Is there a place for preconditioning during cardiac operations in humans?

Effect of helium pre- or postconditioning on signal transduction kinases in patients undergoing coronary artery bypass graft surgery

BACKGROUND

The noble gas helium induces pre- and postconditioning in animals and humans. Volatile anesthetics induce cardioprotection in humans undergoing coronary artery bypass graft (CABG) surgery. We hypothesized that helium induces pre- and postconditioning in CABG-patients, affecting signaling molecules protein kinase C-epsilon (PKC-ε), p38 mitogen activated protein kinase (p38 MAPK), extracellular signal-regulated kinase 1/2 (ERK-1/2) and heat shock protein 27 (HSP-27) within cardiac tissue, and reducing postoperative troponin levels.

METHODS

After ethical approval and informed consent, 125 elective patients undergoing CABG surgery were randomised into this prospective, placebo controlled, investigator blinded, parallel arm single-centre study. Helium preconditioning (3 × 5 min of 70 % helium and 30 % oxygen) was applied before aortic cross clamping; postconditioning (15 min of helium) was applied before release of the aortic cross clamp. Signaling molecules were measured in right atrial appendix specimens. Troponin-T was measured at 4, 12, 24 and 48 h postoperatively.

RESULTS

Baseline characteristics of all groups were similar. Helium preconditioning did not significantly alter the primary outcome (molecular levels of kinases PKC-ε and HSP-27, ratio of activated p38 MAPK or ERK ½). Postoperative troponin T was 11 arbitrary units [5, 31; area-under-the-curve (interquartile range)] for controls, and no statistically significant changes were observed after helium preconditioning [He-pre: 11 (6, 18)], helium postconditioning [He-post: 11 (8, 15)], helium pre- and postconditioning [He-PP: 14 (6, 20)] and after sevoflurane preconditioning [APC: 12 (8, 24), p = 0.13]. No adverse effects related to study treatment were observed in this study.

CONCLUSIONS

No effect was observed of helium preconditioning, postconditioning or the combination thereof on activation of p38 MAPK, ERK 1/2 or levels of HSP27 and PKC-ε in the human heart. Helium pre- and postconditioning did not affect postoperative troponin release in patients undergoing CABG surgery. Clinical trial number Dutch trial register ( http://www.trialregister.nl/ ) number NTR1226.

Volatile Anesthetics and Cardiac Function

All volatile anesthetics have been shown to induce a dose-dependent decrease in myocardial contractility and cardiac loading conditions. These depressant effects decrease myocardial oxygen demand and may, therefore, have a beneficial role on the myocardial oxygen balance during myocardial ischemia. Recently, experimental evidence has clearly demonstrated that in addition to these indirect protective effects, volatile anesthetic agents also have direct protective properties against reversible and irreversible ischemic myocardial damage. These properties have not only been related to a direct preconditioning effect but also to an effect on the extent of reperfusion injury. The implementation of these properties during clinical anesthesia can provide an additional tool in the treatment or prevention, or both, of ischemic cardiac dysfunction in the perioperative period. In the clinical practice, these effects should be associated with improved cardiac function, finally resulting in a better outcome in patients with coronary artery disease. The potential application of these protective properties of volatile anesthetic agents in clinical practice is the subject of ongoing research. This review summarizes the current knowledge on this subject.

Sevoflurane preconditioning in on-pump coronary artery bypass grafting: a meta-analysis of randomized controlled trials

PURPOSE

Sevoflurane preconditioning (SevoPreC) has been proved to prevent organ ischemia/reperfusion (I/R) injury in various animal models and preclinical studies. Clinical trials on cardioprotection by SevoPreC for adult patients undergoing coronary artery bypass graft (CABG) revealed mixed results. The aim of this meta-analysis was to evaluate the cardiac effect of SevoPreC in on-pump CABG.

METHODS

Randomized controlled trials (RCT) comparing the cardiac effect of SevoPreC (compared with control) in adult patients undergoing CABG were searched from PubMed, Embase, and the Cochrane Library (up to November 2015). The primary endpoints were postoperative troponin levels. Additional endpoints were CK-MB levels, mechanic ventilation (MV) duration, intensive care unit (ICU) stay, and hospital length of stay (LOS).

RESULTS

Six trials with eight comparisons enrolling a total of 384 study patients reporting postoperative troponin levels were identified. Compared with controls, SevoPreC decreased postoperative myocardial troponin levels [standardized mean difference (SMD) = -0.38; 95 % CI, -0.74 to -0.03; P = 0.04; I ² = 63.9 %]. However, no significant differences were observed in postoperative CK-MB levels [weighted mean difference (WMD) = -1.71; P = 0.37; I ² = 37.7 %], MV duration (WMD = -0.53; P = 0.47; I ² = 0.0 %), ICU stay (WMD = -0.91; P = 0.39; I ² = 0.9 %), and hospital LOS (WMD = 0.08; P = 0.86; I ² = 8.0 %).

