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Ann Thorac Surg 2001;72:1566-1571
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Metabolic changes and myocardial injury during cardioplegia: a pilot study

^a Department of Cardiothoracic Surgery, Austin & Repatriation Medical Centre, Melbourne, Australia
^b Department of Intensive Care, Austin & Repatriation Medical Centre, Melbourne, Australia
^c Department of Anesthesia, Austin & Repatriation Medical Centre, Melbourne, Australia
^d Department of Laboratory Medicine, Austin & Repatriation Medical Centre, Melbourne, Australia

Accepted for publication June 28, 2001.

* Address reprint requests to Dr Bellomo, Department of Intensive Care, Austin & Repatriation Medical Centre, Studley Rd, Heidelberg, Victoria 3084, Australia
e-mail: rb{at}austin.unimelb.edu.au

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Background. The timing, nature, and severity of both increased cardiac troponin I (cTn-I) levels and myocardial injury during ischemic arrest with cardioplegia are unknown. To define them more accurately, we studied myocardial metabolic activity and the release of markers of myocardial cell injury into the coronary sinus before, during, and after cardioplegia.

Methods. We simultaneously measured creatine kinase, creatine kinase-MB, cTn-I, lactate, phosphate, and blood gases in coronary sinus and systemic arterial blood from 12 patients before cardiopulmonary bypass, after removal of the aortic cross-clamp, and after discontinuation of cardiopulmonary bypass. We also measured coronary sinus flow and transmyocardial fluxes of all analytes and calculated myocardial oxygen consumption, myocardial carbon dioxide production, and myocardial energy expenditure.

Results. Myocardial lactate release increased 10-fold after removal of the aortic cross-clamp (p = 0.012) and was accompanied by a surge in myocardial phosphate uptake (p = 0.056). These events were associated with only partial cardioplegia-induced suppression of myocardial oxygen consumption (p = 0.0047), myocardial carbon dioxide production (p = 0.0022), and myocardial energy expenditure (p = 0.0029). Simultaneously, coronary sinus cTn-I levels increased from a mean of 0.76 to 2.43 ng/mL after removal of the aortic cross-clamp, and 2.51 ng/mL after cardiopulmonary bypass (p = 0.014), leading to an increase in arterial cTn-I concentration from 0.18 to 0.98 and 3.01 ng/mL (p = 0.0002). Thus, cTn-I release across the myocardium was absent at baseline, became detectable (p = 0.012) after removal of the aortic cross-clamp, and correlated with cross-clamp and pump times. Similar changes occurred with creatine kinase-MB.

Conclusions. Metabolic myocardial stress occurs during ischemic arrest with cardioplegia and is associated with inadequate suppression of metabolism and with a surge in cTn-I and creatine kinase-MB release, which is maximal after removal of the aortic cross-clamp. These changes are likely to represent structural myocardial cell injury.

	Introduction

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The diagnosis of myocardial cell injury during coronary artery bypass grafting (CABG) is difficult. Until recently, only major myocardial cell necrosis (infarction) could be detected by electrocardiographic criteria, echocardiography, scintigraphy, and the measurements of markers of muscle damage, such as creatine kinase (CK), creatine kinase-MB (CK-MB), and myoglobin. However, serial electrocardiographic examination is insensitive [1] and cannot easily quantify injury, and interpretation of arterial levels of CK or myoglobin is hampered by concomitant release of enzymes or myoglobin from skeletal muscle after median sternotomy [2, 3]. Even the use of isoenzymes such as CK-MB or the CK-MB to total CK ratio is hindered by this phenomenon and by limited sensitivity [4–7]. More recently, cardiac troponin I (cTn-I) has been identified as a specific and sensitive marker of myocardial cell injury even in the perioperative setting [8, 9], including CABG [10, 11]. It could thus be used to study the pathogenesis of myocardial injury during CABG. The combined assessment of the nature and timing of the release of markers of metabolic stress and cTn-I would help elucidate the pathogenesis of such injury. It would also clarify the meaning of increased levels of cTn-I postoperatively. We hypothesized that cTn-I occurs during ischemic arrest with cardioplegia and that such release is associated with biochemical changes of metabolic stress. We tested this hypothesis using several laboratory investigations in a cohort of patients undergoing CABG.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Twelve patients undergoing primary CABG by a single surgeon were studied. Informed consent was obtained and the Human Research Ethics Committee of the institution gave approval for the study.

Before commencement of cardiopulmonary bypass (CPB), the coronary sinus (Cs) was cannulated with a self-inflating retrograde cardioplegia cannula (Research Medical Inc, Midvale, UT). The operative procedure was standardized with all distal and proximal anastomoses constructed under the single aortic cross-clamp.

