Diagnostic uncertainties during assessment of serial coronary stenoses: an in vitro study

Currently, the diagnosis of coronary stenosis is primarily based on the well-established functional diagnostic parameter, fractional flow reserve (FFR: ratio of pressures distal and proximal to a stenosis). The threshold of FFR has a "gray" zone of 0.75-0.80, below which further clinical intervention is recommended. An alternate diagnostic parameter, pressure drop coefficient (CDP: ratio of trans-stenotic pressure drop to the proximal dynamic pressure), developed based on fundamental fluid dynamics principles, has been suggested by our group. Additional serial stenosis, present downstream in a single vessel, reduces the hyperemic flow, Q˜h, and pressure drop, Δp˜, across an upstream stenosis. Such hemodynamic variations may alter the values of FFR and CDP of the upstream stenosis. Thus, in the presence of serial stenoses, there is a need to evaluate the possibility of misinterpretation of FFR and test the efficacy of CDP of individual stenoses. In-vitro experiments simulating physiologic conditions, along with human data, were used to evaluate nine combinations of serial stenoses. Different cases of upstream stenosis (mild: 64% area stenosis (AS) or 40% diameter stenosis (DS); intermediate: 80% AS or 55% DS; and severe: 90% AS or 68% DS) were tested under varying degrees of downstream stenosis (mild, intermediate, and severe). The pressure drop-flow rate characteristics of the serial stenoses combinations were evaluated for determining the effect of the downstream stenosis on the upstream stenosis. In general, Q˜h and Δp˜ across the upstream stenosis decreased when the downstream stenosis severity was increased. The FFR of the upstream mild, intermediate, and severe stenosis increased by a maximum of 3%, 13%, and 19%, respectively, when the downstream stenosis severity increased from mild to severe. The FFR of a stand-alone intermediate stenosis under a clinical setting is reported to be ∼0.72. In the presence of a downstream stenosis, the FFR values of the upstream intermediate stenosis were either within (0.77 for 80%-64% AS and 0.79 for 80%-80% AS) or above (0.88 for 80%-90% AS) the "gray" zone (0.75-0.80). This artificial increase in the FFR value within or above the "gray" zone for an upstream intermediate stenosis when in series with a clinically relevant downstream stenosis could lead to misinterpretation of functional stenosis severity. In contrast, a distinct range of CDP values was observed for each case of upstream stenosis (mild: 8-10; intermediate: 47-54; and severe: 130-155). The nonoverlapping range of CDP could better delineate the effect of the downstream stenosis from the upstream stenosis and allow for the accurate diagnosis of the functional severity of the upstream stenosis.