Haemoglobin, as a main oxygen carrier, plays an important role in supplying oxygen to tissues. When haemoglobin decreases, body may increase cardiac output to maintain the normal metabolic demands of tissues, which increases work load of heart, and result in myocardial damage [4, 5].
The association between haemoglobin concentration and clinical outcomes
The relationship between hemoglobin concentrations and cardiovascular outcomes has been reported in a broad cohort of patients with ACS [15, 17, 23, 24]. But the association between low hemoglobin concentrations and adverse cardiovascular outcomes has not been settled.
Al Falluji et al  examined a database of discharge abstract information in patients admitted with myocardial infarction included 30,341 patients hospitalized in 1986 and (prethrombolytic era, n = 15,584) and 1996 (thrombolytic era, n = 14,757), anemia in this study were not found to have a higher mortality. In contrast, in a large database study of elderly patients with AMI, a powerful, albeit unadjusted, relationship between hematocrit on admission and all-cause 30-day mortality was found, there was a dose-response effect, with progressively lower survival rates with more profound degrees of anemia. Similar results was also reported in the research by Sabatine et al, which included 25419 patients with ST-segment elevation myocardial infarction (STEMI) and 14 503 patients with NSTE from 16 Thrombolysis In Myocardial Infarction trials, and a reverse J-shaped relationship between hemoglobin concentrations and clinical outcomes was found: in patients with STEMI, the cardiovascular mortality increased as hemoglobin concentrations fell below 14 g/dL and hemoglobin values rose above 17 g/dL, compared with hemoglobin concentrations between 14 and 15 g/dL; in patients with NSTE ACS, the cardiovascular death, myocardial infarction, or recurrent ischemia increased as the hemoglobin fell below 11 g/dL and hemoglobin values rose above 16 g/dL, compared with those with hemoglobin 15 to 16 g/dL.
But in Chinese AMI, the relationship between hemoglobin concentrations and cardiovascular outcomes has not been reported up till now. In this research of Chinese AMI, from single medical center, a reverse J-shaped (Figure 1) relationship between hemoglobin concentrations and clinical outcomes was also found, with patients at either end of the hemoglobin spectrum being more likely to have adverse clinical outcomes, significantly for those with haemoglobin concentrations <140 g/L, insignificantly but tendentiously for with haemoglobin concentrations >150 g/L. Compared with patients with haemoglobin concentrations 141-150 g/L, those with haemoglobin concentrations <140 g/L had more cardiac death, cardiogenic shock and heart failure, and those with haemoglobin concentrations >150 g/L also had more cardiac death, cardiogenic shock and postinfarction angina. Through Sabatine and his colleagues have reported a similar results, this report adjusted some important risk factors for MACEs, such as CK-MB and LVEF, which were not adjusted in Sabatine and his colleagues' research. Otherwise, our research had different clinical outcomes, which included cardiogenic shock and heart failure.
The worse outcomes observed in patients with AMI with either end of the hemoglobin spectrum might be explained by theory that anemia could decrease oxygen delivery to tissues, therefore attenuate the ability of collateral flow from nearby patent vessels to limit the extent of myocardial necrosis and peri-infarct ischemia, meanwhile anemia increase myocardial oxygen demand through necessitating a higher work load; and that higher haemoglobin concentration would increase blood viscosity, which decrease oxygen delivery to tissues .
The factors related to haemoglobin concentration
Anemia is common in elder people, about 12.5% of patients aged 71 years or older had anemia . Anemia accounted for 24% of geriatric hospitalized population in some reports . A positive relationship between haemoglobin concentration and serum albumin concentration was reported in the elderly . A direct relationship between haemoglobin concentration and serum albumin level was found in hemodialysis patients [28, 29]. Age was also found to be significantly associated with hemoglobin concentrations (P < 0.001) . But the association between age and haemoglobin concentration has not been reported in AMI patients.
In this paper, the factors related to haemoglobin concentrations were analysed. Age, diastolic pressure, TG, albumin and LVEF were found to be significantly associated with hemoglobin concentration (zero-order correlation probability < 0.01); After careful controlling these relevant factors, age, creatinine, and albumin were significantly associated with hemoglobin concentration (partial correlation probability < 0.01). The older, those with higher creatinine level and those with lower albumin level were more likely to have lower hemoglobin concentrations. Hemoglobin concentration falls with age and creatinine increase, and rises with albumin increase.
There are several possible explanations for these findings. First, elderly people might easily suffer from anemia because hematogenous function declines with aging [30–32]. Second, albumin is a marker of nutrition. Low albumin level means protein-calorie malnutrition to some extent [33, 34]. Shortened red blood cells survival, decreased erythropoietin secretion by kidney, and concurrent deficiencies of iron, pyridoxine and folate have been reported to contribute to anemia .
Although the association between hemoglobin concentrations and adverse cardiovascular events was demonstrated in this and other researches [17, 18, 35], the beneficial effect of transfusion on clinical outcomes remain to be settled.
The studies of transfusion effect on clinical outcomes of anemia have yielded conflicting results. In elderly patients with AMI, transfusion appeared to be beneficial if the hematocrit was < 33% . In contrast, Transfusion was reported to have adverse impact on the prognosis of acute coronary syndromes in some nonrandomized trails. Transfusion in anemic patients admitted with acute coronary syndrome led to a significant increase in 30-day recurrent MI or death [19, 20], especially for NSTE ACS . Therefore, transfusion effect on clinical outcomes of anemia remains to be further investigated.
The limitation of this study
As this is a retrospective study, some potential limitations of this study should be considered. The present study was from single large Chinese medical center, which might limit generalizability of our findings. Management strategy was not included in variables. There may be some differences among groups in management strategy, as was reported in some studies . Differences in treatment among the groups might have the potential to confound our analyses. The cause of anemia in patients in the present study was not known, the different causes might have residual confounding. Because the active bleeding was not included in this study, the anemia exactly means chronic anemia. Although we cannot rule out the possibility that we were unable to adjust for this known and other unknown confounders, given the breadth of covariates adjusted for in the present analyses, the impact is likely to be small.
Since this data was from single large center, which avoid the bias from different centers, unity of data was guaranteed.