CONCLUSION

Available evidence from the present systematic review and meta-analysis suggests that sevoflurane preconditioning may reduce troponin levels in on-pump CABG. Future high-quality, large-scale clinical trials should focus on the early and long-term clinical effect of SevoPreC in on-pump CABG.

Chronic Tempol treatment restores pharmacological preconditioning in the senescent rat heart

Cardioprotective effects of anesthetic preconditioning and cyclosporine A (CsA) are lost with aging. To extend our previous work and address a possible mechanism underlying age-related differences, we investigated the role of oxidative stress in the aging heart by treating senescent animals with the oxygen free radical scavenger Tempol. Old male Fischer 344 rats (22–24 mo) were randomly assigned to control or Tempol treatment groups for 2 or 4 wk (T×2wk and T×4wk, respectively). Rats received isoflurane 30 min before ischemia-reperfusion injury or CsA just before reperfusion. Myocardial infarction sizes were significantly reduced by isoflurane or CsA in the aged rats treated with Tempol (T×4wk) compared with old control rats. In other experiments, young (4–6 mo) and old rats underwent either chronic Tempol or vehicle treatment, and the levels of myocardial protein oxidative damage, antioxidant enzymes, mitochondrial Ca2+ uptake, cyclophilin D protein, and mitochondrial permeability transition pore opening times were measured. T×4wk significantly increased MnSOD enzyme activity, GSH-to-GSSH ratios, MnSOD protein level, mitochondrial Ca2+ uptake capacity, reduced protein nitrotyrosine levels, and normalized cyclophilin D protein expression in the aged rat heart. T×4wk also significantly prolonged mitochondrial permeability transition pore opening times induced by reactive oxygen species in old cardiomyocytes. Our studies demonstrate that 4 wk of Tempol pretreatment restores anesthetic preconditioning and cardioprotection by CsA in the old rat and that this is associated with decreased oxidative stress and improved mitochondrial function. Our results point to a new protective strategy for the ischemic myocardium in the high-risk older population.

The mitochondrial permeability transition pore and its role in anaesthesia-triggered cellular protection during ischaemia-reperfusion injury

This review summarises the most recent data in support of the role of the mitochondrial permeability transition pore (mPTP) in ischaemia-reperfusion injury, how anaesthetic agents interact with this molecular channel, and the relevance this holds for current anaesthetic practice. Ischaemia results in damage to the electron transport chain of enzymes and sets into play the assembly of a non-specific mega-channel (the mPTP) that transgresses the inner mitochondrial membrane. During reperfusion, uncontrolled opening of the mPTP causes widespread depolarisation of the inner mitochondrial membrane, hydrolysis of ATP, mitochondrial rupture and eventual necrotic cell death. Similarly, transient opening of the mPTP during less substantial ischaemia leads to differential swelling of the intermembrane space compared to the mitochondrial matrix, rupture of the outer mitochondrial membrane and release of pro-apoptotic factors into the cytosol. Recent data suggests that cellular protection from volatile anaesthetic agents follows specific downstream interactions with this molecular channel that are initiated early during anaesthesia. Intravenous anaesthetic agents also prevent the opening of the mPTP during reperfusion. Although by dissimilar mechanisms, both volatiles and propofol promote cell survival by preventing uncontrolled opening of the mPTP after ischaemia. It is now considered that anaesthetic-induced closure of the mPTP is the underlying effector mechanism that is responsible for the cytoprotection previously demonstrated in clinical studies investigating anaesthetic-mediated cardiac and neuroprotection. Manipulation of mPTP function offers a novel means of preventing ischaemic cell injury. Anaesthetic agents occupy a unique niche in the pharmacological armamentarium available for use in preventing cell death following ischaemia-reperfusion injury.

Protection by remote ischemic preconditioning during coronary artery bypass graft surgery with isoflurane but not propofol - a clinical trial

BACKGROUND

Remote ischemic preconditioning (RIPC) of the myocardium by limb ischemia/reperfusion may mitigate cardiac damage, but its interaction with the anesthetic regimen is unknown. We tested whether RIPC is associated with differential effects depending on background anesthesia. Specifically, we hypothesized that RIPC during isoflurane anesthesia attenuates myocardial injury in patients undergoing coronary artery bypass graft (CABG) surgery, and that effects may be different during propofol anesthesia.

