| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
J Thorac Cardiovasc Surg 2005;129:41-45
© 2005 The American Association for Thoracic Surgery
Cardiopulmonary Support and Physiology |
a The Department of Cardiothoracic Surgery Hadassah University Hospital, Jerusalem, Israel
b The Joseph Lunenfeld Cardiac Surgery Research Center, Hadassah University Hospital, Jerusalem, Israel
Received for publication September 5, 2003; revisions received March 15, 2004; accepted for publication March 25, 2004.
* Address for reprints: Niv Ad, MD, Cardiothoracic Surgery Department, Hadassah University Hospital, Jerusalem 91120, Israel
nivadmd{at}hotmail.com
 |
Abstract |
|---|
 Top Abstract MethodsResultsDiscussionReferences |
|---|
METHODS: Slices of right atrial trabeculae from 50 patients undergoing elective cardiac surgery were surperfused with oxygenated glucose-containing phosphate-buffered saline solution. After 30 minutes of stabilization, the sections were exposed to 90 minutes of simulated ischemia (nitrogenated phosphate-buffered saline solution without glucose) followed by 90 minutes of reoxygenation (reintroduction of the oxygenated solution). Mitochondrial viability and response were measured by staining with 3-[4.5 dimethylthiazol 2-yl]-2,5-diphenyltetrazolium bromide. The magnitudes of mitochondrial recovery after simulated ischemia and 28 possible risk factors for postoperative atrial fibrillation were entered into univariate and multivariate models.
RESULTS: There were no deaths in this group of patients. Nineteen patients (38%) had postoperative atrial fibrillation. Interestingly, no difference in baseline (before simulated ischemia) mitochondrial function was documented between patients who had postoperative atrial fibrillation and those who did not. An independent predictor for postoperative atrial fibrillation was the degree of mitochondrial dysfunction in response to simulated ischemia, as measured by the intensity of the staining.
CONCLUSION: This study has identified for the first time an association between mitochondrial dysfunction in response to ischemia and postoperative atrial fibrillation. This finding improves our understanding of the pathophysiology of postoperative atrial fibrillation and may eventually lead us to identify candidates for selective preoperative or early postoperative prophylactic treatment.
The mechanism for the development of postoperative atrial fibrillation (POAF) is unclear. It is known that POAF involves reentry, resulting from disparate refractoriness between adjacent atrial areas,4 but the reasons for these dissimilarities are unknown. Risk factors associated with the development of POAF include advanced age, prolonged preoperative atrial conduction duration, chronic obstructive pulmonary disease, excess circulating catecholamines, electrolyte imbalance, atrial myocardial ischemia, sudden ß-blocker withdrawal, prolonged aortic crossclamping, and right atrial manipulation.1-7 One of the confusing aspects of POAF is the fact that it does not develop in roughly 60% of patients having cardiac surgery, despite the presence of one or more of these risk factors in nearly all of them, suggesting an inherent preoperative risk of postoperative development in a subgroup of patients. This may be specifically suggested by the finding that prolonged preoperative atrial conduction duration is associated with POAF.6 It has previously been shown that high lipofuscin content and atrial myolysis found in atria before cardiopulmonary bypass (CPB) may also be associated with vulnerability to POAF.8,9 Taken together, these factors indicate that the preoperative status of a patient's atria is a major determinant of the risk of development of POAF, in addition to the stress and ischemia induced by the surgical procedure itself.
The association between advanced age and risk of atrial fibrillation after cardiac surgery is striking but specifically unclear. It has been attributed to age-related atrial changes such as atrophy and fibrosis,1,10 but this association has never been directly examined.
