Myocardial stunning is ubiquitous after cardiopulmonary bypass (CPB) in cardiac surgery. Despite marked improvement in surgical techniques and cardioplegic solutions, up to 90% of patients undergoing coronary artery bypass grafting (CABG) experienced decreased ejection fractions and/or cardiac index (CI) postoperatively . Moreover, the overall mortality exceeds 30% in patients with acute ischaemic syndrome, advanced age and decreased myocardial reserve . The significance of myocardial dysfunction is believed to be associated with impaired high-energy production and utilization, inadequate myocardial perfusion, free radical injury and altered calcium homeostasis . In the hope of preventing postoperative ventricular dysfunction and improving overall outcome, recent advances in caring for high-risk cardiac patients centre upon optimization of cardio-protection by introducing cardiac preconditioning as a synergistic adjunct to current strategies.
Ischaemic preconditioning (IPC) as originally described by Murry is defined as a rapid, adaptive response to a brief ischaemic insult that improves the tolerance of the myocardium to a subsequent period of more prolonged ischaemia . Experimental evidence accumulated to date show that IPC helps better preserve the myocardial energy state, delays onset of irreversible cell injury, limits infarct size and induces better recovery of contractile function and fewer reperfusion dysrhythmia [5–9]. Although the precise mechanism of IPC is not clearly defined, the development of brief oxidative stress presumably leads to adaptive modification of the heart through a chain of reactions including generation of intracellular mediators , activation of signal transducers  and modification of gene expression [12,13].
More recently, extensive research focuses on identifying clinical compounds that could pharmacologically mimic the cardio-protective effects of IPC to avoid an ischaemic-type preconditioning stimulus in high-risk patients in whom any additional myocardial ischaemic injury can adversely affect postoperative outcome. Isoflurane is a popular and widely used volatile anaesthetic among patients undergoing surgical procedures. Many researchers have shown that volatile anaesthetics can exert early preconditioning to reduce myocardial infarction through a signal transduction pathway that is remarkably similar to that observed during IPC [14,15]. Activation of adenosine receptors , protein kinase C , inhibitory guanine regulatory proteins  and mitochondrial and sarcolemmal adenosine triphosphate regulated potassium (KATP) channels  are implicated in anaesthetic-induced preconditioning. Experimental studies demonstrate that volatile anaesthetic, when administered immediately before an ischaemic interval, decreases high-energy phosphate utilization during ischaemia, preserves cellular ultrastructure, and improves recovery of postischaemic myocardial contractility [20,21]. Whether these experimental approaches will yield reproducible clinical benefits in patients with stunned myocardium remains to be determined.
In the present study, we investigated the phenomenon of preconditioning the myocardium with isoflurane to determine whether or not its cardio-protective effects during the ischaemic-reperfusion period result in reduced myocardial dysfunction or infarct after CPB. It is our belief that administering isoflurane via the CPB circuit before aortic cross-clamping is an important adjunct to our current cardioplegic technique, representing an important and clinically accessible component of myocardial protection.
After approval by the institutional review committee and written informed consent were obtained from the patients, 40 patients with stable angina and multi-vessel disease undergoing elective CABG surgery were enrolled in this prospective, randomized, placebo-controlled study. Patients with acute (<1 week) myocardial infarction, unstable angina, left ventricular aneurysm or very poor left ventricular function (ejection fraction = 25%), significant valvular disease, chronic obstructive pulmonary disease, advanced renal or hepatic dysfunction and those taking sulphonylurea anti-diabetic drugs or theophylline preparations were excluded from participation. Patients' medications were continued up to the morning of surgery.
Anaesthesia and surgical procedures
The conduction of anaesthesia and surgery were similar in all patients. For induction of anaesthesia, a standard protocol with diazepam (0.2 mg kg−1), fentanyl (5–10 μg kg−1), and pancuronium (0.2 mg kg−1) was used. After intubation, all patients were ventilated with a mixture of air and oxygen; but no volatile anaesthetics were administered until the onset of CPB in patients selected for treatment. Anaesthesia was maintained with fentanyl (3–5 μg kg−1 h−1), propofol (2–6 μg kg−1 h−1), midazolam (0.1 μg kg−1 h−1) and pancuronium (40 μg kg−1 h−1). A pulmonary artery catheter was placed after induction of anaesthesia. Following mid-sternotomy and routine preparation, CPB (non-pulsatile roller pump, membrane oxygenator and arterial line filter) was started under full heparinization.