METHODS

In a randomized, single-blinded, placebo-controlled prospective study, serum troponin I concentration (cTnI) (baseline, and 1, 6, 12, 24, 48, and 72 h postoperatively) were measured during isoflurane/sufentanil or propofol/sufentanil anesthesia with or without RIPC (three 5-min periods of intermittent left upper arm ischemia with 5 min reperfusion each) in non-diabetic patients (n = 72) with three-vessel coronary artery disease (ClinicalTrials.gov NCT01406678).

RESULTS

RIPC during isoflurane anesthesia (n = 20) decreased the area under the cTnI time curve (cTnI AUC) (-50%, 190 ± 105 ng/ml × 72 h vs. 383 ± 262 ng/ml × 72 h, P = 0.004), and the peak (7.3 ± 3.6 ng/ml vs. 11.8 ± 5.5, P = 0.004) and serial (P < 0.041) postoperative cTnI when compared to isoflurane alone (n = 19). In contrast, RIPC during propofol anesthesia (n = 14) did not alter the cTnI AUC [263 ± 157 ng/ml × 72 h vs. 372 ± 376 ng/ml × 72 h (n = 19), P = 0.318] or peak postoperative cTnI (10.1 ± 4.5 ng/ml vs. 12 ± 8.2, P = 0.444). None of the patients experienced harm or side effects from the intermittent left arm ischemia.

CONCLUSION

Thus, RIPC during isoflurane but not during propofol anesthesia decreased myocardial damage in patients undergoing CABG surgery. Accordingly, effects of RIPC evoked by upper limb ischemia/reperfusion depend on background anesthesia, with combined RIPC/isoflurane exerting greater beneficial effects under conditions studied.

Drugs mediating myocardial protection

The occurrence of myocardial ischaemia will result in either reversible or irreversible myocardial dysfunction. Even when revascularization is successful, some reperfusion injury may occur that transiently impairs myocardial function. Therefore, treatment should not only be directed towards prompt restoration of myocardial blood flow but measures should also be taken to prevent or alleviate the consequences of myocardial reperfusion injury. Over the years, various strategies have been developed. The present contribution reviews a number of these strategies focusing on pharmacological treatments that have been developed to address myocardial reperfusion injury.

Anaesthesia and myocardial ischaemia/reperfusion injury

Anaesthetists are confronted on a daily basis with patients with coronary artery disease, myocardial ischaemia, or both during the perioperative period. Therefore, prevention and ultimately adequate therapy of perioperative myocardial ischaemia and its consequences are the major challenges in current anaesthetic practice. This review will focus on the translation of the laboratory evidence of anaesthetic-induced cardioprotection into daily clinical practice.

Preconditioning, anesthetics, and perioperative medication

Activation of endogenous signal transduction pathways, by a variety of stimuli including ischemic and anesthetic pre- and post-conditioning, protects myocardium against ischemia and reperfusion injury. Experimental evidence suggests that adenosine-regulated potassium channels, cyclooxygenase-2, intracellular kinases, endothelial nitric oxide synthase, and membrane bound receptors play critical roles in signal transduction, and that intracellular signaling pathways ultimately converge on mitochondria to produce cardioprotection. Disease states, and perioperative medications such as sulfonylureas and COX-2 antagonists, could have adverse effects on cardioprotection by impairing activation of ion channels and proteins that are important in cell signaling. Insights gained from animal and clinical studies are reviewed and recommendations given for the use of perioperative anesthetics and medications.

Inosine and equilibrative nucleoside transporter 2 contribute to hypoxic preconditioning in the murine cardiomyocyte HL-1 cell line

The purine nucleoside adenosine is a physiologically important molecule in the heart. Brief exposure of cardiomyocytes to hypoxic challenge results in the production of extracellular adenosine, which then interacts with adenosine receptors to activate compensatory signaling pathways that lead to cellular resistance to subsequence hypoxic challenge. This phenomenon is known as preconditioning (PC), and, while adenosine is clearly involved, other components of the response are less well understood. Flux of nucleosides, such as adenosine and inosine, across cardiomyocyte membranes is dependent on equilibrative nucleoside transporters 1 and 2 (ENT1 and ENT2). We have previously shown in the murine cardiomyocyte HL-1 cell line that hypoxic challenge leads to an increase in intracellular adenosine, which exits the cell via ENT1 and preconditions via A1 and A3 adenosine receptor-dependent mechanisms. However, the role and contribution of inosine and ENT2 are unclear. In this study, we confirmed that ENT1 and ENT2 are both capable of transporting inosine. Moreover, we found that hypoxic challenge leads to a significant increase in levels of intracellular inosine, which exits the cell via both ENT1 and ENT2. Exogenously added inosine (5 μM) preconditions cardiomyocytes in an A1 adenosine receptor-dependent manner since preconditioning can be blocked by the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (1 μM) but not the A3 adenosine receptor antagonist MRS-1220 (200 nM). These data suggest that cardiomyocyte responses to hypoxic PC are more complex than previously thought, involving both adenosine and inosine and differing, but overlapping, contributions of the two ENT isoforms.

