R.J. Whitbourn* ¹,  M. Hayase, P.J. Fitzgerald and P.G. Yock

Cardiology Department, St Vincent's Hospital ¹, Vic. & Stanford University, USA.

Fractional flow reserve (FFR), (mean distal coronary pressure ¸ mean arterial pressure at peak hyperaemia) is an index for assessment of coronary stenosis severity.  Coronary flow reserve (CFR), derived from the ratio of hyperaemic to resting coronary blood flow velocities, also describes myocardial blood flow.  However, where diseases affect the myocardiac resistance vessels, CFR may not accurately reflect the effect f an epicardial stenosis on myocardial perfusion.  Relative flow velocity reserve (RFVR) is the ratio of target vessel CFR ¸ reference (normal) vessel CFR and theoretically should account for generalised microvessel dysfunction.  We designed a study to define the relationship between FFR, CFR, RFVR and coronary stenosis severity, in the presence of myocardial microvascular dysfunction.  Seven canines underwent determination of FFR using a pressure-recording guidewire (Wavewire^TM Cardiometrics, CA.), coronary flow velocities, CFR and RFVR, using a Doppler Flowire^TM (Cardiometrics, CA.) and percent stenosis severity using intravascular ultrasound.  Stenoses of varying  severity (0-100%) were achieved by placement of a coronary balloon occluder around the circumflex artery at thoracotomy.  A solution  containing ~ 3 x 10⁶, 25mm diameter microspheres was injected into the left coronary artery, to produce increasing levels of microvascular dysfunction (m'sphere 1 & 2). Coefficients of correlation (R) between FFR, CFR, RFVR and % stenosis were determined.

Conclusions: In this model of coronary epicardial and microvascular disease, the FFR and not CFR or RFVR, continued to accurately describe coronary stenosis severity in the presence of microvessel dysfunction.  These results suggest that where small vessel disease co-exists, FFR is a more accurate index of coronary stenosis severity.

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