Cardiovascular disease (CVD) is a serious threat to human health, and early diagnosis and risk stratification of CVD will help to diagnose and treat these patients in an accurate and timely manner. Currently, the discovery and study of more cardiac biomarkers (BMs) provide new insights into the diagnosis and prognosis of CVD, which are briefly reviewed.
1. Markers of cardiac injury
(1) Myocardial enzyme profile and troponin
Previously, cardiac enzymes were limited to the diagnosis of acute myocardial infarction (AMI), but since the application of cardiac troponin (cTns) in clinical practice, there has been a deeper understanding in the diagnosis of myocardial necrosis, risk stratification of CVD, and prognosis. In 2000, the markers tested to define AMI in the United States included as specific markers of myocardial necrosis: cTns (cTnI, cTnT), creatine kinase isoenzyme (CK-MB), without the use of older non-specific cardiac markers such as creatine kinase, glutamate transaminase, glutamic aminotransferase, lactate dehydrogenase [1]. The diagnosis of AMI with elevated cTns has resulted in a lower diagnostic threshold for AMI and has greatly increased the number of AMI patients in the clinic.
Myoglobin, a ferrous hemoglobin protein present in cardiac and skeletal muscle, is the earliest marker of myocardial injury and can be detected 1 to 2 h after myocardial injury, but is not cardiac specific. It is also elevated in renal failure, skeletal muscle injury, trauma, and other diseases. Due to its high sensitivity, a negative test can be used for early exclusion of AMI.
CK-MB is rapidly released 4-6 h after myocardial injury and is more cardiac specific than myoglobin, but also contains 5% of the skeletal muscle component and is therefore elevated in non-cardiac conditions. Prior to the use of cTns, CK-MB was the “gold standard” for the diagnosis of AMI and was important in determining the extent of myocardial infarction (MI) and reinfarction.
The cTns is currently the most specific and sensitive biomarker for the diagnosis of myocardial injury and myocardial necrosis, and has important clinical applications in the risk stratification of acute coronary syndrome (ACS). cTns is elevated 4 to 12 h after myocardial injury and remains elevated for 4 to 10 d. cTns enters the blood stream after irreversible ischemic myocardial cell injury and remains elevated for a longer period of time. Because cTns has the advantage of a long diagnostic “window”, it can also be used as a diagnostic indicator of ST-segment elevation myocardial infarction (STEMI) in its more advanced stages and has a high sensitivity for the diagnosis of micro-MI. cTnI has a high myocardial specificity due to its unique amino acid sequence and is so far the only myocardial protein specific to the myocardium. There is no evidence that regenerating or diseased skeletal muscle in humans or animals expresses cTnI or detectable mRNA for cTnI. cTnI is sensitive to the presence of small focal reversible myocardial injury. cTnT is also present only in cardiac myocytes, and cross-reactivity with skeletal muscle TnT is <5% when measured with monoclonal antibodies, but in the blood of some dialysis patients with renal failure (renal failure), cTnT is elevated. cTnT is elevated, and cTnT isoforms are present in transverse muscle in conditions such as polymyositis and progressive myotonic dystrophy.
Myocardial tissue necrosis of 1 g can be detected by cTns, and even minimal elevation of cTns is associated with myocardial necrosis and increased proximal and distal mortality [2]. As early as 1997, Vecchia et al [3] reported elevated levels of cTns in patients with progressive heart failure (heart failure) without clinical AMI. cTnT was later reported by Missov et al [4] to be elevated in patients with heart failure, with levels paralleling the severity of heart failure. They suggested that elevated cTnT was associated with leakage of immunoreactive cTnT from the free pool of cardiomyocytes, and that cTnT was even detectable in 25-33% of patients with severe acute decompensated heart failure. most investigators believe that elevated cTnT reflects the presence of progressive myocardial necrosis in patients with end-stage heart failure, indicating a worse prognosis. sato et al [5] studied 60 patients with dilated cardiomyopathy , with persistently elevated cTnT concentrations despite conventional treatment in 17 cases, and these patients had larger hearts, lower left ventricular ejection fraction (LVEF), and lower survival rates. horwich et al [6] studied 238 patients with severe heart failure excluding myocarditis and AMI, and found 117 (49.1%) with elevated cTnI by the latest highly sensitive cTnI assay, whose blood flow poorer kinetics and lower LVEF. In conclusion, cTns may be elevated in patients with progressive severe heart failure, and although the mechanism is unclear, it is associated with prognosis, and most investigators believe it is related to low levels of myocardial necrosis.
