iFR is a resting index of stenosis severity that is measured without a
vasodilator. It is the ratio of distal and proximal pressures over the
wave-free period, a specific part of diastole during which coronary
flow velocity is naturally at its highest [10]. This higher flow velocity
allows iFR to assess higher pressure gradients across stenoses than possible
by using the complete cardiac cycle whilst also preserving the key
characteristic of constant flow [9]. As a result iFR has a greater ability to
identify small gradients pertinent to the assessment of a diffusely
diseased vessel [9,12].