Tuesday, August 14, 2012

Standard for Evaluation of Pulmonary Embolism

Guidelines for Evaluation of Patients with Chest Pain where Pulmonary Embolism Suspected.
Pennsylvania residents will be interested to know the published recommendations from the US Department of Health & Human Services regarding evaluation of patients with suspected pulmonary embolism. This publication establishes the appropriateness of initial radiologic examinations for patients with acute chest pain caused by suspected pulmonary embolism. This guideline sets the expected standard of care for workup of a patient with possible pulmonary embolism.

www.guidelines.gov reports that over 290,000 cases of fatal pulmonary thromboembolism (PE) occur in the United States each year. Worse yet, additional cases may not be diagnosed because the symptoms of chest pain, shortness of breath, tachycardia, etc, are nonspecific and may mimic other pulmonary or cardiac conditions. Unsuspected (and undetected) PE continues to be a frequent autopsy finding.

Diagnostic efforts in radiology are aimed at: (1) reaching an acceptable level of diagnostic certainty of PE to warrant anticoagulant therapy, using the least invasive tests, and (2) excluding other reasons for the patient's symptoms.

There are a myriad of diagnostic tools that can be used to determine whether a given patient is, in fact, suffering from PE ex. X-ray, computed tomography angiography (CTA), Ultrasound, VQ scans, pulmonary angiography and magnetic resonance angiography (MRA). However, many of these diagnostic tools are unnecessary or impractical in the standard workup of suspected PE. The Guidelines report sets forth the critical tests that should always be performed to help establish or rule out a diagnosis of PE.

Chest Radiograph (X-Ray)

The posterior/anterior and lateral chest radiograph is an important initial study in the evaluation of suspected PE. The chest radiograph may eliminate the need for additional radiographic procedures by revealing an alternate reason for acute symptoms, such as pneumonia or a large effusion. A normal chest radiograph does not exclude PE, and there are no specific findings that are sufficient to confirm PE. A recent chest radiograph is required to allow accurate interpretation of an abnormal radionuclide ventilation/perfusion lung scan.

Computed Tomography
Multidetector computed tomography pulmonary angiography (CTPA) is indicated in the evaluation of patients suspected of having a PE. CTPA is now the primary imaging modality for evaluating patients suspected of having acute PE. CTPA has played an increasingly significant role in the diagnosis of PE since the first major clinical study in 1992. Technological advancements in CT, from helical to multidetector, have led to improved resolution of the pulmonary arteries, large and small. Numerous studies have examined the accuracy of CTPA as compared to ventilation/perfusion (V/Q) imaging and conventional angiography.

Multiple studies have shown that CTPA is highly sensitive and specific; discrepancies with conventional angiography are mainly at the subsegmental level where even angiographers tend to have poor inter-observer agreement. Intraobserver and interobserver variability for CTPA have been shown to be very good to the segmental level, better than with V/Q imaging.

The overall accuracy of CTPA appears to be very high, and is even higher when combined with clinical assessment and serum D-dimer testing. A positive CTPA result combined with high or intermediate suspicion on clinical assessment has a high positive predictive value. In patients with low clinical suspicion and a negative CTA, acute PE can safely be ruled out. In addition, the adjunctive use of CT venography with CTPA improves the sensitivity of detecting DVT, with similar specificity, thereby increasing the overall accuracy of the diagnosis of thromboembolic disease, as compared to an isolated diagnosis of PE.

CTPA also has fewer "nondiagnostic" studies than V/Q scans. The false negative rate of CTPA is very low. Outcome studies have shown no adverse outcomes in patients with a negative CTPA who were not subsequently treated. Another study has shown CTPA to be cost-effective in conjunction with lower extremity duplex exams. The combination of multidetector CTPA and high-specificity D-dimer testing has very high positive and negative predictive values. In addition, CTPA may occasionally demonstrate pathology other than PE that may be responsible for the patient's symptoms.

CTPA can also identify signs of right ventricular dysfunction that may have prognostic significance or implications for treatment (e.g., need for the institution of thrombolytic therapy vs. conventional anticoagulation alone). Measurements of right ventricular enlargement and reflux of contrast to the inferior vena cava have been used among other indices to gauge right ventricular dysfunction and predict patient mortality. Recent technological advancements such as electrocardiogram (ECG)-gated CT and dual-source CT have allowed accurate evaluation of the pulmonary vasculature, thoracic aorta, and coronary arteries on a single CT study. This so-called "triple rule-out" CT protocol has been shown to be feasible, although it has yet to be proven useful or cost-effective through large-scale clinical trials. It is possible that the "triple rule-out" CT will become routine in the evaluation of certain patients with acute chest pain in the future.

In general, the data indicate that multidetector CTPA (MDCTPA) is more accurate than single-slice CT or other studies, such as V/Q scans. Conventional CT with contrast material (not performed as dedicated CTPA) is generally not indicated in the routine workup of acute chest pain thought to be secondary to acute PE.

The rest of the above-referenced diagnostic tools are typically only used to further narrow the highly-accurate results from X-Ray and CTPA.

In summary:

  • PE remains a common and important condition.
  • A chest radiograph cannot exclude or confirm PE, but is important (as a complementary study) as it can guide further investigations and suggest alternative diagnoses.
  • In general, any test that can confirm either DVT (i.e., lower-extremity venous duplex) or PE is sufficient. Only certain studies, however, have sufficient accuracy to exclude PE.
  • Multislice CT pulmonary angiography is the current standard of care to confirm or exclude PE.
  • V/Q scanning appears to also have high overall accuracy.
  • In pregnancy, with radiation a particular concern, the choice between perfusion scanning and CTPA depends on local equipment and expertise as well as patient factors (normal chest radiograph, ability to breathhold). 
Based on the recommendations of the US Department of Health & Human Services, a patient presenting with chest pain with suspected PE must undergo both a chest x-ray and CTPA. If these tests are not performed and the patient dies or has complications as a result of undiagnosed PE malpractice may have occurred.

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