Chapter 24: Adjunctive Diagnostic Techniques: Fractional Flow Reserve

INTRODUCTION

Although coronary angiography remains the reference standard for diagnosing critical epicardial coronary artery disease (CAD), it has a number of limitations. Identifying functionally significant intermediate coronary narrowings, characterizing plaque morphology, detecting moderate diffuse intimal thickening, evaluating the results of percutaneous coronary intervention (PCI), and assessing the status of the microvasculature all remain challenging with coronary angiography alone.^1-6 Technologic advancements have made several adjunctive diagnostic techniques available that allow the interventional cardiologist to address the limitations of coronary angiography and more thoroughly assess the coronary circulation.

In this chapter we will focus on the role of the coronary pressure wire to measure fractional flow reserve (FFR) and guide decisions regarding coronary revascularization. The background and derivation of FFR were already covered in Chapter 6. We will briefly review the concept of FFR and then detail how to measure FFR, outline the clinical indications for FFR assessment, and present data supporting its role in the cardiac catheterization laboratory.

HISTORICAL BACKGROUND

The significance of intracoronary pressure gradients and their reflection of the severity of underlying CAD has been appreciated for many years.⁷ However, it was not until the advent of miniaturized pressure sensors that allowed a wire-based approach to measuring coronary pressure and not until the development and validation of the FFR index by Pijls and De Bruyne that intracoronary pressure measurements have become a routine procedure in many catheterization laboratories.^8,9

DERIVATION OF FFR

FFR is defined as the fraction of maximal blood flow perfusing the myocardium in the presence of stenosis compared with the maximal blood flow that would reach the myocardium in the theoretical absence of the stenosis. Because the measurement is made at maximal vasodilation, resistance is minimized and flow becomes proportional to pressure. In a normal epicardial artery, there is little pressure decrement along the vessel, and distal pressure is roughly equal to proximal pressure. Therefore, in a diseased vessel, one can estimate the distal flow or pressure in the theoretical absence of the disease by measuring the proximal pressure. FFR then can be calculated easily in a diseased vessel by dividing the distal pressure, measured with a coronary pressure wire, by the proximal pressure, measured with the guiding catheter during peak hyperemia.⁸

Compared with other techniques for determining the functional significance of a coronary narrowing, measuring FFR has several inherent advantages. It has an absolute normal value of 1.0 and a narrow range of normal values extending from 0.94 to 1.0.¹⁰ Because the measurements are made at maximal vasodilation, the effects of resting hemodynamics are eliminated, making FFR an extremely reproducible index, independent of blood pressure and heart rate.¹¹ Compared with measuring CFR to evaluate intermediate coronary lesions, FFR is advantageous because it is epicardial artery-specific and independent of the microcirculation (Table 24-1). As described in ...