1. Instantaneous Wave-free Ratio: a Word of Caution or Reliable Parameter?

    Changqing Yang Guoxin Fan, Xiaolong Qi, Shisheng He, Tongji Hospital, Tongji University School of Medicine Shanghai China

    TO THE EDITOR: We take great interest in the paper (1) by Nijjer et al with regard to instantaneous wave-free ratio (iFR) assessing improvement in coronary haemodynamics after percutaneous coronary intervention (PCI). However, we have some concerns about the invasive, pressure-only index, iFR.

    iFR, a novel resting index without hyperemia, is calculated over five heartbeats as the ratio of distal to proximal coronary pressures during the diastolic. The assumption is that the resistance during a particular part of diastole will be as low as the average resistance during the complete heart cycle in hyperemia and not be influenced by adenosine infusion.2

    Nevertheless, assumption is assumption, whilst numerical equation makes sense. Fluid-dynamics equation elucidates that iFR is able to predict the severity of stenosis (e.g. a 70% long LAD stenosis) only when friction is the predominant cause of energy loss within the stenosis.(2) That is to say, a short 50% left main stenosis, in which separation and turbulent flow are responsible for the energy loss, creates a negligible resting gradient with an extremely large hyperemic gradient. In the recent Resolve registry (3), a poor correlation was found between iFR and fractional flow reserve (FFR). Only if iFR was <_0.82 as="in=" _24="of=" the="_1539="_1539"" patients="patients" could="could" hyperemia="hyperemia" be="be" omitted="omitted" to="to" achieve="achieve" a="a" _95="_95" certainty="certainty" making="making" correct="correct" decision="decision" whether="whether" or="or" not="not" revascularize.="revascularize." so="so" our="our" question="question" raised="raised" again="again" is="is" ifr="ifr" equivalent="equivalent" ffr="ffr" _4="_4" it="it" even="even" instantaneously="instantaneously" measured="measured" name="name" suggests="suggests" totally="totally" independent="independent" pharmacological="pharmacological" vasodilatation="vasodilatation" because="because" calculated="calculated" an="an" average="average" value="value" and="and" strongly="strongly" influenced="influenced" by="by" hyperemia.2="hyperemia.2" we="we" really="really" appreciate="appreciate" this="this" prospective="prospective" observational="observational" study="study" applying="applying" assess="assess" improvement="improvement" coronary="coronary" haemodynamics="haemodynamics" after="after" pci.="pci." found="found" that="that" change="change" intervention="intervention" _0.20="_0.20" _0.21="_0.21" was="was" similar="similar" _0.22="_0.22" _0.15="_0.15" p="p" surely="surely" based="based" on="on" data="data" presented="presented" might="might" used="used" objectively="objectively" document="document" following="following" pci="pci" manner="manner" ffr.1="ffr.1" however="however" may="may" have="have" highly="highly" variable="variable" measurement="measurement" clinical="clinical" practice="practice" almost="almost" unachievable="unachievable" create="create" true="true" resting="resting" condition="condition" obscure="obscure" determine="determine" what="what" extent="extent" some="some" present.="present.">

    1 Nijjer SS, et al. Improvement in coronary haemodynamics after percutaneous coronary intervention: Assessment using instantaneous wave- free ratio. Heart. 2013

    2. Pijls NH. Fractional flow reserve to guide coronary revascularization. Circ J. 2013; 77: 561-569.

    3. A. J. Resolve: A multicenter study to evaluating the diagnostic accuracy of ifr compared to ffr. J Am Coll Cardiol. 2013;

    4. Fan GX and Xu YW. Is the instantaneous wave-free ratio equivalent to fractional flow reserve? J Am Coll Cardiol. 2013; 62: 943.

    Conflict of Interest: None declared
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