Coronary heart disease (CHD) has become the primary cause of death worldwide in both developed and developing countries.1 In China, the mortality rates of CHD in urban and rural residents were 122.04/100,000 and 115.32/100,000, respectively, in 2017.2 The Global Burden of Disease study reported that ischemic heart disease led to approximately 1.7 million deaths in China in 2016.3 CHD has become the leading cause of death in the Chinese population and a serious public health problem. In particular, acute coronary syndrome (ACS) is the chief and serious type of CHD, leading to ventricular remodeling and heart failure.4 Previous studies have also found that patients with ACS have a higher incidence of renal dysfunction.5,6 In recent years, the concept of cardiorenal syndrome (CRS) has been widely recognized, including a series of diseases involving the heart and kidney.7,8 Acute or chronic dysfunction of the heart or kidney may lead to acute or chronic dysfunction of another organ. In 2019, the American Heart Association issued a scientific statement on the classification, pathophysiology, diagnosis, and treatment strategy of CRS, which was divided into 5 subtypes.8 This is very important for the prognosis of patients with type 1 CRS by early detection and the treatment of renal dysfunction in patients with ACS. However, traditional laboratory indicators for kidney function, such as blood urea nitrogen and creatinine, are limited in the early diagnosis of acute kidney injury (AKI) because of their low sensitivity and specificity.9 In addition, diuretic treatment for acute heart failure (AHF) makes the evaluation of renal function more complicated. Recent studies have revealed that markers targeted for renal tubular injury have better early evaluation significance for patients with AKI, especially for patients with AHF, than traditional laboratory indicators. Among them, neutrophil gelatinase–associated lipocalin [neutrophil gelatinase–associated lipocalin (NGAL)], urinary kidney injury molecule-1 (KIM-1), and interleukin-18 (IL-18) are increased in the early stage in patients with AKI and can be used as biological markers of early renal dysfunction.10,11 However, the role of these renal tubular injury markers in type 1 CRS has not been clearly studied. Nicorandil can significantly alleviate the occurrence of angina pectoris,12 reduce the incidence of cardiovascular events, and improve the prognosis in patients with ACS. In patients with heart failure, nicorandil could reduce all-cause mortality, increase cardiac pump function,13 and improve left ventricular diastolic function.14 These protective effects of nicorandil might result from the dual vasodilation of coronary and peripheral arteries to increase coronary blood flow, reduce preload and afterload, and alleviate myocardial injury and cardiac remodeling.15 In addition, some studies have found that nicorandil can provide additional renal protection for chronic kidney disease or significantly reduce the incidence of contrast-induced nephropathy in patients undergoing coronary angiography or percutaneous coronary intervention.16 The possible mechanism of nicorandil is the reduction of vascular and renal dysfunction17 or the neutralization of nephrotoxicity by normalizing the TLR4/MAPK P38/NFκ-B inflammatory cascade.18 Moreover, oral nicorandil reduced cardiac death in hemodialysis patients undergoing coronary revascularization.19 Therefore, it is very interesting to study the effect of nicorandil on renal function in patients with type 1 CRS. This study will explore the effect of nicorandil on cardiac and renal function in Chinese patients with type I CRS caused by ACS.