On-line assessment of myocardial flow reserve

Summary

Myocardial flow reserve (MFR), the ratio between pharmacologically induced hyperaemic flow and baseline flow, is an index of myocardial perfusion. In this study two new techniques were validated by comparing the results with transonic (T) flow measurements. In eight dogs digital (Di) angiographic image processing was used to determine relative flow by maximal contrast density (Dmax) and contrast appearance time (Tarr). In five dogs an 0.46 mm Doppler (Dopp) flow velocity wire was inserted into either the left anterior descending artery (LAD) or the left circumflex branch (LCx) and flow velocity ratios were compared with T flow ratios. T flow and Dopp flow were measured just before injection of non-ionic contrast material (6 ml/s, 7 ml). Hyperaemia was induced by intracoronary administration of 12.5 mg HC1-papaverine. Dmax and Tarr at baseline and hyperaemia were derived from end-diastolic frames after mask-mode background subtraction (Tarr = time from onset of contrast appearance until 50% of Dmax is reached). For each region of interest (ROI), placed over the distal myocardial perfusion bed the average Dmax and Tarr were calculated. From these data DiMFR was obtained. DoppMFR was expressed by the ratio between hyperaemic average peak velocity (APV) and baseline APV. Results: LAD TMFR was 3.2 (±1.5) and LAD DiMFR was 3.6 (±1.0), n = 20, r = 0.70. LCx TMFR was 2.8 (±1.0) and LCx DiMFR was 3.5 (±1.7), n = 20, r = 0.78. The difference between TMFR and DiMFR can be explained by the presence of collateral circulation between LAD and LCx which affected DiMFR measurements more than it did TMFR measurements. After insertion of the Dopp wire a stable velocity signal was obtained in all experiments. The introduction of this wire did not result in a measurable flow reduction. DoppMFR was 2.7 (±0.7) and TMFR was 2.6 (±0.8), n = 12, r = 0.86. Conclusion: DiMFR is a better index of regional myocardial perfusion than TMFR because DiMFR incorporates the contribution of collateral flow. However, it is a rather complex technique with several theoretical limitations. DoppMFR is a simple technique to assess MFR, but its application is limited to large coronary branches and it does not account for the contribution of collateral circulation.