Cardioprotection with Volatile Anesthetics in Cardiac Surgery

Myocardial ischemia during the perioperative period is a major cause of morbidity and mortality after surgery. Experimental data indicate that clinical concentrations of volatile anesthetics protect the myocardium from ischemia and reperfusion injury, as shown by decreased infarct size and more rapid postoperative recovery of contractile function. These anesthetics may also mediate protective effects in other organs, such as the brain and kidney. A number of recent reports have indicated that these experimentally observed protective effects might also be present in the clinical setting. Implementation of such cardioprotection during surgery may provide an additional tool in the treatment and prevention of ischemic cardiac dysfunction in the perioperative period. This review discusses the clinical studies that have focused on the potential cardioprotective effects of volatile anesthetic agents.

Myocardial protection by isoflurane vs. sevoflurane in ultra-fast-track anaesthesia for off-pump aortocoronary bypass grafting

BACKGROUND AND OBJECTIVES

Volatile anaesthetics have gained more popularity recently due to the potential for cardiac protection. Ultra-fast-track anaesthesia implies the immediate extubation after cardiac surgery. The purpose of this prospective randomized double-blind controlled study is to compare the cardioprotective effects of sevoflurane and isoflurane in off-pump cardiac bypass surgery.

METHODS

Forty patients undergoing elective off-pump cardiac bypass surgery with high thoracic epidural analgesia and immediate extubation at the end of surgery were randomized into two groups. During surgery, anaesthesia was provided with either 1 minimum alvelolar anaesthetic concentration of sevoflurane or 1 minimum alvelolar anaesthetic concentration of isoflurane. Troponin-T, creatine kinase-MB, left ventricular wall motion anomalies, time to extubation, respiratory functions and haemodynamic parameters were compared between the two groups by analysis of variance.

RESULTS

All patients were successfully extubated in the operating theatre with minimal postoperative pain. Serial creatine kinase-MB and troponin-T concentrations were not significantly different between the two volatile agents. Haemodynamic stability throughout surgery and contractility was not different between groups. However, extubation time was significantly shorter with sevoflurane (10 +/- 5 min) compared to isoflurane (18 +/- 4 min).

CONCLUSION

This study indicates that during off-pump cardiac bypass surgery, sevoflurane and isoflurane provide the same ischaemic cardioprotective effects. There is no difference for heart contractility and haemodynamic values during and after off-pump cardiac bypass surgery between the two agents. Sevoflurane allows a more rapid recovery from anaesthesia, but this does not translate into better pulmonary function or haemodynamics. Both agents are similar in ultra-fast-track off-pump cardiac bypass surgery.

Sevoflurane preconditioning at 1 MAC only provides limited protection in patients undergoing coronary artery bypass surgery: a randomized bi-centre trial † ‡

BACKGROUND

Volatile agents can mimic ischaemic preconditioning leading to a decrease in myocardial infarct size. The present study investigated if a 15 min sevoflurane administration before cardiopulmonary bypass (CPB) has a cardioprotective effect in patients undergoing coronary surgery.

METHODS

Seventy-two patients were randomized in two centres. The intervention group (S) received 1 MAC sevoflurane administrated via the ventilator for 15 min followed by a 15 min washout before CPB, the control group did not. The primary outcome was the postoperative troponin Ic peak. A biopsy of the atrium was taken during canulation for enzyme dosages. Results are expressed as mean (SD).

RESULTS

Neither troponin Ic nor tissular enzyme measurement exhibited any difference between the groups: peak of troponin Ic was 4.4 (5.6) in S group vs 5.2 (6.6) ng ml(-1) in control group (ns). Intratissular ecto-5'-nucleotidase activity was 7.1 (4.3) vs 8.5 (11.9), protein kinase C activity was 27.1 (15.7) vs 29.2 (28.7), tyrosine kinase activity was 101 (54.1) vs 98.5 (63.3), and P38 MAPKinase activity was 131.1 (76.1) vs 127.1 (86.8) nmol mg protein(-1) min(-1) in S group and control group, respectively (ns). However there were fewer patients with low postoperative cardiac index in S group (11% in S vs 35% in control group, P < 0.05) when considering the per protocol population. In S group, 25% of patients required an inotropic support during the postoperative period, vs 36% of patients in control group (ns).

CONCLUSIONS

This study did not show a significant preconditioning signal after 15 min of sevoflurane administration. The 15 min duration might be too short or the concentration of sevoflurane too low to induce cardioprotection detected by troponin I levels.

