Pulmonary embolism

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Pulmonary embolism (PE) is form of embolism and thromboembolism in which a blockage of the pulmonary artery (or one of its branches), usually when a deep vein thrombosis (blood clot from a vein), becomes dislodged from its site of formation and embolizes to the arterial blood supply of one of the lungs.[1] This process is termed thromboembolism.

Pathophysiology

The development of thrombosis is classically due to a group of causes named Virchow's triad (alterations in blood flow, factors in the vessel wall and factors affecting the properties of the blood). Often, more than one risk factor is present.

Diagnosis

The diagnosis of PE is based primarily on validated clinical criteria combined with selective testing because the typical clinical presentation (shortness of breath, pleuritic chest pain[3][4]) cannot be definitively differentiated from other causes of chest pain and shortness of breath.[5] Patients can present with atypical syndromes such as unexplained exacerbations of chronic obstructive pulmonary disease.[6]

D-dimer may be under-used in patients at low risk of pulmonary embolism.[7] Introduction of computed tomographic pulmonary angiography may have led to overdiagnosis of pulmonary embolism].[8]

Probability scoring

Various clinical prediction rules exist to help diagnose PE, such as the Wells score and the Geneva rule. More importantly, the use of any rule may exclude PE when combined with a normal d-dimer[9] and use of any rule might be associated with reduction in recurrent thromboembolism.[10]

Wells score

History of the Wells score

The most commonly used method to predict clinical probability, the Wells score, is a clinical prediction rule, whose use is complicated by multiple versions being available. In 1995, Wells et al initially developed a prediction rule (based on a literature search) to predict the likelihood of PE, based on clinical criteria.[11] The prediction rule was revised in 1998[12] This prediction rule was further revised when simplified during a validation by Wells et al in 2000.[13] In the 2000 publication, Wells proposed two different scoring systems using cutoffs of 2 or 4 with the same prediction rule.[13] In 2001, Wells published results using the more conservative cutoff of 2 to create three categories.[14] An additional version, the "modified extended version", using the more recent cutoff of 2 but including findings from Wells's initial studies[11][12] were proposed.[15] Most recently, studies (including one by Wells[16]) reverted to Wells's earlier use of a cutoff of ≤ 4 points[13] to create only two categories.[17][16]

Wells score

The Wells score:[18]

  • clinically suspected DVT - 3.0 points
  • alternative diagnosis is less likely than PE - 3.0 points
  • tachycardia (>100 bpm) - 1.5 points
  • immobilization/surgery for 3 days ore more in previous four weeks - 1.5 points
  • history of DVT or PE - 1.5 points
  • hemoptysis - 1.0 points
  • malignancy (treatment for within 6 months, palliative) - 1.0 points
Interpretation of the Wells score

Traditional interpretation[13][14][19]

  • Score >6.0 - High (probability 59% based on pooled data[20])
  • Score 2.0 to 6.0 - Moderate (probability 29% based on pooled data[20])
  • Score <2.0 - Low (probability 4% to 15% based on pooled data[20])

Alternate interpretation[21][13][17][16]

  • Score > 4 - PE likely. Consider diagnostic imaging.
  • Score 4 or less - PE unlikely. Consider D-dimer to rule out PE.

Geneva score

The Geneva score also has several versions including the original version with 7 items that include blood gas and chest radiograph[22] the 'revised' version with 8 items using only signs and symptoms:[23]

Revised Geneva Score
Factor Points
Risk factor
Age > 65 1
Previous PE or DVT 3
Surgery under general anesthesia or lower-limb fracture within 1 month 2
Malignancy (either active or considered cured within 1 year 2
Symptoms
Unilateral lower-limb pain 3
Hemoptysis 2
Clinical signs
Heart rate 75-94 3
Heart rate > 94 5
Pain on lower-limb palpation and unilateral edema 4
Interpretation of the Revised Geneva Score
Points Clinical probability Prevalence of PE
0 -3 Low 8%
4 - 10 Intermediate 29%
> 10 High 74%

More recently, a "Simplified revised version" assigns only one point to each sign and symptom. The simplified version, which gives one point to each of the following:[24]

  • age over 65
  • history of deep venous thrombosis or pulmonary embolism
  • surgery under general anesthesia or lower-limb fracture within 1 month
  • active malignancy
  • unilateral lower-limb pain
  • hemoptysis
  • heart rate between 75 and 94 (give additional point if > 95)
  • pain on lower-limb palpation and unilateral edema

In this version, a patient with a score of 2 or less is unlikely to have a pulmonary embolism during the next three months.

