**By Naritatsu Saito**

As many physicians know, a simple fractional flow reserve (FFR) measurement does not predict the true functional severity of an individual stenosis in multiple sequential coronary stenoses because of the complex fluid dynamic interaction between the stenoses. Theoretical equations to predict the true FFR of individual stenosis in a tandem lesion were developed more than 10 years ago, and the equations have been cited repeatedly in many articles. However, the application of the equations is limited in a tandem lesion. We mathematically derived two novel equations applicable to multiple sequential coronary stenoses with more than 3 stenoses. One is the equation which predicts the true FFR of each stenosis (equation A), and the other predicts the true FFR after releasing a given stenosis in multiple sequential stenoses (equation B).

One may consider that both equation A and equation B are difficult to apply in daily clinical practice since both equations require wedge pressure measurement during the maximum hyperemia. However, both equations are important when considering the theoretical background of the FFR assessment in a sequential lesion. Equation A suggests that the true FFR of individual stenosis is always smaller than the apparent FFR value. The severity of individual stenosis is always underestimated in multiple sequential stenoses. Equation B suggests that the true FFR of entire stenoses after releasing a given stenosis is always smaller than the apparent FFR value obtained by a simple addition of baseline FFR of the entire stenoses plus ∆FFR across the target lesion. A better understanding of the background theory helps to improve the performance of daily practice.

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**A selection of articles on Fractional Flow Reserve available at http://www.invasivecardiology.com**

- Lockie T, Rolandi MC, Piek JJ. Dynamic damping of the aortic pressure trace during hyperemia: the impact on fractional flow reserve measurement. J Invasive Cardiol. 2013 Oct;25(10):549-50.
- Kumar G. Letter to the editor: The influence of right atrial pressure on fractional flow reserve. 2012 Oct;24(10):A43-A44.
- Miller LH, Toklu B, Rauch J, Lorin JD, Lobach I, Sedlis SP. Very long-term clinical follow-up after fractional flow reserve-guided coronary revascularization. J Invasive Cardiol. 2012 Jul;24(7):309-15.
- Cook J, Deshmukh A, Sachdeva R. Hemodynamic findings of spontaneously recanalized coronary thrombus. J Invasive Cardiol. 2012 Jul;24(7):368-9.
- Mizuno A, Nishi Y, Niwa K. Deferring coronary stenting for myocardial bridging by fractional flow reserve and optical coherence tomography. J Invasive Cardiol. 2012 Feb;24(2):E27-9.