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FFR and iFR‐Pullback is used to determine the resting pressure loss caused by each individual stenosis and can be particularly useful when multiple coronary stenoses are present or if there is additional diffuse disease that may modulate the likelihood of hemodynamic improvement after coronary intervention. Performing a pullback assessment provides useful information that can guide an interventional strategy [43]. However, there are significant differences in how this information can be used.

FFR measured at any given location will not represent the final FFR if distal lesions are removed. Hyperemic blood flow is affected by each stenosis within a vessel. More distal stenoses are exposed to lower flow velocities than observed proximally [44]. The change in flow velocity is not easily predicted, as angiographic severity does not equate well to coronary flow; furthermore, even diffuse atherosclerosis between stenoses will cause some flow reduction. Since a pressure‐gradient across a stenosis is determined by the amount of flow velocity, it follows that pressure gradient will change if the flow can be altered [45]. Removing a given stenosis will increase hyperemic flow across the treated segment, such that residual stenoses will have an observable change in the residual pressure gradients. This interaction between pressure and flow between stenoses is referred to “cross‐talk”. In brief, it means, that an FFR value measured at any given location before angioplasty, will not accurately predict FFR after angioplasty. This a limiting factor when interpreting FFR‐pullback [43].

This issue can be resolved by measuring coronary wedge pressure, but this is not a practical solution for typical clinical practice. Instead, FFR pullback should be performed to measure pressure gradients throughout a vessel, and it is recommended that the lesion with the greatest pressure‐gradient is treated first. After this, it is recommended that the FFR measurement and Pullback is repeated again, as there may be a change in significance of the residual stenoses. This is pertinent, as a distal stenosis not previously hemodynamically significant may become significant once a proximal stenosis is relieved.

Resting indices are less prone to artifact by lesion cross‐talk and thus may be preferable when there are multiple coronary lesions. Unlike FFR, the iFR value is less affected by the presence of more distal disease and “cross‐talk” between serial lesions. The iFR value calculated at any given location in the vessel is the iFR value caused by all the stenoses proximal to the pressure sensor. If a stenosis can be removed perfectly, with minimal pressure loss across the treated segment, then it should be possible to predict the post‐PCI iFR value (Figure 7.6) [43].

Figure 7.6 iFR Pullback can be used to predict post‐PCI iFR values. (a) The LAD has a moderate proximal stenosis and a long segment of moderate‐to‐severe disease. iFR is positive for ischaemia. (b) The live beat‐to‐beat iFR pullback suggests the proximal stenosis contributes 0.05 iFR units while the mid‐vessel disease is 0.06. (c) Removing the mid‐vessel disease is predicted to give an iFR of 0.91, above the treatment threshold. (d) Repeat iFR post stenting the mid‐vessel disease confirms an iFR of 0.91; pullback shows that proximal disease continues to contribute 0.05 pressure drop and can be deferred.

When performing an iFR‐Pullback, continuous beat‐to‐beat iFR values are calculated and plotted as continuous line. The shape of the line describes focal and diffuse disease. As the pressure sensor crosses a stenoses, a step in pressure trace will be observed.

Virtually removing a given stenosis can be performed by studying the pressure pullback trace and the numerical values measured. A given stenosis can be discounted from the trace, and the residual pressure gradients will remain to compute a post‐PCI iFR value [22,23]. As the computer algorithms have iterated, it has become easier to remove different segments from the pressure trace to model the hemodynamic effect of a given intervention. In each case, the stented segment would remove the pressure loss in the area of interest, but the residual disease continues to cause the pressure loss. It should be noted that stent expansion may be not be perfect and if a post‐PCI result does not match the predicted iFR result, then attention should be paid to stent optimization. Repeat iFR pullback will reveal relevant residual pressure gradients.

The speed of wire movement will determine if the step‐change is very shallow or very steep. When a consistent pullback speed is utilized, steep steps tend to represent focal disease while more shallow changes represent diffuse disease. Even in focal disease, the pressure steps can be small or large; a single stenosis may in fact cause several different areas of pressure change. If the wire is moved too quickly through the lesions such granularity may be lost, but the important aspect of observing the step will remain true.

The iFR‐Pullback trace is made up of two separate lines: (i) the dotted line, which represents the raw iFR value at the given location, and (ii) the solid line which represents a processed value that prevents the iFR‐Pullback line from falling below what has already been plotted. In many cases, both lines overlap and may not be visible. In some cases, it is possible to see that the dotted line falls significantly at the outlet of a stenosis before returning to the solid line. This represents a pressure‐recovery phenomenon as the sensor moves through a stenosis. It should also be noted that if there is a whipping artefact or ectopic, then the solid line may move artificially. Since it will always continue from the highest value, then these events can make the pressure trace less reliable for calculation of post‐PCI iFR results. In these situations, the dotted line should be assessed as this will return to the true live value.