Cardiopulmonary bypass was performed using a membrane oxygenator (Sorin Monolyth, Biomedica, Mirandola, Italy). The pump prime was a volume of 1,300 to 1,600 mL, depending on the patient’s weight. It consisted of 500 mL of a urea-linked polygeline solution (Hemaccel, Hoechst, Melbourne, Australia; composition: Na⁺, 145 mEq/L; K⁺, 5.10 mEq/L; Ca²⁺, 6.25 mEq/L; Cl^-, 145 mEq/L; traces of phosphate and sulfate ions; 35 g of polygeline/L; and anionic polypeptides to the isoionic point), 900 to 1,100 mL of Ringer’s solution (Baxter, Old Toongabbie NSW, Australia; composition: Na⁺, 147.5 mEq/L; K⁺, 4 mEq/L; Ca²⁺, 4.5 mEq/L; Cl^-, 156 mEq/L), 10,000 U of sodium heparin, and 40 mmol of sodium bicarbonate. Cardioplegia was administered in 800 mL of blood and contained 100 mL of tromethamine (Trometamol, Abbott, Australasia, Kurnell, NSW, Australia), 100 mg of lidocaine HCl, 20 mmol of KCl, and 10 mmol of MgCl₂. Cardioplegia was delivered through the antegrade route and the Cs cannula (approximately 500 mL at a temperature of 14°C to 18°C). Further cardioplegia solution was infused through the Cs cannula at approximately 25-minute intervals. This consisted of 600 mL of blood with 5 mmol of KCl and 20 mmol of MgCl₂ at a temperature of 14°C to 18°C. A "hot shot" was given through the aortic root immediately before aortic cross-clamp removal. This consisted of 600 mL of blood with 5 to 10 mmol of MgCl₂ at 34°C to 36°C. Other intraoperative fluids were administered largely as urea-linked polygeline solution and Hartmann’s solution (Baxter; composition: Na⁺, 129 mEq/L; K⁺, 5 mEq/L; Ca²⁺, 2 mEq/L; and lactate, 29 mEq/L). The pump rate was set at 2.4 L · m^-2 · min^-1, and the minimum core body temperature was 30°C (temperatures measured by means of nasopharyngeal probe).

Blood samples were taken from the Cs and the aorta at predetermined times. These times were as follows:

1. Soon after cannulation, before institution of CPB (pre-CPB).
   
2. On removal of the aortic cross-clamp (after RACC).
   
3. Immediately after the patient was weaned from CPB (post-CPB).
   

The blood samples were analyzed for blood gases (ABL 30 Blood Gas Analyser, Radiometer, Copenhagen, Denmark), lactate (Hitachi 911 Analyzer, Boehringer Mannheim, Indianapolis, IN), phosphate (Hitachi 747 Analyzer, Boehringer Mannheim). Serum CK was measured on the same samples using a Boehringer Mannheim/Hitachi 747 automated analyzer with a sensitivity of 2 IU/L and a 2.5% coefficient of variation. The serum CK-MB activity was measured by immunoinhibiton assay (Boehringer Mannheim CK-MB N-acetyl-cysteine (NAC)-activated kit) using the Boehringer Mannheim Hitachi 911 analyzer (reference range, 0 to 12 IU/L) with a sensitivity of 2 IU/L and a 2.5% coefficient of variation. This assay measures activity and tends to give a higher CK-MB/CK ratio than measurements based on CK-MB body measurements.

Specimens for cTn-I were collected, centrifuged, and stored at -70°C. The samples were thawed and processed in two batches using the Stratus II cTn-I fluorometric enzyme immunoassay (Baxter Diagnostics, Deerfield, IL), with the assay being calibrated before each batch was run. The assay has no cross-reactivity with noncardiac polypeptides. A cutoff value of 0.4 ng/mL was used as the minimal detectable concentration.

Energy expenditure, oxygen consumption, carbon dioxide production, lipid oxidation, and carbohydrate oxidation were calculated using published formulas [12].

The Wilcoxon paired signed rank test was used to perform nonparametric comparisons of paired observations (arterial versus Cs at different times). The Friedman test was used for nonparametric two-way analysis of variance for observations at different times. Transmyocardial changes (fluxes) in measured variables at different times were calculated by subtracting the Cs value from the arterial value and multiplying the difference by the Cs flow. Changes in such fluxes with time were tested for using the Friedman test. If the Friedman test was positive, the Wilcoxon signed rank test was used for post hoc analysis to identify which observations differed from each other. Correlation was tested for using the Spearman rank correlation test. Results are presented as means with standard error of the mean unless otherwise stated. Statistical significance was set at p value less than 0.05.

	Results

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
We studied 12 patients having primary CABG by a single surgeon. Their demographic and clinical features are presented in Table 1.

