Myocardial perfusion reserve by CZT cameras: A journey inside coronary microvascular circulation. Is it time to leave yet?

Excerpt

Myocardial ischemia has been extensively associated with the presence of epicardial coronary artery disease (CAD) and the detection of hemodynamically significant stenosis is crucial in order to guide patient’s management.^1,2 However, a significant proportion of patients referred to invasive coronary angiography will not show obstructive CAD, suggesting that several other factors play an important role in this process.³ Coronary microvascular disease (CMD) comprises a subset of disorders characterized by structural and functional remodeling of the microcirculation, leading to impaired coronary blood flow autoregulation.⁴ In patients with CMD, these processes may lead to absolute decreases in blood flow and consequent myocardial ischemia. The evaluation of coronary physiology based on clinical evaluation is challenging and often invasive tests are needed to guide treatment decisions. The functional severity of epicardial coronary stenosis can be invasively assessed based on the myocardial fractional flow reserve (FFR), a surrogate measure of flow limitation, and lesion-level ischemia.⁵ The invasive evaluation of FFR can be used to rule out the presence of functional significant stenoses, but is not able to directly assess the presence of CMD. The index of microvascular resistance (IMR) is a direct measure of coronary microvascular status reflecting the presence of CMD.⁶ Compared with FFR, less is known about IMR and the clinical benefits related to its reduction. The coronary flow reserve (CFR) is a measure of vasodilator capacity that can be invasively assessed by indicator thermodilution and it reflects the entire coronary vascular status.⁶ All these parameters provide complementary diagnostic and prognostic information. However, noninvasive methods for assessing coronary artery function are available and rapidly evolved during last years. Radionuclide myocardial perfusion imaging (MPI) by single-photon emission computed tomography (SPECT) is a well-validated method for noninvasive evaluation of myocardial perfusion. The presence and severity of abnormal findings at radionuclide MPI is strongly related to the presence and severity of CAD.⁷ Moreover, cardiac MPI by positron emission tomography (PET) is considered the gold standard for noninvasive assessment of absolute myocardial blood flow and myocardial perfusion reserve (MPR), which is able to provide both diagnostic and prognostic information.^8‐11 During last years, the introduction in the clinical practice of cadmium-zinc-telluride (CZT) cameras allowed to perform absolute quantification of MBF and MPR by using SPECT technology.^12,13 The diagnostic ability of CZT SPECT in identifying obstructive CAD has been previously explored.^14,15 However, consistent data exploring the relationship between MPR by CZT SPECT, and invasive measurements of coronary vascular tatus, including IMR, are lacking. …