PERC

The Pulmonary Embolism Rule-out Criteria (PERC) may identify patients who are at such low risk that d-dimer testing is not needed in low (<15% prevalence)[25][26], but not medium (>20% prevalence)[27], risk populations.

Blood tests

In low/moderate suspicion of PE, a normal D-dimer level (shown in a blood test) is enough to exclude the possibility of thrombotic PE.[28][29][30] Unfortunately, many or even most doctors do not explicitly calculate pretest probability when interpreting the results of the d-dimer.[31]

Immunologic tests for d-dimer are generally use immunoassays such as enzyme-linked immunosorbent assay or serologic tests such as agglutination tests. The immunoassays (more specifically, enzyme-linked immunosorbent assay) tend to be more sensitive.[28]

D-dimer tests for pulmonary embolism[28][29]
  sensitivity specificity
Immunoassays
Elisa such as VIDAS™ 95%[28]
96%[29]		 44%[28]
39%[29]
Agglutination tests
Latex agglutination such as Tinaquant™ 89%[28]
96%[29]		 45%[28]
43%[29]
Whole blood hemagglutination test such as SimpliRED™ 78%[28]
87%[29]		 74%[28]
66%[29]

Most patients with a pulmonary embolism have an abnormal alveolar-arterial oxygen gradient.[32]

Imaging

Pulmonary angiography

The gold standard for diagnosing pulmonary embolism (PE) is pulmonary angiography. Pulmonary angiography is used less often due to wider acceptance of CT scans, which are non-invasive.

CT pulmonary angiography

Computed tomography with radiocontrast, also known as computed tomographic pulmonary angiography (CTPA), is increasingly used as the mainstay in diagnosis.[33] Advantages are clinical equivalence, its non-invasive nature, its greater availability to patients, and the possibility of detecting alternative diagnoses[34] from the differential diagnosis when there is no pulmonary embolism. CTPA has progressed to be available with 64 slices, each 0.625 mm thick. These machines take 3-4 seconds to scan and may be gated to the heart beat.

Role in diagnosis

Assessing the accuracy of CT pulmonary angiography is hindered by the rapid changes in the number of rows of detectors available in multidetector CT (MDCT) machines.[35] The PIOPED II study used a mixture of 4 slice and 16 slice scanners and reported a sensitivity of 83% and a specificity of 96%. This study noted that additional testing is necessary when the clinical probability is inconsistent with the imaging results.[36]


Positive predictive value of CT pulmonary angiography (CTA) in the PIOPED II study[36]
Location of embolism Number of patients
with true positive CTA		 Total number of patients
with this finding on CTA		 Total number of patients
with embolism in this location		 Positive predictive value of CTA
Any 150 175 183 86%
Embolism in a main or lobar artery 116 120 Not reported 97%
Segmental vessel 32 47 Not reported 68%
Subsegmental branch 2 8 Not reported 25%
Role in Prognosis

Two systematic reviews[37][38] and two more recent randomized controlled trials[39][16] have studied prognosis after a negative CTPA.

Both systematic reviews concluded that is appears safe to withhold anticoagulation after a negative CTPA.[37][38] However, there may be two limitations to these conclusions. First, only one study in the two reviews has a pretest probability over 40%. Thus, these conclusions may not generalize to patients who are high risk in the three level Wells score. Second, many of the patients in these studies had additional tests such as leg dopplers as part of their evaluation so the results may not address CTPA as an individual test.

A more recent randomized controlled trial used d-dimer along with a mixture of 16 to 64 row detectors and found that adding imaging of the legs was not needed.[39] Among the patients included in this protocol, the prevalence of pulmonary embolism was 20.6%. All patients had either a d-dimer test or a leg ultrasonogram to help exclude pulmonary embolism.

