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Eur J Cardiothorac Surg 2001;19:859-864
© 2001 Elsevier Science NL
a Department of Thoracic and Cardiovascular Surgery, Heinrich-Heine-University Düsseldorf, Moorenstrasse 5, D 40225 Düsseldorf, Germany
b Institute of Clinical Chemistry and Laboratory Diagnosis, Heinrich-Heine-University Düsseldorf, Moorenstrasse 5, D 40225 Düsseldorf, Germany
Received 9 October 2000; received in revised form 21 February 2001; accepted 26 March 2001.
Corresponding author. Tel.: +49-211-8118331; fax: +49-211-8118333
e-mail: petzoldt{at}uni-duesseldorf.de
| Abstract |
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Key Words: Heart-type fatty acid binding protein Marker protein Myocardial damage Coronary artery bypass grafting
| 1. Introduction |
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Heart-type fatty acid binding protein (hFABP) is a small intracellular protein consisting of 132 amino acid residues and weighing 14.5 kDa. It is water soluble and abundant in the cytoplasm. Its physiological role is the transport of hydrophobic long-chain fatty acids from the cell membrane to their intracellular sites of metabolism in the mitochondria, where they enter the citric acid cycle. After membrane damage, it is released to the extracellular space, and due to its small size and physical properties, immediately enters the blood compartment [12].
Since evidence of hFABP release from damaged myocardium was established in an ischemic working rat heart model, several groups have studied the release pattern in patients with myocardial infarction [1316]. They demonstrated plasma concentration curves that were markedly different from those of other marker proteins, with a very early peak and a rapid decline thereafter. However, due to the lack of a test with appropriate availability and tissue specificity, hFABP measurement has so far not achieved widespread clinical use. Moreover, its value in cardiac surgery remains to be elucidated with only very few data published in the recent medical literature [17,18].
Recently, a direct sandwich enzyme-linked immunosorbent assay (ELISA) test has been developed [19], and a commercially available kit has been introduced to the Japanese market using two distinct types of murine antihuman monoclonal antibodies specific for hFABP. Using this kit, we tested the release pattern of hFABP and its relevance to myocardial injury in patients undergoing coronary artery bypass grafting (CABG) with and without evidence of perioperative myocardial infarction, and compared it to other marker proteins of myocardial ischemia.
| 2. Material and methods |
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Standardized anesthesia procedure included induction with Thiopentane (13 mg/kg) and fentanyl (35 mg/kg) and neuromuscular blockade using pancuronium bromide (0.10.15 mg/kg). Anaesthesia was maintained with Etrane via inhalation, or, during extracorporeal circulation (ECC), via continuous infusion.
After median sternotomy and preparation of the IMA, the ascending aorta and superior and inferior caval veins were cannulated and extracoporeal circulation (ECC) was established. A vent catheter was routinely inserted via the right superior pulmonary vein. Blood temperature was reduced to 30°C. At first, all distal anastomoses were performed using either hypothermic cardiac arrest with cold antegrade cristalloid (Bretschneider) cardioplegia (17 patients) or intermittent aortic crossclamping (15 patients). During the last peripheral anastomosis, rewarming was begun. After final release of the aortic crossclamp, if necessary, the heart was electrically defibrillated and allowed to beat during completion of the proximal anastomoses in partial aortic occlusion. After reaching a core temperature of 34.5°C, the patients were weaned from ECC. Positive inotropic support was given when systolic blood pressure fell below 90 mmHg and vasodilators were administered when preload, as measured by left atrial pressure, rose to more than 15 mmHg.
After meticulous hemostasis, chest tubes were inserted and placed retrosternally, intrapericardially and intrapleurally. The thorax was then closed as usual and the patient transferred to the intensive care unit.
2.2. Analysis
Serial venous blood samples were collected on hospital admission, before ECC, immediately before aortic crossclamping, 5 min after declamping, 60 min after declamping, 1 h after admission to the intensive care unit, 6 h after admission to the intensive care unit, and on the 1st, 2nd, and 10th postoperative day for biochemical analysis as indicated below. They were drawn into vacuum tubes containing dry heparin. The tubes were placed immediately into ice and plasma was separated by centrifugation and stored in plastic tubes at -75°C until further usage. Continuous basic clinical and hemodynamic monitoring included systemic blood pressure, right atrial pressure, heart rate, the administration and dosage of inotropic support, single lead ECG rhythm surveillance and peripheral oxygen saturation.
Electrocardiogram: a 12 lead ECG was recorded before surgery, 1, 6, and 12 h after admission to the intensive care unit, and on the 2nd and 10th postoperative day. ECG records were analyzed by an experienced cardiologist blinded to the actual clinical situation in collaboration with two medical students specially trained in ECG analysis. Diagnostic criteria for myocardial ischemia were defined as follows: new development of Q waves of equal to or more than 0.4 mm, major ST-segment elevation or depression, or R-wave reduction of more than 25% in at least two leads. Bundle branch blocks were not considered as indicating ischemia [20,21].
