Answer to Question #4392 Submitted to "Ask the Experts"

I am a respiratory therapist and work in a hospital setting. I am 10 weeks pregnant. One of my patients had a VQ scan the other day at the bedside. He became disconnected from his ventilator and I was the only person in the vicinity. I ran in to reconnect him during the test. I don't know what I was thinking! After the test I was at the bedside for approximately 10 minutes. I also had another patient in the same room whom I visited once. I was told later that the patient was still radioactive and that was not advisable. Can you give me some idea of how risky this behavior was to my fetus?

It sounds like we actually have two questions. First, let's first talk about the VQ scan. It is a ventilation and perfusion scan. For the ventilation portion, which is usually done before the perfusion, about 10 mCi (millicuries) of xenon-133 gas is administered. The patient breathes it in, holds it, then exhales it. Sometimes they are asked to rebreathe it as well. And, sometimes, the gas is administered intravenously dissolved in saline. In the case you describe, it sounds like the xenon gas was administered through a ventilator.

The second part is the perfusion study where approximately 5 mCi of technetium-99m macroaggregated albumin is administered intravenously.

What I can't tell from your description of the event is how soon after the initial inhalation of xenon-133 the ventilator come apart. If it was more than a couple minutes, there would be no exposure to you. It is even questionable whether you would receive any exposure if it was right after the first inhalation. For the sake of argument, though, let's make some conservative assumptions. If we say you could actually have inhaled all of the xenon-133 gas that was administered to the patient (10 mCi), the fetus could have received a total radiation dose of up to 0.02 rad (Wagner et al.). This dose is too low to cause any potential birth defects or a miscarriage. [Note: rad is "radiation absorbed dose" and is a unit used to describe the amount of radiation energy deposited in tissue.]

The second possible question might be what your exposure would be just from sitting next to the patient for a while after he had the VQ scan. Well, the xenon-133 is exhaled as we've already discussed so is no longer term exposure hazard to someone visiting the patient. The technetium-99m will stay with the person for several hours. The dose rate at one foot from the patient who has 5 mCi of technetium-99m in them is about 0.0005 rad/h. So for every hour you stayed at the bedside, you would receive up to 0.0005 rad and the fetus would receive nothing. This also is essentially a negligible dose. [For comparison, natural background radiation exposes all of us to about 0.36 rem of radiation exposure annually; a two-view chest x ray is about 0.015 rem.]

Bottom line, there is no additional risk for you or the fetus from these activities.

Kelly Classic
Certified Medical Health Physicist

Reference
Wagner LK, Lester, RG, Saldana, LR. Exposure of the pregnant patient to diagnostic radiations. Madison, Wisconsin: Medical Physics Publishing; 1997.