Does long-term high-altitude exposure reduce myocardial injury and incidence of acute kidney injury following cardiac surgery? A propensity score matched study

Objective Chronic high-altitude exposure has been shown to reduce ischemia-reperfusion injury in animal experiments. The objective was to evaluate the clinical protective effect of long-term high-altitude hypoxic exposure for patients undergoing cardiac surgery with cardiopulmonary bypass. Methods this A propensity-score-matched analysis was to estimate the and

Does long-term high-altitude exposure reduce myocardial injury and incidence of acute kidney injury following cardiac surgery? A propensity score matched study to apply in clinical practice [1]. Remote ischemia preconditioning (RIPC) has been shown to be protective in animal studies and feasible in clinical practice, however, the results of recent clinical studies have been controversial [2][3][4][5]. Therefore, the protective effect of chronic ischemic and hypoxic preconditioning has garnered much attention [6].
The high-altitude residents live in chronic and potent hypoxic environment. The cardioprotective effect of chronic high-altitude hypoxia against various manifestations of acute I/R injury has been con rmed for a long time [7]. Animal experiments have shown that adaptation to chronic hypoxia can increase the tolerance to myocardial infarct, contractile dysfunction and ventricular arrhythmias [8,9]. Coincidentally, the mechanisms of protection conferred by long-term high-altitude hypoxia (long-lasting effects) and preconditioning (short-lived preconditioning) seem to share the similar signaling pathways, including hypoxia-inducible factor-1 (HIF-1) and nuclear factor E2-related factor (2Nrf2) [6,10]. The facts that make them different are that the myocardial protective effect of long-term high-altitude hypoxia lasted 5 weeks longer than that of preconditioning [11] and long-term high-altitude hypoxia also affects the expression of some proteins associated with maintaining oxygen homeostasis [12].
Many epidemiological surveys showed that patients with coronary heart disease who have been exposed to high altitudes for a long time had better survival than those in plain areas [13][14][15] Exclusion criteria included: (1) patients born in low-altitude areas but living in high-altitude areas after adulthood; (2) patients who reside between 1500 meters and 2500 meters above sea level; (3) incomplete electronic records (including data used for propensity score matching, primary and secondary end points); (4) pregnancy; (5) emergency; (6) beating heart surgery.
A total of 2085 patients were enrolled in this study (Fig. 1). Detailed demographic information and surgical procedures, including complications and intensive care unit (ICU) data, were collected from the electronic medical record system.

Outcome Assessment And De nition Of Postoperative Complications
Outcome assessment and de nition of postoperative complications The primary outcomes include myocardial injury assessed by postoperative creatine kinase (CK)-MB and high-sensitivity cardiac Troponin I (hs-TnI) levels, as well as the incidence of acute kidney injury (AKI) assessed by KDIGO criteria [17]. Other outcomes such as requirement of intra-aortic balloon pump (IABP) support, new on-set stroke, in-hospital mortality, duration of mechanical ventilation, length of stay in ICU, and in-hospital length of stay were also assessed. Myocardial injury was assessed by the levels of CK-MB and hs-TnI upon arrival at the ICU. AKI was de ned according to KDIGO 2012 CKD Guideline as an increase in serum creatinine of ≥ 0.3 mg/dl (≥26.5 µM) within 48 h and an increase in serum creatinine of ≥ 1.5 times baseline known or presumed to have occurred within the prior 7 days [17]. New on-set stroke in this study was de ned as a patient who had been diagnosed by neurology or ICU doctors according to the patient's physical signs and brain computed tomography (CT) scan to ensure the accuracy of diagnosis. Duration of mechanical ventilation was calculated from the time of extubation in the ICU and the time of arrival at the ICU, and additional duration of mechanical ventilation if there was a record of re-intubation.

Statistical analysis
The data were analyzed with a SPSS 23.0 software package. Continuous data were expressed as mean ± standard deviation or median (25% percentile, 75% percentile), and compared with an unpaired t-test and Mann-Whitney U test, respectively. Categorical data were shown as frequency or percentages and compared with the χ 2 test or Fisher's exact test between the two groups. P < 0.05 was declared signi cantly.