Desflurane and Sevoflurane in Cardiac Surgery: A Meta-Analysis of Randomized Clinical Trials

OBJECTIVES

The authors performed a meta-analysis to investigate whether the cardioprotective effects of volatile anesthetics translate into decreased morbidity and mortality in patients undergoing cardiac surgery.

BACKGROUND

It is commonly believed that the choice of the primary anesthetic agent does not result in different outcomes after cardiac surgery. Recent evidence, however, has indicated that volatile anesthetics improve postischemic recovery at a cellular level, in isolated hearts, in animals, and in humans.

METHODS

Four investigators independently searched BioMedCentral and PubMed. Inclusion criteria were random allocation to treatment and comparison of a total intravenous anesthesia regimen versus an anesthesia plan including desflurane or sevoflurane performed on cardiosurgical patients. Exclusion criteria were duplicate publications, nonhuman experimental studies, and no outcome data. The endpoints were the rate of perioperative myocardial infarction and hospital mortality.

RESULTS

The search yielded 22 studies, involving 1,922 patients. Volatile anesthetics were associated with significant reductions of myocardial infarctions (24/979 [2.4%] in the volatile anesthetics group v 45/874 [5.1%] in the control arm, odds ratio [OR] = 0.51 [0.32-0.84], p for effect = 0.008, and p for heterogeneity = 0.77) and mortality (4/977 [0.4%] v 14/872 [1.6%], OR = 0.31 [0.12-0.80], p for effect = 0.02, and p for heterogeneity = 0.88).

CONCLUSIONS

Desflurane and sevoflurane have cardioprotective effects that result in decreased morbidity and mortality. The present data show for the first time that the choice of an anesthetic regimen based on administration of halogenated anesthetics is associated with a better outcome after cardiac surgery.

The effects of volatile anesthetics on cardiac ischemic complications and mortality in CABG: a meta-analysis

PURPOSE

Coronary artery bypass graft surgery (CABG) is associated with cardiac complications, including ischemia, acute myocardial infarction (AMI), and death. Volatile anesthetics have been shown to have a preconditioning-like effect. This systematic review assesses the effects of volatile anesthetics on cardiac ischemic complications and morbidity after CABG.

METHODS

Data were obtained, without language restriction, from searches of MEDLINE, Science Citation Index, PubMed, and reference lists. We included only prospective randomized controlled trials evaluating volatile anesthetics during CABG. Two reviewers independently abstracted data on myocardial ischemia, acute myocardial infarction (AMI), and death. Treatment effects were calculated as odds ratio (OR) with 95% confidence intervals (CI) for binary data, and weighted mean difference (WMD) with 95% CI for continuous data.

PRINCIPAL FINDINGS

Thirty-two studies (2,841 patients) were included. In comparison with iv anesthesia, volatile anesthetics were associated with reduced all-cause mortality (OR, 0.65; 95% CI, 0.36-1.18; P = 0.16). Enflurane was associated with increased AMI (OR, 1.34; 95% CI, 0.68-2.64; P = 0.40), whereas sevoflurane and desflurane reduced cardiac troponin I (cTnI) at six hours, 12 hr, 24 hr [WMD, -1.45; 95% CI (-1.73, -1.16); P < 0.00001], and 48 hr after operation.

CONCLUSION

This meta-analysis demonstrates sevoflurane and desflurane reduce the postoperative rise in cTnI. Sevoflurane-mediated reduction in cardiac troponin was associated with improved long-term outcomes in one study. This meta-analysis was not able to show that these positive effects on troponin were translated into improved clinical outcomes. Well-designed large randomized control trials are needed to further elucidate the differential cardio-protective effects of volatile anesthetics.

Myocardial protection with volatile anaesthetic agents during coronary artery bypass surgery: a meta-analysis

Previous studies have investigated the role of volatile anaesthetic agents in myocardial protection during coronary artery bypass graft (CABG) surgery, and some have identified beneficial effects. However, these studies have been too small to identify a significant effect on myocardial infarction (MI) or mortality. We undertook a systematic overview and meta-analysis of all randomized trials comparing volatile with non-volatile anaesthesia in CABG surgery. We identified 27 trials that included 2979 patients. There was no significant difference in myocardial ischaemia, MI, intensive care unit length of stay or hospital mortality between the groups (all P>0.05). Post-bypass, patients randomized to receive volatile anaesthetics had 20% higher cardiac indices (P=0.006), significantly lower troponin I serum concentrations (P=0.002) and lesser requirement for inotropic support (P=0.004) compared with those randomized to receive i.v. anaesthetics. Duration of mechanical ventilation was reduced by 2.7 h (P=0.04), and there was a 1 day decrease in hospital length of stay (P<0.001). Some of these outcomes were based on a smaller number of trials because of incomplete data, largely because the individual trials focused on one or more surrogate endpoints. We found some evidence that volatile anaesthetic agents provide myocardial protection in CABG surgery, but larger adequately powered trials with agreed, defined outcomes need to be done to fully assess a possible beneficial effect of volatile anaesthetic agents on the risk of MI and mortality.