Susceptibility to POAF may depend on the preoperative metabolic status of the atria, and this may be reflected by the mitochondrial response to postischemic reperfusion injury. Directly or indirectly, mitochondria are important targets for virtually all types of injurious stimuli, including hypoxia and toxins. The damage is commonly expressed as the formation of high-conductance channels, the so-called mitochondrial permeability transition. This, together with findings of an association between deletion of mitochondrial DNA aging and atrial fibrillation, has led to the assumption that impaired mitochondrial function in atrial cardiomyocytes may be related to atrial fibrillation after cardiac surgery.11,12 This study was designed to examine mitochondrial function in response to simulated ischemia-reperfusion (I/R) stress as a potential predictor for the development of atrial fibrillation after surgery.
|
Methods |
|---|
|
TopAbstractMethodsResultsDiscussionReferences |
|---|
Right atrial appendages were obtained from 50 patients undergoing elective cardiac surgery with CPB, but before CPB was initiated (Table 1). The protocol in use for simulated I/R of isolated human atrial sections was adopted from studies by Ghosh and colleagues.13 The specimens were placed in test tubes containing oxygenated, heparinized samples of patients' blood and immediately transferred to the laboratory. The specimens were rinsed with glucose-containing phosphate-buffered saline solution (G-PBS), and trabeculae were gently separated longitudinally.
|
Staining solution
The 3-[4,5-dimethythiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) and diazoxide were obtained from Sigma Chemical Co. The MTT staining solution was prepared by dissolving 0.5 mg MTT in 1 mL G-PBS.
Simulated I/R
Initially, atrial sections (5-10 mg each) were placed in containers with 20 mL G-PBS and incubated for 30 minutes for equilibration at 37°C in a cell culture incubator. After aerobic equilibration, the study sections were rinsed in phosphate buffered saline solution without glucose (PBS) and subjected to I/R, whereas the aerobic control sections remained in the incubator in oxygenated G-PBS for the entire experiment. Ischemia (37°C) was simulated by placing the sections (2 sections per 35-mm culture plate) in a sealed Plexiglas (Elf Atochem North America, Inc, Philadelphia, Pa) chamber (Figure 1).14 The chamber was vigorously flushed with 100% nitrogen for 5 minutes to replace the oxygen with nitrogen. En route to the chamber, the gas passed two traps: trap 1 contained 1% sodium sulfite and trap 2 contained water. A 2-mL syringe, flushed with nitrogen, was filled with ischemic PBS (without glucose, bubbled for 1 hour with nitrogen). Each well was quickly filled with 0.4 mL ischemic PBS (minimal volume to cover the slices) and corked. The flow of nitrogen was attenuated to approximately 1 bubble/s at the exit trap, and the ischemia was maintained for 90 minutes. At the end of 90 minutes of simulated ischemia, the slices were subjected to 90 minutes of reoxygenation in oxygenated G-PBS at 37°C in a 100% oxygenated environment.
|
MTT was present for the entire period of the reoxygenation. Two atrial sections were incubated in 1 mL G-PBS containing MTT (0.5 mg/mL) at 37°C for 90 minutes. The sections were then transferred to a small test tube containing 3 mL saline solution and shaken for 1 minute to remove excess dye. The sections were wiped on gauze cloth and transferred to 1.5-mL plastic test tubes and frozen overnight. Extraction of the formazan dye into 1 mL dimethylsulfoxide was done with vigorous shaking for 1 hour at 37°C. The colored supernatant was measured spectrophotometrically at 500 nm. The sections were dried in a 90°C oven for 24 hours. Staining results were expressed as optical density/mg dry weight of myocardial tissue. Tissue viability was expressed as the ratio between the I/R optical density and that of the aerobic control specimens normalized to dry weight of myocardial tissue.
Statistical analysis
The distribution of all variables was compared between patients who had POAF and those who did not, with the Fisher exact test used for discrete variables and Student t test for continuous variables. Data are expressed as mean ± SD. Multiple logistic regression analysis for variables with statistical significance in the univariate model was performed to determine the independent predictors for POAF. All statistical analyses were performed with Statistical Package for the Social Sciences version 10.0 (SPSS Inc, Chicago, Ill).
