REVIEW ARTICLE Protective effects of anaesthetics in reversible and irreversible ischaemia reperfusion injury

British Journal of Anaesthesia 82 (4): 622 32 (1999) REVIEW ARTICLE Protective effects of anaesthetics in reversible and irreversible ischaemia reperfusion injury S. Ross and P. Foëx* Nuffield Department of Anaesthetics, Radcliffe Infirmary, University of Oxford, Oxford OX2 6HE, UK Br J Anaesth 1999; 82: 622 32 *To whom correspondence should be addressed Keywords: heart, ischaemia; heart, myocardium; complications, ischaemia reperfusion injury Ischaemia and the subsequent reperfusion of the myocardium can lead to reversible or irreversible injury depending on the severity and duration of the preceding ischaemia. Reversible and irreversible injury have received considerable attention because of their direct clinical relevance in cardiac management. Procedures such as percutaneous transluminal coronary angioplasty (PTCA), coronary artery bypass grafting (CABG) and thrombolytic therapy have been associated with regional contractile dysfunction and myocardial infarction. 26 Acute myocardial ischaemia can also occur during other operative procedures. As anaesthetics are an integral part of these procedures, it is not surprising that these agents have been the subject of close inspection. Several studies have been conducted examining the effects of both i.v. and halogenated anaesthetics on recovery from ischaemia reperfusion injury. These studies have shown that some anaesthetics, particularly potent inhalation anaesthetics, can afford protection against the deleterious consequences of ischaemia reperfusion injury. The purpose of this review is to describe the effects of inhalation and i.v. anaesthetics on ischaemia reperfusion injury, specifically stunning and cellular necrosis and, where protection is evident, propose mechanisms of action. Reversible ischaemia reperfusion injury Studies have suggested that reperfusion of ischaemic tissue may promote potentially harmful pathophysiological reactions. This has been described as reperfusion injury and is defined as cell injury caused by reperfusion itself and not the preceding ischaemia. Manifestations of reversible reperfusion injury include reperfusion arrhythmia, postischaemic contractile dysfunction (stunning) and coronary vascular and microvascular injury. For this review, we will be concentrating on the phenomenon of stunning. Braunwald and Kloner 8 were the first to describe the prolonged recovery of myocardial contractile function after a brief period of ischaemia and reperfusion as stunning. A more accurate description would be a fully reversible yet prolonged depression of mechanical function following brief periods of ischaemia even after myocardial blood flow has been fully restored. 3 The hallmark of this condition is the presence of a flow function mismatch, with normal flow but abnormal function. Initially, various independent theories on the cellular mechanisms for myocardial stunning were proposed. However, there is now a growing consensus that rather than being separate entities, these mechanisms could in fact represent different steps in a chain of reactions that ultimately lead to mechanical dysfunction. Mechanisms responsible for prolonged depression of contractile function in reperfused myocardium are still under debate. However, the main culprits that have been put forward include free radical formation, calcium overload and abnormalities of the microvasculature. To discuss each of these fully is outside the scope of this review and there are many excellent reviews of this subject. 2374 However, the main points are summarized. Mechanisms behind stunning Free radical formation The very large number of studies by independent groups, in different experimental preparations using differing models of stunning, leaves little doubt that radicals play a major role in the pathogenesis of stunning. Studies have shown that both during the ischaemic period and, more importantly, during the first few minutes of myocardial reperfusion, there is a large production of free radicals. 73 Free radicals such as the superoxide radical ( d O 2 ), hydroxyl radical ( d OH) and hydrogen peroxide (H 2 O 2 ) are extremely reactive species that have no specific targets and can attack all cellular components, such as the sarcolemma and British Journal of Anaesthesia

Anaesthetics and ischaemia reperfusion injury sarcoplasmic reticulum. Indeed, clinical studies have shown the production of lipid peroxidation products, implying free radical damage of membrane lipids. 68 Damage and the de novo synthesis of cellular components would certainly relate to the time course of recovery from stunning which may take days or weeks. In vivo studies in chronically instrumented dogs have shown that infusion of free radical scavengers such as superoxide dismutase and catalase can improve recovery of contractile function. 90 The sources of free radicals have not been elucidated completely but include the xanthine oxidase system, oxidation of catecholamines, mitochondrial respiration and activated neutrophils. 2 Calcium overload During ischaemia and reperfusion there are alterations in calcium fluxes resulting in intracellular calcium overloading. Calcium overloading is considered by many to be the ultimate culprit in the development of stunning. By measuring intracellular calcium, investigators have found increased concentrations during ischaemia and into the early reperfusion phase. However, these return to normal levels during late reperfusion. 