Myocardial preconditioning and cardioprotection by volatile anaesthetics

The biological bases and the clinical applications of ischaemic and anaesthetic preconditioning are reviewed. Ischaemic preconditioning is an endogenous defensive phenomenon of the myocardium in which brief periods of ischaemia followed by reperfusion reduce the infarct size induced by longer ischaemic stimuli; both an early and a late phase may be distinguished. In the early phase, the mediators released activate ATP-dependent potassium channels and kinase cascade; these enzymes migrating at the level of various subcellular structures phosphorylate some end-effectors responsible for cardioprotection. Several molecules that are involved in the regulation of cell death during ischaemia-reperfusion injury have been proposed for such a role, including mitochondrial ATP-dependent potassium channels, connexins and cytoskeletal and mitochondrial proteins. In the late phase, the triggers and mediators themselves, plus nitric oxide, are responsible for the genetic reprogramming providing a protective effect via ex-novo synthesis of proteins. Volatile halogenated anaesthetics may induce a particular sort of pharmacological preconditioning, anaesthetic preconditioning, which presents some differences in the biochemical signalling mechanism but is able to give protection comparable to the ischaemic form. Anaesthetic preconditioning presents obvious advantages compared to ischaemic preconditioning, and researchers have tried to take advantage of this in the clinical setting, in cardiac surgical patients, in neuroprotection and to prolong the survival of organs destined for transplantation.

[Myocardial preconditioning with volatile anesthetics. General anesthesia as protective intervention?]

Reduction of the perioperative cardiovascular risk with pharmacological interventions plays a prominent role in routine anesthesia practice. For example, perioperative beta-blockade is well established in anesthesiological treatment of patients. There is a growing body of evidence supporting the cardioprotective effects of volatile anesthetics known as anesthetic-induced preconditioning. There are numerous and complex data from animal studies. The mechanisms of anesthetic-induced preconditioning have been extensively studied but have still not been clearly identified. Initial clinical data show the cardioprotective effects of volatile agents by looking at parameters of myocardial function and laboratory values and therefore, the question of the relevance of these data for routine clinical practice has been raised. This review gives a summary of the currently available data focusing on the mechanisms of anesthesiological preconditioning and clinical studies.

Cardioprotection by volatile anesthetics

Preconditioning describes a very powerful endogenous mechanism by which the heart may be protected against ischemia and reperfusion injury. Transient administration of a volatile anesthetic before a prolonged ischemic episode reduces myocardial infarct size to a degree comparable to that observed during ischemic preconditioning. Many components of the signal transduction pathways responsible for cardioprotection are shared by anesthetic and ischemic preconditioning. Exposure to volatile anesthetics generates small "triggering" quantities of reactive oxygen species (ROS) by directly interacting with the mitochondrial electron transport chain or indirectly through a signaling cascade in which G-protein-coupled receptors, protein kinases, and mitochondrial ATP-sensitive potassium (K(ATP)) channels play important roles. Several clinical studies also suggest that preconditioning by volatile anesthetics exerts beneficial effects in patients undergoing cardiac surgery. This review summarizes some of the recent major developments in the understanding of cardioprotection by volatile anesthetics.

The concept of anaesthetic-induced cardioprotection: clinical relevance

Experimental evidence has clearly demonstrated that volatile anaesthetic agents have direct protective properties against reversible and irreversible ischaemic myocardial damage. These properties have been related to a direct preconditioning effect but also to an effect on the extent of reperfusion injury. The implementation of these properties during clinical anaesthesia can provide an additional tool in the treatment and/or prevention of ischaemic cardiac dysfunction in the perioperative period. In clinical practice, these effects should be associated with improved cardiac function, ultimately resulting in a better outcome in patients with coronary artery disease. This potential application of anaesthetic agents has only recently been explored, and its applicability in clinical practice is the subject of ongoing research. This review summarizes the current knowledge on this subject.

Cardioprotection with volatile anesthetics: mechanisms and clinical implications

Cardiac surgery and some noncardiac procedures are associated with a significant risk of perioperative cardiac morbid events. Experimental data indicate that clinical concentrations of volatile general anesthetics protect the myocardium from ischemia and reperfusion injury, as shown by decreased infarct size and a more rapid recovery of contractile function on reperfusion. These anesthetics may also mediate protective effects in other organs, such as the brain and kidney. Recently, a number of reports have indicated that these experimentally observed protective effects may also have clinical implications in cardiac surgery. However, the impact of the use of volatile anesthetics on outcome measures, such as postoperative mortality and recovery in cardiac and noncardiac surgery, is yet to be determined.