2.3. Biochemical analysis
2.3.1. Heart-type fatty acid binding protein
As previously mentioned, a sandwich enzyme linked immunosorbent assay for the determination of hFABP levels has been developed (Markit-M H-FABP®, Dainippon Pharmaceutical Co. Ltd.) using two distinct murine anti-human FABP specific monoclonal antibodies. Briefly, h-FABP within the test sample bound to a monoclonal anti h-FABP antibody coated on microplate wells. Enzyme labeled anti h-FABP antibody was added to the wells to form a sandwich immune complex. After addition of substrate to start the enzymatic reaction, absorbance was measured at 492 nm and at 620 nm in a microplate reader. (The kit was a generous gift from Dainippon Pharmaceutical Co. Ltd.).
2.3.2. CK and CKMB enzyme activity
For the determination of CK- and CKMB-enzyme activity, a plate analysis test was used (Vitros CK or CKMB Analysenplättchen, Vitros, Johnson & Johnson Clinical Diagnostics). Briefly, serum samples were added to a plate prepared in multilayer technology. The containing enzyme catalyzed the generation of creatin and ATP by phosphorylation of creatinphosphate and ADP. A subsequent colouring reaction was read at a wavelength of 670 nm reflecting the enzyme activity.
2.3.3. Cardiac Troponin I
Cardiac troponin I was measured using a microparticle immunoassay (Troponin I, B3C291, Axsym Systems, Abbot GmbH Wiesbaden Germany). In summary, TnI, contained in serum samples, bound to microparticles, coated with anti-Troponin I. The complexes were bound to a glass fibre matrix again using an antibody reaction and were subsequently visualized and measured by a colouring reaction, as already described.
2.4. Statistical analysis
For each patient, 300 variables were registered and stored on the hard disc of a microcomputer using the commercially available software Excel® (Microsoft). The data were divided into discrete and continuous variables. For the continuous variables mean, median and standard deviation were calculated. For the discrete variables the number of values in each category were calculated and given as a percentage.
The Student's t-test for non-paired values was used for comparison of the continuous variables, and the
2 test for the discrete variables. In addition, an
-adjustment was carried out, using the HOLM-procedure and comparing three groups of different markers. Probability values of P<0.05 were defined as statistically significant and are presented, when appropriate. All statistical calculations including the correlation test, were carried out using the statistic tools of Microsoft Excel®.
| 3. Results |
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3.2. Time course of mean concentration
Each marker protein displayed a typical laboratory pattern. Mean values rose from a baseline concentration preoperatively to a postoperative peak indicating myocardial damage of a certain degree in all cases. Thereafter, concentrations declined and reached baseline values on the 10th day.
One hour after declamping, peak concentration of the h-FABP was assessed, whereas significant changes between baseline and the perioperative course of all ischemic markers were already earlier observed. Patients with perioperative myocardial infarction showed higher values from the beginning and throughout the whole perioperative period with a peak concentration after arrival at the intensive care unit. See Fig. 1.
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3.3. Time to peak level
54.8% of the patients reached hFABP peak concentration at 60 min after declamping, whereas only 16.1% showed peak concentrations of CKMB or TnI at this time point. In contrast, serum concentrations of CKMB or TnI reached their maximum level at 1 h after arrival at the intensive care unit in 80.6% or 61.3% of the patients, respectively. For details see Fig. 4. The appearance of hFABP values is statistically earlier than that of TnI- (P=0.001) or CKMB-(P=0.002) values (
2 test and HOLM-procedure).
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| 4. Discussion |
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Plasma levels of specific marker proteins follow a typical pattern. Small, water soluble molecules are more rapidly washed out and detectable in the serum than proteins with a larger molecular mass, mainly due to the permeability of the endothelial layer [1,12]. Ongoing occlusion or reperfusion are other factors with impact on the kinetics and the total amount of protein release from an injured myocardial area [1,3].
HFABP has been shown to be highly sensitive in the diagnosis of myocardial infarction and the degree of myocardial damage [1216,24]. It appears very early after the onset of symptoms in myocardial infarction patients. However, there is only little information about hFABP in cardiac surgery patients. HFABP and myoglobin were shown to peak 0.5 h after reperfusion in 57 patients with various cardiac diagnoses and operations, as published by Fransen et al. [17]. Suzuki et al. [18] demonstrated in a group of 20 patients with coronary heart disease, that HFABP peaked 47.3±2 min after release of the aortic cross-clamp [18].