Propensity Score Matching
In order to reduce the possibility of baseline and surgical covariate imbalance, propensity score matching was employed. Each highlander was matched to two lowlanders based on their propensity score, using the greedy matching protocol, and the xed caliper width is 0.020. Replacement was unacceptable in the matching. We used the standardized differences of the covariate means and proportions to assess the balance in the pairing after matching. They achieve an acceptable balance, absolute standardized differences in all baseline characteristics were < 10%. Adjusted baseline and surgical covariates were age, sex, body mass index (BMI), creatinine, left ventricular ejection fraction (LVEF), history of hypertension, history of diabetes, atrial brillation (AF), stroke, myocardial infarction (MI), heart failure (HF), peripheral arterial disease, chronic obstructive pulmonary disease (COPD), history of surgery, type of surgery, and cardiopulmonary bypass duration (Table 1).  Table 2. Based on the propensity score, 128 highlanders were 1:2 matched to 248 lowlanders (eight highlanders only matched to one lowlander). There were no statistically signi cant differences in baseline characteristics and surgical data after matching. Data are presented as the mean (SD) or number (%). Abbreviations: CABG = coronary artery bypass grafting. CHD = congenital heart disease. CPB = cardiopulmonary bypass.
The matched cohort analysis revealed that CK-MB and hs-TnI levels increased more in the lowlander group on arrival at the ICU [70.6 U/L (56.0, 92.6) vs 85.0 U/L (68.5, 113.5), P < 0.001; 6.1 ng/mL (3.3,11.2) vs 7.9 ng/mL (3.6, 14.1), P < 0.05, respectively] (Table 3). However, the incidences of IABP support after surgery, as well as new on-set stroke, were similar between the two matched groups. The highlander group had a lower rate of AKI (13.3% vs 21.8%, P < 0.05) ( Table 3). There was no signi cant difference in the duration of mechanical ventilation, length of stay in ICU, and in-hospital length of stay (P > 0.05) ( Table 3). Data are presented as the mean (SD), median (IQR) or number (%). CK-MB and hs-TnI were tested when patients entered the ICU. The reference ranges of CK-MB and hs-TnI were 0-25 U/L and 0-0.03 ng/mL, respectively. Abbreviations: CK-MB = creatine kinase. hs-TnI = high-sensitivity cardiac Troponin I. IABP = intra-aortic balloon pump. ICU = intensive care unit.

Main ndings
As far as we know, this is a study with the largest sample size to date to investigate the impact of longterm high-altitude exposure on outcomes of cardiac surgery under CPB. In the present study, we found that long-term high-altitude exposure before cardiac surgery could reduce the myocardial injury, e.g., the levels of CK-MB and hs-TnI after surgery compared to the lowlanders. Meanwhile, patients in the highlander group had a statistically signi cant lower risk of AKI than those in the lowlander group. Moreover, long-term high-altitude exposure was associated with lower mortality after cardiac surgery despite no statistical signi cance. These clinical ndings are consistent with previous studies where long-term high-altitude hypoxia has cardioprotective effect in animal models [8]. In addition, our study also showed that long-term high-altitude exposure might be effective for renal protection and decreasing the mortality after cardiac surgery.

Long-term High-altitude Exposure And Myocardial Injury
The cardioprotective effect of long-term high-altitude hypoxia against acute I/R injury has been known for a long time in animals. High-altitude hypoxia increased the tolerance of animal hearts to I/R injury determined as reduction of infarct size, and decreased the incidence and severity of arrhythmias [10]. It is necessary to verify that the cardioprotective effect of high-altitude hypoxic adaption remains in some clinical settings. In this retrospective study, myocardial injury was assessed by CK-MB and hs-TnI levels on arrival at the ICU and requirement of IABP after cardiac surgery. Except for IABP, the levels of CK-MB and hs-TnI were signi cantly different between the two matched groups, indicating that the cardioprotective effect of long-term high-altitude hypoxia also works in I/R injury induced during CPB.
This is consistent with previous experimental results [18], although previous experiments mainly studied ischemic heart disease such as coronary heart disease [19]. Furthermore, studies have indicated that Tibetans, the main component of the highlander group, who live at high altitude for generations are more tolerant to hypoxia, and they have fewer deleterious changes such as elevated hemoglobin and pulmonary hypertension [20]. To maintain heart function in chronic hypoxia environments, high-altitude natives conduct a metabolic adaptation that leads to an altered substrate preference, meaning their myocardial metabolism switches toward the use of glucose as the substrate when oxygen levels are diminished [21]. This change in metabolic substrate can reduce the defects in heart e ciency and function that occur during myocardial I/R.