A second randomized controlled trial was published after the two systematic reviews. This trial included patients with either patients with a Wells score of 4.5 or greater or a positive D-dimer assay result. The prevalence of pulmonary embolism during the initial evaluation was 14%. The trial found that CTPA, especially when multidector scans are used, increase the number of emboli found as compared to Ventilation/perfusion scan.[16] The importance of the increased detection is uncertain, but may be partly overdiagnosis.[40]

Ventilation/perfusion scan

Ventilation/perfusion scan (or V/Q scan or lung scintigraphy), which shows that some areas of the lung are being ventilated but not perfused with blood (due to obstruction by a clot). This type of examination is used less often because of the more widespread availability of CT technology, however, it may be useful in patients who have an allergy to iodinated contrast or in pregnancy due to lower radiation exposure than CT.

For patients with a normal chest x-ray, doing a V/Q scan rather than computed tomographic pulmonary angiography may reduce radiation exposure.[41]

Chest X-ray

Chest X-rays are often done on patients with shortness of breath to help rule-out other causes, such as congestive heart failure and rib fracture. Chest X-rays in PE are rarely normal,[42] but usually lack signs that suggest the diagnosis of PE (e.g. Westermark sign, Hampton's hump).

Magnetic resonance imaging

Magnetic resonance imaging with gadolinium has lower sensitivity than other methods and images are not always technically adequate.[43]

Ultrasonography of the legs

Ultrasonography or Duplex Doppler ultrasonography of the legs may help by diagnosing deep venous thrombosis (DVT) of the legs that may have led to pulmonary embolism. The presence of DVT, as shown on ultrasonography of the legs usually warrants anticoagulation, because of the strong association between DVT and PE.

Examining the legs may be valid approach in pregnancy, in x-rays might cause birth defects in the unborn child. However, a negative scan does not rule out PE, and low-radiation dose scanning may be required if the mother is deemed at high risk of having pulmonary embolism.

Electrocardiogram findings

An ECG may show signs of right heart strain or acute cor pulmonale in cases of large PEs - the classic signs are a large S wave in lead I, a large Q wave in lead III and an inverted T wave in lead III ("S1Q3T3").[44][45] This is occasionally (up to 20%) present, but may also occur in other acute lung conditions and has therefore limited diagnostic value; the most commonly seen sign in the ECG is sinus tachycardia.

Echocardiography findings

In massive and submassive PE, dysfunction of the right side of the heart can be seen on echocardiography, an indication that the pulmonary artery is severely obstructed and the heart is unable to match the pressure. Some studies (see below) suggest that this finding may be an indication for thrombolysis. Not every patient with a (suspected) pulmonary embolism requires an echocardiogram, but elevations in cardiac troponins or brain natriuretic peptide may indicate heart strain and warrant an echocardiogram.[46]

The specific appearance of the right ventricle on echocardiography is referred to as the McConnell sign. This is the finding of akinesia of the mid-free wall but normal motion of the apex. This phenomenon has a 77% sensitivity and a 94% specificity for the diagnosis of acute pulmonary embolism.[47]

Combining tests into algorithms

Recent recommendations for a diagnostic algorithm have been published by the PIOPED II investigators; however, these recommendations do not reflect research using 64 slice MDCT.[48][20] These investigators recommended:

  • Low clinical probability. If negative D-dimer, PE is excluded. If positive D-dimer, obtain MDCT and based treatment on results.
  • Moderate clinical probability. If negative D-dimer, PE is excluded. However, the authors were not concerned that a negative MDCT with negative D-dimer in this setting has an 5% probability of being false. Presumably, the 5% error rate will fall as 64 slice MDCT is more commonly used. If positive D-dimer, obtain MDCT and based treatment on results.
  • High clinical probability. Proceed to MDCT. If positive, treat, if negative, addition tests are needed to exclude PE.

Treatment

In most cases, anticoagulant therapy is the mainstay of treatment. Some patients at risk of bleeding and with low risk of recurrent embolism may have treatment safely withheld.[49] Acutely, supportive treatments, such as oxygen or analgesia, are often required. See respiratory emergencies and critical care.

Anticoagulation

For more information, see: anticoagulant.

In most cases, anticoagulant therapy is the mainstay of treatment. Heparin, low molecular weight heparins (such as enoxaparin and dalteparin), or fondaparinux is administered initially, while warfarin therapy is commenced (this may take several days, usually while the patient is in hospital). Warfarin therapy often requires frequent dose adjustment and monitoring of the INR. In PE, INRs between 2.0 and 3.0 are generally considered ideal. If another episode of PE occurs under warfarin treatment, the INR window may be increased to e.g. 2.5-3.5 (unless there are contraindications) or anticoagulation may be changed to a different anticoagulant e.g. low molecular weight heparin. In patients with an underlying malignancy, therapy with a course of low molecular weight heparin may be favored over warfarin based on the results of the CLOT trial.[50] Similarly, pregnant women are often maintained on low molecular weight heparin to avoid the known teratogenic effects of warfarin.