58 Increased concentrations of calcium during reperfusion are thought to change Ca 2 sensitivity and/ or maximal Ca 2 -activated force of the myofilaments, possibly via activation of protein kinases. 3 In vitro studies using isolated hearts have shown that reperfusion with low calcium containing solutions or solutions containing calcium channel blockers enhance post-ischaemic contractile function. 22 Indeed, in vivo studies in acutely instrumented dogs have shown that dihydropyridines such as nisoldipine protect post-ischaemic myocardium. 24 If calcium is the ultimate culprit in stunning, what are the mechanisms responsible for the increase in intracellular calcium [Ca 2 ] i during ischaemia? Possibilities include alterations in the Na Ca 2 exchanger and decreased influx or increased efflux from the sarcoplasmic reticulum. During ischaemia, the ph inside the cell decreases through accumulation of hydrogen ions [H ]. One mechanism used by the cell to regulate this intracellular acidosis is the Na H exchanger. 52 Abrupt reperfusion of the ischaemic myocardium would be expected to maximally stimulate the Na H exchanger. Stimulation of the Na H exchanger would restore intracellular ph; however, this would be accompanied by a large increase in intracelluar sodium ions ([Na] i ). Therefore, increases in [Ca 2 ] could be attributed to stimulation of the Na Ca 2 exchanger with an increased influx of calcium during reperfusion. 31 The Na Ca 2 exchanger is also the primary mechanism used by the cell to extrude intracellular calcium during diastole. This would also be inhibited because of the increase in [Na] i. Abnormalities of the microvasculature This is a more controversial topic with experimentalists undecided as to whether this plays an important role in stunning. Some studies have shown an increase in vascular resistance and a decrease in vasodilator responsiveness, 4 while others have found no differences. 23 However, studies have shown that increases in flow produced by vasodilators can recruit contractile function, a phenomenon which does not occur to the same extent in normal unstunned myocardium. 81 Indeed, there have been reports of continued lactate production by stunned swine heart. 63 This evidence would suggest that the stunned myocardium is ischaemic, yet studies have shown normal resting flow and distribution. 82 One explanation is the microvascular spasm hypothesis which postulates the presence of foci of impaired microvascular perfusion or increased oxygen consumption. This theory would explain the positive inotropic effects of vasodilators. In summary, the mechanisms behind stunning are numerous and may be linked. The main culprits are free radical formation, calcium overload and impairment of the coronary microvasculature (Fig. 1). As yet no one pharmacological intervention has been shown to completely abolish the effects of reperfusion injury. However, free radical inhibitors, calcium channel blockers and vasodilating agents have been shown to improve post-ischaemic contractile function. Anaesthetics and reversible ischaemia reperfusion injury Studies examining the effects of i.v. and halogenated anaesthetics on post-ischaemic function have been carried out in both in vivo and in vitro experimental models (Table 1). Halogenated anaesthetics The majority of studies have shown improvement in postischaemic metabolic and/or mechanical function with halogenated anaesthetics. This protection is not species-dependent; studies in dogs, 92 pigs 15 and rats 16 have shown similar beneficial effects of isoflurane, halothane and enflurane. Warltier and colleagues 92 performed a particularly detailed study in chronically instrumented dogs. Several groups were studied, examining the effects of 2% isoflurane and 2% halothane, given before and during ischaemia, with the anaesthetics either continued during or terminated before reperfusion. The main results of this study were that pretreatment with halothane or isoflurane resulted in segmental shortening which completely recovered within 3 5 h, whereas in conscious dogs segmental shortening had only recovered to 50% of control values at 5hofreperfusion. In intact swine, Coetzee and Moolman 15 studied the effects of several concentrations of halothane and found that 0.6% or more led to less post-ischaemic systolic dysfunction at the end of reperfusion compared with controls (15% vs 40%). A study by the same group in isolated rat hearts 16 investigated the effects of different concentrations of isoflurane and enflurane given before and after ischaemic arrest, and in a separate set of experiments when given only in the cardioplegic solution during arrest. Ventricular work was improved in the groups receiving isoflurane or enflurane. 623

