The impact of the microvascular resistance on the measures of stenosis severity

The relationship between measures of stenosis and microvascular resistance is of importance due to medical decisions being based on these values. This research investigates the impact of varying microvascular resistance on fractional flow reserve (FFR) and hyperaemic stenosis resistance (hSR). Microvascular resistance is classified using hyperaemic microvascular resistance (hMR). Additionally, hMR using the upstream pressure value (hMR_Pa) has also been calculated and is compared to hMR measured conventionally. Tests were conducted at three different degrees of stenosis (quantified by percent area) in a coronary flow circuit with varying downstream resistance to simulate the microvasculature. Pressure and flow values are recorded across the stenosed section, allowing for calculation of the diagnostic indexes. Results indicate that for a constant degree of stenosis, FFR would increase with increasing microvascular resistance while hSR would remain almost constant. hMR_Pa was found to approach hMR as the stenosis severity decreased, and the pressure gradient decreased. In the results shown here, with sufficiently high downstream resistance, an 84 % stenosis could produce an FFR value over 0.8. This result suggests that there is the potential for misdiagnosis of the severity of stenosis when combined with elevated microvascular resistance. Consequently, decisions on the clinical significance of a stenosis, classified by FFR, need to consider the effect of the microvascular resistance.