Obesity complicates diagnosis

The rising prevalence of obesity has created health dangers for patients and new challenges in diagnostic testing for hospitalists.

The rising prevalence of obesity has created not only health dangers for patients, but also new challenges in diagnostic testing for hospitalists.

Morbidly obese patients often do not fit on magnetic resonance imaging (MRI) or computed tomography (CT) scanning tables, and layers of fat may reduce the quality of ultrasound and X-ray images. A high body mass index also influences B-type natriuretic peptide (BNP) cut-points in the diagnosis of heart failure.

Photo by Thinkstock
Photo by Thinkstock.

However, there are ways around such obstacles, experts said. Hospitalists can learn how to account for obesity when interpreting BNP test results, and become aware of alternative imaging facilities in their areas that have equipment to accommodate morbidly obese patients.

“Manufacturers are now responding,” said Martin Gunn, MBChB, assistant professor of radiology at the University of Washington in Seattle and an author of a May-June 2011 article in Radiographics on imaging challenges presented by obese emergency patients. “They've come out with CT scanners with larger gantry apertures and increased table weight limits, and the scanners are more powerful to see through more obese patients.”

Imaging options

CT and MRI scanners provide the best quality images but neither can be used with patients who don't fit on scanning tables, said Raul N. Uppot, MD, assistant professor at Harvard Medical School and a radiologist at Massachusetts General Hospital who authored a February 2007 American Journal of Roentgenology article that examined the impact of obesity on medical imaging. In those cases, ultrasound and plain X-rays or radiographs become the best options, but often at the expense of image quality.

“With CT and MRI, if you can pass the test of fitting into the machines, you can get a pretty good quality image,” said Dr. Uppot. “Ultrasound and X-ray don't have a weight limit—you can even take the machine to a person's house—but there is a relationship between ultrasound [image quality] and the distance the beams need to travel through.”

Ultrasound beams are attenuated by fat at a rate of 0.63 decibels per centimeter (dB/cm), according to Dr. Uppot's article. Therefore, “the signal that's coming back to you is that much less than what you send out,” he said, resulting in poor image quality.

Of all the imaging modalities, CT scanning is usually the best option if a person can fit on the scanning table, he said. CT scanning tables typically have a higher weight limit than MRI tables (450 lb vs. 350 lb). Outpatient open MRI scanners typically accommodate larger weight limits, but produce lower-quality images. However, manufacturers have now developed high-quality MRI scanners that can accommodate patients up to 550 pounds, Dr. Uppot said.

While not yet available in many hospitals, manufacturers are now offering medical imaging devices to accommodate obese patients, said Dr. Gunn. For example, some scanners on the market today feature aperture diameters—which determine whether a patient can fit into the gantry of the scanner—of up to 90 cm (compared with 60-70 cm in older models) and maximum table loads of up to 650 lb.

Issues to consider

When deciding which imaging tests to order for an obese patient, the first step is measuring his or her weight and girth, said Dr. Uppot. Some patients may meet the weight limit of a scanning table but their girth or diameter may exceed the machine's aperture diameter, preventing them from fitting into the machine.

In addition, “the distribution of fat is very important,” he added. Subcutaneous fat, usually found in women, is more difficult for an ultrasound beam to penetrate compared with intraperitoneal or visceral fat, he explained.

Hospitalists should get a list of the weight limits of their hospital's imaging machines and post it prominently, Dr. Uppot recommended. They should also maintain a list of outpatient facilities that have lower-strength MRI scanners with higher weight limits.

“A hospitalist who knows that their hospital has a scanner that can accommodate a 500-pound patient is better prepared than someone that doesn't know that information and sends a patient to radiology,” he said, “potentially resulting in the procedure being cancelled and the patient getting angry.”

It also helps to have a conversation with a radiologist in cases where you aren't sure of the best option for a particular patient, he added. For example, before referring a patient who exceeds the weight limit for an MRI scanner to get a CT scan instead, check with a radiologist to discuss whether a CT scan is likely to provide the needed information. If there is some doubt, it might be preferable to send the patient to an outpatient facility that has an MRI scanner with a higher weight limit.

In some cases, hospitals take a calculated risk and ignore the insurance-based weight limits for their scanning tables, experts said. While exceeding the weight limits risks breaking the scanner's table or motor, the cost of fixing or replacing the equipment is typically much less than the cost of a potential lawsuit for misdiagnosis, they noted.

While it is true that a scanning table can bend or break if the listed weight limit is exceeded, tables are typically strength-tested at weights much greater than the limits, according to the Radiographics article co-authored by Dr. Gunn. Therefore, hospitals may be willing to exceed the limits because they consider the risk of breakage or patient injury unlikely.

A persistent myth in the medical community is that morbidly obese patients can be referred to zoos because they have scanners designed for large animals, said both Drs. Uppot and Gunn. However, there is no truth to this theory. Dr. Uppot has contacted numerous zoos across the country and found that their scanners are identical to human scanners, just modified to accommodate animals.

Lowering BNP cut-points

Obesity also reduces the image quality of echocardiograms and chest X-rays, making it difficult to diagnose congestive heart failure (CHF), said Lori Daniels, MD, associate director of the cardiac care unit at the University of California, San Diego. As a result, BNP is a potentially valuable tool for ruling in or ruling out a diagnosis of CHF in patients who present to the ED with shortness of breath.

However, the normal cut-points at which CHF is suspected in non-obese patients have been found not to apply to obese patients, she said. Possible reasons include that the rate that BNP is cleared from the body increases with excessive fat tissue and that obese people produce less BNP than non-obese people.

“If a hospitalist isn't sure about CHF, they should use tests like BNP,” advised Dr. Daniels. “It used to be that people scoffed at using BNP for a clinical diagnosis but now it has more acceptance. Don't rule out heart failure just because the BNP is 60, for example.”

In an article in the May 2006 American Heart Journal, Dr. Daniels concluded that for patients with the highest BMIs (35 or over), a lower BNP cut-point of ≥54 picograms per milliliter (pg/mL) should be used to maintain the same level of diagnostic sensitivity as for non-obese patients, for whom a cut-point of ≥100 pg/mL is recommended.

A physician who fails to adhere to the lower cut-points risks misdiagnosing more than one in five severely obese patients who present with shortness of breath and have a BNP level of less than 100 pg/mL, the article states.

“The major impact [of not adhering to the lower cut-points] is misdiagnosis,” said Dr. Daniels. “If you get a low-ish BNP and decide it's not heart failure, the patient might go on to be treated for other things. They might be given steroids thinking it is a COPD exacerbation, or antibiotics. If it is actually heart failure, those things won't help and might even hurt. Misdiagnosis also leads to prolonged hospital stays and increased morbidity and mortality.”

Similarly, failing to refer morbidly obese patients to appropriate imaging alternatives can have negative repercussions, said Dr. Uppot. Four years ago, he conducted an informal study looking at what happened to 14 patients who came to the ED with an acute condition and could not fit onto the hospital's scanners.

“It turned out that about one-third had to go for surgery to make a diagnosis,” he said. “It suggests that patients who can't get imaging because they can't fit on a table may end up going straight to surgery.”