View larger version (25K): [in this window] [in a new window]   Fig 1. Histogram showing changes in lactate release. Lactate release increased significantly after release of the aortic cross-clamp (post-RACC) compared with baseline (pre-CPB; p = 0.012) but then decreased after weaning from cardiopulmonary bypass (post-CPB; p = 0.049).

View larger version (20K): [in this window] [in a new window]   Fig 2. Histogram showing changes in phosphate uptake. The myocardium was phosphate neutral before (pre-CPB) and after cardiopulmonary bypass (post-CPB), but became a significant extractor of phosphate after release of the aortic cross-clamp (post-RACC; p = 0.011), although the difference in transmyocardial phosphate fluxes among the different times narrowly failed to achieve significance (p = 0.059).

View larger version (25K): [in this window] [in a new window]   Fig 3. Histogram showing the changes in arterial (art) and coronary sinus (Cs) cardiac troponin I (cTn-I) concentration at baseline (pre-CPB), after aortic cross-clamp release (post-RACC), and after weaning from cardiopulmonary bypass (post-CPB). The changes in arterial concentration from baseline are highly significant (p = 0.0002), as are the changes in coronary sinus cardiac troponin I (p = 0.01). However, although arterial concentrations continued to rise after cardiopulmonary bypass (p = 0.009) compared with aortic cross-clamp release, coronary sinus cardiac troponin I did not rise further.

View larger version (22K): [in this window] [in a new window]   Fig 4. Histogram showing changes in myocardial cardiac troponin I release. There was an increase from baseline (pre-CPB) in cardiac troponin I release after aortic cross-clamp removal (post-RACC; p = 0.012). This surge in release was followed by a return to a state of no measurable release after cardiopulmonary bypass (post-CPB).

Myocardial CK-MB release was also detectable after RACC at a rate of 1.12 ± 0.52 IU/min but was absent at baseline and after CPB. The changes in myocardial CK-MB release with time were significant (p = 0.05). Such myocardial CK-MB release was associated with an increase in Cs concentration from 36.1 ± 9 to 122 ± 42 IU/L after RACC (p = 0.003) and then stabilized at 140 ± 46 IU/L after CPB (not significant). Arterial CK-MB concentrations, however, showed an increase from 34.3 ± 10 to 96.8 ± 32 IU/L (p = 0.003), which continued after CPB to reach 145 ± 46 IU/L (p = 0.02).

There was no electrocardiographic evidence of myocardial ischemia in any of the patients. However, there was a detectable rise in cTn-I in the arterial blood of 2 patients (one at 3 and the other at 1.2 ng/mL) before CPB, and myocardial cTn-I release could be demonstrated in 2 more patients also before CPB. None of these 4 patients had an arterial CK-MB concentration of more than 5 IU/L, but in 3 of them, myocardial CK-MB release could be detected, suggesting undiagnosed preoperative ischemia. Finally, 6 patients had cTn-I levels more than 3 ng/mL (laboratory cutoff point for the diagnosis of myocardial infarction) by the time of weaning from CPB.

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
The pathogenesis of cTn-I release and myocardial cell injury in the setting of CABG is only partly understood. Yet such understanding is important in the assessment of the adequacy of cardioplegia and the safety of cardiac operations [11]. The first difficulty lies in the accuracy of the diagnosis of myocardial injury during CABG and is caused by the lack of sufficiently sensitive and specific markers of myocardial cell injury. However, a regulatory protein called cTn-I now appears to be highly specific and sensitive for cardiac injury [13–17]. It is not surprising, therefore, that peripheral blood cTn-I has recently been used to detect myocardial cell injury in the setting of cardiac operation [10, 11, 18]. In this setting, systemic levels otherwise diagnostic of myocardial infarction are seen in many patients. These patients do not have diagnostic increases in CK or CK-MB concentrations, or electrocardiographic changes of ischemia. The meaning and nature of such cTn-I increases, however, is unclear. Do patients develop true but mild myocardial infarction during operation? Do they have release of cTn-I in relation to removal of the atrial appendage? Is cTn-I release induced by inadequate protection during cardioplegia? Is the release related to changes in membrane permeability or to structural cell injury? We can now detect myocardial cell injury with greater sensitivity and specificity; however, we do not know the when, how, and why of such injury. Accordingly, our pilot investigation tried to address some of these issues in the belief that increasing our understanding of the timing, mechanisms, and causes of myocardial injury during CPB is important to our ability to prevent it and to interpret postoperative laboratory changes.

The first finding of our investigation is that, during initial antegrade followed by retrograde cardioplegia, there is ongoing myocardial cell injury. This injury is demonstrated by myocardial cTn-I release with RACC. It is possible that such cTn-I release is always secondary to true coronary ischemia. However, the lack of widespread cTn-I release at other times, the lack of discrete or large episodic peaks in release, and the presence of a Cs to arterial gradient during CPB in all but one of the patients under study all suggest otherwise.

