Bupivacaine is widely used in regional anaesthesia. Accidental intravenous (i.v.) administration of an overdose of bupivacaine can cause serious systemic toxic effects, such as severe dysrhythmias and cardiac arrest . Ropivacaine, a newer local anaesthetic with a chemical structure and local anaesthetic efficacy similar to bupivacaine, is less cardiotoxic than bupivacaine .
Clonidine, an α-2 adrenoceptor agonist with systemic effects, e.g. bradycardia, hypotension and antidysrhythmic properties , is frequently used in premedication . There are few studies of the effects of clonidine on bupivacaine cardiotoxicity [5,6] and none on ropivacaine, and there are no studies concerning the effects of clonidine on resuscitation from cardiac arrest associated with overdose of local anaesthetics. The effects of pre-treatment with clonidine on ropivacaine- and bupivacaine-induced cardiotoxicity may differ because of the different cardiotoxic potencies of these local anaesthetics. The aim of this study was to evaluate how pre-treatment with clonidine influences the cardiovascular effects of bupivacaine or ropivacaine overdose and the effects on resuscitation in anaesthetized rats.
The study was approved by the Animal Ethics Committee of Osmangazi University Medical School. Thirty-two adult, female Sprague-Dawley rats (250-300 g) were divided into four groups: Group 1, saline pre-treatment (2 mL kg−1 in 2.5 min) with ropivacaine infusion (3 mg kg−1 min−1); Group 2, clonidine pre-treatment (5 μg kg−1 in 2.5 min) plus ropivacaine infusion (3 mg kg−1 min−1); Group 3, saline pre-treatment (2 mL kg−1 in 2.5 min) plus bupivacaine infusion (3 mg kg−1 min−1); Group 4, clonidine pre-treatment (5 μg kg−1 in 2.5 min) plus bupivacaine infusion (3 mg kg−1 min−1).
The rats were anaesthetized with an intraperitoneal injection of thiopental sodium (60 mg kg−1) and ketamine (30 mg kg−1). The trachea was cannulated and mechanical ventilation of the lungs instituted (tidal volume: 3 mL, respiratory rate: 56 breaths min−1) with a rodent ventilator (U go Basil, Italy). A cannula was inserted into the right femoral vein to administer drugs and the right femoral artery was cannulated to measure arterial pressure. The electrocardiogram (ECG), invasive arterial pressure and heart rate (HR) were recorded continuously with a data acquisitions system (M100®; Biopac Systems Inc, Goleta, CA, USA). Body temperature was measured using a rectal probe and maintained at 37 ± 0.5°C with an external heat lamp.
Arterial blood gases were measured (ABL 700®; Radiometer, Copenhagen, Denmark) during a 30 min stabilization period following the surgical preparation. Saline 0.9% was infused to compensate for blood withdrawn for analysis. Lung ventilation was adjusted to maintain PaCO2 at approximately 3.3-4.7 kPa. After this period, clonidine or saline 0.9% was administered i.v. in a volume of 2 mL kg−1 over 2 1/2 min. Fifteen minutes later, an i.v. infusion of bupivacaine 0.5% or ropivacaine 0.5% was started at the rate of 3 mg kg−1 min−1 using a calibrated electronic infusion pump (MAY INF 9601®; Commat, Turkey). The local anaesthetic was infused until asystole. The drugs were then stopped, mechanical ventilation continued with 100% O2, and resuscitation measures initiated. Three doses of epinephrine (10 μg kg−1 in a volume of 2 mL kg−1) were given at 30 s intervals. If a spontaneous heart beat was not restored, external cardiac massage was initiated. Further doses of epinephrine were given after 150 and 210 s. Resuscitation was stopped after 5 min, if asystole persisted. If a spontaneous heart beat resumed and arterial pressure was >80 mmHg, the rats were observed for 30 min. The resuscitation results were scored on a three-point scale (0: no response to resuscitation; 1: HR restored with epinephrine injections and cardiac compression; 2: HR restored with epinephrine injections alone).
At the end of the experimental procedures, the recorded data were evaluated and the following events were calculated: time to first QRS alteration (increase of QRS complex duration by more than 20%); time to first dysrhythmia (first dysrhythmia accompanied by an abnormal systole on the arterial pressure trace); time to 25%, 50% and 75% reduction of HR relative to baseline; time to 25%, 50% and 75% reduction of mean arterial pressure (MAP) relative to baseline and time to asystole (FST) defined as the absence of pressure pulse on the arterial pressure trace.
Statistical analysis of variables over time was performed using a one-way ANOVA after testing for normal distribution with the Kolmogorov-Smirnov test. Comparisons between groups were based on Duncan's significant difference test. Resuscitation results were analysed by the U-test. All results are presented as mean ± SD. P < 0.05 was considered significant.
Baseline arterial pressure, HR and values for PaCO2 (Group 1: 4.1 ± 0.2 kPa; Group 2: 4.3 ± 0.24 kPa; Group 3: 4.2 ± 0.25 kPa and Group 4: 4.08 ± 0.3 kPa) were similar among all groups before the experiments.
Clonidine significantly reduced MAP and HR (P < 0.01) before bupivacaine or ropivacaine infusion (Fig. 1), while there was no change in the saline pre-treated rats.