Sometimes, anticoagulation may be done as an outpatient.[51][52][53][54]

Duration of treatment

Regarding the duration of anticoagulation, see embolism and thrombosis: treatment.

Vena cava filter

If anticoagulant therapy is contraindicated and/or ineffective an inferior vena cava filter may be implanted[55]; however, the risk-benefit is uncertain.[56]

A superior vena cava filter can be used for upper extremity thrombosis; however, the median survival is approximately one month.[57][58] A retrievable filter has been used.[59]

Thrombolysis

For more information, see: Thrombolysis.

Clinical practice guidelines address the management of severe forms of embolism and thrombosis which may require thrombolysis.[60] An algorithm from the guidelines is online at http://circ.ahajournals.org/content/123/16/1788/F2.large.jpg. Massive PE causing hemodynamic instability (marked decreased oxygen saturation, tachycardia and/or hypotension) is an indication for thrombolysis, the enzymatic destruction of the clot with medication.

For submassive PE, some advocate thrombolysis if right ventricular dysfunction can be demonstrated on echocardiography.[61] However, an uncontrolled study suggests that submassive saddle PEs may not require thrombolysis.[62]

Thrombolysis can be given for severe PEs when surgery is not immediately available or possible (e.g. periarrest or during cardiac arrest). The only trial that addressed this issue had 8 patients; the four receiving thrombolysis survived, while the four who received only heparin died.[63] The use of thrombolysis in moderate PEs is still debatable. The aim of the therapy is to dissolve the clot, but there is an attendant risk of bleeding or stroke.[64]

Surgical management of PE

Surgical management of acute pulmonary embolism (pulmonary thrombectomy) is uncommon and has largely been abandoned because of poor long-term outcomes. However, recently, it has gone through a resurgence with the revision of the surgical technique and is thought to benefit selected patients.[65]

Chronic pulmonary embolism leading to pulmonary hypertension (known as chronic thromboembolic hypertension) is treated with a surgical procedure known as a pulmonary thromboendarterectomy.

Prognosis

Mortality from untreated PE is said to be 26% based on the outcome in patients who received placebo treatment.[66] The rate of 26% in the placebo group is probably an overstatement, given that the technology of the day may have detected only severe PEs.

With treatment, emboli resolve.[67] About half of emboli cannot be imaged after 10 days and about 80% cannot be imaged after one month.[67]

Prognosis depends on the amount of lung that is affected (embolic burden)[68] and on the co-existence of other medical conditions; chronic embolisation to the lung can lead to pulmonary hypertension. There is controversy over whether or not small subsegmental PEs need to be treated at all[69] and some evidence exists that patients with subsegmental PEs may do well without treatment.[70][36]

Predicting complications

The PESI and Geneva prediction rules can estimate mortality and so may guide selection of patients who can be considered for outpatient therapy.[71]

Complications are more likely if heart rate is 100 bpm or more and if the D-dimer concentration > 3,000 microg/ml or more.[72]

Evaluation for underlying causes for recurrence

After a first PE, the search for secondary causes is usually brief. Only when a second PE occurs, and especially when this happens while still under anticoagulant therapy, a further search for underlying conditions is undertaken. This will include testing ("thrombophilia screen") for Factor V Leiden mutation, antiphospholipid antibodies, protein C and S and antithrombin levels, and later prothrombin mutation, MTHFR mutation, Factor VIII concentration and rarer inherited coagulation abnormalities.

