Role of Spiral Computed Tomography in the Diagnosis of Pulmonary Embolism in the Emergency Department

STATE OF THE ART

Role of Spiral Computed Tomography in the Diagnosis of Pulmonary Embolism in the Emergency Department

From the Department of Radiology, UPMC Health System– Presbyterian, Pittsburgh, PA. Received for publication November 3, 1998. Revision received January 5, 1999. Accepted for publication February 8, 1999. Editor’s Note: This article continues a series of special contributions addressing state-of-the-art techniques, topics, or concepts. State-ofthe-art articles will be featured in Annals on a regular basis in the next several volumes. Hoechst Marion Roussel provided honoraria to the authors of this state-of-the-art article. Annals is grateful to Hoechst Marion Roussel for their continued support of emergency medicine. Address for reprints: J Michael Holbert, MD, UPMC Health Systems–Presbyterian, Department of Radiology, 200 Lothrop Street, Pittsburgh, PA 15213; 412-647-7288, fax 412-647-7795; E-mail holbertjm @radserv.arad.upmc.edu. Copyright © 1999 by the American College of Emergency Physicians.

0196-0644/99/$8.00 + 0 47/1/97702

J Michael Holbert, MD Philip Costello, MD Michael P Federle, MD

Recently a debate has developed in the medical community as radiologists in some centers suggest the selective substitution of spiral computed tomography (CT) for ventilation-perfusion (V/Q) nuclear medicine imaging as a screening test for the diagnosis of acute pulmonary embolism. Proponents of spiral CT argue that it is more accurate than the usual practice of combining the (V/Q) scan and the physician’s best clinical judgment. V/Q scans classify patients into groups according to the probability of pulmonary emboli, whereas the thrombus is visible with spiral CT. Opponents point out that large-scale patient outcome studies using spiral CT have not been completed, but such information is available for (V/Q) scans. Most clinicians are familiar with the strengths and limitations of an assessment that relies primarily on the (V/Q) scan, because this examination has been available for many years. Although spiral CT does not perform as well as pulmonary arteriography in detecting subsegmental emboli, the importance of smaller peripheral emboli is controversial. This review explores the advantages and disadvantages of investigations currently available for the diagnosis of acute pulmonary embolism from the perspective of the emergency physician, presenting the view that spiral CT is likely to have an increasingly important place in patient evaluation. [Holbert JM, Costello P, Federle MP: Role of spiral computed tomography in the diagnosis of pulmonary embolism in the emergency department. Ann Emerg Med May 1999;33:520-528.] INTRODUCTION

Acute pulmonary embolism is perplexing. It strikes frequently, affecting about 500,000 people per year in the United States,1 and it kills, causing more than 50,000 deaths annually in the United States.2 Thirty percent of patients with suspected pulmonary embolism are seen first in the emergency department.3 Many patients have no signs or symptoms, and the presentation is often non-

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specific, ranging from mild dyspnea to sudden, massive chest pain. Clinical problems from relatively insignificant heartburn to life-threatening aortic dissection can simulate pulmonary embolism. Until now, physicians have lacked an effective means of triage for these patients. The classical approach to the patient with a suspected pulmonary embolism has been unsatisfying. Ventilationperfusion (V/Q) scanning has been used as a screening test, and pulmonary angiography has been recommended when the diagnosis remained unclear. High-probability (V/Q) scans in the setting of a high clinical suspicion, or normal scans in the case of a low clinical suspicion, are very accurate for the diagnosis or exclusion of acute pulmonary emboli. However,V/Q scanning leaves the question of possible acute pulmonary embolism unanswered in about three fourths of patients.3 Few of these patients have pulmonary angiography, a procedure viewed as highly diagnostic, but invasive, costly, and often unavailable. V/Q scanning is seldom helpful in establishing the cause for symptoms in patients who do not have acute pulmonary embolism, but two thirds of patients referred for evaluation do not have pulmonary embolism.3 The diagnosis and treatment of acute pulmonary embolism should be viewed more generally as the diagnosis and treatment of thromboembolic disease. Proven or suspected acute pulmonary embolism warrants a search for deep vein thrombosis (DVT) in the lower extremities, the principal source for potentially life-threatening acute pulmonary emboli. Ordinarily, if 1 form of thromboembolic disease (DVT or acute pulmonary embolism) is diagnosed, the diagnosis of the other form is not pursued. Patients undergo anticoagulation for treatment of both manifestations of thromboembolic disease; patients with acute pulmonary embolism also receive supportive care. In most cases of acute pulmonary embolism, the clot undergoes autothrombolysis and is not treated directly. For the small group of patients with hemodynamic instability, thrombolytic therapy may be indicated. Spiral computed tomography (CT) has recently emerged as a possible adjunct or alternative to (V/Q) scanning for evaluating patients with suspected pulmonary embolism.4-7 With a (V/Q) scan the presence of an embolism is inferred on the basis of a V/Q mismatch, but spiral CT allows direct visualization of a thrombus.7 Although spiral CT has some limitations, it can detect central and segmental pulmonary emboli effectively. It is rapid, widely available, and relatively less expensive than (V/Q) scanning or pulmonary angiography. Spiral CT is also capable of demonstrating a broad range of thoracic conditions that can mimic acute pulmonary embolism. If large clinical stud-