Ross and Foëx Fig 1 Mechanisms of reversible ischaemia reperfusion injury. Table 1 Effect of potent inhalation and i.v. anaesthetics on post-ischaemic recovery Author Reference Agent Species Result Boutros et al. (1997) [7] Halothane/isoflurane Rat Improved mechanical function Buljubasic et al. (1992) [9] Halothane Guinea-pig Improved mechanical function Buljubasic et al. (1993) [10] Halothane Guinea-pig Improved metabolic function Coetzee et al. (1991) [14] Halothane Rat Improved metabolic and mechanical function Coetzee and Moolman (1993) [15] Halothane Pig Improved mechanical function Coetzee et al. (1993) [16] Isoflurane/enflurane Rat Improved metabolic and mechanical function Coetzee (1996) [17] Propofol Pig No improvement in mechanical function Freedman et al. (1985) [28] Enflurane Rat Improved metabolic and mechanical function Kanaya and Fujita (1994) [41] Isoflurane Dog Improved metabolic and mechanical function Kanaya et al. (1995) [42] Isoflurane Dog Improved metabolic function Kersten et al. (1996) [45] Isoflurane Dog Improved mechanical function Kersten et al. (1997) [46] Isoflurane Dog Improved mechanical function Lochner et al. (1994) [53] Halothane Rat Improved structural recovery Mattheussen et al. (1993) [60] Halothane/isoflurane Rabbit No change in mechanical or metabolic recovery Marijic et al. (1990) [59] Halothane/isoflurane Guinea-pig Improved mechanical recovery with halothane Nakayama et al. (1997) [70] Isoflurane Dog Improved metabolic recovery Oguchi et al. (1995) [72] Enflurane/isoflurane/ Rat Improved metabolic recovery with isoflurane and enflurane sevoflurane/halothane Ross et al. (1998) [76] Propofol Dog No improvement in mechanical recovery Sahlman et al. (1995) [77] Halothane/isoflurane Rat No improvement in mechanical or metabolic recovery Schlack et al. (1996) [79] Halothane Rat Improved mechanical and metabolic function Stowe et al. (1995) [83] Halothane Guinea-pig Improvement in mechanical function Warltier et al. (1988) [92] Halothane/isoflurane Dog Improvement in mechanical function White et al. (1994) [94] Halothane Dog Improvement in mechanical function In studies where protection was evident with halogenated anaesthetics, the agents were present either during the preischaemic period and/or during the ischaemic period. Thus the pharmacological effects of these agents would still have been present during the initial stages of reperfusion. Where no improvement in either metabolic or mechanical function was observed with halogenated anaesthetics, the studies were carried out in isolated models. Mattheussen and colleagues, 60 in isolated rabbit hearts, administered halothane or isoflurane during the pre-ischaemic period. Reperfusion was performed with perfusate devoid of isoflurane or halothane and thus the potential benefits of these agents would have been lost at a time when the major mediators of stunning are induced. In a study conducted by Sahlman and colleagues 77 in rat hearts, a short period of hypoperfusion was used. Although evidence of myocardial dysfunction was evident during the hypoperfusion period, full recovery of both mechanical and metabolic function in the control group was achieved within 3 min of reperfusion. The severity and duration of ischaemia may not have been enough for the protective effects of these agents to be observed. The degree of protection afforded by each of the halogenated anaesthetics is debatable. There have been very few studies where more than two halogenated anaesthetics have been studied. Oguchi and colleagues 72 examined the effects of halothane, enflurane, isoflurane and sevoflurane on mechanical and metabolic function in isolated rat hearts. They found no difference in mechanical recovery between the groups. However, they found greater post-ischaemic ATP concentrations with isoflurane and enflurane at 1.0 MAC. Again in isolated rat hearts, Boutros, Wang and Capuano 7 examined the effects of halothane and isoflurane on ischaemia and reperfusion and found improved mechanical and metabolic recovery compared with controls, but no difference between halothane and isoflurane. Marijic and colleagues, 59 in isolated guinea-pig hearts, studied the effects of halothane and isoflurane on hypoxic and reoxygenation injury and found that the systolic pressure generated in hearts exposed to halothane was greater than 624

Anaesthetics and ischaemia reperfusion injury in hearts exposed to isoflurane. In addition, diastolic compliance was increased in the halothane compared with the isoflurane and control groups. There are several confounding factors when assessing the protective potency of anaesthetics. Volatile anaesthetics affect electrical and mechanical function and alter coronary flow to different degrees. 84 Using a model in which mechanical function was unaffected by preload, afterload, and extrinsic autonomic and humoral influences, Stowe and colleagues 84 showed, using varying concentrations of halothane, isoflurane and enflurane, that halothane and enflurane produced greater decreases in cardiac mechanical function and oxygen consumption compared with isoflurane. Coronary flow did not decrease proportionately to mechanical function and oxygen consumption resulting in a relative improvement in oxygen balance. Therefore, when comparing volatile anaesthetics in vivo, their variable effects on myocardial oxygen demand and supply must be borne in mind as they may affect the severity of ischaemia and thereby myocardial tissue trauma. In summary, potent inhalation anaesthetics such as isoflurane, halothane and enflurane exhibit protective properties such that, after a short period of ischaemia, metabolic and/ or mechanical recovery are improved. I.v. anaesthetics At variance with potent inhalation anaesthetics, i.v. anaesthetics have shown little evidence of cardioprotection