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It would be comforting to think that when patients arrive for contrast-enhanced CT scanning, all necessary preparations have been made to ensure their safety. In fact, diagnostic imaging staff often have only the sketchiest information about patients until they walk in the door.
Instead of advance planning, the safe administration of contrast media relies largely on detailed questioning and careful observation by radiology nurses and technologists-and a commitment by radiologists to change plans at a moment's notice.
"We depend on the information that's given to us by the doctor's office and what we get from the computer if they are inpatients," said Joanna Po, R.N., administrative director of nursing in the division of diagnostic imaging at M.D. Anderson Cancer Center. "However, the majority of the time, with ambulatory patients, we screen them as they come in."
Last-minute screening can lead to last-minute surprises. The radiology nurses Po supervises-more than 100 in all-use detailed questionnaires to identify patients who might be at increased risk for an adverse reaction to contrast media. The questionnaires delve into everything from the patient's clinical history, including allergies, asthma, diabetes, kidney disease, and other potentially complicating medical conditions, to current medications and food intake within the previous three hours. Still, careful follow-up questioning is often necessary, as patients may not fully understand the questions or remember important information.
"Sometimes we'll ask if they've had a contrast media reaction and they'll talk about penicillin," Po said. "So we'll say, 'Did you ever have an x-ray with injection of a dye or iodine?' and they'll say, 'Oh, yeah. I remember now,' and they'll tell us they had a reaction."
Today, with near-universal use of nonionic contrast media, adverse reactions are rare. A study by researchers at the University of California, Los Angeles (AJR 2001;176:1385-1388), reviewed data from more than 90,000 examinations with iodinated contrast media between 1985 and 1999. Investigators noted an adverse reaction rate of 6% to 8% when only ionic contrast was used. That rate plummeted to 0.2% when only nonionic contrast was used.
More than 90% of the adverse reactions in the UCLA study appeared to be allergy-related. While no one takes the possibility of allergies lightly, the list of risk factors that signal the need for premedication has been whittled down over the years.
"We used to premedicate anyone who had any allergy. Now we premedicate only for a contrast allergy, an iodine allergy," said Dr. Giovanna Casola, director of body imaging at the University of California, San Diego.
When radiology nurses or technologists learn during the screening process that a patient is at high risk for an allergic reaction to contrast material, everything stops and the examination is rescheduled. The patient must not only take several doses of prednisone and Benadryl during the 24 hours before the examination but must also arrange for transportation to and from the hospital.
"If patients are premedicated with Benadryl, they need to be driven to the hospital; they have to be accompanied," Casola said. "You have to keep little issues like that in mind, too."
Although a history of allergy-related asthma doesn't warrant premedication with prednisone and Benadryl, it does signal the need for closer monitoring, because the injection of contrast material can trigger an asthma attack, Po said. Nurses at M.D. Anderson ensure that patients with asthma have an inhaler nearby during contrast administration, and they are prepared to give the patient an injection of epinephrine if needed.
The potential for life-threatening adverse reactions, however rare, has raised thorny issues about where and when it is safe to use iodinated contrast. Many radiologists are uncomfortable with the idea of administering contrast without a physician and full Code Blue team onsite. At the same time, satellite imaging centers are becoming increasingly common.
M.D. Anderson opened an outpatient center about three years ago. Located a mile from the main hospital, the outpatient center offers MR, CT, and PET studies. In addition to technologists, it is staffed with three registered nurses with advanced cardiac life support certification, as well as licensed vocational nurses and nurses aides.
Imaging center staff reduce the risk of an adverse contrast reaction through careful patient screening at the time
of scheduling. Patients may not be scanned at the outpatient center if they have had a previous allergic reaction to contrast material, are physically debilitated or unable to stand on their own, need regular monitoring of vital signs, require supplemental oxygen, are on seizure precautions, or are severely diabetic. Despite careful patient selection, however, it has been necessary to call 911 twice since the outpatient center opened, Po said.
"We do not have our Code Blue team cover that area. In the event that a patient develops problems, we call 911," she said. "In these cases, both were new patients, so the screening was limited, and they had not seen their doctor in the clinic yet."
