In clinical cardiac Rubidium-82 (82Rb) PET studies, globally impaired flow reserve is a relevant marker for predicting short-term cardiovascular events, and it may be used for integration with currently established functional and morphologic test results and for guidance of preventive measures, especially in the absence of regional flow-limiting disease, according to a study in the May issue of the Journal of Nuclear Medicine.
Current noninvasive tests for coronary artery disease (CAD) detect atherosclerosis or regional ischemia. However, according to the study authors, global myocardial flow reserve is not routinely identified, although it may be an additional marker of disease development and progression.
For the clinical work-up of suspected or known stable coronary artery disease, Kenji Fukushima, MD, from the division of nuclear medicine at Johns Hopkins University in Baltimore, and colleagues studied 275 individuals with rest–dipyridamole 82Rb myocardial perfusion imaging using PET. In addition to clinical measures of regional perfusion and function, they used an experimentally validated approach to quantify global myocardial flow reserve. Follow-up was obtained for 362 days.
The researchers found that myocardial blood flow and flow reserve showed “significant correlation” to systemic and cardiac hemodynamics and a weak association with risk factors such as age and history of hyperlipidemia.
Flow reserve was expectedly lower in subjects with regional ischemia (1.70 vs. 2.31 in those without), but a wide range was observed in those without regional perfusion abnormalities, according to the authors. The researchers used a composite endpoint of hard and soft events to determine that flow reserve below the median was predictive of adverse outcome in the overall population and in subjects with normal regional perfusion (178 patients), whereas stress flow was predictive only in the overall population.
The age-adjusted multivariate analysis confirmed regional perfusion defects (relative hazard, 2.51) and low global flow reserve (relative hazard, 2.93) as independent predictors of cardiac events.
“Although impaired flow reserve is a marker of microvascular dysfunction in the absence of flow-limiting epicardial disease, the situation is different and more complex in the presence of obstructive, flow-limiting CAD,” Fukushima and colleagues wrote. “In this setting, global flow reserve reflects a composite marker of the degree of flow limitation by macroscopic stenosis and the degree of microvascular dysfunction. These can be distinguished only if information about coronary anatomy is available.”
They concluded that the findings of their limited-size, retrospective analysis support the notion that implementation of absolute quantification of myocardial flow reserve may augment clinical myocardial perfusion imaging, by improving the identification of individuals at higher risk of adverse events within the first one to two years after the test.