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.