during stunning. In acutely instrumented dogs, White and colleagues 94 compared a group anaesthetized with fentanyl and a group anaesthetized with halothane and found greater protection with halothane. Also, in acutely instrumented dogs, Ross and colleagues 76 compared the effects of propofol infusion with those of fentanyl and found no difference in recovery between groups. In swine, Coetzee 17 compared a group anaesthetized with halothane and groups anaesthetized with propofol and found greater protection with halothane. However, these studies do not conclusively prove that i.v. anaesthetics do not afford protection, but show that compared with another anaesthetic, cardioprotection is minimal. The difficulty in studying the effects of these i.v. agents, specifically in vivo, is the frequent lack of comparison with a non-anaesthetized control group, as many studies have been conducted using an acutely instrumented model requiring basal anaesthesia. In contrast, studies using isolated heart models do not require basal anaesthesia. A recent study by Ko and colleagues examining the potential protective effects of propofol in isolated rat hearts found that propofol protected ventricular function after global ischaemia. 49 The pressure developed by the ventricle reached 55 and 76 mm Hg in groups treated with propofol 30 and 100 mmol litre 1, respectively, whereas the developed pressure was 39 mm Hg in the control group. Clearly, further work in both in vivo and in vitro models on the potential cardioprotective effects of i.v. anaesthetics is merited. Mechanisms behind protection against reversible injury Preservation of ATP concentrations During ischaemia, adenosine triphosphate (ATP) is degraded to metabolites such as adenosine, hypoxanthine and inosine (purines) which readily diffuse across cell membranes and are washed out into the coronary sinus at the time of reperfusion. 27 Restoration of ATP requires de novo synthesis of purines, a relatively slow process compared with rephosphorylation of ATP catabolites. 98 One possible mechanism that has been proposed to account for the protection offered by volatile anaesthetics is the preservation of myocardial energy stores, allowing the myocardium to recover initially to a greater degree. In isolated rat hearts, Freedman and colleagues 28 measured ATP and creatine phosphate (CP) concentrations and found these were similar between both treated and control groups of hearts before and during ischaemia. However, there were higher concentrations of CP and ATP in enflurane treated hearts at the end of reperfusion. Coetzee and colleagues, 14 in isolated rat hearts, showed similar myocardial ATP concentrations during arrest in halothane treated and in control hearts. However, ATP concentrations in halothane treated hearts were significantly greater after reperfusion. In rat hearts, Oguchi and colleagues 72 compared the effects of halothane, enflurane, isoflurane and sevoflurane and found that enflurane and isoflurane hearts had higher ATP concentrations at the end of reperfusion compared with controls. The ATP sparing effects of isoflurane have also been established in in vivo studies. Kanaya and colleagues 42 measured ATP concentrations after ischaemia in the presence and absence of isoflurane and found higher concentrations in the isoflurane treated groups. The protective effects of halogenated anaesthetics may be caused by their negative inotropic and chronotropic actions. Reductions in heart rate, systolic pressure and therefore rate pressure product would lower myocardial energy demand and oxygen consumption, thereby reducing myocardial metabolic requirements. In this setting, it would be difficult to separate an antiischaemic action from a reduction in reperfusion injury. However, heart rate and mean arterial pressure were artificially controlled to match untreated groups thereby eliminating a reduction in cardiac work as a variable. 42 The authors could not exclude enhanced oxygen supply during reperfusion as coronary flow was not measured and isoflurane is a known vasodilator. Similarly, Takahata, Ichihara and Ogawa, 87 studying the effects of sevoflurane, found that with matching haemodynamic variables the ischaemiainduced metabolic changes were significantly attenuated by sevoflurane compared with controls. The mechanism by which isoflurane and possibly other halogenated anaesthetics preserve ATP is not known but probably involves ATP synthesis and/or use. While volatile anaesthetics may preserve myocardial ATP concentrations, the contribution this 625

Ross and Foëx makes to improved mechanical function is debatable. Studies have shown that stunned myocardium still retains the ability to respond to inotropic stimulation, implying that preservation of energy stores may not play an important role. 36 Alterations in calcium fluxes One of the primary culprits proposed for post-ischaemic mechanical dysfunction is alteration in calcium fluxes. Ischaemia and reperfusion are associated with increases in intracellular calcium. 57 Increases in intracellular calcium may change the sensitivity of the contractile apparatus such that there is a decrease in myofilament calcium responsiveness. The effects of general anaesthetics on the cardiovascular system are consistent with the effect of these substances on calcium channels (e.g. inhibition of SA node automaticity, 5 prolongation of A V conduction time 1 and depression of myocardial contractility). 