The CPB circuit was primed with Ringer's lactate (20 mL kg−1), sodium bicarbonate (1 mL kg−1, 7.5% w/v), and mannitol (20% w/v, 5g kg−1). Once bypass was running at a full flow (2.4L min−1 m−2) with the heart depressed, patients were randomly assigned to the control or volatile isoflurane preconditioning groups (ISO group). In the ISO group, isoflurane (2.5 minimum alveolar concentration (MAC)) was added to the gas mixture in the oxygenator for 15 min, followed by 5 min of isoflurane-free bypass before aortic cross-clamping. Patients experiencing severe hypotension (mean arterial pressure < 50 mmHg) after isoflurane administration were excluded to keep away from the possible effect of IPC. Patients in the control group received a time-matched (20 min) period of isoflurane-free CPB. After aortic cross-clamping, modified St Thomas cardioplegic was delivered via the aortic root and coronary sinus to achieve cardiac arrest. The cardioplegic was administered at regular intervals to maintain cardioplegia until completion of the bypass graft.
The core body temperature was allowed to drift to moderate hypothermia of 28–29°C. After completion of vessel anastomosis, the final warming cardioplegic was infused before removal of the aortic cross-clamp, and the heart resumed beating. While the institutional CPB weaning practice was a renal dose of dopamine (2 μg kg−1 min−1), an increased dopamine dose of 5 μg kg−1 min−1 was used when the CI 10 min after termination of CPB was below 2.0 L min−1 m−2.
Determination of biochemistry markers
Blood samples for troponin I (TnI) were obtained before induction of anaesthesia and at 6 h, 1 and 2 days postoperatively. All the ischaemic markers were immediately measured using the Immulite analyser (Turbo, DPC, Los Angeles, CA, USA), using a sandwich chemiluminescence detection method.
Assessment of cardiac function
Using a continuous cardiac output (CCO) monitoring system (Baxter Swan-Ganz® CCO/SVO2 model 744H-7.5F; Baxter/Edwards Critical-Care, Irvine, CA, USA), haemodynamic parameters, including heart rate (HR), MAP, central venous pressure (CVP), mean pulmonary arterial pressure (MPAP), pulmonary capillary wedge pressure (PCWP), CI, stroke volume index (SVI) and systemic and pulmonary vascular resistance indices (SVRI, PVRI) were recorded after anaesthetic induction, 15 min after cessation of CPB, 6 h after arrival in the intensive care unit (ICU), and on the first postoperative day. The variables were measured in triplicate by individuals who were unaware of intraoperative isoflurane administration.
Perioperative pharmacological inotropic support
Administration of high-dose dopamine for post-CPB inotropic support was recorded after CPB completion and 24 h after surgery in the ICU by independent observers blinded to the study protocol.
Clinical outcome analysis
After surgery, patients were evaluated daily for the occurrence of adverse events, such as new myocardial infarction, cerebrovascular insult or renal dysfunction, which were diagnosed by the intensive care physician who were not aware of the objective or the hypothesis of the study. Long-term end-points, including length of stay in ICU, length of hospital stay and the time requirement of extubation were also recorded.
All data are expressed as means ± SDs. Patient characteristics and CPB data were analysed using χ2 (or Fisher's exact test) and unpaired t-tests. U-test was used to analyse the differences in the serial TnI data between the two groups of patients. Differences in serial haemodynamic profiles, both within the same group and between groups were analysed using paired t-tests or analysis of variance for repeated measures. Significance was set at P < 0.05. Statistical analyses were performed using the SPSS statistical software package, version 10.0 (SPSS Inc., Chicago, IL, USA).
Salient patient characteristics and the intraoperative data are summarized in Table 1. The study groups were similar with respect to all parameters before CPB. Complete revascularization and uneventful surgery was achieved for all patients.
MAP and HR increased after surgery. The changes in HR, MAP, CVP, MPAP and PCWP were similar in both groups (Table 2). PVRI was significantly increased after surgery in both groups. In the control group, there was no significant change in CI after CPB. In the ISO group, CI increased significantly from 2.1 ± 0.4 to 2.7 ± 0.7 and 2.6 ± 0.6L min−1 m−2 at 15 min after CPB and 6 h after surgery, respectively (P < 0.05; Table 2). The SVI was significantly higher in the isoflurane group after CPB and 6 h after surgery as compared to the control subjects (P < 0.05).
Cardiac TnI analysis
The postoperative release of cardiac TnI was consistently lower in the isoflurane group than that in the control group (Fig. 1). The mean TnI level at 24 h after surgery was significantly reduced in the isoflurane group compared with the control group (P = 0.04).