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ies confirm the accuracy of spiral CT, it has the potential to revolutionize the approach to the patient with suspected acute pulmonary embolism. In this article we compare the advantages and disadvantages of V/Q imaging, pulmonary arteriography, Doppler ultrasound, and spiral CT. The limitations of spiral CT, V/Q imaging, and pulmonary arteriography for the diagnosis of subsegmental emboli are discussed. Finally, we suggest a protocol for effective screening of patients with suspected acute pulmonary emboli. CURRENT DIAGNOSTIC OPTIONS Ventilation-perfusion scan

The multiinstitutional Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) trial has established the usefulness of V/Q scans in evaluating patients with possible pulmonary emboli. 3 The landmark PIOPED investigation was performed to determine the sensitivity and specificity of V/Q scanning for the diagnosis of acute pulmonary embolism. The rigorous design of the PIOPED trial has made it the most widely quoted and respected study of the role of V/Q scanning in acute pulmonary embolism. In the PIOPED study, V/Q scans were grouped into 4 categories according to the probability that the findings were related to an acute pulmonary embolism (nearnormal/normal, low-, intermediate-, or high-probability). The level of clinical suspicion was very important. The PIOPED investigators established that a patient with a low clinical suspicion and a normal/near-normal or low-probability V/Q scan has a 2% to 4% chance of having a clinically important acute pulmonary embolism. 3 A patient with a high-probability clinical impression and high-probability V/Q scan has a 96% probability of acute pulmonary embolism.3 Thus clinicians were confident in withholding anticoagulation for patients with normal/near-normal or low-probability V/Q scans and treating patients with high-probability V/Q scans with anticoagulation. However, many physicians remain dissatisfied with the use of V/Q scans for the evaluation of acute pulmonary embolism. V/Q scans do not demonstrate a pulmonary embolus directly, but rather show its secondary effects on the pulmonary vascular bed. Secondary effects can be more difficult to interpret. The interobserver disagreement in the PIOPED trial for intermediate- and low-probability V/Q scans was 25% and 30%, respectively. 3