References

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  2. ↑ Segal JB, Brotman DJ, Necochea AJ, et al. (June 2009). "Predictive value of factor V Leiden and prothrombin G20210A in adults with venous thromboembolism and in family members of those with a mutation: a systematic review". JAMA 301 (23): 2472–85. DOI:10.1001/jama.2009.853. PMID 19531787. Research Blogging.
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  13. ↑ 13.0 13.1 13.2 13.3 13.4 Wells P, Anderson D, Rodger M, Ginsberg J, Kearon C, Gent M, Turpie A, Bormanis J, Weitz J, Chamberlain M, Bowie D, Barnes D, Hirsh J (2000). "Derivation of a simple clinical model to categorize patients probability of pulmonary embolism: increasing the models utility with the SimpliRED D-dimer.". Thromb Haemost 83 (3): 416-20. PMID 10744147.
  14. ↑ 14.0 14.1 Wells PS, Anderson DR, Rodger M, Stiell I, Dreyer JF, Barnes D, Forgie M, Kovacs G, Ward J, Kovacs MJ (2001). "Excluding pulmonary embolism at the bedside without diagnostic imaging: management of patients with suspected pulmonary embolism presenting to the emergency department by using a simple clinical model and d-dimer". Ann Intern Med 135 (2): 98-107. PMID 11453709.
  15. ↑ Sanson BJ, Lijmer JG, Mac Gillavry MR, Turkstra F, Prins MH, Büller HR (2000). "Comparison of a clinical probability estimate and two clinical models in patients with suspected pulmonary embolism. ANTELOPE-Study Group". Thromb. Haemost. 83 (2): 199-203. PMID 10739372.
  16. ↑ 16.0 16.1 16.2 16.3 16.4 Anderson DR, Kahn SR, Rodger MA, Kovacs MJ, Morris T, Hirsch A, Lang E, Stiell I, Kovacs G, Dreyer J, Dennie C, Cartier Y, Barnes D, Burton E, Pleasance S, Skedgel C, O'Rouke K, Wells PS (2007). "Computed tomographic pulmonary angiography vs ventilation-perfusion lung scanning in patients with suspected pulmonary embolism: a randomized controlled trial". JAMA 298 (23): 2743-53. DOI:10.1001/jama.298.23.2743. PMID 18165667. Research Blogging.
  17. ↑ 17.0 17.1 van Belle A, Büller H, Huisman M, Huisman P, Kaasjager K, Kamphuisen P, Kramer M, Kruip M, Kwakkel-van Erp J, Leebeek F, Nijkeuter M, Prins M, Sohne M, Tick L (2006). "Effectiveness of managing suspected pulmonary embolism using an algorithm combining clinical probability, D-dimer testing, and computed tomography". JAMA 295 (2): 172-9. DOI:10.1001/jama.295.2.172. PMID 16403929. Research Blogging.
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  20. ↑ 20.0 20.1 20.2 20.3 Stein PD, Woodard PK, Weg JG, Wakefield TW, Tapson VF, Sostman HD, Sos TA, Quinn DA, Leeper KV, Hull RD, Hales CA, Gottschalk A, Goodman LR, Fowler SE, Buckley JD (2007). "Diagnostic pathways in acute pulmonary embolism: recommendations of the PIOPED II Investigators". Radiology 242 (1): 15-21. DOI:10.1148/radiol.2421060971. PMID 17185658. Research Blogging.
  21. ↑ Geersing, G.-J.; P. M. G. Erkens, W. A. M. Lucassen, H. R. Buller, H. t. Cate, A. W. Hoes, K. G. M. Moons, M. H. Prins, R. Oudega, H. C. P. M. van Weert, H. E. J. H. Stoffers (2012-10-04). "Safe exclusion of pulmonary embolism using the Wells rule and qualitative D-dimer testing in primary care: prospective cohort study". BMJ 345 (oct04 2): e6564-e6564. DOI:10.1136/bmj.e6564. ISSN 1756-1833. Retrieved on 2012-10-05. Research Blogging.
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  28. ↑ 28.0 28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8 Stein PD, Hull RD, Patel KC, et al (April 2004). "D-dimer for the exclusion of acute venous thrombosis and pulmonary embolism: a systematic review". Ann. Intern. Med. 140 (8): 589–602. PMID 15096330. [e] ACP Journal Club review Cite error: Invalid <ref> tag; name "pmid15096330" defined multiple times with different content Cite error: Invalid <ref> tag; name "pmid15096330" defined multiple times with different content
