessels (sensitivity 12% and specificity 97%).

Chest CT angiography ( CTA) is the accepted diagnostic

modality of choice (Figure 25-1). It is rapid and sensitive

for detecting proximal PEs. The clinical outcome after a

negative CTA is favorable, and the likelihood for subsequent

.6. Figure 25-1 . Computed tomography angiog raphy

with pulmonary embolism.

thromboembolic events is extremely low. CTA is also use ­

ful to identify alternate diagnoses.

V/Q lung scan results are interpreted as normal, low,

intermediate, or high probability for PE. A normal scan

effectively rules out PE with a negative predictive value of

97%. However, this test is infrequently used today except

when specific contraindications to a CTA exist. Although

previously favored for pregnant patients, guidelines now

typically recommend CTA in pregnant patients too .

Lower extremity duplex ultrasound may be used to

diagnose DVT in a patient with a high clinical suspicion of

PE and a negative CTA.

MEDICAL DECISION MAKING

The diagnosis of PE can be elusive, and with growing concerns of excessive testing and resultant radiation, the clinician must determine not only how to work up the patient,

but also which patients need to be worked up. Although

experience and clinical gestalt may reproduce the output of

some decision rules, it is felt that the use of clinical prediction rules is warranted.

The Pulmonary Embolism Rule-Out Criteria (PERC

rule) was prospectively derived and validated to identify

very low-risk patients who do not require diagnostic testing (Table 25- 1). When there is a low clinical gestalt for

PE and all 8 criteria are met (with no contraindications

for use of the rule), then patients are determined to be

very low risk for PE with a 45-day incidence of venous

thromboembolism or death of less than 2%. In these

patients, no further work-up for PE is recommended.

If the PERC rule does not apply, a patient's pretest

probability for PE should be calculated using 1 of 2 rules

(Geneva or Wells) that utilize findings from the history and

physical examination (Box 25- 1 and Table 25-2). The

results risk-stratify the patient into two groups-PE

unlikely or PE likely. Both simplified revised Geneva and

CHAPTER 25