01 FFR Mongrain aimradial2016 - coronary blood flow

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  Regulation of the coronary blood flow: primer for the interventional cardiologist - Rosaire Mongrain
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  • 1. Bio-­Fluid  Mechanics  and     Fractional  Flow  Reserve Rosaire Mongrain Olivier  F.  Bertrand  
  • 2. Disclosure I have no conflict of interest to declare
  • 3. 1.  Assessment  of  a  Stenosis                                            n s h s ne n L h nn D L K ba ba D V A AK V D dx AD DAK P s 08.021.1K profile)  (parabolic          8 2 1 133321333 4 e 22 22 2 2 0 2 2  Ds/Dn or    As/An Diameter  or  Area Geometrical/anatomical                                                      Functional  (Gould)     V=Q/A dQ/dt) Taylor  Series  Expansion  of  the  Pressure  Gradient  Function:
  • 4. 1.  Assessment  of  Stenosis  Severity:  FFR HealthyDiseased
  • 5. Conceptual  Jump   The conceptual jump is an analogy between hydraulic and electrical circuit FLOW                          PRESSURE                    RESISTANCE 1.  Assessment  of  Stenosis  Severity:  FFR
  • 6. Thus  the  pressure  based  calculation  of  FFR  requires:   -­‐ Pao aortic  pressure  [mmHg],   -­‐ Pd distal  stenosis  pressure  [mmHg],   -­‐ Pv venous  pressure  [mmHg],   -­‐ Rmyo,n healthy  myocardial  resistance  [mmHg·∙s·∙ml-­‐1],   -­‐ Rmyo,d diseased  myocardial  resistance  [mmHg·∙s·∙ml-­‐1] 1.  Assessment  of  Stenosis  Severity:  FFR
  • 7. The concept is appealing as the healthy myocardial blood flow is unknown Since the average venous pressure (right atrial pressure) is approximately 5mmHg (De Bruyne 1995), it can be assumed to be negligible (Pijls 1994) and we get: However, given that the FFR is a ratio, neglecting the venous pressure may, in certain situations, induce non-­negligible errors Neglecting Pv could be significant in high Pv patients (hemodialysis up 10-­12 mmHg) nmyoao dmyod RP RP FFR , , / /  2.  FFR  Simplification  (Pv)    
  • 8. The FFR calculation is further simplified by assuming that the diseased and healthy myocardial resistances become equal under induced maximal hyperaemic conditions (in the absence of microvascular disease) Autoregulation  of  coronary  blood   flow  based  on  perfusion  pressure   During maximum physiological dilation, there is a linear relationship between the perfusion pressure and the coronary blood flow coronary  blood  flow   Pappano,  A.J.,  and  Wier,  W.G.,  (2012)  Cardiovascular  physiology,  Elsevier/Mosby,  Philadelphia,  PA. As such, in a clinical setting, under induced hyperaemic conditions (through the injection of a vasodilatory agent), the FFR can be approximated by: 2.  FFR  Simplification  (Rmyo,d – Rmyo,n)    
  • 9. 2.  FFR  Simplification  (Rmyo,d – Rmyo,n)     Autoregulation  of  coronary   blood  flow  based  on  perfusion   pressure   ao d P P FFR  It should be noted that the hydraulic resistance corresponds to the inverse of the slope of auto-­ regulation curve. In other words, the steeper the slope the less resistance. In principle, under hyperemic conditions, the slope would be steeper (even lower resistance but for a relatively narrow coronary perfusion pressure).
  • 10. • In fact, there is another implicit conceptual jump in using hydraulics • Hydraulics is a model lumping of fluid mechanics • In going from fluid mechanics to hydraulics, complex local phenomena are reduced to a simple global resistance R • The effects of phenomena like flow separation, vortices, recirculation, turbulence are all lumped into a unique resistance value R 3.  FFR:  The  measurement  of  Pd ? ao d P P FFR 
  • 11. Recirculation 3.  FFR:  The  measurement  of  Pd ?
  • 12. Obviously, the post-­‐stenotic has to be avoided for the measurement of Pd. Where  is  it  fine  from  a  bio-­‐fluids  point  of  view  to  do  the  measurement  ? ? 3.  FFR:  The  measurement  of  Pd ? Referring  to  a  cartoon  representation  of  a  stenosis  (Wong  1986): ao d P P FFR 
  • 13. From a bio-­‐fluids perspective, we refer to the notion of velocity field recovery In other words, where does the flow become undisturbed past the stenosis ? This is answered with the equation: L/D ~ 0.06 Re (empirical for laminar flow) with L the length before the flow is redeveloped, D the vessel diameter and Re the Reynolds number: (Re ~ 250 for a normal vessel, Re ~ 500-­‐600 for mid range stenosis and Re > 1000 for a severe stenosis) For a mid range stenosis normal vessel, L ~ 0.06 X 500 Re X 3.5 mm ~ 10 cm ! This would be impractical for side branches  DU0 Re  3.  FFR:  The  measurement  of  Pd ?
  • 14. As a result, the measurement of Pd is affected by a certain error due to local dynamic pressure gradients in the post-­stenotic region It should be noted that these dynamic pressure gradients are very sensitive to geometry changes (this includes the presence of wires, catheters, wall deformations, side branches) Also the higher the velocity, the higher the problematic of dynamic pressure gradients In fact, for a fluid, the notion of resistance is linked not only to its viscosity but also to the flow dynamics 3.  FFR:  The  measurement  of  Pd ? ao d P P FFR 
  • 15. Viscous kinetic   The flow also influences the Resistance. In other words, the Resistance is flow dependent. This  can  be  viewed  with  the  Gould  equation: Flow  dependent  Resistance 3.  FFR:  The  measurement  of  Pd ?
  • 16. 4.  What  about  iFR ? However, as discussed above, from a bio-­fluid perspective, higher velocities are associated with higher flow disturbances and higher flow dependent stenosis resistance depence This, in principle, would make the measurement of Pd more problematic to dynamic pressure gradient effects In general, the flow velocity is higher and pressure is lower over the wave free period In principle, it results in lower microvascular resistance during the free period in comparison to the complete cardiac cycle https://en.wikipedia.org/wiki/Instantaneous_wave-­‐free_ratio#/media/File:Coronary_flow_velocity_and_microvascular_resistance_over_the_wave-­‐free_period.jpg
  • 17. • The  transition  from  Flow  to  Pressure  requires  a  hydraulic  model • The  hydraulic  model  is  a  simplification  of  fluid  mechanics • Neglecting  the  venous  pressure  Pv may  not  always  be  precise • The  equality  of  the  myocardial  bed  resistances  is  an  assumption • The  measure  of  the  distal  pressure  Pd can  be  problematic  for   several  reasons  (dynamic  pressure  effects) • From  a  bio-­‐fluids  point  of  view,  the  measurement  of  Pd could  be   more  problematic  with  IFR  (dynamic  pressure  effects) 16 5.  The  measurement  of  FFR   (Summary):  
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