After CPB, 20% (5/20) of the patients in the control group and 15% (3/20) of the patients in the isoflurane group received high-dose dopamine (>5 μg kg−1 min−1) support. There were no statistical differences between the two groups.
There was no adverse effect related to isoflurane administration. Length of stay in ICU, length of hospital stay and time requirement of extubation were not significantly different. Postoperatively, there was one patient death and one patient with pleural effusion in each group.
An increasing number of investigations demonstrate that isoflurane protects against myocardial ischaemia-reperfusion injury. The present study showed that 15-min pre-administration of isoflurane followed by a 5-min washout period resulted in an improved post-bypass CI and reduced TnI release in patients undergoing CABG surgery.
IPC of the myocardium is a proven alternative and powerful cardio-protective strategy. Two distinct windows of protection are produced by IPC: an acute early memory phase limited to 1–3 h after the brief ischaemic stimulus (termed classic preconditioning) , and a longer, more delayed, period emerging after 12–24 h and persisting for up to 72 h (termed second-window preconditioning) [23,24].
Volatile anaesthetics, including isoflurane, provided myocardial protection in experimental animals through a signal transduction cascade that is remarkably similar to the pathways identified in IPC [25–28]. Pre-administration of isoflurane exerted a protective effect by reducing infarct size when the discontinuation of this volatile agent was followed by a washout period before ischaemia. A few small studies have investigated the anaesthetic-induced preconditioning effect of isoflurane in patients undergoing CABG surgery and the data support a cardio-protective effect of isoflurane as evidenced by a trend of consistently lower levels of creatine kinase (CK-MB) and TnI, improved haemodynamic recovery and decreased ST-segment changes [29–31].
Myocardial ischaemia and reperfusion result in dysrhythmia, injury to coronary microvasculature and contractile dysfunction or myocardial stunning. Myocardial stunning after CABG is associated with increased morbidity and mortality in patients with severe multivessel disease and with reduced myocardial function. Previous studies demonstrated that CI remains essentially unchanged or is even lowered immediately after CABG . This contractile dysfunction usually resolved within 24–48 h and it did not appear to be dependent on alterations in preload and afterload . Warltier and colleagues showed that isoflurane improved the functional recovery of stunned myocardium . Our results are in concordance with the findings that isoflurane-pretreated patients had improved recovery of myocardial performance postoperatively, as manifested by better CI and SVI during the first 15 min and 6 h after CPB (P < 0.05). Due to the preload of the heart, manifested as PCWP and CVP, were similar between the groups, the haemodynamic improvement might result from less stunned myocardial function and better recovery of contractility. While CI can be influenced by many other therapeutic interventions, especially inotropic support, because there was no statistically significant difference in post-CPB dopamine support between the groups, this potential source of influence was minimal.
Whether isoflurane is capable of producing second-window preconditioning remains unclear. Kehl and colleagues  demonstrated that isoflurane did not produce a second window of protection when administered 24 h before prolonged myocardial ischaemia in vivo. In agreement with their findings, we found that the changes in CI 24 h after CPB did not significantly differ, suggesting that isoflurane did not confer delayed preconditioning protection in CABG patients. Conversely, Tanaka and colleagues  demonstrated that exposure to isoflurane 24 h before coronary artery occlusion and reperfusion reduced experimental myocardial infarct size in rabbits, indicating a second window of preconditioning. The discrepancy may be due to variations in experimental design and more likely resulted because the late window of protection seems to occur at distinct times in different species . Future research is needed to characterize the time and duration of the early and second windows of anaesthetic-induced preconditioning.
We used TnI, a sensitive marker of cellular necrosis, to evaluate myocardial cellular injury in our study. Unlike the myocardial fraction of CK-MB, it is not released from skeletal muscle during surgery and is normally present in the plasma in very low concentrations, thus providing a wide diagnostic window. The results showed consistently lower release of TnI in the ISO group. Compared to the controls, the mean TnI level was significantly reduced in the ISO group at 24 h after surgery. Our results support the improved myocardial protection that isoflurane confers, as demonstrated by previous studies [29,30].