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Beyond interobserver variation is a bigger problem. From the PIOPED study, appropriate therapy was clear only for near-normal/normal or high-probability scans, but the low- and intermediate-probability groups made up 34% and 39%, respectively or nearly three fourths of the patients referred for V/Q scanning.3 Although patients in the low- and intermediate-probability groups usually do not receive further testing, 16% of patients with low-probability and 33% of patients with intermediateprobability V/Q scans had pulmonary emboli proven by angiography. 3 This means that some patients with pulmonary emboli are untreated, whereas others without pulmonary emboli receive inappropriate anticoagulation. The consequences of failure to diagnose acute pulmonary emboli in the low-probability group may not always be apparent to an acute care physician. However, in a group of 77 patients with poor cardiopulmonary reserve who had low-probability V/Q scans but did not receive anticoagulation, 7.8% had autopsy-confirmed death by pulmonary embolism within 3 months of evaluation.8 If a patient has symptoms of acute pulmonary embolism along with leg pain or swelling or has a low- or intermediate-probability V/Q scan, gray-scale and color Doppler venous ultrasound has been used to look for DVT. 9,10 Impedance plethysmography, which uses changes in electrical impedance as a noninvasive means of measuring blood flow through the deep veins of the lower extremities, has also been used, 8,11-13 but its use is no longer wide-spread. Contrast venography is excellent in demonstrating DVT, 14 but it is invasive, painful, and carries a risk of at least 1% to 2% of inducing DVT, although a substantially higher rate of induced DVT has been reported in some series.15 DVT often causes leg swelling that limits intravenous access. The role of Doppler ultrasound is important, because recurrent embolism from DVT is a major risk to patients with a missed diagnosis of pulmonary embolism. DVT can be occult, so the first sign of thromboembolic disease often is acute pulmonary embolism. If DVT is proved, the patient is usually treated with anticoagulation, regardless of the V/Q scan interpretation. Although Doppler ultrasound and impedance plethysmography are valuable treatment guides for thromboembolic disease, disease can be present although noninvasive testing demonstrates no abnormality. With impedance plethysmography up to 50% of patients with acute pulmonary embolism do not have demonstrable DVT. 16,17 Some thrombi develop in deep pelvic veins that ultrasound cannot assess adequately, and other thrombi have already migrated to the lungs.

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Pulmonary angiography

In the classical evaluation of suspected acute pulmonary embolism, pulmonary angiography has been recommended when a strong clinical suspicion exists and the diagnosis is uncertain after V/Q scanning and Doppler ultrasound. Pulmonary angiography depicts thrombi directly, and no other modality has equal spatial resolution or demonstrates subsegmental emboli as well. Wedge angiography can show pulmonary emboli in vessels as small as 1.5 mm.18 Although pulmonary angiography is the “gold standard” for the diagnosis of acute pulmonary embolism, it has several shortcomings. Although pulmonary angiography is usually considered definitive, it has an interobserver agreement of only 81%.19 Pulmonary angiography is invasive with a mortality of about .3%19 and major complications in 1% to 3% of patients.19-21 Patients are often so ill that physicians are even reluctant to refer patients with intermediate-probability V/Q scans for pulmonary angiography, although one third of these patients have pulmonary emboli.3 In 1 study, less than half of patients with moderate or indeterminate-probability V/Q scans underwent angiography.22 Spiral computed tomography

Although pulmonary angiography is unsurpassed at depicting small pulmonary emboli, spiral CT is excellent for visualization of clot within the central or segmental pulmonary arteries. Pulmonary emboli are visible as filling defects in the center or periphery of the artery, outlined by contrast material flowing around the clot (Figure 1). In the segmental and subsegmental pulmonary arteries, the clot can completely occlude or even enlarge the artery. Acute pulmonary emboli were detected with conventional CT,23,24 but the potential of spiral CT for diagnosing pulmonary emboli became apparent with the introduction of spiral CT,25 a major advance in CT scanner technology.26 With conventional CT, the patient lies on a scanner table that is intermittently advanced. An x-ray tube revolves around the patient after each halt in table motion, but the tube is started and stopped with each advance. With spiral CT the x-ray tube describes a helix relative to the patient by revolving constantly as the table advances continually. Without the necessity of starting and stopping the tube repeatedly, the same patient volume can be scanned much faster. This reduces scan time substantially, limiting artifact from patient motion and improving the efficiency of contrast material use. A study can be completed in less than half an hour, including

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patient transport time, starting an intravenous line, and image review. Recent improvements in CT scanner technology have reduced the actual scan time for a pulmonary embolism study to 20 seconds or less, decreasing breathholding requirements. Spiral CT has some limitations, however. Visualization of smaller arteries can be affected by problems with technique or suboptimal vessel opacification. Larger emboli can still be diagnosed even in technically limited studies. Breathing motion affects the peripheral arteries more; the central and segmental arteries can still usually be evaluated well even in studies limited by patient breathing. With the newest CT scanners the very rapid scan times mitigate the effect of patient breathing. A ventilator-dependent patient can be paralyzed and respiration can easily be suspended for the time necessary to perform the spiral CT, usually yielding a satisfactory examination. Remy-Jardin et al 27 reported 4% of studies as technically inadequate and 10% as nondiagnostic. These problems are substantially less common with current-generation equipment and updated scanning techniques. False-positive and false-negative study results can occur because of interpretation errors. Breathing artifacts, incomplete opaci-

