Injections during diastole reduce the CTFC by 6 frames. It is therefore important to standardize use of nitrates in studies that involve CTFC measurement, or to at least confirm that nitrate use is well balanced across arms of the trial. Administration of nitrates, which cause enlargement of the artery and the volume to be filled with dye significantly, increases the CTFC by approximately 6 frames. Alterations in catheter size do not affect CTFC measurements. Varying the force of dye injection may change the frame count by up to 2 frames, which is a relatively small and insignificant difference from a clinical trial perspective. Differences between observers are less than 0.75 frames and the correlation between observers is 0.97-0.99. Highly reproducible results are obtained with very low interobserver and intraobserver variability. For example, in the use of images acquired at 15 frames/s, frame counts are multiplied by a factor of 2 to derive the CTFC. Centers that use image acquisition at speeds other than the most widely used frame rate in the US of 30 frames/s need to adjust CTFC assessments accordingly. The number of frames from the first frame to the last frame when dye enters the standardized distal landmark is counted. ![]() The standardized distal landmarks are taken as the first branch of the posterolateral artery for the right coronary artery, most distal branch of the obtuse marginal branch for the circumflex, and the distal bifurcation for the LAD (also known as the 'whale's tail' branch of the LAD). The first frame taken for measurements is the frame in which dye touches both borders of the coronary artery and moves forward with at least 70% opacification of the vessel lumen. The frame count number after adjustment for vessel length is given the term 'corrected TIMI frame count'. ![]() A correction factor is required to compensate for the longer length of the left anterior descending artery (LAD) compared with the circumflex and right coronary arteries (the number of frames required for dye to traverse the LAD is divided by 1.7). In the CTFC method, the number of frames required for dye to reach a standardized distal landmark is counted.
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