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Spiral CT Angiography for Acute PE: Where Do We Stand?

Victor F. Tapson, MD, FCCP, Duke University Medical Center
March, 2005

Review: Moores LK, Jackson WL Jr, Shorr AF, Jackson JL. Meta-Analysis: Outcomes in Patients with Suspected Pulmonary Embolism Managed with Computed Tomographic Pulmonary Angiography. Annals of Internal Medicine 2004;141:866-874.

Dr. Lisa Moores, and colleagues, from Walter Reed Army Medical Center, and the Uniformed Services University of the Health Sciences, have recently conducted a meta-analysis to better clarify the utility of computed tomographic pulmonary angiography (CTPA) in the setting of suspected pulmonary embolism (PE), particularly with regard to outcome when the CTPA is negative. The study, published on December 7, 2004 suggests that in the setting of suspected PE and a negative CTPA, it appears to be safe to withhold anticoagulation.

CTPA refers to a CT scan with contrast to specifically evaluate the pulmonary arteries. Spiral CT scanning (i.e., CTPA) has become the procedure of choice for suspected acute PE except in setting of renal insufficiency or significant allergy. Limitations of the ventilation-perfusion (VQ) scan have long plagued clinicians as the VQ scan is most commonly nondiagnostic, even in the setting of ultimately proven PE. A major advantage of the VQ scan, however, is that when it is negative it is essentially foolproof. The same cannot be said for the CT. Importantly, a negative CT is also different from a nondiagnostic CT. Data suggest that patients with nondiagnostic CT scans who do not receive anticoagulation do have relatively high rates of recurrence and so these patients cannot be handled with the same confidence as those with negative a CT. So, how confident should we be with a negative CT scan for acute PE?

The sensitivity for large emboli (segmental or larger, and particularly lobar or larger) is exceptionally good and is not the primary concern. Emboli that are subsegmental are less obvious and may be missed. The same is true, however, with pulmonary arteriography, the gold standard test for acute PE. The Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED), published in JAMA 15 years ago, revealed that interobserver variability for these smaller emboli even with arteriography is far from perfect. The question then becomes, do small emboli matter? The answer is that they probably don't in and of themselves, and this meta-analysis supports the fact that the absence of PE, or better, the absence of detectable PE portends a good outcome without anticoagulation. The potential value of utilizing a pretest clinical probability estimate in patients with suspected PE was demonstrated in the PIOPED study and was found very useful in combination with VQ scanning. Less data are available for pretest probability and the CT scan. Certainly though, clinicians should include their own pretest probability, however, determined in the decision process. Patients seen in emergency rooms with dyspnea frequently undergo CT scanning to rule out PE, so as to not miss this potentially deadly disease, and concerns are heightened by the threat of malpractice. Patients included in many clinical trials often include those who may be at very low risk for PE but a scan may be done to be absolutely sure. These are often patients presenting to the emergency room and not complex hospitalized patients. If a patient, for example, develops sudden onset dyspnea, has some degree of hypoxemia, and has unexplained pleuritic chest pain, and has risk factors for deep vein thrombosis, then a negative CT may not be enough to rule out PE. Fortunately, a CT does often discover another potential reason for the symptoms, again enhancing its utility compared with the VQ scan. A patient in the above setting should certainly be considered for lower extremity ultrasound for better certainty. A negative ELISA D-dimer test would also add to the confidence level substantially but is unusual, at least in typical medical and surgical patients.

A further concern about a negative CT negating the need for further testing is that a patient may have one or more very small emboli which would perhaps be inconsequential. If these had embolized from a large thrombus still in the leg, the prognosis may well be much worse. No good clinical trial has randomized patients to anticoagulation versus none based upon remaining thrombus in the leg, using a sensitive leg test such as MRI or venography. This would be interesting.

The authors offer that the combined data offered by the meta-analysis give us a reasonably good comfort level, which is encouraging. While we know that the CT is not 100% sensitive, the outcome data do look quite good, but we need to take into account the limitations of meta-analysis. The limitations of the study are outlined by the authors and they acknowledge, for example, that follow-up was not complete for all studies. They indicate that "the role of CPTA without concomitant lower extremity imaging is still undefined." It would appear wise to individualize patients to some degree. Additional testing should be considered, for example, if clinical suspicion is high, or if significant risk factors are present. Patients in whom risk and clinical suspicion is ultimately deemed low might be appropriate candidates for no further testing and no treatment. The PIOPED 2 data, will be published soon and will give us better insight into the utility of combining CTPA with CT venography (one test to look at both the legs and the lungs). There is still a role for the art of medicine when considering PE.

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