We acknowledge several potential limitations of the present study. Firstly, patients taking sulfonylurea anti-diabetic drugs were excluded because previous studies proved oral hypoglycaemic agents are inhibitors of the KATP channel, and both ischaemic and anaesthetic preconditioning are abolished by sulfonylurea [38,39]. Due to coronary artery disease and myocardial infarction occur with increased frequencies among diabetic patients, preconditioning of the diabetic myocardium may differ considerably from preconditioning of non-diabetic myocardium. The role of isoflurane-induced preconditioning in diabetic hearts needs to be thoroughly investigated before any conclusion can be drawn as to its applicability in the clinical situation. Secondly, the present study only examined the effects of isoflurane at 2.5 MAC in one cycle therefore we could not show dose-related effects of isoflurane during CABG. Furthermore, despite several cycles of IPC being shown to improve outcome compared to only one cycle , it remains to be established whether or not anaesthetic preconditioning can be enhanced by administering the volatile anaesthetic in several cycles of exposure interspersed with corresponding washout intervals. It may also be severely constrained by practical considerations to allow enough time for sufficient washout to occur in vivo during anaesthesia and surgery. Thirdly, different anaesthesia protocols could change the results. Nonetheless, in our study, the two groups underwent the same anaesthesia protocol, thus minimizing the potential influence of differing protocols. Fourthly, the CI and SVI differences are rather short-lived, suggesting only an early protective effect of isoflurane-induced preconditioning. Larger scale studies are required to validate these findings and to determine whether isoflurane could confer a longer protection at clinically relevant concentrations. Finally, owing to cost and practical considerations, it is hard for us to increase our sample size. A small sample size may have low statistical power, introduce important biases and the results may not be robust. Further investigation with a larger sample size will be required to determine how isoflurane preconditioning can be best achieved in the clinical setting.
In summary, the current results indicate that a 15-min pre-administration of isoflurane (2.5 MAC), followed by a 5-min washout period before aortic cross-clamping appeared to confer significantly early improvement in haemodynamic performance and significantly reduced release of TnI of human beings hearts after CABG. Further studies are needed to investigate how anaesthetic preconditioning can be best achieved in the clinical setting.
This work has been supported by a research grant from the Kaohsiung Veterans General Hospital (VGHKS 93-01).
1. Khuri SF, Farhat T, Zankoul F et al
. Intraoperative assessment of the stunned versus infarcted myocardium with the simultaneous use of transesophageal echocardiography and the measurement of myocardial pH: two case studies. J Cardiac Surg
(Suppl 3): 403–409.
2. Bounous EP, Mark DB, Pollock BG et al
. Surgical survival benefits for coronary disease patients with left ventricular dysfunction. Circulation
3. Ely SW, Berne RM. Protective effect of adenosine in myocardial ischemia. Circulation
4. Murry CE, Jennings RB, Reimer KA. Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium. Circulation
5. Cave AC. Preconditioning induced protection against post-ischemic contractile dysfunction: characteristics and mechanisms. J Mol Cell Cardiol
6. Kaukoranta PK, Martti PK, Ylitalo KV, Kiviluoma KT, Peuhkurinen KJ. Normothermic retrograde blood cardioplegia with or without preceding ischemic preconditioning. Ann Thorac Surg
7. Eltchaninoff H, Cribier A, Tron C, Derumeaux G, Koning R, Hecketsweiller B et al
. Adaptation to myocardial ischemia during coronary angioplasty demonstrated by clinical, electrocardiographic, echocardiographic, and metabolic parameters. Am Heart J
8. Ghosh S, Standen NB, Galinanes M. Preconditioning the human myocardium by stimulated ischemia studies on the early and delayed protection. Cardiovasc Res
9. Walker DM, Walker JM, Pugsley WB, Pattison CW, Yellon DM. Preconditioning in isolated superfused human muscle. J Mol Cell Cardiol
10. Toller WG, Kersten JR, Pagel PS, Warltier DC. Ischemic preconditioning, myocardial stunning and anesthesia. Curr Opin Anaesthesiol
11. Schulz R, Cohen MV, Behrends M, Downey JM, Heusch G. Signal transduction of ischemic preconditioning. Cardiovasc Res
