Propofol infusion anaesthesia in dogs pre-medicated with medetomidine

Ten laboratory beagles pre-medicated with medetomidine (40 μg/kg bodyweight [bwt]) were anaesthetised using a rapid injection of propofol, followed by propofol infusion. A loading dose of 4 mg/kg bwt of propofol was administered intravenously (iv) as a bolus and, immediately after, a 60 min iv propofol infusion (150 μg/kg bwt/min) was initiated. After a transient increase, mean arterial blood pressure decreased significantly below the pre-propofol level. However, the lowest values recorded (115 ± 11 mmHg) remained within the physiological limits. Heart rate increased significantly (from 41 ± 7.3 to 58 ± 11 beats/min) after initiation of the propofol infusion. No significant changes were seen in respiratory frequency; pO₂ decreased transiently; minimum values (10 ± 2.3 kPa) recorded 5 mins after initiation of the propofol infusion differed significantly from the starting level. pCO₂ increased significantly and the highest values recorded were 6.1 ± 0.35 kPa. Accordingly, pH decreased reaching the lowest level (pH 7.29) 15 mins after initiation of the propofol infusion. The analgesic effect of the present combination was not studied, but the absence of the palpebral and pedal reflexes suggested a surgical stage of anaesthesia. Therefore, propofol infusion in beagles pre-medicated with medetomidine proved to be a promising anaesthetic regimen but, if used clinically, oxygen-enriched inspired air should be used.     

Doxapram infusion during halothane anaesthesia in ponies

Doxapram, 0.05 mg/kg bodyweight/min, was infused during the second hour of 2 h halothane anaesthesia in six ponies. Two of the ponies were anaesthetised on a second occasion as controls and given 5 per cent dextrose in place of the doxapram. Respiratory depression typical of halothane anaesthesia in ponies developed in the first hour of anaesthesia and continued during the second hour in the control animals. During doxapram infusion arterial carbon dioxide tension decreased and pH increased. Arterial blood pressure increased but there was no change in pulse rate, the electrocardiogram or arterial oxygen tension. Anaesthesia lightened during doxapram infusion necessitating an increase in the vapouriser setting in order to prevent arousal. Recovery from anaesthesia appeared unaffected by the doxapram infusion.     

Effect of medetomidine infusion on the anaesthetic requirements of desflurane in dogs

The objective of this paper was to evaluate the effect of constant rate infusion of medetomidine on the anaesthetic requirements of desflurane in dogs. For this, six healthy dogs were studied. Measurements for baseline were taken in the awake, unsedated dogs, then each dog received intravenously (i.v.) three anaesthetic protocols: M (no medetomidine infusion), M0.5 (infusion of medetomidine at 0.5 microg/kg/h, i.v.) or M1 (infusion of medetomidine at 1 microg/kg/h, i.v.). All dogs were sedated with medetomidine (2 microg/kg, i.v.) and measurements repeated in 10 min. Induction of anaesthesia was delivered with propofol (3 mg/kg, i.v.) and maintained with desflurane for 90 min to achieve a defined surgical plane of anaesthesia in all cases. After tracheal intubation infusion of medetomidine was initiated and maintained until the end of anaesthesia. Cardiovascular, respiratory, arterial pH (pHa) and arterial blood gas tensions (PaO(2), PaCO(2)) variables were measured during the procedure. End tidal desflurane concentration (EtDES) was recorded throughout anaesthesia. Time to extubation, time to sternal recumbency and time to standing were also noted. Heart rate and respiratory rate were significantly decreased during sedation in all protocols compared to baseline values. Mean heart rate, mean arterial pressure, systolic arterial pressure, diastolic arterial pressure, respiratory rate, tidal volume, arterial oxygen saturation, end-tidal CO(2), pHa, PaO(2), and PaCO(2) during anaesthesia were similar for all protocols. EtDES for M (8.6 +/- 0.8%) was statistically higher than for M0.5 (7.6 +/- 0.5%) and M1 (7.3 +/- 0.7%) protocols. Infusion of medetomidine reduces desflurane concentration required to maintain anaesthesia in dogs.     

Infusion of a combination of propofol and medetomidine for long-term anesthesia in ponies

OBJECTIVE: To determine the minimal infusion rate of propofol in combination with medetomidine for long-term anesthesia in ponies and the effects of atipamezole on recovery. ANIMALS: 12 ponies. PROCEDURE: Ponies were sedated with medetomidine (7 microg/kg of body weight, IV). Ten minutes later, anesthesia was induced with propofol (2 mg/kg, IV). Anesthesia was maintained for 4 hours, using an infusion of medetomidine (3.5 microg/kg per hour, IV) and propofol at a rate sufficient to prevent ponies from moving after electrical stimulation. Arterial blood pressures and blood gas analysis, heart rates, and respiratory rates were monitored. For recovery, 6 ponies were given atipamezole (60 microg/kg, IV). Induction and recovery were scored. RESULTS: Minimal propofol infusion rates ranged from 0.06 to 0.1 mg/kg per min. Mean arterial blood pressure was stable (range, 74 to 86 mm Hg), and heart rate (34 to 51 beats/min) had minimal variations. Variable breathing patterns were observed. Mean PaO2 (range, 116 to 146 mm Hg) and mean PaCO2 (range, 48 to 51 mm Hg) did not change significantly with time, but hypoxemia was evident in some ponies (minimal PaO2, 47 mm Hg). Recovery was fast and uneventful with and without atipamezole (completed in 20.2 and 20.9 minutes, respectively). CONCLUSIONS AND CLINICAL RELEVANCE: Infusion of a combination of medetomidine and propofol was suitable for prolonged anesthesia in ponies. Recovery was rapid and uneventful. A combination of propofol and medetomidine may prove suitable for long-term anesthesia in horses. Monitoring of blood gases is essential because of potential hypoxemia.     

