1 Service dAnesthésie Réanimation and 2 Laboratoire de physiologie Lyon Nord, Hôpital cardio-vasculaire Louis Pradel, Avenue Doyen Lépine, F-69500 Lyon Bron, France *Corresponding author
This work was presented at the 23rd annual meeting of the Society of Cardiovascular Anesthesiologists, Vancouver, Canada, May 2001 (Anesth Analg 2001; 92: SCA39).
Accepted for publication: April 30, 2002
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Abstract |
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Methods. Anaesthetized open-chest rabbits underwent 30 min of coronary artery occlusion followed by 3 h of reperfusion. Before this, rabbits were randomized into one of five groups and underwent a treatment period consisting of either no intervention for 45 min (control; n=10), or 30 min of 1 MAC halogenated anaesthetic inhalation followed by 15 min of washout. End-tidal concentrations of halogenated agents were 3.7% for sevoflurane (n=11), 1.4% for halothane (n=9), 2.0% for isoflurane (n=11), and 8.9% for desflurane (n=11). Area at risk and infarct size were assessed by blue dye injection and tetrazolium chloride staining.
Results. Mean (SD) infarct size was 54 (18)% of the risk area in untreated controls and 40 (18)% in the sevoflurane group (P>0.05, ns). In contrast, mean infarct size was significantly smaller in the halothane, isoflurane, and desflurane groups: 26 (18)%, 32 (18)% and 16 (17)%, respectively (P<0.05 vs control).
Conclusions. Halothane, isoflurane and desflurane induced pharmacological preconditioning, whereas sevoflurane had no significant effect. In this preparation, desflurane was the most effective agent at preconditioning the myocardium against ischaemia.
Br J Anaesth 2002; 89: 48691
Keywords: anaesthetics volatile; heart, myocardial function
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Introduction |
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Material and methods |
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A total of 73 New Zealand White rabbits of either sex (2.02.5 kg) were premedicated with xylazine 5 mg kg1 i.m. and anaesthetized with ketamine hydrochloride 50 mg kg1 i.m. Anaesthesia was maintained by continuous infusion of a mixture of ketamine 3 mg kg1 h1 and xylazine 1.5 mg kg1 h1. Adequate depth of anaesthesia was ensured before any surgical procedures by the absence of pedal and palpebral reflexes. Following tracheostomy, the animals lungs were ventilated mechanically (Servo ventilator 900B, Siemens-Elema, Sweden and SA2 ventilator, Dräger, Lübeck, Germany) with 100% oxygen. Tidal volume was set at 15 ml kg1 and the respiratory rate to 35 bpm. Ventilation was adjusted to maintain end-tidal CO2 E'CO2 in the physiological range. End-tidal gas concentrations were measured continuously (gas analyser Capnomac Ultima, Datex, Helsinki, Finland). Body temperature, recorded through a thermistor inserted into the oesophagus, was maintained between 39.0 and 40.5°C by means of a servo-controlled heating element incorporated into the operating table. Limb lead II of the ECG was monitored continuously by means of subcutaneous needle electrodes. Systemic arterial pressure was monitored using a Gould pressure transducer connected to a 1-mm fluid-filled catheter inserted in the right carotid artery. The right internal jugular vein was catheterized with a 1 mm catheter to infuse fluids and drugs. Hetastarch 5 ml kg1 h1 was infused continuously via this intravenous cannula. Fentanyl 50 µg i.v. was injected before thoracotomy to provide adequate analgesia.
The heart was exposed via a left thoracotomy. The first large marginal branch of the circumflex artery was identified and a 5/0 Dexon suture was passed around this artery, approximately halfway between the apex and the base. The suture ends were threaded through a small vinyl tube to make a snare to perform further coronary occlusion and reperfusion. After the surgical procedure, a 15-min stabilization period was allowed.
In all groups, the coronary artery was occluded for 30 min. Myocardial ischaemia was confirmed by the appearance of a regional cyanosis on the epicardium distal to the snare, akinesia or bulging in this area, and a marked ST segment elevation on the ECG. After 30 min, the snare was released and reperfusion allowed for a period of 3 h. Reperfusion was visually confirmed by the appearance of hyperaemia. The thread passed around the marginal artery was left in place.
