Effect of Patient-Specific Coronary Flow Reserve Values on the Accuracy of MRI-Based Virtual Fractional Flow Reserve

The purpose of this study is to investigate the effect of varying coronary flow reserve (CFR) values on the calculation of computationally-derived fractional flow reserve (FFR). CFR reflects both vessel resistance due to an epicardial stenosis, and resistance in the distal microvascular tissue. Patients may have a wide range of CFR related to the tissue substrate that is independent of epicardial stenosis levels. Most computationally based virtual FFR values such as FFRCT do not measure patient specific CFR values but use a population-average value to create hyperemic flow conditions. In this study, a coronary arterial computational geometry was constructed using magnetic resonance angiography (MRA) data acquired in a patient with moderate CAD. Coronary flow waveforms under rest and stress conditions were acquired in 13 patients with phase-contrast magnetic resonance (PCMR) to calculate CFR, and these flow waveforms and CFR values were applied as inlet flow boundary conditions to determine FFR based on computational fluid dynamics (CFD) simulations. The stress flow waveform gave a measure of the functional significance of the vessel when evaluated with the physiologically-accurate behavior with the patient-specific CFR. The resting flow waveform was then scaled by a series of CFR values determined in the 13 patients to simulate how hyperemic flow and CFR affects FFR values. We found that FFR values calculated using non–patient-specific CFR values did not accurately predict those calculated with the true hyperemic flow waveform. This indicates that both patient-specific anatomic and flow information are required to accurately non-invasively assess the functional significance of coronary lesions.

DISENGAGE Registry

Background: During fractional flow reserve (FFR) measurement, the simple presence of the guiding catheter (GC) within the coronary ostium might create artificial ostial stenosis, affecting the hyperemic flow. We aimed to investigate whether selective GC engagement of the coronary ostium might impede hyperemic flow, and therefore impact FFR measurements and related clinical decision-making. Methods: In the DISENGAGE (Determination of Fractional Flow Reserve in Intermediate Coronary Stenosis With Guiding Catheter Disengagement) registry, FFR was prospectively measured twice (with GC engaged [FFR eng ] and disengaged [FFR dis ]) in 202 intermediate stenoses of 173 patients. We assessed (1) whether ΔFFR eng –FFR dis was significantly different from the intrinsic variability of repeated FFR measurements (test-retest repeatability); (2) whether the extent of ΔFFR eng –FFR dis could be clinically significant and therefore able to impact clinical decision-making; and (3) whether ΔFFR eng –FFR dis related to the stenosis location, that is, proximal and middle versus distal coronary segments. Results: Overall, FFR significantly changed after GC disengagement: FFR eng 0.84±0.08 versus FFR dis 0.80±0.09, P <0.001. Particularly, in 38 stenoses (19%) with FFR values in the 0.81 to 0.85 range, GC disengagement was associated with a shift from above to below the 0.80 clinical cutoff, resulting into a change of the treatment strategy from medical therapy to percutaneous coronary intervention. The impact of GC disengagement was significantly more pronounced with stenoses located in proximal and middle as compared with distal coronary segments (ΔFFR eng –FFR dis , proximal and middle 0.04±0.03 versus distal segments 0.03±0.03; P =0.042). Conclusions: GC disengagement results in a shift of FFR values from above to below the clinical cutoff FFR value of 0.80 in 1 out of 5 measurements. This occurs mostly when the stenosis is located in proximal and middle coronary segments and the FFR value is close to the cutoff value.

Optimization of Balloon Obstruction for Simulating Equivalent Pressure Drop in In-Vivo Conditions

The study of hemodynamics in an animal model associated with coronary stenosis has been limited due to the lack of a safe, accurate, and reliable technique for creating an artificial stenosis. Creating artificial stenosis using occluders in an open-chest procedure has often caused myocardial infarction (MI) or severe injury to the vessel resulting in high failure rates. To minimize these issues, closed-chest procedures with internal balloon obstruction were often used to create artificial stenosis. However, it should be noted that the hemodynamics in a blood vessel with internal balloon obstruction as opposed to physiological stenosis hasn’t been compared. Hence, the aim of this research is to computationally evaluate the pressure drop in balloon obstruction and compare with that in physiological stenosis. It was observed that the flow characteristics in balloon obstruction are more viscous dominated, whereas it is momentum dominated in physiological stenosis. Balloon radius was iteratively varied to get a pressure drop equivalent to that of physiological stenosis at mean hyperemic flow rates. A linear relation was obtained to predict equivalent balloon obstruction for physiological stenosis.

Misinterpretation of Stenosis Severity in the Presence of Serial Coronary Stenoses

Diagnosis of the functional severity of an epicardial coronary stenosis using parameters like Fractional Flow Reserve, FFR (ratio of distal to proximal pressure of a stenotic region), might be affected in the presence of an additional downstream stenosis. In order to assess this effect, we have performed an in-vitro experiment which is used to validate a computational study. Three combinations of serial stenoses were tested: 80%-64%, 80%-80% and 80%-90% area stenosis (AS). The physiological mean hyperemic flow (flow at maximal arterial dilatation) values were obtained using an in-vitro experimental set-up. These flow rates were used as steady flow inputs by time-averaging the spatially averaged flow pulse over two cardiac cycles for the computational study. FFR values were calculated at hyperemic flow using both the experimental and numerical pressure data. As the downstream severity increased from 64% AS to 80% AS, hyperemic coronary flow decreased from 136.4 ml/min to 126.4 ml/min. Flow decreased further to 90.7 ml/min with a downstream severity of 90% AS. FFR of the intermediate stenosis increased from 0.76 to 0.79 and further to 0.88 as the downstream stenosis increased from 64% to 80% with a final severity of 90% AS. Similarly, numerically obtained FFR values increased to 0.83, 0.80 and 0.92 for the corresponding cases indicating an error within 7% of the experimental values. These results indicate that the presence of a downstream stenosis might lead to a clinical misinterpretation of the upstream stenosis severity.