Second, Cs cTN-I levels were positively correlated with lactate levels, and cTn-I release occurred in conjunction with myocardial lactate release and phosphate uptake. These findings strongly suggest that metabolic stress or inadequate cell protection are a more likely explanation for the pathogenesis of cTn-I release and the myocardial injury it represents. This interpretation is also in keeping with recent data from patients with normal coronary arteries undergoing aortic valve replacement [8]. In these patients arterial cTn-I concentrations correlated with aortic cross-clamp time and duration of cardioplegia. It is also in keeping with data showing a differential impact on cn-I levels of combined cardioplegia versus retrograde cardioplegia alone in selected CABG patients [19]. The type and duration of cardioplegia are important determinants of myocardial cell injury. Using the Langendorff technique, investigators have been able to demonstrate that cTn-I release follows a different pattern from that of CK and CK-MB after ischemia-reperfusion [20] with an early and a late peak. Only the late peak is associated with CK and CK-MB release, suggesting that the early release of troponins may be related to membrane leakiness rather than cell death. Later, if injurious stimuli persist, apoptosis and cell death may occur. In keeping with all these observations, cTn-I release also correlated with cross-clamp and pump times in our study.

Other information obtained during this investigation also supports the view that inadequate protection during cardioplegia participates in the pathogenesis of myocardial cell injury. The average suppression in myocardial oxygen consumption during cardioplegia in our patients was less than 60% with several patients experiencing a less than 50% reduction. Equally, myocardial carbon dioxide production and myocardial energy expenditure continued at more than 30% of baseline values during the same period.

Although no biochemical definition of adequate cardioplegia exists, in their aggregate, these findings suggest that myocardial metabolism was inadequately suppressed by initial antegrade followed by retrograde cardioplegia. This lack of metabolic suppression may cause continued adenosine triphosphate consumption and depletion. Continued metabolic activity during cardioplegia is confirmed by the findings of lactate release and phosphate uptake. Sustained adenosine triphosphate depletion is likely to result in bioenergetic failure and structural cell injury. Structural damage is also suggested by the concomitant detection of myocardial CK-MB release during cTn-I release. Because of the size of the CK-MB enzyme (molecular weight, 84,000 d), its release must imply major structural damage to the cell membrane of myocardial cells.

Other findings, however, suggest that mechanisms beyond cardioplegia may also be responsible for myocardial injury during CABG. For example, in this heterogeneous group, 4 patients had detectable cTn-I release even before cardioplegia was instituted. Such release occurred with no electrocardiographic changes of ischemia in pain-free patients undergoing operation and would not have otherwise been identified. This finding supports the view that true but silent coronary ischemia may occur in the immediate perioperative period and contribute to the pathogenesis of myocardial injury during CABG. Furthermore, enzyme release may result from other events such as inadequate blood flow in regional areas of the myocardium, the effect of defibrillation, the manual handling of the heart, and the cutting of intramyocardial vessels during operation. However, given the timing and nature of our aggregate observations, we believe that inadequate protection during cardioplegia remains the most important pathogenetic event.

Our study is a pilot investigation, and, as such, it is small in size and deals with a particular group of patients. Furthermore, our approach to myocardial protection was determined on the basis of administration of intermittent retrograde tepid blood cardioplegia. The findings, therefore, may not be applicable to other centers or other patient cohorts or patients treated with a different regimen of myocardial protection. The statistical power of the study is limited, and we may have failed to demonstrate findings that would be visible in larger groups. Measurements were only taken on three occasions. More frequent testing would have been more informative. However, before proceeding to more frequent measurements during cardioplegia, with the inevitable impact on the duration of operation, it was thought necessary to perform a pilot investigation first.

In conclusion, despite the above limitations, we have demonstrated that unsuspected myocardial injury occurs before cardioplegia in some patients and during cardioplegia in almost all patients. We have also shown that such injury leads to cTn-I and CK-MB release, suggesting structural cell damage. We have demonstrated that cTn-I release occurs, whereas myocardial oxygen consumption and energy expenditure are inadequately suppressed, and is associated with lactate production and phosphate uptake as well as with cross-clamp and pump times. These findings support the notion that postoperative increases in cTn-I are unlikely to reflect the adequacy of grafting and more likely to reflect the adequacy of cardioplegia.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
This work was supported in part by the Austin Hospital Anaesthesia and Intensive Care Trust Fund.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Jai S. Raman Brian F. Buxton Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Raman, J. S. Articles by Buxton, B. F. Search for Related Content PubMed PubMed Citation Articles by Raman, J. S. Articles by Buxton, B. F. Related Collections Myocardial protection