Asian Cardiovasc Thorac Ann 2007;15:502-506
© 2007 Asia Publishing EXchange Ltd
Cardiac Troponin I Concentrations During On-Pump Coronary Artery Surgery
Fabio Capuano, MD,
Caterina Simon, MD,
Antonino Roscitano, MD,
Gianluca Sclafani, MD,
Euclide Tonelli, MD,
Riccardo Sinatra, MD
Division of Cardiac Surgery, St. Andrea Hospital, University of Rome "La Sapienza", Rome, Italy
For reprint information contact: Fabio Capuano, MD, Tel: 39 06 3377 5310, Fax: 39 06 3377 5483, Email: capmd{at}katamail.com, Division of Cardiac Surgery, St. Andrea Hospital, Via di Grottarossa 1035-1039, 00188 Rome, Italy.
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ABSTRACT
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Perioperative myocardial infarction remains a frequent complication after coronary artery bypass grafting, and is associated with a poor prognosis. This retrospective study compared cardiac troponin I concentrations after on-pump bypass grafting in 2 groups of patients: 100 operated on using a single-clamp technique to perform anastomoses, and 80 operated on using a double-clamp technique. Postoperative cardiac troponin I levels were not significantly different between groups. It was concluded that the double-clamp technique did not reduce the incidence of myocardial infarction after elective on-pump coronary artery bypass grafting, and use of a single clamp is safe with no adverse effect on postoperative outcome.
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INTRODUCTION
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Perioperative myocardial infarction (MI) remains a frequent complication after coronary artery bypass grafting (CABG), with an incidence of 3% to 21%.1,2 This wide variation can be attributed to differences in criteria for detection of perioperative MI. Patients are usually monitored by frequent biochemical analysis and electrocardiogram (EKG) recordings. The 12-lead EKG alone cannot confirm perioperative MI, although it is simple and reproducible.3 Detection of abnormalities in segmental wall motion by transthoracic 2-dimensional echocardiography has been used to confirm cardiac injury after surgery, but it may be less sensitive than biochemical analysis.4 Two consensus statements by cardiology groups have recommended the use of biochemical markers for diagnosis of acute MI.5,6 Previously, creatine kinase-MB isoenzyme (CK-MB) was used for diagnosis of MI in both cardiac surgery and acute coronary syndromes; however, CK-MB is not 100% cardiospecific.7 Cardiac troponin I (cTn-I) is highly specific for myocardial tissue, with no cross-reactivity with the skeletal muscle isoform.8 Thus it has supplanted CK-MB as the biomarker of choice for detection of cardiac injury.9 Elevated cTn-I is associated with ischemic EKG changes after CABG, and a poor overall prognosis.10 Cardiac surgery per se induces an increase in plasma cTn-I, even in the absence of ischemic damage. This increase may depend on the type of aortic clamping used to perform anastomoses during on-pump CABG: proximal and distal anastomoses constructed during a single period of aortic cross clamping; or distal anastomoses constructed during a single period of aortic cross clamping and proximal anastomoses during partial occlusion of the ascending aorta while rewarming (double-clamp technique). Surgeons trained using the 2-clamp technique have been hesitant to change to the 1-clamp technique because of concerns that prolonged ischemic time may increase the incidence of perioperative MI. Surgeons trained using the 1-clamp technique argue that it allows more uniform cardioplegia delivery while the grafts are constructed. To determine whether increased plasma cTn-I after CABG might be associated with the longer period of ischemia during the 1-clamp technique, we compared cTn-I levels after on-pump CABG in patients operated on using either the 1 or 2-clamp technique.