  29. ↑ 29.0 29.1 29.2 29.3 29.4 29.5 29.6 29.7 Di Nisio M, Squizzato A, Rutjes AW, Büller HR, Zwinderman AH, Bossuyt PM (February 2007). "Diagnostic accuracy of D-dimer test for exclusion of venous thromboembolism: a systematic review". J. Thromb. Haemost. 5 (2): 296–304. DOI:10.1111/j.1538-7836.2006.02328.x. PMID 17155963. Research Blogging. Cite error: Invalid <ref> tag; name "pmid17155963" defined multiple times with different content
  30. ↑ Gupta RT, Kakarla RK, Kirshenbaum KJ, Tapson VF (August 2009). "D-dimers and efficacy of clinical risk estimation algorithms: sensitivity in evaluation of acute pulmonary embolism". AJR Am J Roentgenol 193 (2): 425–30. DOI:10.2214/AJR.08.2186. PMID 19620439. Research Blogging.
  31. ↑ Smith C, Mensah A, Mal S, Worster A (November 2008). "Is pretest probability assessment on emergency department patients with suspected venous thromboembolism documented before SimpliRED D-dimer testing?". CJEM 10 (6): 519–23. PMID 19000347. [e]
  32. ↑ McFarlane MJ, Imperiale TF (1994). "Use of the alveolar-arterial oxygen gradient in the diagnosis of pulmonary embolism". Am. J. Med. 96 (1): 57–62. PMID 8304364. [e]
  33. ↑ Stein PD, Kayali F, Olson RE (May 2004). "Trends in the use of diagnostic imaging in patients hospitalized with acute pulmonary embolism". The American journal of cardiology 93 (10): 1316–7. DOI:10.1016/j.amjcard.2004.02.022. PMID 15135716. Research Blogging.
  34. ↑ Kim KI, Müller NL, Mayo JR (March 1999). "Clinically suspected pulmonary embolism: utility of spiral CT". Radiology 210 (3): 693–7. PMID 10207469. [e]
  35. ↑ Schaefer-Prokop C, Prokop M (2005). "MDCT for the diagnosis of acute pulmonary embolism". European radiology 15 Suppl 4: D37-41. PMID 16479644. [e]
  36. ↑ 36.0 36.1 36.2 Stein PD, Fowler SE, Goodman LR, Gottschalk A, Hales CA, Hull RD et al. (2006). "Multidetector computed tomography for acute pulmonary embolism.". N Engl J Med 354 (22): 2317-27. DOI:10.1056/NEJMoa052367. PMID 16738268. Research Blogging. Review in: ACP J Club. 2006 Nov-Dec;145(3):76 Cite error: Invalid <ref> tag; name "pmid16738268" defined multiple times with different content
  37. ↑ 37.0 37.1 Quiroz R, Kucher N, Zou KH, et al (April 2005). "Clinical validity of a negative computed tomography scan in patients with suspected pulmonary embolism: a systematic review". JAMA : the journal of the American Medical Association 293 (16): 2012–7. DOI:10.1001/jama.293.16.2012. PMID 15855435. Research Blogging.
  38. ↑ 38.0 38.1 Moores LK, Jackson WL, Shorr AF, Jackson JL (December 2004). "Meta-analysis: outcomes in patients with suspected pulmonary embolism managed with computed tomographic pulmonary angiography". Annals of internal medicine 141 (11): 866–74. PMID 15583229. [e]
  39. ↑ 39.0 39.1 Righini M, Le Gal G, Aujesky D, et al (April 2008). "Diagnosis of pulmonary embolism by multidetector CT alone or combined with venous ultrasonography of the leg: a randomised non-inferiority trial". Lancet 371 (9621): 1343–52. DOI:10.1016/S0140-6736(08)60594-2. PMID 18424324. Research Blogging.
  40. ↑ Ranji SR, Shojania KG, Trowbridge RL, Auerbach AD (2006). "Impact of reliance on CT pulmonary angiography on diagnosis of pulmonary embolism: a Bayesian analysis". J Hosp Med 1 (2): 81-7. DOI:10.1002/jhm.71. PMID 17219477. Research Blogging.
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  42. ↑ Worsley D, Alavi A, Aronchick J, Chen J, Greenspan R, Ravin C (1993). "Chest radiographic findings in patients with acute pulmonary embolism: observations from the PIOPED Study.". Radiology 189 (1): 133-6. PMID 8372182.
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  51. ↑ Wells PS, Anderson DR, Rodger MA, et al (April 2005). "A randomized trial comparing 2 low-molecular-weight heparins for the outpatient treatment of deep vein thrombosis and pulmonary embolism". Arch. Intern. Med. 165 (7): 733–8. DOI:10.1001/archinte.165.7.733. PMID 15824291. Research Blogging.
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