In addition, elevated cTns is also seen in patients without ACS in renal failure, and an acute increase in cTns levels from baseline is associated with increased mortality [7,8]. Elevated cTns in patients with renal insufficiency may be associated with decreased renal clearance, and the mechanism is unclear. It has also been hypothesized that in renal failure the elevated cTns originates from skeletal muscle, but the evidence is insufficient. The presence of microscopic MI in patients with renal failure has been confirmed by pathological evidence. cTnT is mildly elevated because about 6-8% of cTnT and 3-5% of cTnI are present in the free form in the cytoplasm, and free cTns are released into the blood earlier during acute myocardial injury. cTnT is higher in the free form than cTnI in the cytoplasm because of its larger molecular weight. Thus, it is more commonly elevated than cTnI.
Due to the above-mentioned release and clearance kinetics of these BMs, a combination of multiple markers is currently recommended to achieve a sensitivity of nearly 100% for the diagnosis of AMI. Myocardial ischemia and unstable angina (UA) cannot be determined by markers of myocardial necrosis such as CK-MB and cTns, and the diagnosis of AMI requires the combination of clinical evidence of ACS [1].
(2) Ischemia-modified albumin
Ischemia-modified albumin (IMA) is a new ischemic marker and is the first cardiac marker approved by the FDA for the detection of myocardial ischemia.IMA is produced due to a conformational change of N-terminal albumin induced by a low pH environment during ischemia, which is manifested by a reduced ability of serum proteins to bind exogenous cobalt ions (CO2+). The main mechanisms include endothelial and extracellular hypoxia, acidosis, free radical damage, disruption of the cell membrane energy-dependent Na+-K+ pump and increased free CO2+. imA not only detects myocardial ischemia, but also increases rapidly after an episode of myocardial ischemia. in patients with suspected ACS without ECG changes, imA has a high sensitivity but not a high specificity for identifying established UA. imA is detected in ischemia The sensitivity is significantly higher than that of cTns, but the positive predictive value is low, and its clinical specificity needs to be confirmed by clinical studies. Bhagavan et al [9] reported that the sensitivity and specificity of IMA in combination with cTns for the diagnosis of myocardial ischemia were 88% and 94%, and the positive and negative predictive values were 92% and 91%, respectively, but the ability of IMA to differentiate the presence of MI in patients with myocardial ischemia was poor. IMA can also be used as a diagnostic tool for percutaneous transluminal coronary angioplasty (PTCA). IMA is also a sensitive early indicator of transient ischemia with percutaneous transluminal coronary angioplasty (PTCA), which can be elevated minutes after PTCA and return to baseline at 6 h, and can evaluate the collateral circulation of the coronary arteries associated with PTCA[10] .
The IMA level is significantly higher in patients with ACS than in those with stable coronary heart disease (CHD) and normal patients, and Aparci et al [11] determined the IMA cutoff value (477 U/ml) by ROC curve analysis. The sensitivity and specificity of this value in predicting 1-year mortality were 70% and 82%, respectively.
Elevated IMA is also seen in patients with tumors, infections, end-stage renal disease, liver disease, and cerebral ischemia. Although IMA may be a marker for rapid detection of ACS, further studies are needed.
2. Cardiovascular inflammation and related markers
Atherosclerosis (AS) is an inflammatory disease, and the whole process of atheromatous plaque development and rupture leading to ACS is considered to be an inflammatory response to injury. Epidemiology has confirmed that inflammation-related biomarkers such as C-reactive protein (CRP), metallo-matrix proteases (MMPs), and interleukin-6 (IL-6) are elevated in the serum of patients with ACS and have been used as predictors of future cardiovascular events.
