Appropriate Ordering of Medical Imaging Studies in Cases of Clinically Suspected Pulmonary Embolism

Any patient with creatinine level of 2.0 mg/dL or less and/or eGFR of 30 ml/min or greater can have CTPA to evaluate for PE without undue risk of CIN

If clinical evaluation (e.g. Well’s Score assessment followed by D-Dimer assay [1] ) of a patient with suspected pulmonary embolism (PE) warrants the use of a medical imaging to establish the diagnosis then CT Pulmonary Angiography (CTPA) continues to be the method of choice for initial evaluation [2]
Pulmonary embolism credit James Heilman, MD

CTPA has an excellent negative predictive value (NPV) in the setting of low to intermediate clinical probability for PE (96% and 89% respectively) [3] and it’s excellent positive predictive value (PPV) in the setting of intermediate to high clinical probability for PE (92% and 96% respectively) 2, but also because it is readily available and can show alternate diagnoses.

With regard to the use of CTPA versus Radionuclide Ventilation Perfusion Lung Imaging V/Q scan in the setting of clinically suspected PE, CTPA will be used instead of V/Q scan, in most situations. While a V/Q scan is often ordered in patients with renal insufficiency or with allergy to iodinated contrast agents, the latest revision of the “ACR Manual on Contrast Media”, Version 9, 2013 [4] has the following important conclusions:

  • The “…risk of contrast induced nephrotoxicity (CIN) from intravenous iodinated contrast media is sufficiently low such that a [creatinine] threshold of 2.0 mg/dl in the setting of stable chronic renal insufficiency is probably safe for most patients”.
  • The eGFR calculated using the MDRD formula appears to underestimate true GFR. Nonetheless, various studies (n=209 to 451) showed that the rate of contrast induced nephropathy (CIN) (serologically measurable as opposed to clinically significant) in patients with eGFR of >45 ml/min was 0–3%, for those with 30–45 ml/min was 3–5%, while for those with <30 ml/min was 7–12%.
  • Regarding patients with previous index case of an allergic reaction to iodinated contrast, pre­ medication regimens “…have demonstrated a decrease in overall adverse events …but no decrease in the incidence of repeat severe adverse events …” and that breakthrough reactions (i.e. those that occur with contrast administration despite pre-medication) are “…most often similar to the index reaction…” and "…patients with a mild index reaction have an extremely low risk of developing a severe breakthrough reaction.’’

Based on the above, and especially considering that the quoted data is based on the use of 100ml of iodinated contrast and not on the significantly lower doses that we now commonly use, any patient with creatinine level of 2.0 mg/dL or less and/or eGFR of 30 ml/min or greater can have CTPA to evaluate for PE without undue risk of CIN. Medical Imaging will take whatever steps necessary to mitigate the risk of CIN in patients whose values lay near the limits of these parameters. Likewise, any patient with previous mild to moderate allergic reactions to iodinated contrast may receive CTPA after receiving the standard pre-medication regimen. This includes the emergency setting where an “urgent” pre-medication regimen (IV Solumedrol 40 mg q4h until contrast given plus IV diphenhydramine 50 mg 1hour prior to contrast injection) can be used. Patients with index case reaction characterized as severe (i.e anaphylaxis) will still be evaluated with V/Q scan rather than CTPA.

Lastly, according to the ACR Appropriateness Criteria-Acute Chest Pain-Suspected Pulmonary Embolism, in pregnant women with suspected PE, CTPA and perfusion only radionuclide lung scan have equal appropriateness values and nearly the same Relative Radiation Level (RRL). If the CXR is entirely normal and lower extremity venous Doppler US is normal then either study can be ordered. If the CXR has any abnormalities, then CTPA should be ordered. [5]
V/Q imaging credit Westgate EJ, FitzGerald GA

Presented by

  • Phil Gilroy, M.D. – McHenry Radiologists & Imaging Associates
  • Spiro Gerolimatos, M.D., Medical Director – Medical Imaging
  • John Heinrich – Director, Medical Imaging
  • Robin Rausch – Assistant Director, Medical Imaging

References

  1. ACR Appropriateness Criteria-Acute Chest Pain-Suspected Pulmonary Embolism, last review 2011.
  2. 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. Wells PS, Anderson DR, Rodger M, Stiell I, Dreyer JF, Barnes D, Forgie M, Kovacs G, Ward J, Kovacs MJ. Ann Intern Med. 2001 Jul 17;135(2):98–107.
  3. Wells Clinical Prediction Rule for Pulmonary Embolism Dino W. Ramzi, M.D., C.M., and Kenneth V. Leeper, M.D., Emory University School of Medicine, Atlanta, Georgia Am Fam Physician. 2004 Jun 15;69(12):2829–2836
  4. Computed Tomographic Pulmonary Angiography vs Ventilation-Perfusion Lung Scanning in Patients With Suspected Pulmonary Embolism; David R. Anderson, MD; Susan R. Kahn, MD; Marc A. Rodger, MD; Michael J. Kovacs, MD; Tim Morris, MD; Andrew Hirsch, MD; Eddy Lang, MD; Ian Stiell, MD; George Kovacs, MD; Jon Dreyer, MD; Carol Dennie, MD; Yannick Cartier, MD; David Barnes, MD; Erica Burton, BSc; Susan Pleasance, BScN; Chris Skedgel, MSc; Keith O’Rouke, PhD; Philip S. Wells, MD JAMA. 2007;298(23):2743–2753. doi:10.1001/jama.298.23.2743.
  5. Spiral Computed Tomography for Acute Pulmonary Embolism U. Joseph Schoepf, MD; Samuel Z. Goldhaber, MD; Philip Costello, MD Circulation. 2004; 109: 2160–2167 doi: 10.1161/​01.CIR.0000128813.04325.08

Footnotes