Acta anaesth. scand. 1979, 23, 107-1 11

The Potentiation of Halothane Anaesthesia by Clonidine S. KAUKINEN and K. PYYKKO Department of Biomedical Sciences, University of Tampere and Tanipere Central Hospital, Tampere, Finland

The antihypertensive agent, clonidine, has a marked sedative effect. We studied whether clonidine also deepens halothane anaesthesia. Eight rabbits were anaesthetized with and without clonidine premedication in a cross-over study. Clonidine premedication (50 ,ug/kg subcutaneously) was administered three times daily for 3 days. Tolerance to pain during halothane anaesthesia was tested by compressing the ear with a vessel clamp. Halothane concentrations were determined by gas chromatography. The rabbits premedicated with clonidine tolerated painful stimuli without reactions at lower halothane concentrations in arterial blood and inspired air than unpremedicated rabbits. MAC calculated from blood concentrations was 1.29% for unpremedicated and 1.09% for clonidine-premedicated rabbits. The results suggest that clonidine diminishes the anaesthetic requirement in halothane anaesthesia. Received 29 June, accepted f o r publication 4 September 1978

Clonidine produces its antihypertensive therapy must thus be continued throughout action mainly by reducing the central the day of operation in order to avoid the & sympathetic outflow. This is attributed to possible withdrawal syndrome (BRODSKY et al. 1979). Because stimulation of the central alpha-adrenergic BRAVO1976, KAUKINEN & of its sedative effect it could be expected that receptors (VAN ZWIETEN1973, KOBINCER PICHLER1974). I n addition to the antihyper- clonidine also potentiates the action of tensive effect, this central action causes anaesthetics. Clonidine prolongs the sleeping sedation, a frequent side effect of clonidine. time in chloral hydrate-anaesthctized rats & TAYLOR 1969). However, in This sedative effect prolongs sleep and sup- (LAVERTY presses R E M sleep in animals (KLEINLOGELrelation to clinically used anaesthctics, there et al. 1975) and also in man (AUTRETet al. are no studies about the effect of clonidine on 1977). The behavioral depression of clonidine the depth of anaesthesia. The present paper is seen also in EEG synchronization in differ- reports the potentiation of halothane anaesent animal species (FLORIOet al. 1975). thesia by clonidine in rabbits. Clonidine produces analgesia in rats (PAALzow & PAALZOW 1976), which may also be due to its stimulatory effect on the alpha- MATERIAL AND METHODS adrenergic receptors in the lower brain-stem The experiments were carried out on eight male rabI tructures. bits, weighing from 2600 to 3600 g. Each animal was Clonidine has a short duration of action. anacsthetized with halothane (Anestanm, Leiras Ltd., The abrupt cessation of clonidine therapy can Turku, Finland) with and without clonidine premediinduce a withdrawal syndrome which consists cation in a cross-over manner with an interval of 14 days. As premedication, 50 pg/kg of clonidine (cloniof a hypertensive crisis and other symptoms dine hydrochloride, Star Ltd., Tampere, Finland) was of sympathetic overstimulation (HUNYOR et given at 8 a.m., 4 p m . and 12 p.m. subcutaneously in al. 1973, GOLDBERC et al. 1977). Clonidine a volume of 0.5 ml/kg for 3 days. This dose is the lowest




which is known to induce sedation in rabbits: in the behaviour of the animals and also in EEG (FLORIO et al. 1975). The animals werc anaesthetized about 1 hour aftcr the last injection of clonidine. The induction of anaesthesia was carried out with halothane in air (5 limin), using a tightly fitting face mask and a Rees anaesthetic system. Halothane was delivered from a Fluoteca vaporizer Mark I1 in a concentration of I .5 -2.5 vol.%. Anaesthesia was maintained by the same system. I t was checked by capnography in the pilot experiments that this system did not produce carbon dioxide retention. After the animal was asleep, the central auricular artery was cannulated for the collection of blood samples. Thereafter, halothane concentration in the vaporizer was adjusted to 1.5% and anaesthesia was maintained with this concentration for 30 min. At the end of this period the tolerance of the animal to pain stimuli was checked by compressing the tip of the ear with a crile-type of vessel clamp. If the rabbit reacted to the compression by moving its ear, extremities or body, the halothane concentration in the vaporizer was increased by 0.25%. After a stabilization period of 30 min, the tolerance to the pain stimuli was checked again. This procedure was repeated until a halothane concentration was achieved under which the animal tolerated pain without any movements. At that time a blood sample was taken for determination of the halothane concentration. Also, a gas sample from the inside of the face mask was collected for halothane determination. This sample contained a mixture of fresh gas from the anaesthetic machine and of the expired air which would then be reinhaled. Thus the sample consisted of the actual inspired gas. The respiration rate of the rabbit was also measured. For the determination of minimum alveolar concentration (MAC), the halothane concentration in the vaporizer was decreased gradually by decrements of 0.25% for 30 min until the animal again reacted to pain stimuli. Second blood and gas samples were then collected for determination of halothane concentrations, and the rabbit was allowed to recover from anaesthesia. After 14 days the experiment was repeated, but this time without clonidine premedication if the rabbit had received it the first time, and vice uci-sa. 'The mean of the halothane concentrations in the first and second samples was considered to be the lowest concentration at which the rabbit tolerated the pain stimuli used. MAC was calculated from the concentrations in the blood, assuming the blood-gas partition coefficient of halothane for rabbits to be 3.94 (HERBERT et al. 1972). 1968, MAPLESON The blood halothane concentration (pgiml) was analyzed by the mrthod of ALLOTet al. (1971), with some modifications. Immediately after the sampling, 1 ml hcparinized blood was injected into a glassstoppcrcd titbe containing 1 ml carbon tetrachloride (with a trace impurity of chloroform of lOOG200 mg/l).

