41% -67% of patients with acute ST-segment elevation myocardial infarction (STEMI) have severe stenosis in non-culprit vessels (\> 50% diameter stenosis). Compared with patients with single-vessel lesion, these patients with multi-vessel disease have a worse survival rate after percutaneous coronary intervention (PCI). Previous studies have shown that they not only receive more revascularization than the latter but also have a higher incidence of heart failure and more frequent electrical instability after myocardial infarction.

Early guidelines from European Society of Cardiology and the American College of Cardiology/American Heart Association discourage treating the non-culprit vessels in the acute phase. However, these recommendations are given mainly based on some small-sample retrospective studies.

With the use of second-generation drug-eluting stents and novel antithrombotic drugs, the clinical benefits from primary PCI and elective PCI have been greatly improved. Results from some small-sample prospective randomized studies have demonstrated that complete revascularization on STEMI patients with multi-vessel diseases is superior to a strategy of culprit-only revascularization. Especially, more recent two trials (PRAMI and CVLPRIT) further confirmed that complete revascularization in the acute phase can produce beneficial clinical results compared to culprit-only revascularization. However, stenting for these lesions in the two studies was decided based on angiographic findings regardless of whether the lesion caused myocardial ischemia or symptoms. Moreover, coronary angiography may underestimate and overestimate the functional severity of the lesion. Stent implantation preventively will lead to overtreatment, increasing additional cost and risk. Finally, not all such studies demonstrated positive findings. For example, the PRAGUE-13 study comparing angiography-guided complete revascularization with culprit-only treatment did not find that complete revascularization has more advantages. Therefore, the angiography-guided strategy for complete revascularization in STEMI patients with multi-vessel lesions remains questionable.

The clinical value of flow fractional reserve (FFR) as a gold standard for the assessment of coronary function in ischemia has been well confirmed in coronary intervention. Recently, three studies based on coronary functioning have shown that FFR-guided complete revascularization is superior to the strategy of only treating culprit lesion. However, clinical benefit of FFR is mainly from reducing the incidence of subsequent revascularization but not hard endpoints such as death. On the other hand, the time of treatment for non-culprit vessel was not completely consistent and not all patients in the FFR group received FFR measurements in these studies. Heterogeneities from these studies may weaken the clinical benefit of FFR in guiding complete revascularization in STEMI patients. Although the current guidelines recommend that non-culprit lesions be allowed for treatment when emergency PCI is performed on specific patients (Class IIa, Level A), it is still necessary to conduct studies targeted on these issues to further clarify the value of coronary functional approach in revascularization on non-culprit vessel of patients with STEMI. FFR also has certain limitations: first, FFR is an invasive test, which requires a pressure guide wire, in turn inevitably increases the cost of operation and may cause additional procedure-associated risks; secondly, drugs such as adenosine are required to maximize hyperemia of coronary artery, and these adverse drug reactions may increase the incidence of adverse clinical events, especially in the acute state of myocardial infarction.

Quantitative flow ratio (QFR) is a novel angiography-based method accurate assessing coronary physiological functions for deriving FFR without the use of pressure wire and induction of hyperemia. The three-dimensional reconstruction of the coronary arteries is performed through two angiographic images with an acquisition angle difference of \> 25 °. The QFR value is finally calculated based on the flow velocity obtained by a method of frame rate counting. In the past five years, a vast number of studies confirmed QFR is highly consistent with FFR in the diagnosis of myocardial ischemia, and the diagnostic accuracy of QFR is significantly higher than that of quantitative coronary angiography. Recent studies have also shown that, compared with conventional angiography-guided PCI, QFR-guided PCI for patients with stable angina pectoris has significantly reduced adverse cardiovascular events such as death and revascularization. Especially, QFR and FFR can accurately assess the coronary physiological function status of non-culprit lesions in emergency PCI patients, and its effectiveness is consistent with FFR applied to patients with stable coronary heart disease. Although previous studies provided the theoretical support, there are still poor data on QFR-guided non-culprit revascularization on STEMI patients with multi-vessel disease. Thus, we hypothesized that strategies for QFR to assess all blood flow limiting lesions and guide revascularization during emergency PCI would lead to better short-term and long-term clinical outcomes for STEMI patients with multiple vessel lesions, including improved left ventricular function, less secondary revascularization, less frequency of hospitalization, lower medical costs.