Pre? and postconditioning during cardiac surgery

In spite of improved myocardial protection, postoperative arrhythmias and cardiac failure are still important problems causing morbidity and mortality in cardiac surgery. Ischemic preconditioning has been widely investigated experimentally with the purpose of identifying new therapeutic agents, but we have not unraveled the underlying mechanisms and we are not able yet to exploit them pharmacologically in clinical practice. Studies of preconditioning in cardiac surgery provide conflicting results, but the majority of studies show that ischemic preconditioning is an effective adjunct to myocardial protection in cardiac surgery. Interventions aimed at modifying reperfusion, or postconditioning, have the advantage that they also can be used after the ischemic insult has occurred, i.e. also in situations with "non-scheduled" ischemia. Postconditioning, as preconditioning, needs pharmacological mimics to be used routinely in settings of cardiac surgery or other human interventions. Possible common signaling pathways of the two phenomena are discussed, and suggested directions for clinical studies are outlined.

The adenosine transporter, mENT1, is a target for adenosine receptor signaling and protein kinase Cepsilon in hypoxic and pharmacological preconditioning in the mouse cardiomyocyte cell line, HL-1

Brief exposure of the heart to hypoxia results in less cellular damage after subsequent hypoxia, an effect known as preconditioning (PC). PC has been widely studied but is still not fully understood. Adenosine (Ado), adenosine receptors, and protein kinase C (PKC) have been implicated as integral components of PC. Adenosine (nucleoside) transporters (NTs) facilitate flux of Ado across cell membranes, but their role in PC is unknown. Therefore, we used the murine cardiomyocyte cell line, HL-1, and asked if there was feedback regulation of NTs by Ado, Ado receptors, and PKC following either hypoxic or pharmacological PC. Activation (by specific agonists) of A1 or A3 Ado receptors or PKC resulted in PC in HL-1. The A1 (but not A3) receptor is coupled to PKCepsilon, and activation of PKCepsilon (by specific peptide agonist) resulted in PC. Moreover, PKCepsilon stimulates Ado uptake via the predominant NT in HL-1, mouse equilibrative nucleoside transporter 1 (mENT1). Studies in primary neonatal mouse cardiomyocytes confirmed our observations in HL-1 cells. Hypoxic challenge led to a rapid increase in, and efflux of, intracellular Ado from cells, which was blocked by NT inhibitors (dipyridamole/nitrobenzylthioinosine). Moreover, NT inhibition during hypoxia or PC was highly protective, suggesting that Ado loss contributes to decreased cell viability. Our data suggest that hypoxic challenge causes an efflux of Ado via ENTs, activation of A1 and/or A3 receptors, signaling through PKCepsilon, and activation of ENT1. Since Ado is required for ATP synthesis on reperfusion, this feedback regulation of mENT1 would promote reuptake of Ado.

Inhibition of glucose uptake in murine cardiomyocyte cell line HL-1 by cardioprotective drugs dilazep and dipyridamole

Inhibition of adenosine reuptake by nucleoside transport inhibitors, such as dipyridamole and dilazep, is proposed to increase extracellular levels of adenosine and thereby potentiate adenosine receptor-dependent pathways that promote cardiovascular health. Thus adenosine can act as a paracrine and/or autocrine hormone, which has been shown to regulate glucose uptake in some cell types. However, the role of adenosine in modulating glucose transport in cardiomyocytes is not clear. Therefore, we investigated whether exogenously applied adenosine or inhibition of adenosine transport by S-(4-nitrobenzyl)-6-thioinosine (NBTI), dipyridamole, or dilazep modulated basal and insulin-stimulated glucose uptake in the murine cardiomyocyte cell line HL-1. HL-1 cell lysates were subjected to SDS-PAGE and immunoblotting to determine which GLUT isoforms are present. Glucose uptake was measured in the presence of dipyridamole (3-300 microM), dilazep (1-100 microM), NBTI (10-500 nM), and adenosine (50-250 microM) or the nonmetabolizable adenosine analog 2-chloro-adenosine (250 microM). Our results demonstrated that HL-1 cells possess GLUT1 and GLUT4, the isoforms typically present in cardiomyocytes. We found no evidence for adenosine-dependent regulation of basal or insulin-stimulated glucose transport in HL-1 cardiomyocytes. However, we did observe a dose-dependent inhibition of glucose transport by dipyridamole (basal, IC(50) = 12.2 microM, insulin stimulated, IC(50) = 13.09 microM) and dilazep (basal, IC(50) = 5.7 microM, insulin stimulated, IC(50) = 19 microM) but not NBTI. Thus our data suggest that dipyridamole and dilazep, which are widely used to specifically inhibit nucleoside transport, have a broader spectrum of transport inhibition than previously described. Moreover, these data may explain previous observations, in which dipyridamole was noted to be proischemic at high doses.