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PATIENTS AND METHODS
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This retrospective study was performed at the Department of Cardiac Surgery, St. Andrea Hospital, during the 14-month period from February 2005 through March 2006. There were 180 consecutive patients, 147 (81.6%) men and 33 (18.4%) women, with a mean age of 63.6 ± 8.5 years, who underwent elective isolated primary CABG on cardiopulmonary bypass. The patients were classified into 2 groups: 100 had the 1-clamp technique to construct distal and proximal anastomoses, with cardioplegia delivered on completion of each proximal anastomosis; 80 had the 2-clamp technique with the aortic clamp removed after construction of the distal anastomoses and replaced during proximal anastomoses. Segregation was based on the degree of aortic calcification: patients in the 1-clamp group were found to have a calcified aorta that was not amenable to safe application of a side-biting aortic clamp. Disease of the ascending aorta was suspected by epicardial scanning, and the quality of the aorta was assessed digitally by the surgeon at operation. Patients with active infection or malignancy, uncontrolled diabetes, hepatic disease, cardiogenic shock, recent (72 hours) MI, percutaneous angioplasty failure, emergency surgery, a ventricular pacemaker, reoperation or combined coronary and valve operations were excluded from the study. Table 1
shows the demographic and preoperative clinical characteristics of the 180 patients, and indications for the surgical procedure based on angiographic findings. In the early postoperative period, all patients were followed up in our intensive care unit (ICU). To detect perioperative MI, arterial blood samples (2–3 mL) were obtained 15 min after aortic cross clamping (T1), 15 min after cross clamp release (T2), on ICU admission (T3), 8 hours postoperatively (T4), 24 hours postoperatively (T5), and 48 hours postoperatively (T6). Cardiac troponin I was assayed in plasma by a microparticle enzyme immunoassay using the AxSym system (Abbott Laboratories, Abbott Park, IL, USA), which has an upper limit of normal of 0.04 ng·mL–1. A 12-lead EKG was recorded preoperatively, immediately on admission to the ICU, and every day thereafter until discharge to an unmonitored setting. Postoperative echocardiography was performed within 4 days, using a transthoracic Acuson Sequoia C256 system (Acuson Corporation, Mountain View, CA, USA) with a 3V2C probe. Two-dimensional images were obtained in the parasternal short and long axis views, apical 2 and 4-chamber views, and subcostal views, as recommended by the American Society of Echocardiography.11 Electrocardiograms and echocardiograms were analyzed by 2 experienced clinicians unaware of the clinical and biochemical information. Perioperative MI was defined as: new Q waves 
40 ms in at least 2 adjacent EKG leads;12 alteration of the ST segment in 2 consecutive leads in the same ventricular territory; and increased plasma cTn-I > 12.5 ng·mL–1 in the first 24 hours postoperatively. Ongoing refractory angina requiring intravenous nitrate therapy for control was regarded as unstable angina. Hospital mortality was defined as death occurring during the hospitalization. Low cardiac output was defined as the need for intraaortic counterpulsation or inotropic drugs for > 24 hours. The operations were carried out by one surgeon using a similar operative technique in all cases. Radial and pulmonary arterial catheters were introduced under local anesthesia. After standard anesthesia with a neuromuscular blocking agent, a median sternotomy was performed, followed by aortic and right atrial cannulation. Each patient underwent routine epiaortic ultrasound scanning and mapping of the ascending aorta to detect atherosclerotic disease. When severe calcification of the ascending aorta was found, the patient was assigned to the 1-clamp group intraoperatively. Myocardial protection was achieved by intermittent infusion of tepid (30°C) hyperkalemic blood cardioplegia into the aortic root. The interval between infusions never exceeded 20 min. All patients underwent multiple grafting using saphenous vein and internal mammary artery grafts: left internal mammary artery in all patients, and right internal mammary artery in 11 (11%) in the 1-clamp group and in 8 (10%) in the 2-clamp group. In the 2-clamp group, distal anastomoses were made during total aortic occlusion, and proximal anastomoses were constructed after release of the aortic cross clamp and after applying a second partially occluding aortic clamp. In the 1-clamp group, all anastomoses were constructed during a single period of aortic occlusion. Intraoperative graft flow measurement (Cardiomed; Medistim, Oslo, Norway) was routinely carried out after cardiopulmonary bypass just before sternal closure, under stable hemodynamic conditions for each graft, to test graft flow and quality of the anastomoses. Unless bleeding, intravenous chlorzoxazone was started within 4 hours, and oral aspirin within 24 hours after surgery. Statistical analyses were performed using SPSS Statistical Package 13.0 (SPSS, Inc, Chicago, IL, USA). Continuous variables are expressed as mean ± standard deviation. The unpaired Student’s t test was used to compare continuous variables, and categorical data were analyzed using the chi-squared test or Fisher’s exact test, as appropriate. We used exact p values unless p < 0.05, as the measure of evidence.