(1) C-reactive protein
Cytokines and inflammatory mediators produced by the local inflammatory response within atheromatous plaques can promote hepatic synthesis of acute phase reactants, including CRP, fibrinogen, and serum amyloid A protein, and studies have demonstrated an association with increased cardiovascular risk.CRP has a modulator effect by activating complement, promoting phagocytic activity, stimulating tissue factor expression on the surface of monocytes, and other immunomodulatory functions It is important in the pathophysiology of tissue injury, inflammatory response. Inflammatory cells have CRP receptors, and CRP activates cells through their receptors and damages blood vessels through direct infiltration or cytokine production.
Since CRP levels in healthy humans are usually <3 mg/L, a highly sensitive assay [high-sensitivity C-reactive protein (hs-CRP)] is used for screening of CVD. For risk assessment of CVD, hs-CRP <1.0 mg/L is considered low risk, 1.0 to 3.0 mg/L is considered intermediate risk, and >3.0 mg/L is considered high risk. If hs-CRP >10 mg/L indicates the presence of other inflammatory conditions, further review is required. hs-CRP is considered to be the strongest predictor of cardiovascular risk, but widespread clinical use must address the standardization of assay methods and reference values [12], and its current role in clinical practice in terms of diagnosis, prognosis, or therapeutics needs further confirmation.
The recently published MONICA/KORA Augsburg cohort study predicted all-cause mortality, fatal CVD, and CHD by testing hs-CRP in 3,620 middle-aged men, showing that those with hs-CRP >3.0 mg/L were twice as likely as those with hs-CRP <1.0 mg/L, with hazard ratios (HR) of 1.88, 2.15, and 1.74, respectively. another study from the same cohort predicted all-cause mortality, fatal CVD, and CHD by testing hs-CRP >2.0 mg/L in more than 2,000 healthy middle-aged men, with hazard ratios (HR) of 1.88, 2.15, and 1.74, respectively. Another study from the same cohort found that CRP and IL-6 levels were significantly higher in the CHD-onset group compared to the non-onset group by following more than 2,000 middle-aged healthy individuals for a mean of 11 years, while IL-18 levels did not show significant differences between the two groups, suggesting that increased CRP and IL-6 concentrations in this group are independent predictors of future CHD events [13]. However, the extent to which increased hs-CRP concentrations are predictive of CVD still needs further study, and the inflammatory status of the study group affects its relationship with CVD events. Excessive hs-CRP concentrations (>10 mg/L) tend to be associated with inflammatory status, while even in healthy individuals groups there is a higher incidence of inflammatory status, higher CRP concentrations are not linearly related to CVD events, and the evaluation of CVD risk needs to be done with caution [14].
(2) Metallo-matrix proteases
Plaque rupture is associated with inflammation because inflammatory cells are important regulators of plaque stability. Inflammatory cells can produce substances such as MMPs, interleukins (IL), and tumor necrosis factor, which interact to degrade the extracellular matrix. Endogenous inhibitory factors called tissue inhibitors of metalloproteinases (TIMPs) are attached to the active site of MMPS to regulate their activity. Monocytes/macrophages secrete MMPs in an enzymatic state, and cytokines such as IL-1 and tumor necrosis factor-α secreted by vascular smooth muscle cells, T lymphocytes, and endothelial cells have a stimulatory effect on the gene expression of MMPs, which specifically bind to various components of the extracellular matrix and degrade the extracellular matrix, weakening the plaque fibrous cap, while inhibiting vascular smooth muscle cell proliferation and promoting Blankenberg et al [15] found that serum MMPs levels were strongly associated with the risk of fatal coronary artery disease independently of other traditional risk factors for CVD in a study of 1,127 patients with diagnosed coronary artery disease over 4 years. .
MMP-1, MMP-2 and MMP-9 are significantly increased in patients with AMI and UA. oxidized low-density lipoprotein (ox-LDL) induces the secretion of MMP-1, MMP-2 and MMP-9 by endothelial cells on the inner surface of the plaque and in the neovascular wall within the plaque during local ischemia and hypoxia in AS. components, and in rupture-prone plaque shoulders, enhanced collagen breakdown is closely associated with MMPs produced by macrophages.Naruko et al [16] showed a correlation between MMP-9 and future cardiovascular mortality, with implications for cardiovascular disease prognosis after correcting for the effects of other inflammatory factors such as CRP, fibrinogen, IL-6 and IL-18.Robertson et al [17] measured plasma MMP-9 concentrations in patients with ACS using percutaneous coronary intervention with a distal protection device, and showed localized increases in MMP-9, IL-6, and ox-LDL concentrations in the “criminal” lesioned vessels of the coronary arteries, thus confirming their role as “recognition” markers of unstable plaques. The increased plasma concentrations of MMP-9, IL-6, and ox-LDL in the “offender” vessels of the coronary arteries may be due to the release of unstable plaques.
