1Institut für Klinische Anaesthesiologie, Heinrich-Heine-Universität Düsseldorf, Postfach 10 10 07, D-40001 Düsseldorf, Germany. 2Physiologisches Institut I, Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany.*Corresponding author
Accepted for publication: August 21, 2000
Abstract
Ischaemic preconditioning can protect the myocardium against ischaemic injury by opening of the adenosine triphosphate (ATP)-sensitive potassium (KATP) channel. Isoflurane is also thought to open this channel. The present investigation tested the hypothesis that pre-ischaemic treatment with isoflurane mimics ischaemic preconditioning (producing chemical preconditioning) and thereby protects the myocardium against ischaemic injury in an isolated rat heart model. Control hearts underwent 30 min of global no-flow ischaemia followed by 60 min of reperfusion. The hearts of the preconditioning group underwent two 5 min periods of no-flow ischaemia interspersed with 5 min of reperfusion before the sustained ischaemia. In three additional groups, hearts were subjected to 15 min of 1.5 minimal alveolar concentration (MAC) of isoflurane (ISO-1), 15 min 3 MAC (ISO-2) or 25 min 1.5 MAC (ISO-3) of isoflurane followed by 5 min washout before the global ischaemia. Left ventricular (LV) developed pressure and creatine kinase release were measured as variables of myocardial performance and cellular injury, respectively. Recovery of LV developed pressure was improved after ischaemic preconditioning [after 60 min reperfusion, mean 63 (SEM 6)% of baseline] compared with the control group [18 (4)% P<0.01] but not by isoflurane, independently of concentration or duration of administration [ISO-1, 17 (2)%, P=0.99 vs control; ISO-2, 12 (3)%, P=0.64; ISO-3, 4 (1)%, P=0.06]. Total creatine kinase release over 1 h of reperfusion was not significantly different between control [251 (36) U g1 dry weight] and all isoflurane groups [ISO-1, 346 (24) U g1, P=0.30; ISO-2, 313 (33) U g1, P =0.73; ISO-3, 407 (40) U g1, P=0.03]. These results indicate that pre-ischaemic administration of isoflurane does not cause anaesthetic-induced preconditioning in the isolated rat heart.
Br J Anaesth 2001; 86: 26971
Keywords: anaesthetics volatile, isoflurane; heart, ischaemia; model, heart; rat
Brief ischaemic periods render the myocardium resistant to myocardial injury from subsequent ischaemic insults, a process called ischaemic preconditioning. The ATP-sensitive potassium (KATP) channel plays a major role in the mechanism of ischaemic preconditioning, and volatile anaesthetics have been shown to act on this channel.1 In 1997, Cope and colleagues first demonstrated that preadministration of some volatile anaesthetics followed by a washout period protects isolated rabbit myocardium against infarction and mimics the ischaemic preconditioning phenomenon (pharmacological or chemical preconditioning).2 A similar protective effect was found in vivo in dogs3 and rabbits.4 5 In isolated rat hearts, volatile anaesthetics have been shown to be cardioprotective if administered before ischaemia without preischaemic washout.6 However, without preischaemic washout a substantial anti-ischaemic effect of isoflurane cannot be excluded. We investigated whether chemical preconditioning-like effects of isoflurane can be induced in isolated rat hearts subjected to global ischaemia. We chose this model to prevent systemic effects of isoflurane and changes in collateral blood flow influencing ischaemic injury. To exclude anti-ischaemic effects, isoflurane was washed out 5 min before ischaemia.
