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Plein and Motwani1, in their editorial comment on a study by Ebersberger et al.2 in the journal, state that because the ischaemia threshold of fractional flow reserve (FFR) was initially ‘validated’ in patients with unequivocal stress-induced ischaemia by stress SPECT/echocardiogram (ECHO),3 subsequent studies using FFR as a reference standard to assess the accuracy of other non-invasive methods of assessment of stress-induced myocardial ischaemia such as CMR ‘are somewhat paradoxical and create questionable circular arguments’.

I suggest an alternative conclusion based on the following reasoning: the initial ‘validation’ study3 was really to determine the ischaemia threshold of FFR in patients with large and severe enough stress-induced ischaemic defects unequivocally detected by a known technique, stress SPECT/ECHO. Once this was determined, FFR outperforms stress SPECT/ECHO in at least three scenarios, where it is currently useful clinically. First, the pressure wire can inherently accurately measure FFR in smaller vessels supplying smaller myocardial segments than in the initial validation study, below the detection limits of either stress SPECT/ECHO of high quality without artefact. Secondly, in patients susceptible to artefacts with either SPECT/ECHO, the accuracy of FFR remains unaffected. Thirdly, in balanced three-vessel obstructive coronary artery disease (CAD) underestimated by SPECT because it measures intervessel relative flow reserve, the accuracy of FFR remains unaffected because it measures vessel-specific relative (to assumed maximal) flow reserve.

Issue Section:
Letters to the Editor
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