Normal Coronary Flow Reserve (CFR) Values
A normal CFR is ≥2.7 in adults with chest pain syndromes and cardiovascular risk factors undergoing cardiac catheterization with angiographically normal vessels, though early animal studies suggested values of 3.5 to 5.0. 1
CFR Values in Different Clinical Contexts
Adults with Cardiovascular Risk Factors
- In patients with chest pain syndromes and CAD risk factors but angiographically normal coronary arteries, the normal CFR is 2.7, which reflects some degree of patient variability and underlying microvascular disease 1
- This lower value compared to animal models (3.5-5.0) indicates that cardiovascular risk factors inherently impair microvascular function even without visible epicardial disease 1
Adults with Specific Comorbidities
- Patients with essential hypertension and normal coronary arteries have reduced CFR, partly due to myocardial hypertrophy and abnormal microvasculature 1
- Patients with aortic stenosis and normal coronary arteries also demonstrate reduced CFR, related to hypertrophy and microvascular abnormalities 1
Clinical Thresholds for Abnormal CFR
Guideline-Based Cutoffs
- CFR <2.0 is definitively abnormal and indicates impaired coronary vascular reserve, representing the inability to increase coronary flow above 2 times resting flow 1, 2, 3, 4
- CFR ≤2.0 to ≤2.5 (depending on methodology) serves as the marker of microvascular dysfunction per American Heart Association and American College of Cardiology guidelines 2
- A Doppler-derived CFR <2.5 in non-obstructive CAD indicates an abnormal microcirculatory response, corresponding to thermodilution-derived CFR <2.5 per European Society of Cardiology guidelines 2
Prognostic Implications
- Each 0.1 unit reduction in CFR is associated with proportionally increased mortality (HR 1.16 per 0.1 unit) and MACE (HR 1.08 per 0.1 unit) 5
- Abnormal CFR is associated with 3.78-fold higher all-cause mortality and 3.42-fold higher MACE across diverse patient populations 5
Relative CFR (rCFR) Normal Range
- The normal range for relative CFR (rCFR) is 0.8 to 1.0, calculated as the ratio of CFR in the target vessel to CFR in a normal reference vessel 1
- rCFR is independent of aortic pressure and rate-pressure product, making it useful when an adjacent non-diseased coronary artery is available 1
- rCFR >0.8 may have prognostic value similar to negative stress testing 1
Important Physiological Factors Affecting CFR
Hemodynamic Influences
- Tachycardia reduces CFR by 10% for each 15-beat increase in heart rate, as it increases basal flow and decreases hyperemic flow 1
- CFR is altered by changes in basal or hyperemic flow, which are influenced by hemodynamics, loading conditions, and contractility 1
Measurement Location Variability
- CFR varies significantly along the length of the coronary artery, particularly in patients with impaired TIMI myocardial perfusion grade (0/1), where distal CFR is 0.11 units higher than proximal CFR (P=0.026) 6
- In patients with normal TIMI perfusion grade (2/3), there is no significant difference between proximal and distal CFR 6
Clinical Interpretation Caveats
Limitations of CFR as a Standalone Measure
- CFR is a combined measure of both epicardial and microvascular resistance, so when abnormal, it cannot distinguish which component is affected 1
- Clinicians are reluctant to use CFR as the sole indicator of lesion significance except when it is normal 1
- CFR is best used to assess the microcirculation in the absence of epicardial artery narrowings, not for assessing stenosis significance 1
When to Combine CFR with Other Indices
- Combining CFR with FFR and IMR provides complementary diagnostic information on epicardial CAD and microvascular function 1
- In patients with non-obstructive atheroma (FFR >0.8), an impaired CFR and increased IMR (≥25) indicates coronary microvascular disease 1, 2
- Discordance between normal FFR and abnormal CFR may indicate diffuse atherosclerotic CAD causing "low-flow" ischemia and may be prognostically important 1