Medetomidine and atipamezole in sheep: disposition and clinical effects

MAC-sparing effect of nitrous oxide in sevoflurane anesthetized sheep and its reversal with systemic atipamezole administration

INTRODUCTION

Nitrous oxide (N2O) is an anesthetic gas with antinociceptive properties and reduces the minimum alveolar concentration (MAC) for volatile anesthetic agents, potentially through mechanisms involving central alpha2-adrenoceptors. We hypothesized that 70% N2O in the inspired gas will significantly reduce the MAC of sevoflurane (MACSEVO) in sheep, and that this effect can be reversed by systemic atipamezole.

MATERIALS AND METHODS

Animals were initially anesthetized with SEVO in oxygen (O2) and exposed to an electrical current as supramaximal noxious stimulus in order to determine MACSEVO (in duplicates). Thereafter, 70% N2O was added to the inspired gas and the MAC re-determined in the presence of N2O (MACSN). A subgroup of sheep were anesthetized a second time with SEVO/N2O for re-determination of MACSN, after which atipamezole (0.2 mg kg-1, IV) was administered for MACSNA determinations. Sheep were anesthetized a third time, initially with only SEVO/O2 to re-determine MACSEVO, after which atipamezole (0.2 mg kg-1, IV) was administered for determination of MACSA.

RESULTS

MACSEVO was 2.7 (0.3)% [mean (standard deviation)]. Addition of N2O resulted in a 37% reduction of MACSEVO to MACSN of 1.7 (0.2)% (p <0.0001). Atipamezole reversed this effect, producing a MACSNA of 3.1 (0.7)%, which did not differ from MACSEVO (p = 0.12). MACSEVO did not differ from MACSA (p = 0.69). Cardiorespiratory variables were not different among experimental groups except a lower ETCO2 in animals exposed to SEVO/N2O.

CONCLUSIONS

N2O produces significant MACSEVO-reduction in sheep; this effect is completely reversed by IV atipamezole confirming the involvement of alpha2-adrenoreceptors in the MAC-sparing action of N2O.

The bioavailability of medetomidine in eight sheep following oesophageal administration

There is sound evidence that medetomidine is an effective analgesic for acute pain in sheep. In this study, 15 μg kg(-1) of medetomidine was administered intravenously, and into the oesophagus, in a cross-over study, using eight sheep. Following intravenous administration, medetomidine could be detected in the plasma of these sheep for 120-180 min but following oesophageal administration, medetomidine could not be detected in the plasma of any sheep at any of 17 time points over four days. It is suspected that this is due to high first pass metabolism in the liver. Consequently, we conclude that future studies investigating the use of analgesics in orally-administered osmotic pumps in sheep should consider higher doses of medetomidine (e.g. >100 μg kg(-1)), further investigations into the barriers of medetomidine bioavailability from the sheep gut, liver-bypass drug delivery systems, or other α2-adrenergic agonists (e.g. clonidine or xylazine).

A comparison of two different ketamine and diazepam combinations with an alphaxalone and medetomidine combination for induction of anaesthesia in sheep

AIMS

To investigate the perceived adverse effects of a particular batch of ketamine during induction of anaesthesia in sheep and to assess if any adverse effects would make intubation more difficult for the veterinary students.

METHODS

Thirty adult sheep (mean bodyweight 74.5 (SD 9.4) kg) were randomly assigned to one of six groups of five sheep. Sheep in Groups A and B received I/V 0.5 mg/kg diazepam and 10 mg/kg ketamine (Ketamine Injection; Parnell Laboratories NZ Ltd, of the suspect batch); those in Groups C and D received I/V 0.5 mg/kg diazepam and 10 mg/kg ketamine (Ketalar; Hospira NZ Ltd.), and those in Groups E and F received I/V 2 μg/kg medetomidine and 2 mg/kg alphaxalone. In Groups A, C and E, intubation was by an experienced anaesthetist, and in Groups B, D and F intubation was by a veterinary student. Time from injection to successful intubation, the ease of intubation, saturation of haemoglobin with oxygen (SpO₂) and partial pressure of oxygen in arterial blood (PaO₂) were measured before the sheep were connected to an anaesthetic machine and allowed to breath oxygen. Times to extubation, holding its head up and standing, maximum and minimum heart rates, respiratory rates, maximal end tidal CO₂, and the quality of recovery were then recorded.

RESULTS

There were no measurable differences in outcomes between sheep in Groups A and B compared with C and D. Time to intubation was slightly shorter for the experienced anaesthetist than the student, but the difference was not significant. The sheep in Groups E and F took less time to recover than those in Groups A-D (p<0.05), but there were no significant differences between the groups in either the ease of induction or quality of recovery. Most sheep in Groups E and F showed minor excitatory effects, mainly at induction, which did not interfere with induction. Respiratory rates were lower in Groups E and F than Groups A-D (p<0.01), but SpO₂ was higher in Groups E and F than A and B (p<0.05).

