The time interval between myocardial infarction and normalisation of baseline coronary blood flow and maximal hyperaemic blood flow remains uncertain and may differ between patients. The impact of these temporary blood flow changes on iFR may resolve before the impact on FFR and the impact on iFR may need more than 2 weeks to resolve [4,5,6,7,8].
In the current study, the time interval between STEMI and nonculprit stenosis evaluation did not impact the overall agreement between iFR and FFR. However, among cases with disagreement between follow-up iFR and FFR, iFR was more likely than FFR to indicate hemodynamic significance < 5 days after STEMI whereas FFR was more likely than iFR to indicate hemodynamic significance after ≥ 5 days. The observations after ≥ 5 days probably resemble observations in stable conditions more closely, i.e., FFR is more often significant than iFR with similar outcomes of revascularisation guided by iFR and FFR [9, 10]. Within < 5 days after STEMI, both iFR and FFR may be affected, but in opposite directions, and the optimal method for nonculprit stenosis evaluation in this setting remains undetermined [4,5,6,7,8]. Also, the optimal time point for making this assessment remains to be established . Different methods can be applied taking timing and potential bias of the used method in relation to timing into consideration . In the acute or subacute setting, baseline flow may be increased and hyperemic flow may be decreased which may yield decreased iFR (overestimation of stenosis significance) and increased FFR (underestimation of stenosis significance) and these changes are expected to normalize over time although the time frame for this normalization is undetermined and may vary between patients [4,5,6].
In conclusion, in staged nonculprit stenosis evaluation after STEMI, iFR and FFR has an overall agreement that is comparable to that observed in stable patients. However, the time interval between STEMI and follow-up evaluation may impact agreement between iFR and FFR.