fication of vessels, and confusion with pulmonary veins or hilar lymph nodes are sources of error, but these problems are less troublesome with more observer experience. 28 The need for contrast material can limit the use of spiral CT. The most common relative contraindications for evaluating pulmonary emboli with spiral CT are contrast material allergy and renal failure. A commonly used dosage of contrast material for spiral CT is 150 mL of 60% iodine. If follow-up angiography is needed, digital pulmonary angiography is usually readily accomplished with 100 mL of contrast material, a dosage that is well within the safe limits of a total of 300 mL of contrast material. SPIRAL COMPUTED TOMOGRAPHY COMPARED WITH PULMONARY ANGIOGRAPHY AND V/Q SCAN

Several studies have compared spiral CT scan with pulmonary angiography for the diagnosis of acute pulmonary embolism (Table).27,29-31 Data from these studies, with a total of 184 patients, show that spiral CT has a sensitivity range of 53% to 100% and a specificity range of 78% to 97%. When all pulmonary arteries are considered,

Figure 1.

A, Spiral CT of the chest shows a saddle embolus in the left and right pulmonary arteries (arrows). B, A more caudal image shows thrombi as filling defects in the interlobar pulmonary arteries outlined by contrast material (arrows).

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the range of sensitivity and specificity is relatively broad. Some of the differences can be explained by differences in equipment, scanning protocols, contrast material administration, study design, and radiologist experience. When only central or segmental emboli are considered, the sensitivity and specificity of spiral CT approach 90% in most studies.4 A recent article by Drucker et al29 found a sensitivity of 53% to 60% and a specificity of 81% to 97% in evaluating the ability of 2 groups of readers to detect all pulmonary emboli. The authors suggested that 4 factors could have contributed to a lower sensitivity for spiral CT than earlier studies. These included more liberal patient selection criteria, a possibly suboptimal scanning protocol, possibly suboptimal contrast material administration protocol, and the lack of use of a workstation that could improve viewing of images. Experience has shown that the relatively thick slices that were used by Drucker et al limit the diagnosis of subsegmental pulmonary emboli. Our current protocol calls for a 2-mm slice thickness compared with the 5-mm sample that was used by Drucker et al.29 Interactive workstation interpretations are increasing with the changeover of radiology departments from film-based imaging to electronic imaging. The results of a much larger group of patients who were included in the European Multicenter Trial (ESTIPEP) were presented at the 1998 Scientific Program of the Radiological Society of North America. Ten European academic centers participated in the study and 401 patients completed the protocol. Clinical probability was estimated and all patients underwent spiral CT and V/Q scan. Pulmonary angiography was performed in cases of low- and intermediate-probability V/Q scans and contradictory spiral CT and V/Q scan diagnosis. Compared with

pulmonary angiography, spiral CT had a sensitivity of 88% and a specificity of 94%. The interobserver agreement was 72% for spiral CT, much better than the interobserver agreement of 39% for V/Q scanning and 46% for pulmonary angiography. Data from this large, multiinstitutional trial will probably define the role of spiral CT in pulmonary embolism as the PIOPED study determined the place of the V/Q scan. The PIOPED study highlighted a problem with defining sensitivity and specificity of V/Q scanning.3 For highprobability V/Q scans, the specificity is high at 97%, but the sensitivity is only 41%. Combining the 3 abnormal categories of low-, intermediate-, and high-probability V/Q scans yields a sensitivity of 98% for patients with clinically important pulmonary embolism, but the specificity is only 10%. Because considering all abnormal V/Q scans as positive is so nonspecific, it may be more appropriate to compare the sensitivity and specificity of high-probability V/Q scans to spiral CT. Mayo et al 32 examined a group of 139 patients who underwent V/Q scanning, spiral CT, and angiography. They found a sensitivity of 87% and a specificity of 95% for spiral CT, and a sensitivity of 65% and a specificity of 94% for high-probability V/Q scans. Some proponents of V/Q scanning have emphasized the sensitivity of low- or intermediate-probability V/Q scans, particularly for detecting subsegmental emboli. However, few patients with low- or intermediate-probability V/Q scans and a high clinical suspicion are referred for angiographic confirmation; even fewer with a low clinical suspicion are referred.22 Unfortunately, without a high clinical suspicion or angiographic confirmation of pulmonary embolism, patients with low- or intermedi-

Table.

