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VQ to the Rescue! Utilizing Ventilation Perfusion Scanning to evaluate for Pulmonary Embolus

Case Report: A 53 year old woman with a history of DVT, hypertension, hyperlipidemia, type II DM and stage 3 CKD, presents to the ED with sudden onset pleuritic chest pain and dyspnea that began 2 hours prior to presentation.

  • On exam, she is tachycardic with a heart rate of 108.

  • Her EKG reveals sinus tachycardia.

  • Her pre-test probability for PE according to the Wells’ Criteria is 6 which puts her in a moderate risk group.

  • Labs were remarkable for electrolyte derangements of azotemia and a GFR of 35.

  • Considering her CKD, you decide to keep the patient overnight for a planar V/Q scan.

  • The V/Q scan result comes back as follows: Intermediateprobability of pulmonary embolus.

Figure 1 – V/Q planar scan of two patients with a normal and abnormal perfusion pattern

(Source:https://www.slideshare.net/MedicineAndHealthCancer/gerber-pulmonary-embolism)

Clinical Question: What is the optimal management of a patient with a moderate pre-test probability for pulmonary embolism and an intermediate probability ventilation-perfusion (V/Q) scan result?

How to interpret a V/Q scan?

  • The possible outcomes of the planar V/Q scan are classified in terms of probability of PE, and it is important to interpret these results within the context of the patient’s pre-test probability:

  • Normal V/Q scan: effectively rules out PE [1].

  • Very low probability V/Q scan: Non-diagnostic, reliably rules out PE in patients with low pre-test probability [2].

  • Low probability V/Q scan: Non-diagnostic.

  • Intermediate probability V/Q scan: Non-diagnostic.

  • High probability V/Q scan: Considered diagnostic for PE [3].

  • The planar V/Q scan provides diagnostic value in approximately 46% to 73.5% of scans [1, 3].

  • A Single Photo Emission Computed Tomography(SPECT) V/Q scan may also be performed.

  • It generates a 3-dimensional image and allows for new ways to display data.

  • Compared to planar V/Q scans, it has been shown to have a higher sensitivity, specificity, and accuracy, and a lower rate of non-diagnostic results [4].

  • The nuclear medicine department must be equipped with a modern hybrid scanner to perform V/Q SPECT with CT

Summary of Evidence:

  • A retrospective cross-sectional study on the current management of PE based on pre-test probability examined 122adults from a single tertiary care hospital over the year 2016-2017 and found the following results: [5]

  • 33 (27%) patients were anticoagulated.

  • 9 (7.4%) patients had high pre-test probabilities, of these:

  • 3 (2.5%) had a low probability V/Q scan and 1 (33%) was anticoagulated.

  • 3 (2.5%) had an intermediate probability V/Q scan and all were anticoagulated.

  • 3 (2.5%) had a high probability V/Q scan and all were anticoagulated.

  • The remaining results can be found below in Table 1.

  • The small sample size (122 participants) and single center study limit the generalizability of findings.

  • The retrospective design prevents insight into the anticoagulation decision-making process for each patient.

  • It is unclear if nuclear medicine physicians who interpreted the V/Q scans had knowledge of pre-test probabilities.

  • Participants lacking a pre-test probability score had one assigned to them retrospectively, and the Wells’ score has a subjective component for “whether PE is the #1 diagnosis or equally likely”.

  • Physicians are interpreting a V/Q result of intermediate probabilityas positive for PEin patients with moderate or high pre-test probability.

  • Accordingly, they are initiating therapeutic anticoagulation in these patients.

  • Currently, there are no randomized controlled trials that investigate the outcomes of therapeutic anticoagulation vs. no anticoagulation in this patient subgroup.

  • Without such data, little can be drawn about the optimal management of these patients.

  • In the PIOPED trial conducted in 1990, 322 patients had intermediate-probability scans and of those, 105 (33%) had pulmonary embolism [6].

  • The intermediate probabilityV/Q scan interpretive category may not be not well understood by clinicians.

Table 1 – Prevalence of patients managed with anticoagulation according to pre-test probability and V/Q scan results

Recommendations

  • Given the lack of evidence, it would be appropriate to interpret the intermediate probability V/Q result as positive for PE in the patient population with high pre-test probability [7].

  • Evaluate your patient for the risks and benefits of therapeutic anticoagulation

  • Involve your patient in the decision-making process and empower them with the knowledge to make an informed decision.

  • Consider the need for further imaging studies such as SPECT V/Q scan, duplex US of the lower extremities or CT angiography.

  • If available at your institution, obtain SPECT V/Q imaging which has a higher sensitivity and specificity than a planar V/Q scan [8].

  • If duplex ultrasound reveals venous thrombosis, begin anticoagulation therapy

  • Evaluate the risks and benefits of obtaining a CT angiography in your patient

  • Nuclear medicine physicians should use caution in assigning intermediate probability reads, and consider modifying the terminology used to communicate the results of a V/Q scan.

References

1. Anderson D, et al. Computed tomographic pulmonary angiography vs. ventilation-perfusion lung scanning in patients with suspected pulmonary emboli: a randomized controlled trial. JAMA 2007; 298: 2743-53.

2.Gottschalk, A., Stein, P., Sostman, H., Matta, F. and Beemath, A. (2007). Very Low Probability Interpretation of V/Q Lung Scans in Combination with Low Probability Objective Clinical Assessment Reliably Excludes Pulmonary Embolism: Data from PIOPED II. Journal of Nuclear Medicine, 48(9), pp.1411-1415.

3.Sostman, H., Stein, P., Gottschalk, A., Matta, F., Hull, R. and Goodman, L. (2008). Acute Pulmonary Embolism: Sensitivity and Specificity of Ventilation-Perfusion Scintigraphy in PIOPED II Study. Radiology, 246(3), pp.941-946.

4.Roach, P., Schembri, G., & Bailey, D. (2019). V/Q Scanning Using SPECT and SPECT/CT.

5.Aghajafari, P., Civelek, A. and Rowe, S. (2018). Clinical management of pulmonary embolism based on pre-test probability and ventilation perfusion study results. J Nucl Med, 59(no. supplement 1 1610).

6.Pioped Investigators. (1990). Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). Jama, 263(20), 2753.

7.Rosenow III, E. C. (1995, February). Venous and pulmonary thromboembolism: an algorithmic approach to diagnosis and management. In Mayo Clinic Proceedings (Vol. 70, No. 1, pp. 45-49). Elsevier.

8.Phillips, J., Straiton, J. and Staff, R. (2015). Planar and SPECT ventilation/perfusion imaging and computed tomography for the diagnosis of pulmonary embolism: A systematic review and meta-analysis of the literature, and cost and dose comparison. European Journal of Radiology, 84(7), pp.1392-1400.

 

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