Instantaneous Wave-Free Ratio (iFR) in Diagnosing Coronary Artery Stenosis
The instantaneous wave-free ratio (iFR) is strongly recommended for evaluating the functional severity of intermediate coronary stenoses to guide revascularization decisions, with a Class I, Level A evidence recommendation from major cardiology societies. 1
What is iFR?
iFR is a physiological assessment method that:
- Measures the pressure gradient across a coronary stenosis during the "wave-free" period in diastole
- Calculated as the ratio of distal coronary pressure to aortic pressure during this specific diastolic period
- Does not require administration of vasodilators like adenosine (unlike FFR)
- Uses a diagnostic cut-off value of ≤0.89 to indicate hemodynamically significant stenosis
Clinical Role of iFR in Coronary Stenosis Assessment
When to Use iFR
iFR is recommended in the following scenarios:
- Patients with angina or anginal equivalent symptoms
- Angiographically intermediate stenoses (typically 40-90% for non-left main stenoses; 40-70% for left main stenoses)
- Undocumented ischemia (no prior stress test showing ischemia)
- To guide decision-making for percutaneous coronary intervention (PCI)
Clinical Decision-Making Algorithm
- Identify intermediate coronary stenosis on angiography (40-90% for non-left main; 40-70% for left main)
- Perform iFR measurement
- Interpret results:
- iFR ≤ 0.89: Hemodynamically significant stenosis → Proceed with PCI
- iFR > 0.89: Non-significant stenosis → Defer PCI (recommended)
Evidence Supporting iFR Use
Major clinical trials have established iFR's role:
- DEFINE-FLAIR and iFR-SWEDEHEART trials demonstrated that iFR-guided revascularization is non-inferior to FFR-guided revascularization for clinical outcomes 1, 2
- Deferring PCI when iFR > 0.89 is associated with low rates of major adverse cardiac events (MACE) 1
- iFR-guided strategy may be superior to FFR-guided strategy in patients with mild to intermediate stenosis for predicting MACE 3
Comparison with FFR
iFR offers several advantages over FFR:
- No need for adenosine administration
- Less patient discomfort (significantly fewer patients report chest discomfort during the procedure) 2
- Shorter procedural time
- Similar diagnostic accuracy (pooled diagnostic accuracy of 0.81) 4
However, there are important considerations:
- iFR is influenced by clinical factors including gender, diabetes mellitus, aortic stenosis, inflammation, and renal function 3
- Mismatch between iFR and FFR occurs in approximately 18% of lesions 3
Clinical Practice Guidelines
Both American and European guidelines strongly support iFR use:
ACC/AHA/SCAI (2022): Class I, Level A recommendation for using iFR or FFR to guide PCI decisions in patients with angina and angiographically intermediate stenoses 1
ESC (2024): Class I, Level A recommendation for coronary pressure assessment to evaluate functional severity of intermediate non-left main stenoses prior to revascularization; Class IIa, Level A for left main stenoses 1
Common Pitfalls and Caveats
Borderline values: Values near the cut-off (0.89) may warrant additional testing or clinical judgment
Left main assessment: For left main stenosis, consider combining iFR with IVUS for more comprehensive assessment 1
Discordance with angiography: Visual assessment of stenosis severity correlates poorly with functional significance - only ~35% of 50-70% stenoses and ~80% of 71-90% stenoses are functionally significant 1
Avoid routine assessment of all vessels: Systematic and routine wire-based coronary pressure assessment of all coronary vessels is not recommended (Class III recommendation) 1
Alternative indices: Other resting indices (Pd/Pa, dPR, RFR) have varying degrees of accuracy compared to iFR but lack robust clinical outcome data 1
In conclusion, iFR provides a reliable, adenosine-free method for assessing the functional significance of coronary stenoses and should be incorporated into clinical decision-making for revascularization in patients with intermediate coronary stenoses.