12. Baxter GF, Ferdinandy P. Delayed preconditioning of myocardium: current perspectives. Basic Res Cardiol
13. Kloner RA, Jennings RB. Consequences of brief ischemia: stunning, preconditioning, and their clinical implications: Part 2. Circulation
14. Hawaleshka A. Ischemic preconditioning: mechanisms and potential clinical applications. Can J Anaesth
15. D'Attellis N. Anesthetic preconditioning: target the right patients. Anaesthesiology
, 2003; 98
16. Hanouz JL, Yvon A, Massetti M, Lepage O, Babatasi G, Khayat A et al
. Mechanisms of desflurane-induced preconditioning in isolated human right atria in vitro
17. Toller WG, Gross ER, Kersten JR, Pagel PS, Gross GJ, Warltier DC. Sarcolemmal and mitochondrial adenosine triphosphate-dependent potassium channels: mechanism of desflurane-induced cardioprotection. Anaesthesiology
18. Toller WG, Kersten JR, Gross ER, Pagel PS, Warltier DC. Isoflurane preconditions myocardium against infarction via activation of inhibitory guanine nucleotide binding proteins. Anaesthesiology
19. Stadnicka A, Kwok WM, Warltier DC, Bosnjak ZJ. Protein tyrosine kinase-dependent modulation of isoflurane effects on cardiac sarcolemmal K (ATP) channel. Anaesthesiology
20. Kohro S, Hogan QH, Nakae Y, Yamakage M, Bosnjak ZJ. Anesthetic effects on mitochondrial ATP-sensitive K channel. Anaesthesiology
21. Kato R, Foex P. Myocardial protection by anesthetic agents against ischemia-reperfusion injury: an update for anesthesiologists. Can J Anaesth
22. Van Winkle DM, Thornton JD, Downey DM, Downey JM. The natural history of preconditioning: cardioprotection depends on duration of transient ischemic and time to subsequent ischemia. Coronary Artery Dis
23. Kuzuya T, Hoshida S, Yamashita N et al
. Delayed effects of sublethal ischemia on the acquisition of tolerance to ischemia. Circ Res
24. Przyklenk K, Kloner RA. Ischemic preconditioning: exploring the paradox. Prog Cardiovasc Dis
25. Kersten JR, Schmeling TJ, Pagel PS, Gross GJ, Warltier DC. Isoflurane mimics ischemic preconditioning via activation of KATP
channels: reduction of myocardial infarct size with an acute memory phase. Anaesthesiology
26. Cason BA, Gamperl AK, Slocum RE, Hickey RF. Anesthetic-induced preconditioning: previous administration of isoflurane decreases myocardial infarct size in rabbits. Anaesthesiology
27. Cope DK, Impastato WK, Cohen MV, Downey JM. Volatile anesthetics protect the ischemic rabbit myocardium from infarction. Anaesthesiology
28. Ismaeil MS, Tkachenko I, Gamperl AK, Hickey RF, Cason BA. Mechanisms of isoflurane-induced myocardial preconditioning in rabbits. Anaesthesiology
29. Belhomme D, Peynet J, Louzy M, Launay JM, Kitakaze M, Menasche P. Evidence for preconditioning by isoflurane in coronary artery bypass graft surgery. Circulation
30. Tomai F, De Paulis R, Penta de Peppo A et al
. Beneficial impact of isoflurane during coronary bypass surgery on troponin I release. G Ital Cardiol
31. Haroun-Bizri S, Khoury SS, Chehab IR, Kassas CM, Baraka A. Does isoflurane optimize myocardial protection during cardiopulmonary bypass
? J Cardiothorac Vasc Anaesth
32. Lee HT, LaFaro RJ, Reed GE. Pretreatment of human myocardium with adenosine during open heart surgery. J Card Surg
33. Kloner R, Przyklenk K, Kay G. Clinical evidence for stunned myocardium after coronary artery bypass surgery. J Card Surg
34. Warltier DC, Al-Wathiqui MH, Kampine JP, Schmeling WT. Recovery of contractile function of stunned myocardium in chronically instrumented dogs is enhanced by halothane or isoflurane. Anaesthesiology
35. Kehl F, Pagel PS, Krolidowski JG et al
. Isoflurane does not produce a second window of preconditioning against myocardial infarction in vivo
. Anaesth Analg
36. Tanaka K, Ludwig LM, Krolidowski JG et al
. Isoflurane produces delayed preconditioning against myocardial ischemia and reperfusion injury. Anaesthesiology
37. Schwartz LM, Sebbag L, Jennings RB, Reimer KA. Duration and reinstatement of myocardial protection against infarction by ischemic preconditioning in open chest dogs. J Mol Cell Cardiol
38. Grover GJ, Garlid KD. ATP-sensitive potassium channels: a review of their cardioprotective pharmacology. J Mol Cell Cardiol
39. Tanaka K, Kehl F, Gu W et al
. Isoflurane-induced preconditioning is attenuated by diabetes. Am J Physiol Heart Circ Physiol
40. Sandhy R, Diaz RJ, Mao GD, Wilson GJ. Ischemic preconditioning: differences in protection and susceptibility to blockade with singlecycle versus multicycle transient ischemia. Circulation