Long-term High-altitude Exposure And Aki
Renal protection strategies such as ischemic/hypoxic preconditioning (mainly in rodents) have been published decades ago and their extraordinary effectiveness has raised the hope for powerful clinical tools to prevent acute kidney injury [22]. So far, translation to the clinical medicine has not been successful. Nevertheless, some clinical studies using RIPC strategies have drawn positive conclusions [23,24]. As described earlier in this study, the mechanism of long-term high-altitude hypoxia is similar to that of ischemia preconditioning [10]. This may be the reason why patients in the highlander group had a signi cantly lower risk of AKI than those in the lowlander group. It is worth noting that the protective effect of long-term high-altitude hypoxia may weaken or even disappear after reaching the plain area and breaking away from the hypobaric hypoxia environment. This might explain the diminished difference in renal replacement therapy between the two groups at long term follow up.
Mechanisms Of Organ-protective Effects Induced By Longterm High-altitude Exposure Although the protective effect of chronic high-altitude hypoxia has been veri ed by many experiments, its mechanism needs further study. In high-altitude hypoxia environment, sympathetic system is activated [25]. Catecholamines contribute to the induction of cardioprotection by long-term high-altitude hypoxia.
Studies have shown that the cardioprotective effect in a dog model of intermittent hypoxia can be completely prevented by administration of the 1-adrenoceptor antagonist metoprolol before each hypoxic session. Reactive oxygen species (ROS) also plays a very important role. Milano et al [26] suggested that repeated reoxygenation could produce better myocardial protection than chronic hypoxia. However, the duration of hypoxia exposure in native highlanders is much longer than that of repeated reoxygenation or chronic hypoxia administration. The intensity of hypoxia exposure might produce a su cient mechanism of myocardial or other organic protection in highlanders. Furthermore, the endogenous production of nitric oxide (NO) and its therapeutic effects have been con rmed by a number of studies in I/R [27]. Application of NO or NO donors before heart ischemia can reduce the severity of myocardial IR injury, thereby reducing the infarct size and endothelial dysfunction. Coincidentally, levels of NO were elevated in the circulation of Tibetans to increase the oxygen supply to cells when united with increased ventilation [20]. Nevertheless, the mechanisms of organ-protective effects induced by long-term high-altitude exposure need to be comprehensively investigated in the future.

Limitations
The present study has several limitations. First, the sample size of high-altitude patients is limited because of the relatively few populations in high altitude areas. Furthermore, as a retrospective study, some complications such as the occurrence of arrhythmia, might be missing for data collection. Therefore, we chose the myocardial injury markers collected on arrival at the ICU to evaluate the myocardial injury. Since not every patient had follow up cardiac enzymes after ICU arrival, we were unable to collect longer follow up laboratory data. Finally, we identi ed the long-term high-altitude exposure with improved surgical outcome after cardiac surgery, however, we could not identify the exact mechanism behind this, and if this was due to hypoxia/ischemia preconditioning. Further study in human myocardial tissue samples may help us to understand the mechanism.

Conclusions
This study demonstrated that long-term high-altitude exposure was associated with lower levels of CK-MB and hs-TnI, and a lower risk of AKI after cardiac surgery, suggesting that long-term high-altitude exposure may be bene cial for reducing I/R organic injury after cardiac surgery with CPB.  Figure 1 Study inclusion criteria.