178 Chapter 8 heterogeneity in both resting and hyperemic myocardial blood flow in healthy individuals, as measured by PET.29 Additionally, heterogeneity has been observed in invasively obtained microvascular resistance in patients with obstructive CAD30, as well as in invasively obtained CFR among post-allograft coronary transplant patients. Recently, Hillier et al.31 used breathing-enhanced oxygenation-sensitive cardiac magnetic resonance imaging (OS-CMR) in women with ANOCA, revealing regional disparities in myocardial oxygenation, thus indicating heterogeneity in myocardial oxygenation among patients with ANOCA. Nonetheless, this study is the first to investigate ANOCA patients concerning the heterogeneity of coronary flow across different coronary territories. In our study, we observed disparate outcomes across various coronary territories concerning MPR. Specifically, a significantly lower MPR observed in the RCA territory in ANOCA patients compared to the matched healthy controls was found, whereas no significant difference was observed in the LAD and RCX territories. It is worth noting that the RCA is usually only tested for coronary spasms if no spasm occurs in the LAD. In our study, spasm provocation in the RCA was only performed in 7 ANOCA patients, and CMD measurements were limited to the LAD. Therefore, the observed lower MPR in the RCA may be related to a higher prevalence of coronary spasms or CMD in the RCA territory, although this cannot be conclusively determined in our study. As global differences in MPR have been identified in ANOCA before32, using fully QP allows for non-invasive measurements of coronary perfusion and MPR in all coronary territories, including different coronary segments. Incorporating QP assessment by CMR and acquiring information regarding myocardial perfusion in all coronary territories may serve as a step toward a more accurate diagnosis in ANOCA. MPR in ANOCA endotypes Previous studies have investigated the use of QP in obstructive CAD or in CMD patients.9, 33 Rahman et al.18 and Thomson et al.32 demonstrated that (semi-)quantitative perfusion stress CMR accurately detects CMD (defined as a CFR < 2.5) and correlates to CFT results. However, it is important to note that both studies have exclusively included CMD patients, leaving a gap in understanding the applicability of QP-CMR in the broader ANOCA population. For QP to be established as a reliable diagnostic tool, it is crucial to understand its utility across the entire ANOCA population. Our study addressed this by including ANOCA patients, irrespective of endotypes. Interestingly, no differences in MPR distribution were found among the different stratified ANOCA endotypes, suggesting that MPR abnormalities are not exclusive to CMD but are independently regulated, contrary to previous literature. Additionally, we observed some overlap in MPR values between ANOCA patients and healthy controls. This underscores the challenge of using MPR alone to differentiate between ANOCA patients and healthy individuals, especially given the lack of consensus about a clear cutoff. However, if reliable MPR thresholds are established in future studies,
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