Combining fractional flow reserve and the index of microcirculatory resistance for prognosis prediction in renal artery stenosis interventions: a feasibility sub-study of FAIR-pilot trial

Abstract Background Interventional therapies for renal artery stenosis (RAS) was debatable in previous randomized trials. Fractional flow reserve (FFR), while useful in coronary interventions, does not assess microcirculatory status, which may be critical in RAS. The index of microcirculatory resistance (IMR) provides additional microvascular information, which has not been used in RAS patients. Purpose To investigate the feasibility of IMR measurement in RAS patients and the association with the outcome. Methods FAIR-pilot study is a multicenter pilot randomized controlled trial exploring the application of FFR to guide the renal artery stenting in patients with renovascular hypertension (NCT05732077). A pressure-temperature sensor guidewire was used to measure the IMR. To derive mean transit time at rest, thermodilution curves were obtained by 3 injections of 4 mL of room temperature saline down the renal artery. Hyperemia was induced using dopamine 50μg/kg bolus to the renal artery. The IMR was calculated as distal arterial pressure (Pd) x mean transit time during hyperemia (TmnH) (Figure 1). Prognostic evaluation was based on improvements in postoperative blood pressure, which was assessed using both ambulatory blood pressure monitor (ABPM) and anti-hypertensive medication number, along with renal function, as determined by the estimated glomerular filtration rate (eGFR). Results The IMR was measured among five patients enrolled in FAIR-pilot study. As shown in Table 1 and 2, stent implantation was conducted on five lesions across patients 1 to 4 based on renal FFR < 0.8, with each experiencing varying degrees of improved blood pressure control subsequently. Patient 5 did not receive a stent and was lost to follow-up. At three-month, the recorded decrease in systolic blood pressure ranged from a maximum of 34 mmHg to a minimum decrease of -6 mmHg among the four patients. The antihypertensive medication number showed a maximum reduction of 2 and no change as the minimum. We observed a correlation between baseline IMR and baseline renal function. No significant change in IMR was noted after interventional procedures. Preliminary findings suggest that IMR measurement is feasible in patients with RAS, while the ∆T (temperature change time) was very small, which potentially increasing the measurement bias. Conclusion The study suggests measuring IMR in patients with RAS is feasible and can offer information into the renal microvasculature. However, the small ∆T encountered during measurement may introduce increased bias, which should be considered when interpreting results.