Invasive Measures of Myocardial Perfusion and Ischemia

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Abstract

Until recently, our understanding of coronary artery disease (CAD) has been largely based on a purely anatomical approach as derived from the invasive angiogram. The confirmation of the diagnosis of ā€œsignificantā€ CAD, the assessment of its extent, the risk stratification of patients, the therapeutic decisions, the definition of study end-points, and the validation of non-invasive testing, all mainly relied on ā€œeyeballingā€ the angiogram, i.e. a subjective evaluation of the presence of at least 50% (or 70%) diameter stenosis.With the development of invasive, wire-based, means to quantify coronary pressure and flow with high spatial resolution, one realized that purely angiographic metrics correlated poorly with functional information.

Currently, it is admitted that both anatomical and functional information are needed to define CAD and to optimize its management.

In the present review, we summarize the main characteristics of invasive functional indices of ischemia and perfusion.

Section snippets

Definition

Fractional flow reserve (FFR) has been defined19 as the ratio of maximal myocardial blood flow in the presence of the stenosis divided by the theoretically normal maximal myocardial blood flow (i.e. in the absence of the stenosis). Stated another way, FFR informs the operator to what extent the presence of the stenosis limits the maximum achievable flow and, thus, to what extent the restoration of the epicardial conductance will increase myocardial perfusion. FFR can uniquely quantify

Definition

Coronary flow reserve (CFR) has been introduced by K Lance Gould2 who pioneered the era of applied coronary physiology or ā€œphysiological thinkingā€ about CAD. CFR is defined as the extent to which myocardial or coronary flow can increase above its baseline value as obtained during maximal pharmacological hyperemia. CFR quantifies the ratio of stress to rest flow in the diseased vessel. Functional assessment of ischemia using perfusion imaging produces global myocardial flow misdistribution data

Definition

Hyperemic stenosis resistance (HSR) is an index of the resistance to flow opposed by the epicardial narrowing.41 It has been proposed by the groups of Spaan and Piek. Theoretically it should provide the most accurate description of the functional consequences of a given stenosis as it takes into account both the pressure gradient and the flow (velocity).

Calculation

HSR is the ratio of the transstenotic pressure gradient divided by the coronary blood flow velocity under conditions of maximal hyperemia. The

Definition

The index of microvascular resistance (IMR) was introduced by Fearon44 as the first means to quantify the resistance of the coronary microvasculature. IMR measures the resistance to myocardial flow specifically related to the microvascular compartment.

Calculation

IMR is calculated from the ratio of distal coronary pressure, Pd, and the thermodilution-derived mean transit time during maximal micro- and macrovascular vasodilation, Tmn. The latter can be obtained by a bolus injection of 3ā€“4Ā mL of saline at

Definition

The instantaneous wave-free ratio (iFR) was introduced by Sen and Davies.50 It can be defined as the ratio of resting distal coronary pressure to aortic pressure, during diastole, when resistance is allegedly ā€˜naturallyā€™ constant and minimized.

In contrast to all other indices, iFR does not aim at measuring any physical characteristics (flow, pressure or resistance) of the coronary circulation.

Calculation

iFR is calculated as the mean pressure distal to the stenosis during the diastolic wave-free period (Pd

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    Disclosure information: Dr. Adjedj is supported by a research grant from the FƩdƩration Francaise de Cardiologie. Dr. Toth has no conflicts of interest. Dr. De Bruyne reports that his institution receives grant support and consulting fees on his behalf from St. Jude Medical.

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