Methods and results
The experiments were performed in accordance with the regulations of the German Animal Protection Law and local institutional regulations. The exact protocol has been described in detail previously.7 In brief, hearts were excised from anaesthetized male Wistar rats (300350 g body weight) and mounted in a Langendorff perfusion system. Retrograde perfusion via the aorta was initiated in constant pressure mode (80 mm Hg) with oxygenated modified KrebsHenseleit buffer. The heart rate of the isovolumetric beating hearts was maintained at 375 beats min1 by atrial pacing. A hollow-fibre oxygenator was integrated into the perfusion system and isoflurane was administered by an agent-specific vaporizer into the fresh gas supply of the oxygenator. A Datex infrared gas analyser controlled the delivered vapour concentration continuously. After preparation, a stabilization period of 20 min was allowed. The experimental programme consisted of three phases: intervention (20 or 30 min); global no-flow ischaemia (30 min); and reperfusion (60 min). Control hearts received no further treatment in the intervention phase. In a second group (PC), ischaemic preconditioning was induced by two 5 min periods of no-flow ischaemia, each followed by 5 min of reperfusion. The hearts of the isoflurane groups received 15 min 1.5 minimal alveolar concentration (MAC) (ISO-1), 15 min 3 MAC (ISO-2) or 25 min 1.5 MAC of isoflurane (ISO-3), followed by 5 min washout before global ischaemia (1 MAC=2.3% Vol in the rat). Left ventricular (LV) developed pressure [LV peak systolic pressure minus LV end-diastolic pressure (LVEDP)], LVEDP, oxygen consumption and creatine kinase release were measured as variables of myocardial performance and cellular injury, respectively. Data are expressed as mean (SEM). Statistical analysis was performed by two-way analysis of variance (ANOVA) for time and treatment effects. If an overall significant difference was found, comparison was made for each time using one-way ANOVA followed by the TukeyKramer post test (between groups) or Dunnetts post test (vs baseline) when appropriate. P<0.05 was regarded as significant.
A total of 40 hearts were included in the statistical analysis (five groups, n=8 each). Baseline values were similar in all groups. Postischaemic function of control hearts was markedly depressed and LV developed pressure recovered to only 18 (4)% of baseline values (P<0.01) after 60 min reperfusion (Fig. 1). In the PC group, the two 5 min periods of ischaemia had almost no influence on haemodynamic variables after 5 min of washout, except for LV developed pressure which decreased from 117 (7) mmHg to 88 (7) mmHg (P=0.01 PC vs baseline). The preconditioning protocol resulted in better preservation of myocardial performance after 30 min of global ischaemia in comparison with control hearts. LV developed pressure finally reached 63 (6)% of the baseline value (P<0.01 vs control). Isoflurane could not mimic the myocardial protection produced by preconditioning. LV developed pressure recovered to 17 (2)% of baseline (ISO-1, P=0.98 vs control), 12 (3)% (ISO-2, P=0.64 vs control) and 4 (1)% (ISO-3, P=0.06 vs control), similar to the control hearts and significantly lower in comparison with PC (P<0.01 vs all isoflurane groups). LVEDP increased in control hearts during ischaemia to a peak value of 36 (4) mm Hg after 25 min, indicating myocardial contracture (Table 1). During early reperfusion, there was a further increase in LVEDP, which reached a maximum of 100 (6) mm Hg (P<0.01 vs baseline) at 5 min of reperfusion (reperfusion contracture). At the end of the experiment, LVEDP in control hearts was still elevated [60 (7) mm Hg, P<0.01 vs baseline]. Ischaemic preconditioning reduced the degree of myocardial contracture in the ischaemic period [maximum LVEDP 23 (6) mm Hg] and in the reperfusion phase. After 60 min of reperfusion, LVEDP was only slightly elevated in the PC group [22 (7) mm Hg, P=0.73 vs baseline] and was significantly lower than the control. The extent of myocardial contracture was not affected by isoflurane administration. LVEDP increased to 60 (6) (ISO-1, P=1.00 vs control), 65 (4) (ISO-2, P=0.99 vs control) and 78 (5) mm Hg (ISO-3, P=0.25 vs control) at the end of reperfusion, significantly higher than in preconditioned hearts (P<0.01 vs all isoflurane groups). Oxygen consumption during baseline was 1047 (99) µl min1 g1 dry weight, and was similar in all groups. Oxygen consumption was reduced after the reperfusion period in control hearts to 439 (53) µl min1 g1. Preconditioning led to greater oxygen consumption at the end of reperfusion (Table 1), indicating a greater amount of viable tissue in this group. In contrast, oxygen consumption in all isoflurane groups was similar to that in the control group and lower than that in the PC group (Table 1). Isoflurane did not reduce the amount of cell necrosis, as shown by creatine kinase release as an indicator of cellular injury. Cumulative creatine kinase release after 1 h of reperfusion was 251 (36) U g1 dry weight in control hearts. Creatine kinase release was not reduced by pre-ischaemic administration of isoflurane (Table 1).