Spiral CT versus angiography. Investigators

No. of Patients

Remy-Jardin et al31 (1992) Goodman et

al30

(1995)

Remy-Jardin et al27 (1996) 29

Drucker et al (1998) interpreted by 2 groups

Sensitivity (%)

Specificity (%)

42

100*

96*

20

86*

75

63 91*

92* 89 78*

60 53

81 97

47

*

For central pulmonary emboli. Other percentages are for all pulmonary emboli.

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ate-probability scans may not receive anticoagulants even though they have pulmonary emboli.8,22,33 Therefore the relatively high sensitivity of an abnormal V/Q scan for detecting subsegmental emboli is so nonspecific that it has little treatment value. Several authors have emphasized the limitation of spiral CT in detecting subsegmental emboli,27,30,34 but the importance of this shortcoming is unclear. Pulmonary emboli split into an average of 6 to 8 fragments20,31,35 and in most patients 1 or more of these fragments is large enough to be detected at spiral CT. Isolated pulmonary emboli are probably infrequent, but the true prevalence of isolated pulmonary emboli is unknown. Pulmonary angiography is the standard for detection of subsegmental emboli and only patients with an uncertain diagnosis undergo angiography. Variations of the prevalence of isolated subsegmental emboli in different studies are reflections of different patient populations, selection bias, and study design.34,36 The least biased study was probably the PIOPED trial,36 where 5.6% of pulmonary emboli were limited to the subsegmental arteries.3 In 4 studies comparing spiral CT with pulmonary angiography, 0% to 36% of pulmonary emboli were isolated and subsegmental.27,29,30,34 Spiral CT demonstrated 0% to 50% of these isolated subsegmental emboli. Although pulmonary angiography is the diagnostic standard for subsegmental emboli, even angiography has difficulties with peripheral emboli. In the PIOPED study the interobserver agreement for subsegmental emboli was only 66%,19 showing that in clinical practice some subsegmental emboli must be missed. In an evaluation of 125 consecutive angiograms, Diffin et al37 found that by unanimous consensus, 4% of study patients had isolated subsegmental pulmonary embolism. Consensus review changed the initial diagnosis in 30% of patients with only subsegmental emboli. When all subsegmental emboli in all patients were reviewed, consensus interpretation changed the initial diagnosis on a per-embolus basis in 37% of subsegmental emboli. The clinical importance of subsegmental emboli is controversial and is probably dependent on the patient’s cardiopulmonary reserve. In healthy patients, subsegmental emboli are probably of no clinical consequence. Because there was only a 66% interobserver agreement for subsegmental pulmonary emboli in the PIOPED trial, some subsegmental pulmonary emboli must have been missed, yet pulmonary embolism was only seen in .6% in 1 year of surveillance after angiography.19 In another study, 147 patients with a negative pulmonary arteriogram and no anticoagulation had 6 months of follow-