The tube was shaken for 2 niin and thereafter ccntrifuged for 3 min at 2000 g. The lower phase was transferred into another tube and the halothane concentration was determined by gas chromatography. The halothane concentration in inspiratory air (v01.x) was determined by the method of WHITEet al. (1970). Halothane was collected by a [email protected] pump for 5 min from the inside of the face mask into a tube containing activated charcoal at a flowrate of 80 ml/min. Halothane vapour was desorbed by extraction with 10 ml of carbon tetrachloride overnight and the concentration was analyzed by gas chromatography. For the gas chromatographical analysis, a [email protected] Aerograph model 2 100 gas chromatograph equipped with a glass column (length 150 cm, internal diameter 6 mm) packed with 1.5% OV-101 on Chromosorbe G (AW-DMCS, 100/120 mesh) was used. The carrier gas was nitrogen (30 ml/min), and a flame-ionisation detector was employed with a hydrogen flow of 30 mI/min and an air flow of 500 ml/min. T h e temperatures were as follows: oven 5O"C, injector 75°C and detector 250°C. The sample volume was 1 pl. Each sample was determined in triplicate. The ratio of halothane and of chloroform (internal standard) peaks was determined and compared to the standards. T h e results are presented as means+s.e. mean. T h e statistical significances of the differences between the means were evaluated by Student's t-test for paired variates.

RESULTS After the administration of clonidine, the rabbits appeared calm and sedated. The clonidine-prernedicated rabbits fell asleep with halothane more easily and rapidly than those without clonidine medication. The prernedicated rabbits tolerated the pain stimuli produced by the compression of the ear a t a lower halothane concentration, both in arterial blood and in inspired air, than the animals without clonidine medication (Table 1). Halothane concentrations in the alveoli (MAC), as calculated from the blood concentrations, were in agreement with the concentrations in inspired air, being about 0.5 vol.O,r, lower in both groups than the latter values. The MAC-level of anaesthesia was achieved in the clonidine-prernedicated rabbits with 14.5% lower setting in the vaporizer than in the rabbits without prernedication. I n the eight rabbits studied, there was only one in whom clonidine medication had no


Table 1 I-Ialotlrane concentrations in arterial blood and inspired air and respiratory frequency at the moment when the rabbits tolerated thc pain stimuli without reactions. MAC is calculated from blood conrentrations. The results are means k s.e. mean.