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RESULTS
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The intraoperative variables are summarized in Table 2. Aortic cross clamp times were longer in the 1-clamp group, but mean cardiopulmonary bypass time was similar in both groups. Table 3 shows cTn-I concentrations; the preoperative cTn-I level was 0 ng·mL–1 in both groups. The maximum postoperative levels of cTn-I did not show a significant difference between groups. There was no difference in peak cTn-I levels in respect of the number of grafts (Figure 1). Postoperative outcomes are summarized in Table 4. None of the variables differed significantly between the 2 groups. The causes of death were low cardiac output in 2 patients and pneumonia in another.
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Figure 1. Plasma cardiac troponin I (cTn-I) concentrations 15 min after aortic cross clamping (T1), 15 min after cross clamp release (T2), on admission to intensive care unit (T3), 8 hours (T4), 24 hours (T5), and 48 hours postoperatively (T6). (A) The whole study population. (B) Patients undergoing 2-vessel bypass grafting. (C) Patients undergoing 3-vessel bypass grafting. (D) Patients undergoing 4-vessel bypass grafting. DCT = double-clamp technique, SCT = single-clamp technique.
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DISCUSSION
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Cardiac troponin I is a myofibrillar protein regulating the interaction of actin and myosin, without calcium. Three isoform have been isolated: troponin I-fast and troponin I-slow, found exclusively in skeletal muscle fibers; and cTn-I in cardiac muscle, which has 31 more amino acids than skeletal muscle forms. Elevation of plasma cTn-I persists for 5–7 days, permitting flexibility in the timing of blood samples.13 Cardiac troponin I has been shown to be a specific marker of cardiac damage, and it does not increase in the healthy population nor as a result of muscular disease or noncardiac surgery.10 Dissection of the myocardium to expose the intramyocardial arteries, manipulation of the heart, left atriotomy for mitral valve replacement, aortotomy for exposure of the aortic valve, and placement of the pursestring sutures for cannulation cause myocardial lesions, which may explain why cTn-I increases early after cardiac surgery, even in the absence of ischemic myocardial damage. To reduce ischemia, many surgeons use a 2-clamp technique during on-pump CABG. Tsang and colleagues14 showed no significant difference in the incidence of perioperative MI between the 2 groups (1 vs 2-clamp) on the basis of CK-MB levels, but today this result can be confirmed by cTn-I concentrations. Kim and colleagues15 also found no differences between groups in the incidence of perioperative MI detected by EKG (new Q waves in 2 or more leads). A plasma cTn-I level > 11.6 ng·mL–1 24 hours postoperatively is a strong indication of perioperative MI after CABG.16 Peak cTn-I level is an effective way to identify high-risk post-CABG patients, allowing aggressive treatment to reduce the high incidence of adverse outcomes in the long term, such as hemodynamic instability, prolonged intubation time, pulmonary and renal disease. The main result of this study is that maximum postoperative levels of cTn-I did not show a significant difference between the single and double-clamping groups. On the other hand, Grega and colleagues17 demonstrated a significant decrease in the relative risk of adverse cerebral outcomes with the use of a single period of clamping (compared to a 2-clamp technique) after primary CABG. Dar and colleagues18 reported that a single period of clamping improved cerebral protection as measured by the release of S-100 protein. The limitations of the current study are that it was retrospective and involved a small number of patients. Further investigations are needed in a larger population to confirm these data. However, we believe, in the light of cTn-I concentrations, that a single period of clamping is safe with no adverse effects on myocardial protection and postoperative outcome in patients undergoing elective isolated primary on-pump CABG.
Presented at the 16th World Congress of World Society of Cardio-Thoracic Surgeons, August 17–20 2006, Ottawa, Canada.
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ABSTRACT
INTRODUCTION