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up.38 None of these patients died of thromboembolic disease and none had recurrent embolism. However, Remy-Jardin et al27 have reported clinical improvement after anticoagulation in 4 patients with subsegmental emboli and limited cardiopulmonary reserve. The available evidence suggests that use of spiral CT for evaluating possible pulmonary emboli is safe. Ferretti et al39 examined 164 patients with possible acute pulmonary embolism, intermediate-probability V/Q scans, and negative duplex ultrasound of the legs. During 3-month follow-up of 112 patients with negative results after spiral CT for detection of pulmonary embolus, repeat ultrasound demonstrated calf vein thrombi in 3 patients and 3 patients had recurrent pulmonary embolism, resulting in 1 death, for a false-negative rate of 5.4%. This false-negative rate is comparable to a group of studies that followed patients suspected of pulmonary embolism who had negative pulmonary angiograms. Ferretti et al also found that pulmonary embolism was initially present in 24% of patients with intermediate-probability V/Q scans and normal results on duplex leg ultrasound. This illustrates the complementary role of leg ultrasound and spiral CT. Although the study by Ferretti et al39 is a start toward developing data on clinical outcomes, much more information is needed, particularly with regard to outcome of treated or untreated subsegmental pulmonary emboli. Questions such as the prevalence of subsegmental emboli or the sensitivity and specificity of spiral CT for detecting pulmonary emboli are unlikely to be answered directly with the inherent biases in clinical practice and limitations of pulmonary angiography for evaluating subsegmental emboli. Several authors have pointed out the need for large clinical outcome trials, which would address the clinical significance of these issues indirectly.5,40,41 On balance, it seems likely that spiral CT would detect more clinically important emboli than the current clinical practice. There are broader considerations involved in the choice between V/Q scanning and spiral CT. The classical workup for pulmonary embolism is inadequate to establish a definitive diagnosis in most patients. More than two thirds of patients evaluated for acute pulmonary embolism have other diagnoses unlikely to be clarified by V/Q scan or pulmonary angiography and further investigations are needed.3 In the ED, it is important not only to choose a study that gives a reasonable chance of identifying those patients with acute pulmonary embolism, but also one that may provide an alternative diagnosis. Spiral CT has a proven value in diagnosing subtle pneumonia or pneumothorax, pleural or pericardial disease, aortic dissection, acute gastrointestinal

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disorders, and many other conditions that can clinically simulate pulmonary embolism. Van Rossum et al 42 found that spiral CT demonstrated abnormalities in 69% of patients that could account for V/Q scan abnormalities. Ferretti et al 39 identified lung pathology in 11% of patients that could account for the patient presentation and the nondiagnostic V/Q scan. In addition to suggesting alternative diagnoses, spiral CT is cost-effective. A V/Q scan is 1.4 times as expensive as a spiral CT scan; a pulmonary angiogram is 6 to 8 times as expensive as a spiral CT.4 Van Erkel et al43 investigated the cost-effectiveness of various diagnostic algorithms, including combinations of V/Q scan, ultrasound, Ddimer assay, conventional angiography, and spiral CT. Ultrasound of the lower extremities followed by spiral CT was the best strategy when mortality was the primary outcome parameter. A discussion of D-dimer assay is beyond the scope of this article, but initial screening by D-dimer assay, a test for fibrin degradation products, and further evaluation by spiral CT was the best strategy when the determining characteristic was cost per life saved. Van Erkel et al43 found that all the best strategies included spiral CT and suggested that the use of spiral CT was likely to

reduce mortality and improve cost-effectiveness in the evaluation of pulmonary embolism. A critical assumption was that spiral CT had a sensitivity of 85% or greater. For an assumed lower sensitivity of spiral CT, conventional angiography yielded a lower mortality, but did not improve cost-effectiveness. This points out the importance of continued efforts to improve the sensitivity of spiral CT. SUGGESTED ALGORITHM FOR ACUTE PULMONARY EMBOLISM

Any recommendation for evaluating a patient with a suspected pulmonary embolism should be viewed in the unique clinical context of the patient and the medical institution. Sometimes spiral CT will confirm an alternate diagnosis such as aortic dissection in a patient suspected of pulmonary embolism. In these instances, possible DVT is no longer a critical concern. Also, any recommendation for the use of spiral CT carries with it the requisite that spiral CT is conveniently available and that a prompt, accurate diagnosis is rendered. Optimal clinical use requires that technical support staff is available quickly at all hours,

Figure 2.