|
Results |
|---|
|
TopAbstractMethodsResultsDiscussionReferences |
|---|
|
|
|
|
|
Discussion |
|---|
|
TopAbstractMethodsResultsDiscussionReferences |
|---|
MTT has the property of acting as hydrogen acceptor to be reduced by dehydrogenases enzymes. MTT reduction of a viable tissue is related also to the presence of coenzymes nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate. It is a simple and reliable method correlated with ischemic damage and mitochondrial dysfunction in both atrial and ventricular myocardium. In addition, MTT assay can be used to assess myocardial oxygen consumption and energetic index.15 Studies have shown that oxidative stress may lead to compromised mitochondrial redox function as indicated by the MTT assay.13,16,17
This study has pointed out for the first time an association between POAF and mitochondrial dysfunction in response to simulated I/R. It has demonstrated that impaired response of right atrial mitochondria to simulated I/R may predict the development of POAF. Several studies have shown that ischemia or hypoxia followed by reperfusion causes loss of both cytosolic and mitochondrial glutathione, resulting in lack of recovery of contractile activity.18,19 The finding of an association between deletion of mitochondrial DNA aging and atrial fibrillation has led to the assumption that impaired mitochondrial function in cardiomyocytes may be related to atrial fibrillation and aging.11 Such a process leads to bioenergetic deficiency as a result of impaired synthesis in the human atrium. Other studies have shown the association between atrial fibrillation and impaired mitochondrial function in the atrium and alteration in specific adenosine triphosphatase activity, energy metabolism, and upregulation of heart failure markers such as endothelin 1 and atrial natriuretic peptide.11,20 Another report has revealed that mitochondrial dysfunction in the cardiomyocytes may cause impairment of cellular energy metabolism and induced apoptosis.21-23 It is therefore reasonable to hypothesize that susceptibility to POAF may depend on the preoperative metabolic status of the atria and mitochondrial function and that it might be predictable on the basis of morphologic parameters in preoperative histologic atrial specimens and mitochondrial function.
The incidence of atrial fibrillation after off-pump coronary artery bypass grafting surgery is not lower than that after surgery performed with the use of a heart-lung machine. This finding, coupled with the fact that in this study the model was predicted for patients after surgery for valve disease as well as coronary artery bypass grafting, encourages us to support the assumption that this complication is not directly related to the type of surgery or the operative technique but rather to preoperative atrial pathology that is the result of the disease process in a given patient.
Our study has obvious limitations. Its size is limited, and no mechanism to explain the findings has been suggested. Because of the small size, classic parameters involved with POAF such as age and chronic obstructive pulmonary disease did not reach statistical significance. However, it demonstrates an important phenomenon, namely that POAF is associated with mitochondrial dysfunction in response to simulated I/R injury. It has been well discussed in many articles that mitochondria are involved in the ultrastructural changes appearing in the myocardial cells as consequence of oxygen deprivation. Mitochondrial protection during I/R or better mitochondrial function in response to such insult may therefore result in improved myocardial function and may also have antiarrhythmic effect. Studies are now required to dissect the molecular events and changes in gene expression that lead to the increased susceptibility. Such studies may also enhance the identification of preoperative treatments to improve the preoperative metabolic status of the atrial cells and reduce the risk for POAF.
|
References |
|---|
|
TopAbstractMethodsResultsDiscussionReferences |
|---|
This article has been cited by other articles:
|
|
 |
|
  E. Golomb, A. Nyska, and H. Schwalb Occult Cardiotoxicity--Toxic Effects on Cardiac Ischemic Tolerance Toxicol Pathol, August 1, 2009; 37(5): 572 - 593. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Golomb, A. Schneider, E. Houminer, J. Dunnick, G. Kissling, J. B. Borman, A. Nyska, and H. Schwalb Occult Cardiotoxicity: Subtoxic Dosage of Bis(2-chloroethoxy)methane Impairs Cardiac Response to Simulated Ischemic Injury Toxicol Pathol, April 1, 2007; 35(3): 383 - 387. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Sedrakyan, A. Wu, G. Sedrakyan, M. Diener-West, M. Tranquilli, and J. Elefteriades Aprotinin use in thoracic aortic surgery: Safety and outcomes J. Thorac. Cardiovasc. Surg., October 1, 2006; 132(4): 909 - 917. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. V. Podgoreanu and D. A. Schwinn New Paradigms in Cardiovascular Medicine: Emerging Technologies and Practices: Perioperative Genomics J. Am. Coll. Cardiol., December 6, 2005; 46(11): 1965 - 1977. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ANN THORAC SURG | ASIAN CARDIOVASC THORAC ANN | EUR J CARDIOTHORAC SURG |
| J THORAC CARDIOVASC SURG | ICVTS | ALL CTSNet JOURNALS |
