Читать книгу Interventional Cardiology - Группа авторов - Страница 189

The instantaneous wave‐free ratio (iFR) is a resting index of stenosis severity which quantifies the impact a stenosis upon the coronary circulation. A pressure‐only index, it is measured during the wave‐free period in diastole. This is a period where microcirculatory resistance is low, and importantly, stable, compared to other components of the cardiac cycle. When resistance is stable, pressure and flow are linearly related, enabling a resting pressure index to assess stenosis severity without the need of an exogenous vasodilator. iFR can be computed either as a single heartbeat or as an average over five heart beats. The wave‐free period is not significantly affected by beat‐to‐beat variability or atrial arrhythmias. Heart rates typical for performing physiological assessment (40–130 bpm). Automated algorithms can remove incorrectly calculated beats showing wide variability. iFR can also be plotted throughout the vessel during a gentle pressure wire pullback – performed over 30 seconds, it can identify focal and diffuse disease.

It is also possible to measure iFR during administration of a vasodilator such as adenosine (referred to as iFRa). This consistently produces lower iFR values, the majority even lower than FFR [61]. Notably, stability of microvascular resistance remains unchanged meaning there is minimal incremental diagnostic yield by adding adenosine. Indeed, comparing iFR, FFR, and iFRa against a combined pressure and flow based parameter such as hyperemic stenosis resistance (HSR), did not show any added value [62].