Proposed evaluation for suspected pulmonary embolism. V/Q scan, radionuclide ventilation-perfusion scan; US, Doppler ultrasound; Rx, treatment; DVT, deep vein thrombosis; angio, pulmonary angiography; PE, pulmonary embolism. (Modified from Goodman et al.4)

Image Not Available

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that appropriate protocols are followed, and that radiologists are expert at diagnosing acute pulmonary emboli. Goodman et al4 have proposed an algorithm for evaluating a patient with a suspected pulmonary embolism, taking into account the advantages and disadvantages of each imaging test. Figure 2 is a modification of this algorithm. Patients who present to the ED with the diagnosis of possible acute pulmonary embolism are a heterogeneous population4,5,41 and can broadly be divided into 2 groups; 1 group without chronic cardiopulmonary disease typically presents as outpatients in acute distress with normal chest radiographs.41Another group with chronic cardiopulmonary disease often presents as recent inpatients in acute distress with abnormal chest radiographs.4,41 The V/Q scan is a very effective screening test for suspected pulmonary embolism for patients without chronic cardiopulmonary disease and should be performed initially for those patients (Figure 2).4,44 Patients with high-probability scans usually undergo anticoagulation, and an alternative diagnosis is sought in patients with normal/lowprobability V/Q scans and low clinical suspicion.16 If the V/Q scan shows intermediate or low probability and there is a moderate to high clinical suspicion, patients should undergo Doppler ultrasound of the lower extremities with or without a spiral CT scan as noninvasive alternatives to pulmonary angiography.27,32,44 Mayo et al32 found that in 80% of patients with intermediate-probability V/Q scans, spiral CT provided a correct diagnosis. Spiral CT is effective for evaluating suspected pulmonary emboli in patients with intermediate-probability27,32,39,45 or lowprobability27,45 V/Q scans. If spiral CT does not establish an alternative diagnosis, or is normal or indeterminate and there is a high clinical suspicion of acute pulmonary embolism, the patient should undergo pulmonary angiography. For patients with a low clinical suspicion of pulmonary embolism, spiral CT or other tests could be considered to establish an alternative diagnosis rather than performing a V/Q scan as the initial screening test. The V/Q scan is much less effective in evaluating patients with chronic cardiac or pulmonary disease and abnormal chest radiographs as they are more likely to have V/Q abnormalities.4,5,46 In an analysis of the PIOPED data, Lesser et al46 determined that 91% of V/Q scans in patients with chronic obstructive pulmonary disease were of lowor intermediate-probability and that 78% of V/Q scans for patients with any cardiopulmonary disease were low- or intermediate-probability. Many of these patients with nondiagnostic V/Q scans would be expected to undergo pulmonary angiography in the classical algorithm. Patients with chronic cardiopulmonary disease should undergo a spiral CT scan as the initial investigation for pulmonary

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embolism, if they have no symptoms of DVT (Figure 2). Even seriously ill patients usually tolerate spiral CT. Patients with a negative or indeterminate spiral CT scan or those with initial symptoms of DVT should undergo Doppler ultrasound. 4,5 This lowers the possibility of a missed and potentially untreated pulmonary embolus with the chance of recurrent embolism. Depending on the clinical presentation and degree of clinical suspicion, those patients who have a negative Doppler ultrasound and negative spiral CT results would have repeat Doppler ultrasound in 5 days, pulmonary angiography, or additional testing to establish another diagnosis. In summary, the approach of an emergency physician to the evaluation of a patient who presents with a potential pulmonary embolus may be different from an internist or surgeon caring for hospital inpatients. Many of the patients seen in the ED have been previously healthy and are suitable for V/Q scanning as a screening test. These patients often have normal or high-probability V/Q scans, directing the physician either to look for an alternative diagnosis or begin anticoagulation therapy. V/Q scans are less helpful in patients with chronic cardiopulmonary disease. Spiral CT provides a rapid, accurate method to diagnose pulmonary embolism in these individuals by direct clot visualization. Spiral CT has the additional benefit of establishing a conclusive alternative diagnosis in many conditions that can simulate pulmonary embolism, avoiding further testing. In patients with a negative spiral CT scan, Doppler ultrasound should be done to help exclude DVT, the source of potentially fatal recurrent emboli. Because no studies of spiral CT have been done on a restricted population of ED patients or outpatients, projecting the role of spiral CT in the diagnosis of acute pulmonary embolism from studies that include both inpatients and outpatients has some limitations. Gefter and Palevsky41 pointed out that the difference in sensitivity for spiral CT evaluation of pulmonary embolism for inpatients and outpatients has not been examined. Also, the ED patient population has a lower pretest probability of acute pulmonary embolism than unrestricted radiology department referral populations that include postoperative patients and patients with serious preexisting medical conditions. Still, ED or clinic referrals can be expected to make up a substantial number of patients referred to radiology departments for suspected pulmonary embolism. For example, in the PIOPED study, 30% of patient referrals came from an ED or clinic.3 Finally, we must recognize that the diagnosis and management of acute pulmonary embolism are evolving with legitimate variations from one medical practice to another. Spiral CT is a developing technology, not yet available in all practice settings around the clock; V/Q scanning is a more