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The results of the present study demonstrate that isoflurane, regardless of the concentration used and the duration of administration, does not produce pharmacological preconditioning in isolated rat hearts. These findings are in contrast to four recent studies, which can be summarized as follows. (i) Isoflurane mimicked ischaemic preconditioning in a canine model in vivo and in rabbit models in vivo and in vitro with regional ischaemia and reperfusion.2 3 (ii) Cardioprotection was independent of the systemic effects of the volatile anaesthetic (in vitro model) and of increases in collateral blood flow (rabbit model with almost no coronary collateralization).2 5 (iii) The effect could be observed with low (0.6 MAC) and high (2 MAC) concentrations of isoflurane, short (5 min) and long (65 min) durations of administration and different time intervals between isoflurane administration and prolonged ischaemia (530 min).35 (iv) Cardioprotection by isoflurane could be abolished by preadministration of glyburide, a specific blocker of the KATP channel.3 5 These results demonstrate a preconditioning-like effect of isoflurane (pharmacological preconditioning) in different experimental models and with different experimental protocols. However, these effects could not be confirmed in isolated rat hearts. Besides differences in models (regional vs global ischaemia) and protocols (duration of isoflurane administration, pre-ischaemic washout), it is likely that pharmacological preconditioning by isoflurane is species-dependent and the isolated rat heart is not a suitable model for the study of anaesthetic-induced preconditioning. Species differences make it difficult to transfer results of previous experimental studies to possible effects on the human myocardium.
References
1 Kersten JR, Gross GJ, Pagel PS, Warltier DC. Activation of adenosine triphosphate-regulated potassium channels. Mediation of cellular and organ protection. Anesthesiology 1998; 88: 495513[ISI][Medline]
2 Cope DK, Impastato WK, Cohen MV, Downey JM. Volatile anesthetics protect the ischemic rabbit myocardium from infarction. Anesthesiology 1997; 86: 699709[ISI][Medline]
3 Kersten JR, Schmeling TJ, Pagel PS, Gross GJ, Warltier DC. Isoflurane mimics ischemic preconditioning via activation of KATP channels. Reduction of myocardial infarct size with an acute memory phase. Anesthesiology 1997; 87: 36170[ISI][Medline]
4 Cason BA, Gamperl AK, Slocum RE, Hickey RF. Anesthetic-induced preconditioning. Previous administration of isoflurane decreases myocardial infarct size in rabbits. Anesthesiology 1997; 87: 118290[ISI][Medline]
5 Ismaeil MS, Tkachenko J, Gamperl AK, Hickeey RF, Cason BA. Mechanisms of isoflurane-induced myocardial preconditioning in rabbits. Anesthesiology 1999; 90: 81221[ISI][Medline]
6 Ross S, Foëx P. Protective effects of anaesthetics in reversible and irreversible ischaemiareperfusion injury. Br J Anaesth 1999; 82: 62232
7 Schlack W, Preckel B, Stunneck D, Thämer V. Effects of halothane, enflurane, isoflurane, sevoflurane and desflurane on myocardial reperfusion injury in the isolated rat heart. Br J Anaesth 1998; 81: 9139