55 Calcium flux into and out of the heart cell is mediated primarily by calcium channels and by the Na Ca 2 exchanger. There are several types of calcium channel. However, in cardiac cells the predominant calcium channels are L- and T-type channels. These channels can be differentiated by their different voltage thresholds for activation and different kinetics for opening and closing. The T-type calcium channel current activates at more negative membrane potentials and decays rapidly, whereas the L-type calcium current activates at more positive membrane potentials and decays slowly. The L-type current is the major contributor in providing the external Ca 2 required for excitation contraction coupling. Anaesthetics such as halothane, isoflurane and enflurane have been shown to reduce whole cell Ca 2 current in rat, 39 guinea-pig 89 and canine ventricular myocytes. 6 Eskinder and colleagues 25 examined the role of halothane, isoflurane and enflurane on both L- and T-type Ca 2 currents and found that, at equi-anaesthetic concentrations, all produced similar depression of peak Ca 2 currents in canine cardiac Purkinje fibres. Isoflurane and enflurane also reduced dihydropyridine binding to L-type Ca 2 channels. 21 These results indicate that the sarcolemmal Ca 2 channel is a site of action of these anaesthetics. Additional mechanisms by which halogenated anaesthetics may alter calcium flux is through the Na Ca 2 exchanger 34 and through release of Ca 2 from internal stores such as the sarcoplasmic reticulum. 18 Haworth and Goknur 34 showed that halothane, isoflurane and enflurane inhibited Na Ca 2 exchange in rat myocytes and Katsuoka and Ohnishi 44 showed that halothane and enflurane decreased the calcium content of the cardiac sarcoplasmic reticulum by increasing its permeability. One of the mechanisms thought to be responsible for the prolonged depression of contractile function in reperfused myocardium is calcium overloading. From studies showing the beneficial effects of L-type calcium channel blockers such as verapamil and diltiazem in both in vivo and in vitro models, 71 75 movement of Ca 2 through this channel appears to play a predominant role in calcium overloading. However, calcium influx via other mechanisms such as the Na Ca 2 exchanger is also involved. 33 Since anaesthetics depress calcium influx through L-type channels and Na Ca 2 exchange, it seems reasonable to postulate that anaesthetics could be protecting the reperfused myocardium through reduced calcium loading. Hoka, Bosnjak and Kampine, 38 in isolated guinea-pig hearts, showed that halothane significantly depressed calcium accumulation during reperfusion. In a study measuring the binding of a dihydropyridine calcium channel blocker to voltage-sensitive Ca 2 channels, Drenger and colleagues 20 found that there was an increase in the number of binding sites during ischaemia with a reduction in binding sites after reperfusion. An increase in the number of binding sites during ischaemia could explain the increase in calcium influx during early reperfusion. Administration of halothane before ischaemia reduced the number of binding sites during ischaemia. The authors concluded that a reduction in binding sites could result in a reduction in calcium entry during reperfusion. Lochner and colleagues 53 examined calcium content and myocardial structure in isolated rat hearts and found that hearts perfused with halothane-gassed buffer exhibited significant reversal of ischaemic damage and complete ultrastructural repair. Untreated hearts still exhibited severe oedema, contracture and contracture bands. At the end of reperfusion, the hearts exposed to halothane also had lower intracellular Ca 2 content. Du Toit and Opie 22 showed that using a sarcoplasmic reticulum Ca 2 uptake inhibitor, post-ischaemic mechanical recovery was improved in isolated rat hearts. Halothane and enflurane reduced sarcoplasmic reticulum content of calcium whereas isoflurane had relatively little effect. 93 Loss of calcium from the internal stores and ultimately into the extracellular space could protect against calcium overloading. Another potential mechanism by which potent inhalation anaesthetics may protect is through an interaction with the Na H exchange mechanism, as has been shown in an erythrocyte model. 50 Inhibition of this exchange mechanism would preserve intracellular acidosis and so prevent the influx of calcium via the Na Ca 2 exchanger. Inhibition of free radicals Cytotoxic oxygen-derived free radicals have been implicated widely as aetiological agents in stunning. Oxygen metabolites such as the superoxide anion ( d O 2 ), hydrogen peroxide (H 2 O 2 ) and the hydroxyl radical ( d OH) are generated during ischaemia and reperfusion and studies using scavenging enzymes have shown greater recovery of post-ischaemic contractile function. Anaesthetics have been shown to affect free radical activity. Tanguay and colleagues 88 showed a beneficial effect of halothane, isoflurane and enflurane on free radical-induced impairment of cardiac function in isolated rabbit hearts. The effects of free radicals, produced by electrolysis of the perfusate, were directly inhibited by halothane, isoflurane and enflurane, with potency in the 626