Anaesthetics and cardiac preconditioning. Part II. Clinical implications

There is compelling evidence that preconditioning occurs in humans. Experimental studies with potential clinical implications as well as clinical studies evaluating ischaemic, pharmacological and anaesthetic cardiac preconditioning in the perioperative setting are reviewed. These studies reveal promising results. However, there are conflicting reports on the efficacy of preconditioning in the diseased and aged myocardium. In addition, many anaesthetics and a significant number of perioperatively administered drugs affect the activity of cardiac sarcolemmal and mitochondrial K(ATP) channels, the end-effectors of cardiac preconditioning, and thereby markedly modulate preconditioning effects in myocardial tissue. Although these modulatory effects on K(ATP) channels have been investigated almost exclusively in laboratory investigations, they may have potential implications in clinical medicine. Important questions regarding the clinical utility and applicability of perioperative cardiac preconditioning remain unresolved and need more experimental work and randomized controlled clinical trials.

Preconditioning and cardiac surgery

Preconditioning is in experimental studies the most powerful mode of cardioprotection known. The signal transduction pathways involve a variety of trigger substances, mediators, receptors, and effectors. The studies of preconditioning in cardiac surgery provide conflicting results but the majority of studies show that ischemic preconditiong is an effective adjunct to myocardial protection. However, ischemic preconditioning with repeated clamping of the aorta will never get widespread use. If the "preconditioning response" is to be exploited in cardiac surgery, targeting the underlying molecular mechanisms must provide easily applicable techniques or drugs, which are shown in large scale clinical studies to be beneficial.

Preconditioning: myocardial function and energetics during coronary hypoperfusion and reperfusion

BACKGROUND

Ischemic preconditioning (IP) is gaining more acceptance as a protective method in beating heart surgery. Yet it remains controversial whether preconditioning can attenuate myocardial dysfunction during reperfusion after severe coronary hypoperfusion. We examined this issue and also the issue of whether this protection is mediated by adenosine A1 receptors.

METHODS

In isolated, blood-perfused rabbit hearts, the effects of IP (3 minutes of no flow ischemia and 8 minutes of reperfusion) during 30 minutes of coronary hypoperfusion and 60 minutes of reperfusion were investigated. In two groups (n = 8 each) with and without (control group) preconditioning, ventricular function was assessed by load-insensitive measures: slope of the end-systolic pressure-volume relation (Emax), slope of the stroke work/end-diastolic volume relation (Mw), and end-diastolic pressure-volume relation. External efficiency was calculated, and contractile efficiency was assessed using the reciprocal of the myocardial oxygen consumption-pressure-volume area relationship. To investigate the possible role of adenosine, the adenosine A1 receptor antagonist DPCPX (2.5 micromol/L) was administered before preconditioning in a third group (n = 7).

RESULTS

The effects of hypoperfusion on systolic function, diastolic function (dP/dtmin, end-diastolic pressure-volume relation), external efficiency, and contractile efficiency were similar in both the IP and control groups. Lactate efflux was significantly reduced after preconditioning (p = 0.02). During reperfusion, recovery of systolic function and coronary flow were significantly improved in the IP group compared with controls: aortic flow, 85% versus 63% (p = 0.01); dP/dtmax, 91% versus 67% (p = 0.001); pressure-volume area, 97% versus 68% (p = 0.01); Emax, 74% versus 62% (p = 0.03); and Mw, 94% versus 84% (p = 0.04). Release of creatine kinase was reduced in the IP group, 9.6 +/- 1.3 U x 5 min(-1) x 100 g(-1) wet weight, versus controls, 12.7 +/- 2.7 U x 5 min(-1) x 100 g(-1) wet weight (p = 0.04). During reperfusion, contractile efficiency (p = 0.03) and external efficiency (p = 0.02) recovered better in preconditioned than in untreated hearts. Recovery was less pronounced in the DPCPX group compared with the IP group (p, not significant).

CONCLUSIONS

The results, derived from load-insensitive measures, confirm that IP provides protection after episodes of severe hypoperfusion by attenuating systolic dysfunction without improving diastolic dysfunction and reduces the severity of anaerobic metabolism as well as ischemic injury. Contractile efficiency and external efficiency both indicate improved energetics after IP (oxygen utilization by the contractile apparatus). The protective effect, at least in part, is mediated by adenosine A1 receptors.