At the end of the reperfusion period, the coronary artery was briefly re-occluded and diluted Uniperse blue (Ciba-Geigy, Hawthorne, NY, USA) was injected into the jugular vein to delineate the in vivo area at risk. With this technique, the previously non-ischaemic area appears blue whereas the area at risk remains unstained. Anaesthetized rabbits were then injected with potassium chloride (1 g) and the heart was excised and cut into five or six 2-mm-thick transverse slices.
After removing right ventricular tissue, each slice was weighed and identified. The basal surface of each slice was photographed for future measurement of the area at risk. Each slice was then incubated for 15 min in tetrazolium chloride to differentiate infarcted (pale) from viable (red) myocardial area.15 Each slice was then photographed again. Using enlarged projections, the boundaries of the different areas on each slice were traced. The extent of left ventricle (LV) area, area at risk and infarct size were quantified by computerized planimetry (ImageJ software, version 1.01z, National Institutes of Health, USA) and corrected for the weight of the tissue slice. Total weights of area at risk and area of necrosis were then calculated and expressed as weight (g) or as percentages of total LV weight. It was decided prospectively that hearts with a risk region <10% of the LV weight would be excluded from the study.
Experimental groups
Animals were randomly assigned into five groups (Fig. 1). All groups underwent a 30-min coronary artery occlusion and 3 h of reperfusion. Before this prolonged ischaemia, they underwent a 45-min treatment period. After the 15-min stabilization period, the halogenated anaesthetic was added to the inspired gas for 30 min and discontinued 15 min before ischaemia. Volatile anaesthetics were titrated to an end-tidal concentration of 3.7% sevoflurane, 1.4% halothane, 2.0% isoflurane and 8.9% desflurane, corresponding to 1.0 MAC of each anaesthetic in the rabbit.1618 For each animal, end-tidal concentration of halogenated agent was less than 0.1% at the end of the washout period.
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Statistics
Statistical analysis of haemodynamics and E'CO2 was performed using two-way ANOVA with repeated measures on one factor. LV weight and area at risk were analysed by analysis of variance. Effect of pretreatment on percent of risk zone infarcted was analysed by one-way analysis of variance followed by Dunnetts post test when appropriate. Differences in infarct sizes among groups were evaluated by analysis of covariance and post-hoc least significance difference test, with infarct size as the dependent variable and area at risk as the covariant. Statistical calculations were performed using Statistica 5.0 (Statsoft Inc., Tulsa, OK, USA) and GraphPad InStat version 3.00 for Windows 95 (GraphPad Software, San Diego, CA, USA). All values are expressed as mean (SD). P<0.05 was considered significant.
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Results |
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Haemodynamic data
Haemodynamic data, including heart rate and arterial pressure, are summarized in Table 1. Inhalational anaesthetic administration decreased systolic arterial pressure consistently but transiently in all groups. This effect was statistically significant for the sevoflurane and desflurane groups. E'CO2 and body temperature did not differ between the groups.
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These data were confirmed when comparing the weights of the infarct size with area at risk as a covariant, which represents a major determinant of myocardial infarction in the rabbit model (Table 2).
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Discussion |
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Several studies have reported the cardioprotective effects of halogenated anaesthetics against ischaemia-reperfusion, depending on the timing of administration and the ischaemic time. Warltier and colleagues4 and Meissner and colleagues5 showed that halothane, isoflurane and desflurane can attenuate myocardial stunning. All four inhalational anaesthetics can limit infarct size in in vivo preparations, whether they have been administered during a sustained ischaemia or during the reperfusion. When given during a sustained ischaemic period, halothane1 and isoflurane2 decreased infarct size. When given only during the reperfusion period, desflurane and sevoflurane limited myocardial necrosis, whereas isoflurane had no effect.19 A 30-min inhalation of sevoflurane3 or desflurane20 discontinued just before the onset of ischaemia also had significant anti-necrotic effect.
