Stress MPI with Tc-99m SPECT identifies high-risk obese patients

An estimated 65% of U.S. adults are either overweight or obese, according to the National Center for Health Statistics. A pair of studies of U.S. and Korean cohorts recently published in the New England Journal of Medicine (August 24, 2006, Vol. 355:8) holds that being even a few pounds overweight results in a 10% to 50% chance of dying prematurely compared with those who have a normal weight, as determined by body mass index (BMI).

All these excess pounds place a greater strain on the heart and lead to increased cardiovascular complications ranging from coronary artery disease (CAD) to cardiac death. Coronary angiography is the prevailing method for diagnosing CAD, but it is associated with risk, which increases for obese patients.

According to a multinational team of researchers from the U.S. and the Netherlands, identifying an accurate method for risk stratification of obese patients suspected of CAD is critically important to limit invasive procedures for high-risk patients.

"The noninvasive assessment of CAD in obese patients is a clinical challenge," wrote the authors in a study published in last month's Journal of Nuclear Medicine (August 2006, Vol. 47:8, pp. 11302-1306).

The group turned to stress myocardial perfusion imaging (MPI) with a Tc-99m-labeled agent, Tc-99m tetrofosmin (Myoview, GE Healthcare Amersham Biosciences, Chalfont St. Giles, U.K.), to assess its value in predicting mortality and cardiac events in obese patients.

Over a period of six years, from 1996 through 2002, the researchers conducted stress Tc-99m tetrofosmin SPECT studies on 267 consecutive obese patients at their institutions with a successful follow-up on 265 (110 men, 155 women; mean age of 59 years) of the study group. Exercise stress testing was performed on 70 patients using a symptom-limited upright bicycle ergometer, and dobutamine-atropine stress testing was performed on the remaining 195 patients in the study. A computer-averaging of the electrocardiographic complexes was conducted for both types of stress tests.

Approximately one minute before the end of either stress test, the patients were injected with a dose of 370 MBq of Tc-99m tetrofosmin. The resting studies were conducted a minimum of 24 hours after the stress tests and also utilized a 370 MBq dose of Myoview.

The SPECT imaging took place on a triple-head gamma camera system (Prism 3000 XP, Picker, now Philips Medical Systems, Andover, MA) with a low energy, all-purpose collimator. The imaging protocol consisted of 32 projections obtained over a 180° arc from left posterior oblique to right anterior oblique, with an acquisition time of 45 seconds per projection, the authors wrote. Image data was collected in a 64 x 64 matrix and reconstructed utilizing a filtered backprojection algorithm and a ramp reconstruction filter.

The researchers then analyzed the images for a reversible or fixed perfusion defect, which were considered abnormal for purposes of the study.

"A reversible perfusion defect was defined as a perfusion defect on stress images that partially or completely resolved at rest in at least two contiguous segments and slices in the 47-segment model," they wrote. "A fixed perfusion defect was defined as one appearing on two or more contiguous segments and slices on stress images and persisting on rest images."

The scientists found myocardial perfusion abnormalities that were fixed in 62 patients (23%) and reversible in 90 patients (34%). In the 62 patients with fixed defects, 35 had no history of a previous myocardial infarction. Follow-up was conducted through contact with the patients' general practitioners, via a review of hospital records, and verified through civil data registry.

"End points were death from any cause, cardiac death, and hard cardiac events (cardiac death and nonfatal myocardial infarction, defined by cardiac enzyme levels and electrocardiographic changes)," the authors wrote.

The mean follow-up period for the patient cohort was 5.5 years; during this time 41 patients died -- with cardiac death attributed to 22 of these patients. The researchers determined that the annual cardiac death rate was 0.6% in patients with normal perfusion and 3.3% in those with abnormal perfusion exams. In addition, they observed that patients with multiple-vessel distribution of abnormalities had a higher annual death rate (5.5%) than those patients who had a single-vessel distribution.

The researchers also performed a multivariate analysis of the data, and determined that perfusion abnormalities were independently predictive of both cardiac and overall mortality risk with a 95% confidence interval.

"These data indicate that the results of MPI can provide the physician with valuable information regarding the risk status of obese patients and, therefore, assist in the decision on the next management strategy," the authors concluded.

By Jonathan S. Batchelor staff writer
September 1, 2006

Related Reading

SPECT/CT implementation: More than plug and play, June 7, 2006

Cardiac SPECT/CT fusion captures SNM's Image of the Year, June 6, 2006

Ventricular dysfunction not enough to identify sudden cardiac death risk, March 28, 2006

PET, SPECT measures of LVEF have superior predictive value, March 13, 2006

Nitrates enhance tetrofosmin SPECT viability capabilities, September 29, 2005

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