FFR Fundamentals and Measurements

Transcription

1 FFR Fundamentals and Measurements Ghassan S. Kassab Thomas Linnemeier Chair Professor Biomedical Engineering, Indiana University Purdue University Indianapolis

2 Principle of FFR Q S ( P P ) / R P max d v d FFR Q N ( ) / max P a P v R P a Q s max: Hyperaemic myocardial blood flow in presence of stenosis Q N max: Normal hyperaemic myocardial blood flow P d : Distal coronary pressure P a : Aortic pressure P v : Venous pressure (assume 0) R: Hyperaemic myocardial resistance (assume unchanged in stenosis)

3 Bifurcation Lesions A greater discrepancy can exist between anatomical evaluation and FFR since the bifurcation lesion is a combination of three ostial lesions Rev Esp Cardiol. 2006;56(2):183-4.

4 FFR of SB The influence of proximal & distal stenotic lesions must be considered. If a significant stenosis exists at the proximal main branch (MB), SB FFR overestimates the functional severity of the SB lesion due to pressure decrease by the proximal stenosis. If a significant stenosis exists distal to a SB ostial lesion and FFR was measured before that stenosis, FFR underestimates the lesion severity by submaximal flow by the distal lesion.

5 FFR after MB Stenting There is no validated criterion for SB intervention after MB stenting. SB ostial lesion is typically aggravated after MB stenting (SB jailing): spasm, thrombosis, stent struts, plaque shift, and carina shift. Angiographic assessment for jailed SB ostial lesion is difficult: stent radiopacity, image filters, and edge enhancement by digital angiography, incomplete mixing of blood and contrast. Angiographic percent diameter stenosis overestimates the functional severity in general. Hence, FFR is needed. FFR measurements for jailed SB in not recommended in SB with: severe angulation, heavy calcification, diffuse or multiple stenosis.

6 A Technique in the Use of Fractional Flow Reserve in Coronary Artery Bifurcation Lesions Ratcliffe et al, Int J Angiol. 2012;21:59-62.

7 Utility of Virtual Approaches? HeartFlow Segment lesions Computational time 6 hrs Flow assumption normal microvasculature Yoon et al, JACC Cardiovasc Imaging. 2012;5(11):

8 Analytical Model Introduction of a predictive analytical model for pressure drop and myocardial FFR for coronary stenosis derived strictly from stenosis dimensions and hyperaemic flow (i.e., contains no empirical parameters). Validation of the physics-based model through in vitro and in vivo (swine) experiments as well as a finite-element (FE) model. Huo et al, J R Soc. 2012;9(71):

9 Analytical model derived from the general Bernoulli equation (conservation of energy), which considers various energy losses along the length of a lesion. Methods Model Input Variables: Lesion lumen CSA (proximal, distal & minimal CSA along the lesion) Lesion length Hyperaemic volumetric flow rate through the lesion

10 Methods FFR during hyperaemic flow: P FFR P distal a P P v v (2.1) where P a is the mean aortic pressure; P v is the central venous pressure; P proximal and P distal are the hyperaemic coronary pressure proximal and distal to stenosis, respectively. If central venous pressure is assumed to be negligible, the equation can be approximated as: P P p distal a FFR (2.2) P P a a where Δp is the pressure gradient along the axis of vessel proximal to distal position of stenosis.

11 Methods We propose a model to determine Δp specifically for the coronary arteries. Since gravity is negligible in the coronary circulation, the general Bernoulli equation can be written as: P P P P P convective constrictive diffusive expansion (2.3) ΔP convective, ΔP constriction, ΔP diffusive and ΔP expansion are energy losses due to flow convection, sudden constriction, in CSA from proximal normal vessel to stenosis, flow diffusion and sudden expansion in CSA from stenosis to distal normal vessel, respectively.

12

13 Results Comparison of pressure drop between the theoretical model (equation (2.3)) and FE simulation shows a highly significant linear relationship: P P r FE model ( 1) theory

14

15 Conclusions Major Findings: The analytical model is highly accurate as compared to the FE model but provides predictions in real time. Flow pulsatility has a negligible effect on FFR since the measure is based on mean pressure. Stenosis eccentricity or geometry have a negligible effect on FFR in coronary circulation given the small Reynold s number. The stenosis flow pattern along with the degree of area stenosis is an important contributor to the energy loss of sudden expansion. The entrance effect at the inlet of a coronary stenosis contributes significantly to the pressure drop (important to consider for bifurcations).

16 Future Extension of Virtual FFR to Bifurcation Lesions Geometry: Angio, CT, IVUS, OCT Flow: Angiographic measurement (Molloi, Zhou and Kassab. Academic Radiology. 2004;11(7): ) CT (Use of scaling laws for patient-specific flow) Analytical model: real-time FFR