Pre-administration of isoflurane also exerted a protective effect by reducing infarct size when the discontinuation of this volatile agent was followed by a washout period before ischaemia. This isoflurane-induced pharmacological preconditioning has been shown in vitro9 and in vivo.10 11 Similar observations have been made using halothane and enflurane.9 Similar to ischaemic preconditioning, isoflurane-induced preconditioning involves adenosine receptors,12 Gi protein13 and mitochondrial KATP channels.14 Adenosine A1 receptors and protein kinase C can also modulate halothane-induced preconditioning.9
Our study is the first to address the issue of the intensity of the preconditioning effect of these four halogenated agents in the same experimental conditions. We found that the degree of protection differed between the four agents. Sevoflurane failed to significantly reduce infarct size. While halothane and isoflurane appeared very similar, desflurane displayed the most potent cardioprotective effect. Consistent findings have been reported by Toller and colleagues using a canine model.3 The authors showed that discontinuation of a 30-min administration of 1 MAC sevoflurane just before onset of ischaemia decreased infarct size. However, when sevoflurane was discontinued 30 min before ischaemia, the myocardial protective effect disappeared. Two-minute ischaemic preconditioning was not long enough to precondition the heart. However, 2-min ischaemia associated with a 30-min sevoflurane pre-administration and a 30-min washout period decreased infarct size. Preconditioning induced by desflurane has not been shown previously. As with ischaemic preconditioning and isoflurane-induced preconditioning, KATP channels may be responsible for desflurane cardioprotection.20
The reasons for the differences between the four inhalational agents remain unclear. Hypotension induced by volatile agents does not seem to be involved in the volatile-anaesthetic-induced preconditioning, the main mechanism involved being a direct effect on KATP channels.10 In the current study, desflurane and sevoflurane were the most hypotensive agents, with opposite effects on infarct size. This is in agreement with Cope and colleagues,9 who showed a lack of correlation between infarct size and the hypotensive effects of volatile anaesthetics. Although +dp/dt and dp/dt were not measured, it is possible that the different lusitropic and inotropic effects of the four anaesthetics might influence these results.
The basal anaesthetic protocol might influence our results, because ketamine has been shown to block preconditioning21whereas opiates such as fentanyl confer cardioprotection by mimicking ischaemic preconditioning.22 However, all groups received the same anaesthetic protocol, and it is unlikely that this could explain the differences between groups.
Like ischaemic preconditioning,23 some volatile anaesthetics, such as halothane and isoflurane, can inhibit apotosis.24 Although we did not measure tissue ATP concentrations or assess apoptosis, we cannot rule out that sevoflurane pre-administration can be protective by decreasing apoptosis without significantly decreasing infarct size.
One possible explanation of the most pronounced effect of desflurane in our study is an increase in sympathetic activity25 or the release of myocardial catecholamines26 induced by this agent. Indeed, it has been shown that -adrenergic agonists may trigger ischaemic precon ditioning.27 28 Regarding an increase in sympathetic activity, desflurane-induced hypotension was not concomitant of tachycardia in the present study, which might signify a lack of sympathetic activation. Moreover, Pac-Soo and colleagues29 showed in a rabbit model that although low concentrations of isoflurane (1.2%) and desflurane (6%) increased renal sympathetic nerve activity, higher concentrations (such as those used in our study) depressed sympathetic activity. On the other hand, Gueugniaud and colleagues suggested that desflurane may be responsible for myocardial catecholamine release in isolated rat myocardium.26 However, it is not known how myocardial catecholamine release may influence desflurane-induced preconditioning.
Toller and colleagues3 suggested that the lack of effect of sevoflurane might be explained by the low blood gas solubility coefficient (0.62),30 resulting in less residual drug in the myocardium at the end of the washout period than with isoflurane. This does not fit with our data, however, since we demonstrated that desflurane, with the lowest bloodgas partition coefficient (0.49),30 exhibited the most pronounced effect.
We only examined the effects of 1 MAC; however, we cannot rule out that there is a doseresponse curve and that a higher concentration of sevoflurane might be effective in preconditioning the rabbit heart.
In conclusion, our results indicate that halothane, isoflurane and desflurane, but possibly not sevoflurane, can induce pharmacological preconditioning. Further studies are needed to address the selective efficacy and mechanism of action for each of these agents.
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Acknowledgements |
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References |
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