The first sign of ropivacaine toxicity was a widening of the QRS complex in Groups 1 and 2 after 57 ± 18 and 70 ± 17 s, respectively, followed by a 25% reduction of HR (after 173 ± 61 and 171 ± 146 s, respectively) and MAP (after 197 ± 97 and 402 ± 252 s, respectively). Dysrhythmia was a late sign of ropivacaine toxicity and occurred after 399 ± 233 s in Group 1 and 655 ± 112 s in Group 2. Asystole finally occurred after 747 ± 162 s in Group 1 and 1026 ± 284 s in Group 2. Except for the widening of the QRS complex and the 25% reduction of HR, all other signs of ropivacaine cardiac toxicity occurred significantly later in the clonidine pretreated rats (P < 0.05). Haemodynamic and ECG data of the ropivacaine-treated groups are given in Table 1.
Clonidine increased the resuscitation success rate for the rats given ropivacaine. HR was restored in four of the eight animals in Group 1 - three of the animals required external cardiac massage. Spontaneous HR was restored in six of the eight animals in Group 2 with epinephrine alone and none of the animals required external cardiac massage. Resuscitation scores were significantly higher in Group 2 than in Group 1 (Table 2, P < 0.05).
All signs of local anaesthetic overdose occurred significantly earlier in the bupivacaine control group (Group 3) than in the ropivacaine control group (Group 1, P < 0.05). Clonidine did not delay the occurrence of bupivacaine cardiotoxicity in Group 4 (Table 3).
Resuscitation scores did not differ between Groups 3 and 4 (Table 2, P > 0.05). HR was restored in two animals of Group 3 and in one animal of Group 4 after treatment with epinephrine.
Our results showed that bupivacaine was more cardiotoxic than ropivacaine, and the cardiotoxic effects of bupivacaine were not alleviated by pre-treatment with clonidine. De la Coussaye and colleagues  suggested that combined therapy with clonidine and dobutamine could reverse the cardiotoxic effects of bupivacaine in anaesthetized dogs, but they did not test clonidine alone. De Kock and colleagues  reported that the same pre-treatment dose of clonidine significantly reduced the cardiotoxicity of a lower dose of bupivacaine. They noted that the reduction of cardiotoxicity was associated with the antidysrhythmic and central sympathetic outflow depressant effects of clonidine.
The discrepancy between our results and those of De Kock and colleagues may be attributed to the higher dose of bupivacaine used in our study. We preferred to use a higher dose since most life-threatening accidental infusions occur after bolus injections [1,7]. On the other hand, Bruguerolle and colleagues  reported that bupivacaine clearance was decreased in animals pre-treated with clonidine. This may partly explain the non-significant trend of an increase in the cardiotoxicity of bupivacaine in the presence of clonidine pre-treatment. In contrast, clonidine reduced the cardiotoxic effect of ropivacaine. Clonidine-induced bradycardia may help to explain the increased toxic threshold of ropivacaine. Ropivacaine and bupivacaine block the cardiac sodium channels, but the affinity of bupivacaine for the sodium channel is higher than that of ropivacaine. A slower HR before the local anaesthetic infusion allows more time for sodium channels to remain closed or inactive. This might have a greater effect on ropivacaine toxicity than on that of bupivacaine, which has a higher affinity.
Highly lipophilic local anaesthetics, e.g. bupivacaine, interfere with mitochondrial energy metabolism and reduce ATP synthesis [9-12]. This could partially explain some of the toxic effects of local anaesthetics, such as myocardial depression. Eledjam and colleagues  found, in a study on rabbits, that the administration of ATP, 30 min before administration of bupivacaine, almost completely reversed the negative inotropic effects of the bupivacaine. The results of this study suggested that inhibition of energy metabolism might be one of the major factors of bupivacaine cardiotoxicity. Pharmacological differences in lipophilicity of local anaesthetics correlate well with the depression of mitochondrial ATP synthesis . Previous studies reported that ropivacaine disturbed mitochondrial energy metabolism to a lesser extent than bupivacaine [14,15]. In this respect, the lower lipid solubility of ropivacaine could partially explain the lower potency of this molecule in disrupting mitochondrial bioenergetics. Clonidine-induced bradycardia may reduce myocardial ATP demand in animals in the ropivacaine group but this effect may not be that impressive in the bupivacaine group because of higher toxic effect of bupivacaine on the ATP synthesis rate.
Previous studies reported that clonidine increased cytosolic calcium concentrations, smooth muscle tension and cardiac contractility in rats and mice [16,17]. This effect might be involved in the reduced cardiotoxicity of ropivacaine in clonidine pre-treated rats. On the other hand, Mio and colleagues  suggested that bupivacaine attenuated contractility by decreasing the sensitivity of myofilaments to calcium in ventricular muscle of rats. Thus, clonidine pre-treatment may be ineffective in decreasing bupivacaine cardiotoxicity.
Several studies suggested that α-2 adrenergic agonists might be associated with better post-resuscitation myocardial function and survival [19-21]. ATP concentrations are also a very important factor for the success of cardiac resuscitation. The response to vasoactive drugs and cardiac resuscitation are reduced at low ATP concentrations . This may explain the better resuscitation scores in Group 2. In conclusion, our data suggest that clonidine pre-treatment does not reduce bupivacaine cardiotoxicity, but does ameliorate ropivacaine cardiotoxic effects.
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