than the MAC-values previously reported for rabbits: 0.8y0 (DAVIS et al. 1975) and 0.6304 ( M ~ 1977). ~ The K present ~ high ~ MAC~ ~ ~ value may be due to the intensive pain stimulus used: compression of the tip of the ear with a crile-type of vessel clamp. It probably produces more powerful pain than Without Premedicated the clamping of the tail with a bulldog clip prrnirdication with rlonidine which was used in the previous studies. Also Halothane concenour rabbits tolerated the lesser pain caused by tration in arterial the percutaneous cannulation of the auricular 370k 14 3 1 4 k 17** blood (pgiml) artery a t a much lower halothane concentraHalothane conce~ition than the compression with a crile-clamp. tra tion in inspired I n the central nervous system, clonidine 1.83k0.08 1.61 +0.09** air (vol.o/,) MAC (vo1.X) 1.29_+0.04 1.09*0.05** stimulates the post-synaptic alpha-adrenergic Respiratory receptors directly ( A N D ~et N al. 1970, FLORIO 82 3 66 k 4** frequency in min et al. 1975). I n addition, it reduces the release of norepinephrine by activating the ** PiO.01 as compared to the values without clonidinr inhibitory presynaptic alpha-adrenergic reprernedication. ceptors (STARKEet al. 1973, A N U ~ et N al. 1976). I t is, however, not known which of effect on the halothane requirement. Also, these mechanisms is responsible for the respiratory frequency a t the deepest level of sedative effect of clonidine. Clonidine still anaesthesia in the clonidine-treated animals induces sedation in rats after norepinephrine was lower than in those without clonidine stores have been depleted by reserpine or medication. after the synthesis of norepinephrine has been blocked by alpha-methyl-p-tyrosine (FLORIO et al. 1975). This refers to the direct postDISCUSSION synaptic site of action as a cause of sedation. MAC: is considered to be a measure of the O n the other hand, the opposite findings have potencyofan anaesthetic agent. It is the lowest also been reported. After the noradrenergic concentration of an anaesthetic in the alveoli nerve endings and the presynaptic receptors required to inhibit the reactions of the subject in them have been destroyed by 6-hydroxyto painful stimuli (MERKEL& ECER 1963). dopamine plus reserpine, clonidine has been Because of the difficulties of obtaining found to cause hyperactivity in rats instead end-expiratory gas samples for halothane of sedation (ZEBROWSKA-LUPINA et al. 1977). determination in rabbits without tracheo- This would indicate that presynaptic recepstomy, we measured the concentration of tors are also involved in the sedation prohalothane in arterial blood. After the duced by clonidine. T h e sedation can stabilization period of 30 min now used, the be abolished by alpha-adrenergic blocking arterial blood and also the brain are practic- agents, like yohimbine (FLORIO et al. 1975, ally 1000/, saturated with halothane. After AUTRETet al. 1977). This also favours the this time, also, the alveolar concentration of presynaptic site of action because yohimbine halothane rcmains fairly constant (EGER is considered primarily as an antagonist to 1974). Assuming the blood-gas partition the presynaptic alpha-adrenergic receptors coefficient of halothane for rabbits to be 3.94 (ANDBNet al. 1976). ( MAPLESON et al. 1972), the calculated MAC Among the antihypertensive agents, reserfhr the unpremedicated rabbits in this experi- pine and methyldopa also cause sedation. ment was 1.29%. This is substantially higher This agrees with the idea of the presynaptic