CONCLUSIONS

The clinical impression that the batch of Parnell ketamine produced unexpected effects was shown to be incorrect. All the combinations produced anaesthesia that allowed intubation by the veterinary student.

CLINICAL RELEVANCE

All the drug combinations produced satisfactory anaesthesia in sheep, but the alphaxaloneand medetomidine combination resulted in faster recovery.

Transplacental transfer of medetomidine and ketamine in pregnant ewes

The extent of placental transfer of medetomidine and ketamine is unknown in pregnant ewes. Date-mated singleton (n = 8) and twin (n = 8) pregnant merino cross ewes were anaesthetized for Caesarean delivery of preterm lamb fetuses. A combination of medetomidine (20 μg/kg) and ketamine (10 mg/kg) was administered by intravenous injection and surgery performed immediately thereafter. Blood samples were collected from the ewe at one, five and 10 min after intravenous injection and from the umbilical vein of the fetus at delivery. Non-pregnant ewes were also anaesthetized (n = 8). There was no difference in the plasma concentration of medetomidine or ketamine when comparing singleton and twin ewes or pregnant and non-pregnant ewes for the short duration of the study. Fetal plasma concentrations of each drug were comparable to the maternal concentrations at the same time. We conclude that both drugs cross the placenta readily and provide anaesthesia and analgesia for the fetus when it is delivered.

Bioaccumulation of the new antifoulant medetomidine in marine organisms

Biofouling is a huge problem globally and new alternative antifoulants are presently being investigated. One candidate is medetomidine, a commonly used sedative in veterinary medicine, which has been shown to effectively prevent settlement of barnacles. The purpose of this study was to measure uptake, elimination and bioconcentration of medetomidine in Mytilus edulis, Abra nitida, Crangon crangon and periphyton communities to evaluate the risk of bioaccumulation in the marine environment. Bioconcentration factors (BCF) and bioaccumulation factors (BAF) were used to assess the bioaccumulation. The calculations of these factors were based on the distribution of the radiolabelled medetomidine. BCF for C. crangon was 2.8 while M. edulis had a BCF of 134 and the periphyton communities' BCF was 1195 l/kg fresh weight (FW). The concentration of medetomidine in the animals reached steady state after 24-48 h for all test systems except for A. nitida, which never stabilised enough to calculate a bioaccumulation factor (BAF). Elimination from the organism's tissues was rapid for three of the test systems with half-lives between 1 and 24 h. A. nitida had a half-life of 96-120 h. This study demonstrates that the bioconcentration and bioaccumulation of medetomidine differs between aquatic organisms and that microalgal communities in the form of periphyton have the highest bioconcentration factor of the organisms tested.

A 2 -agonists in sheep: a review

OBJECTIVE

To review the use and adverse effects of alpha(2)-agonists in sheep.

STUDY DESIGN

Literature review.

MATERIAL AND METHODS

'Pubmed' of the United States National Library of Medicine and 'Veterinary Science' of CAB International were searched for references relating sheep to alpha(2)-agonists. The bibliographies of retrieved articles were further scrutinized for pertinent references, and relevant articles were selected manually.

RESULTS

Reports on the use of clonidine, xylazine, detomidine, romifidine, medetomidine and dexmedetomidine, MPV-2426 and ST-91 in sheep were found in the literature. Most of the studies described xylazine followed by medetomidine and clonidine. The literature on detomidine and romifidine in sheep was sparse. Reports included pharmacokinetic studies, evaluation of sedative, analgesic, and anaesthetic techniques with or without cardiovascular effects, and experimental investigations of adverse effects (mainly hypoxaemia) including the mechanisms of pulmonary oedema and impaired oxygenation after alpha(2)-agonist administration.

CONCLUSIONS

A(2)-agonists are potent and effective analgesics in sheep. In combination with ketamine, they are frequently used for the induction and maintenance of anaesthesia, in this case analgesia is satisfactory. The degree of hypoxaemia which occurs with all commercially available alpha(2)-agonists is highly variable and depends on individual or breed-related factors; the most severe reactions occur after intravenous (IV) injection and during general anaesthesia. Clinical relevance Subclinical respiratory disease is common in sheep. Rapid IV injection of alpha(2)-agonists without supplementary oxygen should be avoided whenever hypoxaemia may be critical.

Comparison of two pre-anaesthetic medetomidine doses in isoflurane anaesthetized sheep

OBJECTIVE

To compare the sedative, anaesthetic-sparing and arterial blood-gas effects of two medetomidine (MED) doses used as pre-anaesthetic medication in sheep undergoing experimental orthopaedic surgery.