Anaesthetics and ischaemia reperfusion injury order, enflurane halothane isoflurane. A direct and full inhibition of the in vivo production of the hydroxyl radical by halothane was also shown by Glantz and colleagues 29 in dogs. Halothane completely inhibited the initial burst of hydroxyl production observed during early reperfusion. Halothane, isoflurane and enflurane also inhibited the production of superoxide anions in neutrophils, 69 a potential source of free radicals during stunning. However, the effects of anaesthetics on free radicals are not conclusive. Godin and Garnett 30 examined the effects of several agents (carbon dioxide, halothane, pentobarbital or ether) in rats and found no effect of any of these agents on enzymatic antioxidant components. The mechanism by which potent inhalation anaesthetics may inhibit free radicals is not known but may involve reducing intracellular calcium concentrations. During ischaemia, studies have shown a calcium-triggered, protease-dependent conversion of xanthine dehydrogenase to xanthine oxidase, a superoxide radical producing enzyme. 62 Effect on K ATP channels K ATP channels are distributed widely in the cardiovascular system. They are thought to determine the basal level of coronary vascular tone 78 and to play a role in preconditioning against infarction. 12 Studies have shown that the use of K ATP channel openers before ischaemia can protect the post-ischaemic heart. 12 35 This protective effect has been attributed to shortening of action potential duration or attenuation of membrane depolarization. This would result in a reduction in intracellular calcium. 95 Anaesthetics have been shown to have an effect on K ATP channels. Larach and Schuler 51 found that 56% of the coronary vasodilator effect of halothane in isolated rat hearts was eliminated by K ATP channel block by glibenclamide. Similarly, vasodilatation caused by isoflurane in open chest swine 11 and in chronically instrumented dogs 86 was eliminated by glibenclamide. With such profound effects on K ATP channels one could postulate that halogenated anaesthetics could also protect the stunned myocardium through the opening of K ATP channels in a similar manner to K ATP openers such as nicorandil and cromakalim. In an ischaemia and reperfusion study, Nakayama and colleagues 70 showed that pretreatment with glibenclamide eliminated the ATP-sparing effect of isoflurane in dogs, implicating K ATP channels in metabolic protection. Kersten and colleagues, 45 also in dogs, showed that the improved mechanical function afforded by isoflurane was partially blocked by glibenclamide, indicating a role of K ATP channels. Attenuation of recovery was evident after 3 h of reperfusion. In a subsequent study by the same group, 46 isoflurane-induced cardioprotection was shown to be partially mediated by adenosine type-1 receptor (A 1 ) activation. Previous studies have shown that the A 1 -adenosine receptor may be linked to the K ATP channel in myocardial stunning in dogs. 90 97 However, the study by Kersten and colleagues 46 was the first to show that the protective effects of a potent inhalation anaesthetic against stunning may be mediated by an A 1 -adenosine K ATP channel-linked mechanism. In summary, the protective effects of potent inhalation anaesthetics can be attributed to several properties exhibited by these agents (Fig. 2). Free radical scavenging, preservation of energy levels, calcium channel blocking effects and effects on K ATP channels have been demonstrated by halogenated anaesthetics. How much each of these properties contribute to the cardioprotection observed is unclear but, in the same manner that reperfusion injury can be attributed to several mechanisms, the fact that these agents act on a number of pathways can only enhance their protective effects. Protective effects of anaesthetics on infarct size The study of the protective effects of pharmacologically active agents on infarct size has received much attention. This attention can be attributed to its direct clinical relevance given the risk of perioperative myocardial infarction. 56 Given that potent inhalation anaesthetics have a cardiovascular depressant effect, the initial focus of attention was to determine what effects the haemodynamic altering properties of these agents may have on infarct size (Table 2). One of the first of these studies was that of Kissin and colleagues 48 in rat hearts. In this study, animals were exposed to 1% halothane for 3 h after coronary ligation. In addition to finding an increase in infarct size, they also found a decrease in systolic arterial pressure. The increase in infarct size in this study was attributed to unfavourable alterations in haemodynamics, in particular a decrease in systolic arterial pressure. In this study, however, the authors did not measure the area at risk, an essential measurement in the quantification of infarct size. Therefore, the results of this study may have been flawed. Similarly, in the study by Chakrabarty, Thomas and Sheridan, 13 the authors found an increase in infarct size with 1.5% halothane; however the area at risk was not appropriately measured. For accurate quantification of infarct size, the area of infarct and the area at risk in the same subject must be measured and expressed as a percentage, a technique which early infarct studies did not apply. Mergner and colleagues 64 65 studied the effects of fentanyl, sodium pentobarbital and halothane on myocardial infarct size in dogs. These authors found a greater infarct size with halothane anaesthesia associated with a lower mean arterial pressure and greater heart rate. In this study, risk area was assessed. This study would seem to demonstrate a potential detrimental effect of potent inhalation anaesthetics, such as halothane, on infarct size. Blood entering the occluded vascular bed is derived from collateral vessels such that a reduction in perfusion pressure may increase the severity of ischaemia in that region, increasing infarct size. Changes in haemodynamic status and the extent of collateral perfusion have to be taken into account when comparing the effects of agents on infarct size, particularly 627

