Our research results found that the incidence of postoperative AMI in elderly hip fracture patients combined with CHD is 11.1%, and 68.5% patients have atypical clinical manifestations. Age, diabetes, anemia at admission, reginal anesthesia, chronic kidney disease, and intraoperative bleeding are risk factors for postoperative AMI. Prognostic studies have found that the mortality rate of 2-year in AMI patients is higher than that of non AMI patients. What else, among all patients with AMI, the 2-year survival rate of patients with ECG ST-segment elevation is lower than patients without ECG ST-segment elevation.

Our research found that age is a risk factor for postoperative myocardial infarction in patients with hip fractures. The study by Harris et al. [7] in 2022 suggested that older age was associated with the occurrence of AMI during non cardiac surgery. At the same time, a literature in 2013 also mentioned that the incidence rate of perioperative myocardial infarction increased significantly with age growth [8], which is similar to our research findings. For elderly patients, especially those over the age of 80, they often have weakened immunity, degenerative changes in important organ functions, multiple coexisting diseases and decreased reserve and compensatory abilities. These can lead to reduced tolerance for anesthesia and surgery, resulting in an increase of postoperative complications.

Our study results found that diabetes is a risk factor for AMI after hip fracture surgery. Previous studies have shown that diabetes is generally considered to be a risk factor for AMI during perioperative period of non-cardiac surgery [9]. A literature in 2015 mentioned that diabetes did not seem to be a risk factor for other postoperative complications except for postoperative myocardial infarction [10]. Patients with diabetes often have poor cardiovascular status, coronary stenosis is more serious than patients without diabetes, and they often have multi vessel and multi segment lesions. The operation process including anesthesia, tissue damage, bleeding, hypoxia, will put the patients in a state of stress. These factors can cause the increase of catecholamine hormone, platelet activation and hypercoagulable state, ultimately lead to the occurrence of acute complications such as arterial atherosclerotic thrombosis, coronary spasm, autonomic nervous instability, systemic inflammatory reaction and oxidative stress, finally lead to AMI [10]. For patients with diabetes, as their clinical manifestations are often not obvious when acute complications occur, they should monitor blood glucose in a timely manner, control blood glucose, keep their blood glucose up to the standard, reduce the fluctuation of blood glucose, and perform the coronary artery CT examination when possible. Fully grasp the patient's preoperative condition and prevent postoperative AMI.

Our research results found that preoperative hemoglobin and intraoperative bleeding are also risk factors for postoperative AMI. Different from other research, we treated hemoglobin as a continuous variable. The higher hemoglobin level at admission, the lower probability of postoperative myocardial infarction (OR 0.974 95% CI 0.958–0.989). A research published in JAMA in 2007 suggested that preoperative anemia was associated with an increased risk of postoperative cardiac events in elderly patients undergoing major non cardiac surgery [11]. The 2015 meta-analysis mentioned a correlation between preoperative anemia and postoperative AMI [12]. A retrospective study in 2023 showed that preoperative anemia was associated with the development of postoperative complications, such as cardiovascular events [13]. The mechanism can be explained by a decrease in the amount of hemoglobin in the blood and a decrease in the oxygen-carrying capacity of the blood due to excessive bleeding or malnutrition after a hip fracture, ultimately resulting in myocardial hypoxia; Anemia can also cause tachycardia, increase myocardial oxygen consumption, further exacerbating the imbalance between oxygen demand and supply, finally lead to acute myocardial injury/myocardial infarction. The mechanism of intraoperative bleeding causing AMI is similar to anemia at admission. Jungchan Park et al. in 2021 suggested that intraoperative hemoglobin levels below 7 g/L or less than 50% were defined as significant intraoperative bleeding. Compared to the group without significant intraoperative bleeding, significant intraoperative bleeding group had an increased risk of AMI (OR 1.58, 95% CI 1.43–1.75) [14]. Our results treated intraoperative bleeding as a continuous variable, and the risk of AMI was 1.003 (95% CI 1.001–1.004). The mechanism behind this can be explained as excessive bleeding during hip fracture surgery leading to intraoperative anemia, as well as hypotension and hypoperfusion, ultimately leading to myocardial ischemia, hypoxia, and necrosis. Therefore, for patients who develop anemia at admission, timely blood transfusion and nutritional support treatment should be given. Meanwhile, Jungchan Park et al.'s study mentioned that intraoperative hemoglobin levels associated with myocardial injury after non-cardiac surgery were 9.9 g/dL, and maintaining intraoperative hemoglobin levels above 9.9 g/dL can help prevent postoperative AMI [14].

In our research findings, reginal anesthesia was a risk factor for postoperative AMI. Reginal anesthesia can affect the activity of the sympathetic nervous system, hinder vascular contraction function, and lead to intraoperative hypotension, when blood pressure drops below 65 mmHg, it often affects cardiac perfusion in a short period of time, leading to myocardial injury or myocardial infarction. Research has shown that using an average low dose of 6.5 mg spinal anesthetic can effectively generate intraoperative comfort and motor block, and the incidence of hypotension is lower compared to a high dose of 10.5 mg [15].

