Abstract
Background: Extent and severity of myocardial hypoperfusion correlates with patient outcome in CHD. The nuclear substudy of COURAGE indicates patients with ischaemia 10% total myocardial volume benefit from PCI. Radionuclide perfusion imaging allows complete myocardial coverage throughout the R-R interval with tracer uptake regionally distributed in proportion to blood flow. Cardiac Magnetic Resonance (CMR) perfusion sequences provide myocardial coverage at discrete time periods in the cardiac cycle. Myocardial blood flow varies during the cardiac cycle and may impact upon how accurately different perfusion imaging techniques quantify physiologically significant ischaemia. Methods: Simultaneous pressure wire and coronary catheter pressure recordings were sampled in 22 patients. Instantaneous Fractional Flow Reserve (iFFR) and Rest TransStenotic Pressure Ratios (iRTPR) were interrogated at 5 millisecond (ms) intervals. Analysis of these indices were performed representative of Single Photon Emission Computerised Tomography (SPECT) and CMR myocardial perfusion sequences. Comparison with conventional FFR (cFFR) and conventional RTPR (cRTPR) were assessed. Results: Maximum iFFR differed from minimum iFFR by 0.20 / 0.17 over 234 / 131ms. Similarly maximum iRTPR differed from minimum by 0.16 / 0.13 over 247 / 136ms. Difference between maximum and minimum iFFR and iRTPR correlated with cFFR(r 0.85) and cRTPR(r 0.97) respectively. For SPECT, the ratio of cFFR to cRTPR moderately correlated with cFFR(r 0.58). For CMR, hypothetical phase specific slice averaged iFFR differed from cFFR by 0.08 / 0.09 and iRTPR from cRTPR by 0.05 / 0.08 in the diastolic period. The largest difference between period averaged iFFR & iRTPR values was 0.08 / 0.10 and occurred at lower cFFR & cRTPR values during the diastolic period. Conclusions: iFFR, iRTPR, and the difference and ratio between each varies during the cardiac cycle. This has considerable implication for quantification of physiologically significant total myocardial ischaemic burden by myocardial perfusion imaging, particularly pertinent to CMR. Variation of tracer uptake during the cardiac cycle may aid diagnosis in the setting of balanced ischaemia when adopting SPECT.
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