A new stress test protocol that could lead to a reduction in the use of perfusion imaging in low risk patients undergoing SPECT myocardial perfusion imaging for possible angina symptoms has been found to be diagnostically safe. The study, presented at the recent International Conference on Nuclear Cardiology and Cardiac CT (ICNC 11) in Berlin, Germany, [Henzlova et al Abstract no 58} predicted that using exercise ECG stress testing alone in patients with high exercise capacity would have had no adverse effects on their prognosis at five years. In the abstract, Henzlova and colleagues, from the Mount Sinai School of Medicine, New York, US, set out to investigate retrospectively if a provisional injection protocol in which patients who met certain criteria were converted to exercise treadmill tests without imaging maintained diagnostic accuracy and prognostic ability. For the retrospective study, data were reviewed from a total of 24,689 patients who had undergone SPECT MPI .
Single-photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) has been used for over 30 years to detect ischemia in patients with suspected coronary artery disease (CAD). By comparing the heart’s blood flow at rest and during stress (patients exercise on a treadmill, cycle ergometers or undergo pharmacological stress with vasodilators or dobutamine), cardiologists can determine if the myocardium is receiving sufficient blood supply, as well as the location and extent of underlying CAD. While SPECT MPI represents a well established technique, its main disadvantage is that patients are exposed to diagnostic levels of radiation. In recent years intensive efforts have been made to reduce ionizing radiation associated with cardiac imaging
“Our results are reassuring in that there are few patients whose diagnosis of CAD would be missed,” said Milena Henzlova, the first author of the study. “Not only would widespread adoption of this approach reduce radiation exposure, it would also save considerable amounts of time and money. “Because it’s non invasive and many patients with a chest pain syndrome don’t have coronary disease, SPECT MPI is often viewed as a ‘gate keeper’ to coronary angiography,” explained Lane Duvall, an investigator in the study. Other studies have suggested that exercise treadmill testing alone may be sufficient to predict CVD outcome without use of SPECT MPI in low risk patients. Earlier studies had reported that patients who exercise at >10 metabolic equivalents (METS), [the measure of oxygen used by the body during physical activity] during stress testing had a very low prevalence of significant ischemia and very low rates of cardiac events during follow-up. “Withholding isotope injections in these selected patients was found to be diagnostically safe with a small percentage of ‘missed’ abnormal perfusion studies, a very low rate of significant stress perfusion defects and left ventricular ischemia, and a prognosis which was better than their counterparts who were injected with the isotope,” said Duvall. Eliminating the need for imaging in even a small percentage of the SPECT MPI studies performed annually would result in significant cost savings.Total test time would be halved from three hours to roughly one hour.