Assessing coronary artery stenosis severity: In vitro validation of the concept of fractional flow reserve

Fractional flow reserve (FFR) is an index to assess the functional obstruction of an isolated coronary artery stenosis. It can be measured using only pressure measurements proximal (Pa) and distal (Pd) to the stenosis: FFR = P(d)/P(a). We studied the relation of pressure and flow-derived measurements of FFR in a pulsatile, hydraulic model of a coronary artery under physiological aorta pressure (80-110 mm Hg) and coronary flow (140-260 ml/min) conditions. Measurements were done at baseline and for several stenosis levels obtained with an external occluder. We found good correlations (r2 > 0.95) between pressure and flow-derived FFR, irrespective of isolated changes in myocardial resistance or aortic perfusion pressure. The basic assumption, i.e., that myocardial resistance is constant, was identified as most crucial in the validity of the concept of pressure-derived FFR. The agreement between our data and published animal and human studies indicates that this is most probably the case in hyperaemic conditions.