Ross and Foëx Fig 2 Protective effects of potent inhalation anaesthetics. Table 2 Effect of potent inhalation anaesthetics on infarct size Author Reference Agent Species Result Cope et al. (1997) [19] Enflurane, halothane, isoflurane Rabbit Reduction in infarct size Chakrabarty et al. (1991) [13] Halothane Rabbit Increase in infarct size Haessler et al. (1994) [32] Isoflurane Rabbit No change in infarct size Kersten et al. (1997) [47] Isoflurane Dog Reduction in infarct size Kissin et al. (1981) [48] Halothane Rat Increase in infarct size Mergner et al. (1985) [64] Halothane Dog Increase in infarct size Mergner et al. (1985) [65] Halothane Dog Increase in infarct size Schlack et al. (1997) [80] Halothane Rabbit Reduction in infarct size in animals with a high degree of collateral circulation, such as dogs. In both studies by Merger and colleagues, 64 65 collateral blood flow to the occluded region was less in the halothane groups compared with the control groups, indicating that the lower systemic pressure under halothane may reduce blood flow from collaterals. In contrast with these investigations, several studies have shown the protective effects of halogenated anaesthetics on infarct size in dogs 47 and rabbits. 19 80 Cope and colleagues 19 compared the effects of the halogenated anaesthetics halothane, enflurane and isoflurane with those of i.v. agents such as ketamine xylazine, propofol and pentobarbital in an in vivo rabbit model. They found substantial decreases in infarct size with the potent inhalation anaesthetics compared with all of the i.v. anaesthetics, with isoflurane showing the greatest reduction. A reduction in rate pressure product could not account for this protective action as this was comparable between the potent inhalation anaesthetics and the ketamine xylazine group. A similar protective effect was observed in isolated hearts transiently exposed to the potent inhalation anaesthetics before ischaemia, suggesting a preconditioning property of these agents. Similarly, Schlack and colleagues, 80 in rabbits, compared the effects of 1 MAC of halothane with controls anaesthetized with -chloralose and found a significant decrease in infarct size with halothane. The authors also compared these results with those of a group receiving halothane and norepinephrine to determine the effects of inhibiting the negative inotropic and vasodilator effects of halothane. There was no difference in infarct size between those rabbits receiving halothane and norepinephrine and those receiving halothane alone, indicating that in this model changes in haemodynamics have little influence on the effects of halothane. Rabbits have minimal collateral circulation (2%) and any reduction in systemic pressure is likely to have minimal effects on collateral blood flow and therefore on the severity of ischaemia in the occluded region. A recent study by Kersten and colleagues 47 demonstrated a similar protective effect of isoflurane in dogs while also showing a preconditioning effect. Isoflurane reduced infarct size by 50%; a similar degree of protection afforded by a 5-min preconditioning ischaemic period. Isoflurane administration reduced systemic pressure, but myocardial blood flow to the occluded region was comparable between control and isoflurane groups. In addition, using a 30-min washout period of isoflurane before ischaemia, they demonstrated that isoflurane still significantly reduced infarct size. Haemodynamics were comparable between this group and controls. In the same study, this group also showed that isoflurane directly preconditions the heart against infarction via activation of K ATP channels. By blocking K ATP channels with glibenclamide in the group with a 30-min washout period of isoflurane, infarct size was comparable with controls. In summary, it would appear from these studies that halogenated anaesthetics per se protect against irreversible injury. However, caution must be exercised when using 628