STUDY DESIGN

Randomized, prospective, controlled experimental trial.

ANIMALS

Twenty-four adult, non-pregnant, female sheep of various breeds, weighing 53.9 +/- 7.3 kg (mean +/- SD).

METHODS

All animals underwent experimental tibial osteotomy. Group 0 (n = 8) received 0.9% NaCl, group L (low dose) (n = 8) received 5 microg kg(-1) MED and group H (high dose) (n = 8) received 10 microg kg(-1) MED by intramuscular (IM) injection 30 minutes before induction of anaesthesia with intravenous (IV) propofol 1% and maintenance with isoflurane delivered in oxygen. The propofol doses required for induction and endtidal isoflurane concentrations (F(E')ISO) required to maintain anaesthesia were recorded. Heart and respiratory rates and rectal temperature were determined before and 30 minutes after administration of the test substance. The degree of sedation before induction of anaesthesia was assessed using a numerical rating scale. Arterial blood pressure, heart rate, respiratory rate, FE'ISO, end-tidal CO2 (FE'CO2) and inspired O2 (FIO2) concentration were recorded every 10 minutes during anaesthesia. Arterial blood gas values were determined 10 minutes after induction of anaesthesia and every 30 minutes thereafter. Changes over time and differences between groups were examined by analysis of variance (anova) for repeated measures followed by Bonferroni-adjusted t-tests for effects over time.

RESULTS

Both MED doses produced mild sedation. The dose of propofol for induction of anaesthesia decreased in a dose-dependent manner: mean (+/-SE) values for group 0 were 4.7 (+/-0.4) mg kg(-1), for group L, 3.2 (+/-0.4) mg kg(-1) and for group H, 2.3 (+/-0.3) mg kg(-1)). The mean (+/-SE) FE'ISO required to maintain anaesthesia was 30% lower in both MED groups [group L: 0.96 (+/-0.07) %; group H: 1.06 (+/-0.09) %] compared with control group values [(1.54 +/- 0.17) %]. Heart rates were constantly higher in the control group with a tendency towards lower arterial blood pressures when compared with the MED groups. Respiratory rates and PaCO2 were similar in all groups while PaO2 increased during anaesthesia with no significant difference between groups. In group H, one animal developed a transient hypoxaemia: PaO2 was 7.4 kPa (55.7 mmHg) 40 minutes after induction of anaesthesia. Arterial pH values and bicarbonate concentrations were higher in the MED groups at all time points.

CONCLUSION AND CLINICAL RELEVANCE

Intramuscular MED doses of 5 and 10 microg kg(-1) reduced the propofol and isoflurane requirements for induction and maintenance of anaesthesia respectively. Cardiovascular variables and blood gas measurements remained stable over the course of anaesthesia but hypoxaemia developed in one of 16 sheep receiving MED.

Cardiopulmonary effects of dexmedetomidine in sevoflurane-anesthetized sheep with and without nitric oxide inhalation

OBJECTIVE

To determine whether inhaled nitric oxide (NO) prevents pulmonary hypertension and improves oxygenation after i.v. administration of a bolus of dexmedetomidine in anesthetized sheep.

ANIMALS

6 healthy adult sheep.

PROCEDURE

In a crossover study, sevoflurane-anesthetized sheep received dexmedetomidine (2 microg/kg, i.v.) without NO (DEX treatment) or with inhaled NO (DEX-NO treatment). Cardiopulmonary variables, including respiratory mechanics, were measured before and for 120 minutes after bolus injection of dexmedetomidine.

RESULTS

Dexmedetomidine induced a transient decrease in heart rate and cardiac output. A short-lived increase in mean arterial pressure (MAP) and systemic vascular resistance (SVR) was followed by a significant decrease in MAP and SVR for 90 minutes. Mean pulmonary arterial pressure (MPAP) and pulmonary vascular resistance increased transiently after dexmedetomidine injection. The Pao2 was significantly decreased 3 minutes after injection and reached a minimum of (mean +/- SEM) 13.3 +/- 78 kPa 10 minutes after injection. The decrease in Pao2 was accompanied by a sudden and prolonged decrease in dynamic compliance and a significant increase in airway resistance, shunt fraction, and alveolar dead space. Peak changes in MPAP did not differ between the 2 treatments. For the DEX-NO treatment, Pao2 was significantly lower and the shunt fraction significantly higher than for the DEX treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

Inhalation of NO did not prevent increases in pulmonary arterial pressures induced by i.v. administration of dexmedetomidine. Preemptive inhalation of NO intensified oxygenation impairment, probably through increases in intrapulmonary shunting.