In our study group, blood samples were taken at time points applicable in clinical routine. At 60 min after declamping, the operation usually approaches its end, and 1 h after arrival at the intensive care unit, routine patient's condition has normally stabilized, so that the ICU staff has time for sophisticated diagnostic activity. We found the mean peak level as early as 1 h after the onset of reperfusion. 55% of the patients reached their individual peak level at this time point. Thus, hFABP peaked significantly earlier than CKMB or TnI.
The diagnostic test that we applied takes about 1 h and requires the presence of laboratory personnel [19]. A rapid assay, using dry laboratory technology and allowing for quantification after only 20 min, is currently being developed [25].
Interestingly, myocardial infarction patients displayed their peak level only at 1 h after arrival at the intensive care unit and all of the three patients had their individual peak level at this time point. This was also noted by Fransen et al. [17], who observed a time delay up to 12 h for patients with myocardial infarction to reach their individual peak level. Suzuki et al. [18] reported on a pattern of hFABP levels in patients with perioperative myocardial infarction equal to routine cases. However, the myocardial infarction patients they mentioned had valvular or aneurysma surgery, hence, pathogenesis of myocardial damage may have been different from patients with coronary artery surgery alone. In a recent paper, Adams [1] points out that each marker protein has a typical rising and falling pattern in the case of myocardial injury, and that high serum levels in the absence of this pattern may lead to false positive results.
One limitation of our study is the relatively small number of patients with myocardial infarction. With only three patients, it seems not possible to define a time point, when patients with perioperative myocardial infarction can be clearly discriminated from those with uneventful perioperative course. Further studies including more infarction patients are intended to better characterize the course of protein markers in this important patient cohort.
There is general agreement that the amount of released marker protein reflects the degree of myocardial injury. Using human heart tissue samples it could be demonstrated that after myocardial infarction, hFABP is quantitatively released into the plasma allowing estimating the infarction size from hFABP levels [13]. That the release of troponin I from myofibrillar ultrastructures correlates with the amount of structural myocellular injury is not controversial [7,22]. The size of the infarction as assessed by CKMB release correlates closely with the volume of the infarction, the ejection fraction, the frequency of rhythm disturbances and the prognosis [5,21,22]. We found a significant correlation between hFABP and CKMB or TnI peak levels, so we can assume that there is a correlation between the level of hFABP and the degree of myocardial damage. Further studies, as already mentioned, are necessary to prove this observation in cardiologically as well as in surgically treated myocardial infarction patients.
5. Conclusion
The determination of HFABP levels represents a useful tool in the rapid evaluation of perioperative myocardial damage. HFABP peaks earlier than CKMB or TnI, and with the introduction of a rapid assay, perioperative myocardial damage can already be evaluated in the operating theatre.
The diagnosis of perioperative myocardial infarction depends on the kinetics of marker proteins in serial blood samples during the first 2 h after the onset of reperfusion, rather than on clear cut-off levels.
| Footnotes |
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| Appendix A. Conference discussion |
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Dr Petzold: We did not check this. I think we had too few patients to really find a correlation therefore. From literature it is known that there is a correlation. Fatty acid binding protein increases with the cross-clamp time.
Dr Flameng: I was a little bit surprised that you did not find a difference in time to peak from the troponin I and the CK-MB.
Dr Petzold: We were astonished too, but we found it.
Dr F. Beyersdorf (Freiburg, Germany): Was this marker also tested in patients who underwent acute myocardial infarction?
Dr Petzold: We did not do this in Dusseldorf, but in literature it has been tested, and it is known to be a very rapid marker. This was the reason why we undertook this study.
Mr Ashraf (Swansea, UK): Did you have postoperative any ECG protocol in this group of patients? Did you correlate it with any ECG changes with troponin I, CK-MB and free fatty acid?
Dr Petzold: It is very difficult to analyze the postoperative ECG. It is very insecure and it is hardly possible to correlate the ECG with a perioperative infarction. Actually we did our infarction diagnosis of these three patients, in one patient due to ECG changes which were a new Q-wave, and in the two others because of significant changes in the other marker proteins that we knew. But in the other patients we had no ECG changes, which could be significant for perioperative myocardial damage. Of course, we looked for it.
Mr Ashraf: A second question, was there any correlation of your level of ischemic damage with postoperative inotropic support?
Dr Petzold: No, it was not.
Mr Ashraf: Did you look into that?
Dr Petzold: We did that, but there was no correlation.
Dr G. Lutter (Freiburg, Germany): In case the patient has got a high hFABP value, i.e. after myocardial infarction, would you change your procedure out of this information.
Dr Petzold: Up to now it takes about one hour to one hour and a half to obtain the results of fatty acid binding protein, so this is too late to draw a conclusion. But we are working on a rapid test, a rapid assay that could give us the results within 20 minutes, and with this, I think we will have to draw consequences in the operation theatre.
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