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mature technology. The use of spiral CT is compelling, but large-scale multicenter patient trials will provide clinical outcomes and the accuracy of spiral CT for diagnosis of pulmonary embolism. When the importance of subsegmental pulmonary emboli is determined, and as the performance and interpretation of spiral CT improve, we anticipate that spiral CT will assume an increasingly important role in evaluating patients in the ED. Potentially, most patients suspected of having pulmonary embolism could be evaluated with a combination of spiral CT and Doppler ultrasound, where appropriate spiral CT equipment and radiologists proficient in the interpretation of spiral CT are available.47 REFERENCES 1. Weiss K: Pulmonary thromboembolism: Epidemiology and techniques of nuclear medicine. Semin Thromb Hemost 1996;22:27-32. 2. Goldhaber SZ: Thrombolysis for pulmonary embolism. Progr Cardiovasc Dis 1991;34:113-134. 3. The PIOPED Investigators: Value of the ventilation/perfusion scan in acute pulmonary embolism: Results of the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED). JAMA 1990;263:2753-2759. 4. Goodman LR, Lipchik RL, Kuzo RS: Acute pulmonary embolism: The role of computed tomographic imaging. J Thorac Imag 1997;12:83-86. 5. Goodman LR, Lipchik RJ, Kuzo RS: Reply to opinions. J Thorac Imag 1997;12:100-102.

23. Godwin JD, Webb WR, Gamsu G, et al: Computed tomography of pulmonary embolism. AJR Am J Roentgenol 1980;135:691-695. 24. Verschakelen JA, Vanwijck E, Bogaert J, et al: Detection of unsuspected central pulmonary embolism with conventional contrast-enhanced CT. Radiology 1993;188:847-850. 25. Winston CB, Wechsler RJ, Salazar AM, et al: Incidental pulmonary emboli detected at helical CT: Effect on patient care. Radiology 1996;201:23-27. 26. Kalender WA, Seissler W, Klotz E, et al: Spiral volumetric CT with single-breath-hold technique, continuous transport, and continuous scanner rotation. Radiology 1990;176: 181-183. 27. Remy-Jardin M, Remy J, Deschildre F, et al: Diagnosis of pulmonary embolism with spiral CT: Comparison with pulmonary angiography and scintigraphy. Radiology 1996; 200:699-706. 28. Remy-Jardin M, Remy J, Artaud D, et al: Spiral CT of pulmonary embolism: Technical considerations and interpretive pitfalls. J Thorac Imag 1997;12:103-117. 29. Drucker EA, Rivits SM, Shepard JAO, et al: Acute pulmonary embolism: Assessment of helical CT for diagnosis. Radiology 1998;209:235-241. 30. Goodman LR, Curtin JJ, Mewissen MW, et al: Detection of pulmonary embolism in patients with unresolved clinical and scintigraphic diagnosis: Helical CT versus angiography. AJR Am J Roentgenol 1995;164:1369-1374. 31. Remy-Jardin M, Remy J, Wattinne L, et al: Central pulmonary thromboembolism: Diagnosis with spiral volumetric CT with the single-breath-hold technique-comparison with pulmonary angiography. Radiology 1992;185:381-387. 32. Mayo JR, Remy-Jardin M, Müller NL, et al: Pulmonary embolism: Prospective comparison of spiral CT with ventilation-perfusion scintigraphy. Radiology 1997;205:447-452. 33. Khorasani R, Gudas TF, Nikpoor N, et al: Treatment of patients with suspected pulmonary embolism and intermediate-probability lung scans: Is diagnostic imaging underused? AJR Am J Roentgenol 1997;169:1355-1357.

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ANNALS OF EMERGENCY MEDICINE

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