In vivo validation of intra-coronary absolute flow measurements and comparison of reliability between different microvascular indices and the microvascular resistance reserve
Authors: Minten L , Bennett J , Oosterlinck W , Meuris B , Algoet M , Langenaeken T , Bezy S , Mccutcheon K , Dubois C 
Published: November 2023

Abstract

Background
Coronary microvascular dysfunction plays a major role in patients with cardiac ischemia without flow limiting epicardial coronary stenosis, takotsubo cardiomyopathy, aortic valve stenosis and heart failure with preserved ejection fraction. In clinical practice, the index of microvascular resistance (IMR) and coronary flow reserve (CFR), and more recently the absolute coronary blood flow (ABF), absolute coronary resistance (ACR) and the microvascular resistance reserve (MRR) are used as parameters to measure resistance and flow using continuous coronary thermodilution.

Purpose
Firstly, to validate ABF measured with continuous coronary thermodilution in vivo against epicardial doppler-measured flow. Secondly, to assess reliability and reproducibility in-vivo of different microvascular indices.

Methods
For the purpose of this study coronary measurements were conducted in two large animal models and in patients, all with the appropriate ethical committee approval. For the first part of the study, fourteen sheep underwent repeated paired measurements of ABF and ACR (with use of continuous thermodilution) with different saline infusion speeds, as well as IMR, CFR and MRR in multiple coronary vessels. Seven additional sheep underwent these measurements while having simultaneous measurements with epicardial coronary flow probes (gold standard) in an open-chest model. The epicardial flow probes were placed on the proximal left anterior descending and circumflex artery. Maximal hyperemia was induced with adenosine (CFR and IMR) or infusion of saline at a rate of 30ml/min (ABF, ACR and MRR). For the second part of the study, in 41 patients receiving routine microvascular assessment, IMR and CFR measurements were duplicated by a single operator, after re-calibration, to assess the reproducibility of the measurement. The combined animal and human data resulted in the analysis of 408 coronary measurements made in 88 coronary vessels during 65 heart catheterizations.

Results
In sheep, ABF measured with continuous thermodilution (with saline infusion speeds from 5ml/min to 30ml/min) was accurate when compared with epicardial flow measurements. (Table 1, Fig 1A) Notably, the continuous thermodilution measurement had the tendency to slightly overestimate ABF. (Table 1, Fig 1B) The two methods were equivalent within 15% (Two one-sided test: mean difference: -8.73%; 95% CI: -14.40 to -3.06%, p= 0.016). When comparing test-retest reproducibility between the indices, CFR performed the worst while ACR and MRR performed the best. MRR had a significantly better reproducibility than the IMR. (Table 1, Fig 1C).

Conclusion
ABF can accurately be determined by continuous coronary thermodilution. MRR and ACR have a significantly superior reliability and reproducibility compared to IMR and CFR. Unlike ACR, MRR is not vessel or territory-size dependent and is therefore the index of choice for future research and clinical assessment of the microcirculation.