the index procedure是一种什么手术或程序？

procedure是指PCI（经皮冠状动脉介入治疗）或CABG（冠状动脉旁路移植术）。请问the index procedure是一种什么手术或程序？

原文：Coronary procedure-related MI ≤ 48 hours after the index procedure is arbitrarily defined by an elevation of cTn values > 5 times for type 4a MI and > 10 times for type 5 MI of the 99th percentile URL in patients with normal baseline values. 

即这个玩意后48小时内cTn5或10倍的升高就是4a或5型MI。