Apr 24, 2013
What is the best way to evaluate a pregnant patient in whom you suspect pulmonary embolism? There is no definitive guideline, but there is no shortage of opinions. Jeff Kline and I work through the evidence and lack thereof.
What do you tell pregnant patients about how much fetal radiation there is from a radiographic study? Do you use rads, grays, seiverts, micrograys? Here's any easy way to think about it... The threshold we want to avoid is 0.1 gray. 0.1 gray at any time during gestation is regarded as the practical threshold beyond which induction of congenital abnormalities is possible. Do you know how much 0.1 gray is in relation to rest of the universe? Don't worry, nobody else does either. To help with perspective, think of 0.1 gray as $100 or 100 points, we'll use dollars here.
A chest x-Ray is one tenth of a penny.
Ct pulmonary angiogram is 25 to 50 cents.
A V/Q scan is 50 to 75 cents, less with a partial dose V/Q, which is often used in pregnancy. So for CT and V/Q , we'll say 50 cents each.
Background radiation during 9 months gestation: $5.
Amount of radiation to increase the risk of cancer before age 20 by one one-hundredth of a percent, or one in ten thousand, $10.
On the scale of $100, CT and VQ give less than $1 of fetal radiation exposure. Considering the risk of a bad outcome from PE, especially in a pregnant patient, where it is one of the leading causes of maternal mortality, err on the side of the workup.
So what study to do? My approach is to perform a V/Q scan in a pregnant patient with a normal chest x-ray. The caveat to this is early in the first trimester, where the decision may be more emotion than data based (on my part). At this stage of gestation, the fetus is about the size of a cashew nut and, with a V/Q. retained urine in the bladder seems like a lot of focused radiation to the entire fetus. In this patient group, I start with CTPA.
In normal pregnancy, 60% of patients will have a d-dimer above standard threshhold. In the first trimester: negative PERC rule, with the caveat of an increased heart rate of 105 (HR rises in pregnancy) plus a negative d-dimer makes PE unlikely. The d-dimer cutoffs are 50% higher for each trimester. If your regular cutoff is 500 ng/mL, pregnancy corrected cutoffs:
Pulmonary Embolism in Pregnancy Lancet 2010. Higher rate of non-diagnostic CTPA than V/Q in pregnant patients
D-dimer levels in normal pregnancy. Article by Jeff Kline et al
Laboratory studies in pregnancy. Obst Gyn 2009