MMP-9 is also useful in the evaluation of left ventricular remodeling after AMI, and Squire et al [18] reported changes in ventricular function after ST-segment elevation MI in relation to the concentrations of MMP-9 and TIMP-1. MMP-9 produced biphasic peaks on days 1 and 4. In contrast, TIMP-1 decreased from day 1 to day 5 after AMI. higher MMP-9 concentrations suggest a larger left ventricle and higher brain natriuretic peptide (BNP) concentrations.
(3) Cell adhesion molecules
Normal vascular endothelium resists the adhesion of circulating leukocytes, and early atheromatous plaque formation involves endothelial insufficiency and leukocyte aggregation. Adhesion molecules [such as vascular cell adhesion molecule (VCAM-1), intercellular adhesion molecule (ICAM-1), P-selectin, E-selectin, etc.] that mediate the interaction between vascular endothelial cells and leukocytes under normal conditions are not expressed or are lowly expressed; in the inflammatory response, the activated leukocytes adhere to the vascular endothelium and can promote endothelial cell injury, vascular endothelial dysfunction through a series of mechanisms that This results in increased expression of ICAM-1 and others, and extensive infiltration of macrophages and T lymphocytes in the blood through the endothelium to the endothelium. VCAM-1 has specific affinity for monocytes and T lymphocytes, and low-density lipoprotein (LDL) is modified to ox-LDL through oxidation, glycation, aggregation, and immune complex formation, and then phagocytosed by macrophages to form lipids in the intima. aggregates in the intima to form lipid plaques and may increase VCAM expression by triggering inflammation, resulting in an increased local inflammatory cellular response [19].The PRIME study [20] of 9,758 healthy men followed for 10 years found that the occurrence of events such as angina, CHD death, and AMI were significantly correlated with elevated ICAM-1 and CRP.Guray et al [21] found that VCAM-1 expression was significantly increased in ACS and was more predictive of plaque stability.
Other molecules involved in cell adhesion including P-selectin and E-selectin have also shown correlation with cardiovascular risk prediction.
(4) Myeloperoxidase
Myeloperoxidase (MPO) is a pre-inflammatory activating enzyme during LDL oxidation, a heme protease synthesized in the bone marrow prior to granulocyte entry into the circulation and stored in asplenophilic granules. MPO is the most abundant protein in neutrophils, monocytes and some macrophages. Large numbers of monocytes infiltrate, activate and degranulate in coronary atheromatous plaques, while releasing large amounts of MPO, resulting in increased levels of MPO at coronary atheromatous plaque lesions and in the circulation.Zhang et al [22] showed increased MPO activity in blood and leukocytes of patients with coronary artery disease (CAD) and a significant correlation between its activity and the degree of CAD (OR 11.9), independent of risk factors such as age, sex, smoking, diabetes, LDL concentration, and leukocyte count. Studies have shown that MPO plays an important role in LDL oxidation and that MPO has some promise as a predictor of cardiovascular risk and as a diagnosis of suspected ACS. Li Li et al [23] found that there was a gradient in MPO concentration in coronary circulation and body circulation in patients with ACS, suggesting that neutrophils secreted intracellular active substances with blood flow through the vascular bed at the coronary lesion, which participated in the local inflammatory response of the lesion and consumed MPO, resulting in a decrease in MPO content in blood passing through the coronary circulation, thus suggesting that MPO is a response to local inflammation in AS plaques This suggests that MPO is a better indicator of local inflammation in AS plaques.
