The original definition of the TFGs from the TIMI 1 study in 1986 are as follows (1):
Grade 0: No perfusion. No antegrade flow beyond the point of occlusion. Grade 1: Penetration without perfusion. Contrast material passes beyond the area of obstruction but fails to opacify the entire coronary bed distal to the obstruction for the duration of the cineangiographic filming sequence. Grade 2: Partial perfusion. Contrast material passes across the obstruction and opacifies the coronary distal to the obstruction. However, the rate of entry of contrast material into the vessel distal to the obstruction or its rate of clearance from the distal bed (or both) are perceptibly slower than its flow into or clearance from comparable areas not perfused by the previously occluded vessel. Grade 3: Complete perfusion. Antegrade flow into the bed distal to the obstruction occurs as promptly as antegrade flow into the bed proximal to the obstruction, and clearance of contrast material from the involved bed is as rapid as clearance from an uninvolved bed in the same vessel or the opposite artery.
Recently, several groups have begun to define "TIMI grade 3 flow" as opacification of the coronary artery within three cardiac cycles (5,6). As the definition above shows, this is not how TIMI grade 3 flow was originally defined (1). This substantial departure from the original definition of TIMI grade 3 flow will result in much higher rates of TIMI grade 3 flow. In the majority of patients with TIMI grade 3 flow in thrombolytic trials (TIMI 4, 10A, and 10B) (7-9), it requires just under 1 s (26.8 ± 9.1 frames or 0.9 s, n = 693) or one cardiac cycle to traverse the length of the artery. Obviously, increasing the time for dye to go down the artery by a factor of three (i.e., from one to three cardiac cycles) that is permissible to qualify for TIMI grade 3 flow greatly increases the rate of TIMI grade 3 flow. Data from the TIMI Angiographic Core Laboratory suggests that the "three cardiac cycle" definition of TIMI 3 flow, results in an approx 10% increase over the original definition of TIMI grade 3 flow (10).
It is possible that TIMI grade 1 flow may sometimes be classified as TIMI grade 2 flow. In the TIMI Angiographic Core Laboratory, we follow the original definition in classifying TIMI grade 2 flow: the dye must reach the apex of the heart during the duration of filming. It is our experience that TIMI grade 1 flow comes in two varieties: one in which the dye barely penetrates the lesion and the other in which dye penetrates the lesion fairly well but the dye moves down the artery so slowly that the operator stops filming before it reaches the apex. We interpret the original definition of TIMI grade 1
flow very literally, and if the dye is not filmed as it reaches the apex, we classify this as TIMI grade 1 flow. It is unclear if other angiographic core laboratories would classify dye that may reach the apex, but is not filmed reaching the apex, as TIMI grade 2 flow. If they do classify flow in this fashion, then this may account for the higher rates of mortality that have been reported by other angiographic core laboratories for TIMI grade 2 flow.
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