Effect of medetomidine and its antagonism with atipamezole on stress-related hormones, metabolites, physiologic responses, sedation, and mechanical threshold in goats

OBJECTIVE

To evaluate the effects of medetomidine and its antagonism with atipamezole in goats.

STUDY DESIGN

Prospective randomized crossover study with 1 week between treatments.

ANIMALS

Six healthy 3-year-old neutered goats (three male and three female) weighing 39.1-90.9 kg (60.0 +/- 18 kg, mean +/- SD).

METHODS

Goats were given medetomidine (20 microg kg(-1), IV) followed, 25 minutes later, by either atipamezole (100 microg kg(-1), IV) or saline. Heart and respiratory rate, rectal temperature, indirect blood pressure, and mechanical threshold were measured, and sedation and posture were scored and blood samples obtained to measure epinephrine, norepinephrine, free fatty acids, glucose, and cortisol concentrations at baseline (immediately before medetomidine), 5 and 25 minutes after medetomidine administration, and at 5, 30, 60, and 120 minutes after the administration of antagonist or saline. Parametric and nonparametric tests were used to evaluate data; p < 0.05 was considered significant.

RESULTS

Medetomidine decreased body temperature, heart rate, and respiratory rate and increased mean arterial blood pressure, cortisol, and glucose. Recumbency occurred 89 +/- 50 seconds after medetomidine administration. All goats were standing 86 +/- 24 seconds after atipamezole administration whereas all goats administered saline were sedate and recumbent at 2 hours. Tolerance to compression of the withers and metacarpus increased with medetomidine. From 5 to 120 minutes after saline or atipamezole administration, there were differences in body temperature, glucose, and cortisol but none in heart rate or blood pressure. Three of the six goats receiving saline developed bloat; five of six urinated. After atipamezole, four of six goats developed piloerection and all goats were agitated and vocalized.

CONCLUSION

At the doses used, atipamezole antagonized the effects of medetomidine on recumbency, sedation, mechanical threshold, and the increase in glucose. Atipamezole increased the rate of return of cortisol toward baseline, and prevented further decline in rectal body temperature.

CLINICAL RELEVANCE

Atipamezole may be used to antagonize some, but not all effects of medetomidine.

Pharmacokinetics and sedative effects of intramuscular medetomidine in domestic sheep

Intramuscular (i.m.) administration of medetomidine (MED) may avoid the severe pressor effects caused by peripheral actions of MED associated with intravenous (i.v.) dosing. The purpose of this study was to determine the pharmacokinetics, the time course of sedation and occurence of hypoxaemia after i.m. administration of MED in domestic sheep. The MED was injected i.m. at a dose of 30 micro g/kg in nine domestic sheep. Blood was sampled at 0, 5, 10, 20, 30, 40, 60, 120, 180, 240, 360 and 600 min after MED. Sedation was assessed and arterial blood samples were taken before and 35 min after MED application. Mean (SD) pharmacokinetic parameters of i.m. MED were: absorption half-life: 13.2 (7.5) min; terminal half-life: 32.7 (14.9) min; time to peak concentration: 29.2 (8.9) min; peak concentration: 4.98 (1.89) ng/mL; volume of distribution: 3.9 (2.4) l/kg; total body clearance: 81.0 (21.5) mL/(min kg). Peak sedation occurred between 30 and 40 min after injection of MED. The degree of sedation correlated with individual plasma concentrations (rS: 0.926). One animal became hypoxaemic (PaO2 = 54.1 mmHg).

The effects of medetomidine and its reversal with atipamezole on plasma glucose, cortisol and noradrenaline in cattle and sheep

In the present study, we report the effect of medetomidine followed by atipamezole on plasma glucose, cortisol and noradrenaline in calves, cows and sheep. Eight calves, eight lactating dairy cows and eight adult female sheep were included in a crossover trial. The animals were injected i.v. with medetomidine (40 microg/kg), followed 60 min later by atipamezole i.v. (200 microg/kg) or saline. The wash-out period between experiments was 1 or 2 weeks. In every animal, medetomidine induced a marked hyperglycaemia, which was reversed by atipamezole. Cortisol levels increased significantly in cows and sheep, reaching levels 4-8-fold higher than the baseline levels 25-45 min after injection of medetomidine. Atipamezole did not affect the cortisol levels, except in sheep where an increase was observed. Plasma levels of noradrenaline decreased in cows and sheep after medetomidine injection, reflecting the inhibition of sympathetic activity by the drug. After injection of the antagonist, there was a large increase in noradrenaline levels. In conclusion, a high dose of medetomidine does not seem to reduce the overall endocrine stress response in cattle and sheep, which has previously been reported in other species.