Even when contrast exams take place in the hospital, there can be concerns about physician availability. Inpatients must sometimes undergo contrast studies in the middle of the night, long after radiologists have gone home, said Dr. Jay Heiken, director of abdominal imaging at Washington University in St. Louis.
"We require that when one of these studies is done, a physician be called and be available onsite," he said.
RENAL COMPROMISE
Chances are very small that iodinated contrast material will cause kidney damage in most healthy patients. In those with renal insufficiency, however, the risk of contrast-induced nephropathy, even with low-osmolar agents, is perhaps 20% to 25%.
Patients with moderate renal insufficiency-an elderly person whose kidneys have been damaged by long-standing hypertension, for example-might not think of themselves as having kidney disease and therefore won't check the appropriate box on a screening questionnaire.
In such cases, checking the serum creatinine level is crucial. Before beginning an examination that involves iodinated contrast material, Casola checks the creatinine level of all patients over 50 or with a family history of renal disease or a personal history that suggests an increased risk of renal disease. If a patient has a creatinine above 2 mg/dL (1.5 mg/dL if the patient also has diabetes), Casola tries to do the examination with ultrasound or MR. Heiken said he generally uses the same creatinine cutoffs as Casola, but he considers several additional variables.
"You need to take into account other issues: Is this the patient's baseline creatinine, or has it recently been increasing or decreasing? Does the patient have other problems, such as diabetes? How well hydrated was the patient at the time the creatinine was drawn? We need to evaluate each patient individually and make that determination," he said.
At Barnes-Jewish Hospital, where Heiken practices, about 100 patients undergo body CT daily. And about once a day, radiologists unexpectedly discover renal insufficiency and must shift a patient to another form of imaging, do the examination without contrast, or in cases of mild renal insufficiency, reschedule the examination to allow the patient time to become adequately hydrated.
Aggressive prehydration is believed to be among the most effective measures for preventing contrast-induced nephropathy. Another option that may afford renally compromised patients some protection is premedication with acetylcysteine. A study by Tepel and colleagues (NEJM 2000;343:180-184) suggested that taking oral acetylcysteine the day before and the day of CT scanning with low-osmolar contrast material could reduce the risk of acute renal failure by 90%. The results of other studies of acetylcysteine have been sharply mixed, however.
In addition, because acetylcysteine is generally given orally the day before contrast administration, the radiologist becomes aware of a patient's renal insufficiency only on the day of scanning, making use of acetylcysteine impractical.
A study from Charing Cross Hospital in London (J Am Coll Cardiol 2003;41:2114-2118) suggests that same-day intravenous administration of acetylcysteine may offer some protection. The study, which involved patients undergoing cardiac catheterization, showed that the incidence of contrast-induced nephropathy was significantly lower in patients who received intravenous acetylcysteine 30 minutes before and four hours after the procedure, compared with those who underwent intravenous hydration alone.
EXTRAVASATION
As faster image acquisition has boosted contrast injection rates, the potential for extravasation has increased. Even in the best institutions, extravasation can be expected in one of every 250 to 500 patients.
The most practical approach to preventing it is to establish good intravenous access, generally by using a 20-gauge or larger needle placed in a large vein such as the antecubital. Many institutions take the extra precaution of having the radiology technologist remain with the patient, palpating the intravenous site as the injection is started, so that extravasation can be detected early and the injection stopped if necessary.
"It's also a good argument for using low-osmolar contrast agents, because if there is extravasation, it produces much less damage than high-osmolar contrast material," Heiken said.
Routine treatment of extravasation involves elevation of the affected arm, application of compresses to the IV site, and periodic evaluation for vascular or neurological compromise. Beyond that, it's important to recognize the potential for compartment syndrome and to be ready to call a plastic surgeon if the swelling becomes extreme, Heiken said.
"Just like having physicians available for contrast reactions, it's important for radiologists to take responsibility when there is a contrast extravasation and to know what to do to evaluate and treat it," Heiken said.
MS. CARRINGTON is a freelance medical writer in Vallejo, CA.
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