MPO has a predictive value for the prognosis of ACS, and Cavusoglu et al [24] found that baseline MPO levels were significant for the long-term prognosis of ACS patients. By following 193 patients with ACS for 2 years, it was found that the median MPO level at patient enrollment was used as a predictor of shear value (20.34 ng/ml), 88% of those without MI events ≤20.34 ng/ml, while 74% of those with MI events >20.34 ng/ml (P=0.024 9), and the investigators concluded that baseline MPO level was an independent predictor of 2-year MI events in patients with ACS. Mocatta et al [25] found that MPO levels were higher in AMI patients from 24 h to 96 h of admission than in controls (55 ng/ml vs. 39 ng/ml, P<0.001). MPO concentrations above the median level were an independent predictor of mortality (OR 1.8, P=0.034), which was higher when combined with high N-terminal brain natriuretic peptide precursor (N-pro-BNP) concentrations and low LVEF values resulted in higher mortality.
However, although MPO is involved in the inflammatory process in CAD, because neutrophil activity is not ischemia-induced, MPO is at best a marker of plaque instability, not a marker of oxidative stress or injury, and is unlikely to be cardiac specific.
Other proinflammatory enzymes associated with increased cardiovascular risk are lipoprotein-associated phospholipase A2 and pregnancy-associated plasma protein-A.
3. Markers of cardiac function
BNP is a member of the natriuretic peptide family, and has been increasingly used in the diagnosis and treatment of CVD since its discovery in 1988. BNP and N-pro-BNP are most suitable for the diagnosis of patients with suspected heart failure. a multicenter trial conducted by Maisel [26] showed that BNP concentration was highly correlated with NYHA class in heart failure patients. Moreover, BNP was associated with hemodynamic indices such as left ventricular end-diastolic pressure (LVEDP) and pulmonary contraction pressure (PCWP). It is highest in patients with decompensated heart failure, moderately elevated in those with existing left ventricular dysfunction without acute onset, and lowest in those without heart failure or left heart insufficiency. The sensitivity and specificity of the 100 μg/L threshold for heart failure diagnosis was 90%, 76%, and 83% accurate, whereas the 50 μg/L threshold was better for exclusion, with a negative predictive value of 96%.
Since BNP is a neurohormone with a short half-life (18-22 min), its concentration can also be monitored to determine the efficacy and prognosis. Tsutamoto et al [27] concluded that NYHA cardiac function class, PCWP, LVEF, BNP, and atrial peptide (ANP) are independent risk factors for death in patients with chronic congestive heart failure, and Kaplan-Meier survival analysis showed that Berger et al [28] found that BNP was an independent predictor of sudden death (SD) compared with ANP and endothelin in 452 patients with chronic congestive heart failure with LVEF ≤35% at 3 years follow-up, with SD in the BNP <130 pg/ml group being 1 %; while SD in the BNP >130 pg/ml group was 19% (P=0.0001).
Recent studies [29, 30] have shown that N-pro-BNP is highly correlated with BNP plasma concentrations and may be more valuable than BNP in the diagnosis of heart failure because of its long half-life (60-120 min).Both increase with reduced NYHA cardiac function, and N-pro-BNP is more significantly elevated, with no difference in the area under the ROC curve for LVEF ≤40%. The area under the ROC curve was not different when LVEF was ≤40%; when LVEF was ≤50%, the area under the ROC curve was higher for N-pro-BNP than for BNP (0.82 vs. 0.79), thus suggesting that N-pro-BNP is more suitable for the diagnosis of mild to moderate heart failure. pro-BNP concentrations in 353 patients with chronic heart failure showed that the concentration gradient increased through the heart, with N-pro-BNP being more pronounced, and that N-pro-BNP was superior to BNP in the prognostic evaluation of heart failure patients.
Recent studies have also shown that BNP and N-pro-BNP can also be used in the prognostic analysis of CHD patients with stable cardiac function and disease.
4. Summary
The increasing use of cardiac biomarkers has provided new approaches and ideas for clinical diagnosis, and their close association with the pathophysiology of CVD has made them a much sought-after diagnostic and prognostic tool. However, some markers are still controversial in clinical studies due to their lack of specificity, and there is still insufficient evidence to clarify whether some markers are participants or “bystanders” in CVD, and whether their elevated levels in serum or tissues are initiators or secondary factors. In any case, the appearance of each biomarker has evolved throughout the different stages of CVD and has become a “marker” of CVD. The study of cardiac biomarkers with high sensitivity and specificity has become a hot spot for clinical research, and has shown some value for the diagnosis and prognosis of the disease. Further studies are needed to confirm or test the clinical significance of some biomarkers in the future.