Anaesthetics and ischaemia reperfusion injury animal models where a high degree of collateral circulation is evident. In these models, a reduction in systemic pressures with a resultant reduction in collateral blood flow may increase infarct size. Similarly, in these models, changes in work performed by the heart (rate pressure product) may protect the myocardium as a result of reduced myocardial oxygen consumption. Therefore, many factors must be taken into account when determining the effects of anaesthetics on infarct size. Mechanisms of protection against irreversible injury Reduction in infarct size through pharmacological interventions is a controversial topic. Calcium channel blockers such as diltiazem 37 and verapamil 85 have been shown to protect against infarction whether given before ischaemia or just before reperfusion. Favourable haemodynamics could not account for this protective effect. The use of free radical scavengers has also been shown to protect against infarction. 54 91 However, not all calcium channel blockers 40 or free radical scavengers 61 have been shown to be effective. As mentioned previously, potent inhalation anaesthetics exhibit calcium channel blocking and free radical scavenging activities and protection against infarction may be through these mechanisms. The most powerful way to protect the myocardium from infarct is through ischaemic preconditioning. Brief periods of ischaemia followed by reperfusion have been shown to significantly reduce the effects of a longer ischaemic period on infarct size. 66 The preconditioning effect of ischaemia can be mimicked by agents such as K ATP channel openers and adenosine agonists. 67 96 A preconditioning effect of potent inhalation anaesthetics such as isoflurane, halothane and enflurane has also been demonstrated against infarction. 19 Kersten and colleagues 47 showed that the protection afforded by isoflurane was as great as preconditioning through repetitive brief periods of ischaemia. In addition, through the use of the K ATP channel blocker glibenclamide, they showed that preconditioning is mediated by activation of K ATP channels. The preconditioning effect of isoflurane was present 30 min after discontinuation of the anaesthetic, thus indicating a window of protection where, despite the absence of the anaesthetic, the protective effects are still present. Cope and colleagues 19 demonstrated similar preconditioning effects of isoflurane, halothane and enflurane in isolated rabbit hearts after a 10-min washout period. The authors also demonstrated that the preconditioning effect of halothane could be blocked by both an adenosine receptor blocker and a protein kinase C inhibitor. This shows that the same signal transduction mechanisms may be linked to both the protective effects against stunning 46 and those against infarction 19 offered by potent inhalation anaesthetics. How long the window of protection may last, for the preconditioning effects of potent inhalation anaesthetics to be realized, is not known. The first window of protection for ischaemic preconditioning has been shown to last for 90 min after the initial brief ischaemic period, with a second window occurring 24 h later. 66 Whether preconditioning with an agent such as isoflurane may offer a first window of similar duration is not known or indeed if a second window of protection may exist. However, this acute memory phase of preconditioning may be beneficial for the clinician where protective anaesthetics are used before ischaemia develops during or after surgery. Conclusion Ischaemia reperfusion injury is an area which has received considerable attention in the past decade because of its direct clinical relevance. Of relevance to the anaesthetist are several studies conducted on the effects of potent inhalation and i.v. anaesthetics on ischaemia reperfusion injury. The results of these studies have shown that inhalation anaesthetics such as isoflurane, enflurane and halothane are potent cardioprotective agents. The protective properties can be attributed to several factors: preservation of energy levels during ischaemia; alteration of intracellular calcium concentrations; inhibition of free radicals; and interaction with K ATP channels. The fact that these agents act on several pathways that are responsible for ischaemia reperfusion injury may make them better therapeutic candidates than specific agents which act solely on one mechanism. Supporting this, studies have shown that combination therapy with agents that affect different mechanisms confer greater protection than each one individually. 43 The potential beneficial effects of protective anaesthetics cannot solely be limited to cardiac surgery. Many patients with compromised cardiovascular systems experience episodes of perioperative ischaemia during non-cardiac surgery. The use of anaesthetics which exert protection may reduce the incidence and/or effects of perioperative ischaemia experienced by these patients. The preconditioning effects of potent inhalation agents may also be beneficial, particularly for those patients who are susceptible to myocardial infarction during and after surgery. To our knowledge, no clinical studies have yet examined the potential protective effects of potent inhalation anaesthetics against ischaemia reperfusion injury. It would seem reasonable to assume that the protective effects demonstrated in several species may also be observed in patients. Unfortunately, the number of variables encountered in the clinical setting, such as duration of ischaemia, preoperative and postoperative medication, and accurate assessment of myocardial function would make it difficult to ascribe any potential beneficial effects solely to halogenated anaesthetics. However, based on experimental evidence, potent inhalation anaesthetics may prove to be useful pharmacological tools against ischaemia and reperfusion injury. Acknowledgement Mr Sean Ross was in receipt of a MRC studentship. 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