ftinctional effects of clonidine and rcccptoi- blocking agents. Naiiintlllyn-Schmiedeberg’sArch. Pharnzacol. 292,43. T., CATHALA, AUTRET,A., MINZ, M., BEILLEVAIRE, norepinephrine release from nerve endings by H.-P. & SCHMITT, H. (1977) Effect of clonidine on drplcting the transmitter stores. sleep patterns in man. Eicrop. 3. &ti. Pharmacol. 12, Reserpine and methyldopa decrease MAC 319. J. B. & BRAVO,J. J. (1976) Acute postin dogs by 20-30%,, depending on the dose BRODSKY, operative clonidine withdrawal syndrome. Atiesthesiused (MILLER et al. 1968). Clonidine lowered 0lOLQ 44, 5 19. MAC in the present study by 15:/,. T h e dose DAVIS,N. L., NIJNNALLY, R . I,. & MALININ,T. I. now used, 50 pglkg, is the lowest dose which (1975) Determination of the minimum alveolar conhas been shown to have (in rabbits) a sedative centration (MAC) of halothane in the white New Zealand rabbit. Brit. 3. Ariaesth. 47, 341. efkct on the behaviour of the animals and also on EEG (FLORIOet al. 1975). The EGER, E. I. I1 (1974) Anaesthetic Uptake and Action. Williams & Wilkins Co., Baltimore, pp. 77-96. analgesic effect which clonidine has been FLORIO,V., BIANCHI,L. & LONGO,V. G. (1975) A reported to exhibit (PAALZOW & PAALZOW study of the central effects of sympathomimetic drugs: EEG and behavioural investigations on 1976) explains the decrease in MAC in our clonidine and naphazoline. Neuropharmacoloa 14,707. 5 tudies. A. D., RAFTERY, E. B. & WILKINSON, P. (%midine is a short-acting drug: three GOLDBERG, (1977) Blood pressure and heart rate and withdrawal doses daily are usually required to maintain of antihypertensive drugs. Brit. med. 3. 1, 1243. its antihypertensive action. Because the HERBEKT, P. (1968) Determination of halothane conabrupt cessation of clonidine medication can centration by gas chromatography. 3. med. Lab. Technol. 25, 233. induce a withdrawal syndrome (HUNYOR et L., HARRISON, T. S. & al. 1973, GOLDBERG et al. 1977), it has been HUNYOR,S. N., HANSSON, HOORLER, S. W. (1973) Effects of clonidine withrecommended that clonidine should continue drawal : possible mechanisms and suggestions for to be administered on the operation day management. Brit. med. 3. 2, 209. (BRODSKY & BRAVO1976, KAUKINEN et al. KAUKINEN, S., KAUKINEN, L. & EEKOIA,R . (1979) Preoperative and postoperative use of clonidine with 1979). Therefore, it is a common practice to neurolept anaesthesia. Acta anaesth. scand. In press. anaesthetize patients who are receiving G. & SAYERS,A. C:. KLEINLOGEL, H., SCHOLTYSIK, clonidine medication. As found in this study (1975) Effects of clonidine and BS 100-141 on the in experimental animals, it can be expected EEG sleep pattern in rats. Europ. 3. Pharmncol. 33, 159. that clonidine will also diminish the anaesKORINGER, W. & PICISLER, I,. (1974) Evidence for thetic requirement in man. direct a-adrenoceptor stimulation of effector neurons J .~ . in cardiovascular centers by clonidine. E Z I Y O Pharniacol. 27, 151. ACKNOWLEDGEMENTS LAVERTY, R. & TAYLOR, K. M. (1969) Behavioural and biochemical effects of 2-(2,6-dich1orophenyl‘This work was supported by grants from the Foundaamino)-Z-imidazoline hydrochloride (St 155) on the tion of ’Tampere University and from the Medical central nervous system. Brit. 3. Pharmncol. 35,253. Society of Tampere, Finland. J. E. (1977) Determination of MAC for MACKENZIE, halothane, cyclopropane and ether in the rabbit. Brit.3. Anaesth. 49, 3 19. KEFERENCXS MAPLESON, W. W., ALLOTT,P. R . & STEWARD, A. AI.I.OTT,P. R., STEWARD, A. & MAPLESON, (1972) The variability of partition coefficients for (1971) Dcterniination of halothane in gas, blood, and halothane in the rabbit. Brit. 3. Anaesth. 44, 656. tissues by chemical extraction and gas chromatoMERKEL,G. & EGER,E. I. I1 (1963) A comparative graphy. Brit. 3. ,4riaesth. 43, 913. study of halothane and halopropane anesthesia. AND~N N.-E., , CORROIII, H., FUXE,K., HOKPELT,B., Including method for determining equipotency. T . (1970) HBKFELT,T., RYDIN,C. & SVLNSSON, Anesthesiolopy 24, 346. Evidencc for a central noradrenaline receptor MILLER,R . D., WAY, W. L. & EGER,E. I. I1 (1968) stimulation by clonidine. Lge Sci. 9, 513. The effects of alpha-methyldopa, reserpine, guanU. A N D ~ NN.-E., , GRABOWSKA., M. & STR~MBOM, ethidine, and iproniazid on minimum alveolar (1976) Different alpha-adrenoreceptors in the anesthetic requirement (MAC). Anesthesiology 29, ccntral ncrvoiis system mediating biochemical and 1153.

sitc of action of clonidine because reserpine and to somr degree also methyldopa diminish



PAALZOW, G. & PAALZOW, L. (1976) Clonidine antinociceptive activity: effects of drugs influencing central monoaminergic and cholinergic mechanisms in the rat. Nautzvn-Schmiedeberg’s Arch. Pharmacol. 292, 119. STARKE, K. & ALTMANN, K. P. (1973) Inhibition of adrenergic neurotransmission by clonidine : an action on prejunctional a-receptors. NeuropharnzaCOlOQ 12, 339. WHITE,L. D., TAYLOR, D. G., MAUER, P. A. & KUPEL, R. E. (1970) A convenient optimized method for the analysis ofselected solvent vapors in the industrial atmosphere. Amer. industr. H9g. Ass. 3. 31, 225. ZEBROWSKA-~JPINA, I., PKZEGALIfiSKI, E., SLONIEC, M.


& KLEINROK, Z. (1977) Clonidine-induced locomotor hyperactivity in rats. The role of central postsynaptic a-adrenoceptora. ~aui~ri-Sctrmiedeberg’s Arch. Pharmncol. 297, 227. VAN ZWIETEN, P. A. (1973) T h e central action of antihypertensive drugs, mediated via central a-receptors. J . Pharm. Pharniacol. 25, 89. Address :

Seppo Kaukirren, M.D. Tampere Central Hospital SF-33520 Tampere 52 Finland

The potentiation of halothane anaesthesia by clonidine.

Acta anaesth. scand. 1979, 23, 107-1 11 The Potentiation of Halothane Anaesthesia by Clonidine S. KAUKINEN and K. PYYKKO Department of Biomedical Sci...
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