Our research findings suggest that CKD was a risk factor for postoperative AMI. A study published in the Journal of Trauma in 2022 showed that CKD patients who underwent hip fracture surgery had a 1.96 times higher risk of cardiovascular events compared to non CKD patients after adjusting for factors such as age, fracture type, and gender (OR 1.96; 95% CI 1.23–3.12), including pulmonary embolism, angina, myocardial infarction, heart failure, arrhythmia, stroke, and death [16]. In our research results, the risk of postoperative AMI in CHD patients with CKD was 2.332 times higher than that of non CKD patients (95% CI 1.383–3.934). CKD and cardiovascular disease have the same risk factors, which includes hypertension and diabetes. Patients with CKD are in long-term of water and sodium retention, the activation of the renin angiotensin aldosterone system (RAAS) and sympathetic nervous system can cause hypertension, leading to left ventricular hypertrophy, left ventricular enlargement, and diastolic dysfunction. In hip fracture surgery, due to factors such as anesthesia, hypotension, inadequate perfusion, anemia, it is more likely to cause myocardial ischemia and hypoxia damage [17]. On the other hand, CKD often leads to vascular calcification and arteriosclerosis [18], resulting in vascular stenosis, insufficient myocardial blood supply, and increased risk of myocardial ischemia, hypoxia, and necrosis after surgery.

Our results found that the incidence of postoperative AMI in elderly patients with hip fractures and CHD was 11.1%. Among these patients, 68.5% had no typical clinical manifestations of chest pain, 26.1% had ECG ST-segment elevation and 73.9% had non ECG ST-segment elevation. Previous studies have mentioned that for patients without CHD, the incidence of perioperative AMI can reach 3%, while for high-risk patients with CHD, the highest incidence can reach 33% [19]. A retrospective case–control study in 2012 showed that 13.8% of elderly patients with hip fractures experienced perioperative AMI, of which 75% were asymptomatic [20]. A 2022 literature mentioned that the incidence of non cardiac perioperative AMI ranges from 0.01 to 10% [7]. These are all similar to our findings. The main mechanism of myocardial infarction in elderly hip fracture patients is type 2 myocardial infarction, caused by an imbalance between myocardial oxygen supply and demand, rather than plaque rupture leading to thrombosis. Our results can also prove this point of view. Advanced age leads to reduced myocardial cell repair ability and increased vulnerability. Both diabetes and CKD can lead to lumen stenosis. Narrow lumen combined with anemia is more likely to cause imbalance between supply and demand, thus causing AMI. Therefore, correcting anemia is the core method of internal medicine treatment for elderly hip fracture patients.

. One study in 2021 showed that patients over 50 years of age with perioperative myocardial infarction had a 1-year mortality rate of 13.9% and a 3-year mortality rate of 21.7% after orthopedic surgery [21]. Our results showed that 1-year mortality was 10.1% and 2-year mortality was 20.7% in patients with AMI, which was significantly higher than 1-year mortality of 3.2% and 2-year mortality of 11.5% in patients without AMI. Many studies have shown that an increase in perioperative troponin concentration is associated with an increase in long-term mortality [22,23,24]. A 2009 study showed a significant difference in one-year mortality between elderly patients with troponin I > 0.3 and 0.03–0.3 who undergoing emergency orthopedic surgery [25], Chong et al. [26] concluded that troponin I only predicts 1-year mortality, not 2-year mortality, in patients undergoing emergency orthopedic surgery. In our research, there is no significant differences between patients with troponin I levels 0.04–0.4 and troponin I levels > 0.4. We think this may be due to our sample size is small or troponin I level 0.4 is not an appropriate point, which requires further study with large sample size. Meanwhile, our research also found that among AMI patients, patients with ECG ST segment elevation had a higher 2-year mortality rate. It can be explained by the fact that patients without ST segment elevation in ECG have not completely blocked vascular lumens, have smaller area of myocardial necrosis, and can achieve better prognosis after correcting risk factors such as anemia and hypoperfusion. What else, we found that among all patients with AMI, there was no significant difference in 2-year survival between those with significant clinical manifestations and those without, which can be explained by the fact that patients with asymptomatic myocardial infarction are not due to mild condition, what else, these patients often have more comorbidities, such as diabetes, dementia, cerebrovascular diseases, and the use of analgesic and sedative drugs, finally lead to insignificant clinical manifestations. However, such patients also have a higher risk.

Firstly: since this is a retrospective study, some data may have selective bias. Secondly: although the internal validation of the nomogram prediction model demonstrates good discriminability, calibration, and clinical practicality, additional databases are needed for external validation, especially from other distributions. Thirdly: the relatively small number of patients in the AMI group may lead to some controversy in our conclusion. In the future, larger sample size studies are needed to carry out.