Effects of medetomidine premedication on propofol infusion anaesthesia in dogs

Observations of cardiovascular and respiratory parameters were made on six dogs anaesthetized on two separate occasions for 120 minutes with a propofol infusion, once without premedication and once following premedication with 10 μg kg-¹ of intramuscular medetomidine. During anaesthesia the heart rate and cardiac index tended to be lower following medetomidine premedication, while the mean arterial pressure was significantly greater (p<0.05). Although the differences were not statistically significant, the systemic vascular resistance, pulmonary vascular resistance and stroke volume index were also greater in dogs given medetomidine. The mean arterial oxygen and carbon dioxide tensions were similar under both regimens, but in 2 dogs supplementary oxygen had to be administered during anaesthesia to alleviate severe hypoxaemia on both occasions they were anaesthetized. Minute and tidal volumes of respiration tended to be greater in dogs not given medetomidine but medetomidine premedication appeared to have no effect on venous admixture. Dogs given medetomidine received intramuscular atipamezole at the end of the 120 min. propofol infusion; the mean time from induction of anaesthesia to walking without ataxia was 174. min in the unpremedicated dogs and 160 min. in the dogs given atipamezole. The mean blood propofol concentration at which the dogs walked without ataxia was higher in the unpremedicated animals (2.12 ± 0.077 μg. ml-¹ compared with 1.27 ± 0.518 μg. ml-¹ in the premedicated dogs). The oxygen delivery to the tissues was lower after medetomidine premedication (p = 0.03) and the oxygen consumption was generally lower after medetomidine premedication but the difference did not achieve statistical significance. No correlation could be demonstrated between blood propofol concentration and cardiac index, systemic or pulmonary vascular resistance indices, systolic, diastolic or mean arterial blood pressures.     

Minimum alveolar concentration of desflurane in llamas and alpacas

OBJECTIVE: To determine the minimum alveolar concentration (MAC) of desflurane in llamas and alpacas. DESIGN: Prospective study. Animals Six healthy adult llamas and six healthy adult alpacas. PROCEDURE: Anesthesia was induced with desflurane delivered with oxygen through a mask. An endotracheal tube was inserted, and a port for continuous measurement of end-tidal and inspired desflurane concentrations was placed between the endotracheal tube and the breathing circuit. After equilibration at an end-tidal-to-inspired desflurane concentration ratio >0.90 for 15 minutes, a 50-Hz, 80-mA electrical stimulus was applied to the antebrachium until a response was obtained (i.e. gross purposeful movement) or for up to 1 minute. The vaporizer setting was increased or decreased to effect a 10-20% change in end-tidal desflurane concentration, and equilibration and stimulus were repeated. The MAC was defined as the average of the lowest end-tidal desflurane concentration that prevented a positive response and the highest concentration that allowed a positive response. RESULTS: Mean +/- SD MAC of desflurane was 7.99 +/- 0.58% in llamas and 7.83 +/- 0.51% in alpacas. CONCLUSIONS AND CLINICAL RELEVANCE: The MAC of desflurane in llamas and alpacas was in the range of that reported for other species.     

Pharmacokinetics of medetomidine in ponies and elaboration of a medetomidine infusion regime which provides a constant level of sedation.

The pharmacokinetics of intravenous (i.v.) medetomidine (7 mcg kg(-1)) were best described by a two-compartment model in five ponies. Total body clearance was 4 (SD 0.60) 1 kg h,(-1)t(1/2alpha)7. 6 (0.91) minutes and t(1/2beta)51.3 (13.09) minutes. In one pony the one-compartmental model was best fit, and total body clearance was 4. 2 l kg h(-1)and t(1/2)was 11 minutes. Medetomidine plasma levels had fallen below the limits of quantification (0.05 ng ml(-1)) within 4 hours. Medetomidine 5 mcg kg(-1)i.v. followed by an infusion of 3.5 mcg kg h(-1)for two hours provided a constant level of sedation reaching steady state plasma medetomidine levels of 1-1.5 ng ml(-1)within 30 minutes. Sedation was reversed effectively by atipamezole (60 mcg kg(-1)) i.v. The pharmacokinetics of medetomidine make it suitable for prolonged use by infusion, such as is required as part of a total intravenous anaesthetic technique in horses.     

Effects of a constant‐rate infusion of dexmedetomidine on the minimal alveolar concentration of sevoflurane in ponies

Reasons for performing study: Dexmedetomidine has been administered in the equine as a constant‐rate infusion (CRI) during inhalation anaesthesia, preserving optimal cardiopulmonary function with calm and coordinated recoveries. Inhalant anaesthetic sparing effects have been demonstrated in other species, but not in horses. Objectives: To determine the effects of a CRI of dexmedetomidine on the minimal alveolar concentration (MAC) of sevoflurane in ponies. Methods: Six healthy adult ponies were involved in this prospective, randomised, crossover, blinded, experimental study. Each pony was anaesthetised twice (3 weeks washout period). After induction with sevoflurane in oxygen (via nasotracheal tube), the ponies were positioned on a surgical table (T0), and anaesthesia was maintained with sevoflurane (expired sevoflurane fraction 2.5%) in 55% oxygen. The ponies were randomly allocated to treatment D (dexmedetomidine 3.5 µg/kg bwt i.v. [T10–T15] followed by a CRI of dexmedetomidine at 1.75 µg/kg bwt/h) or treatment S (bolus and CRI of saline at the same volume and rate as treatment D). After T60, MAC determination, using a classic bracketing technique, was initiated. Stimuli consisted of constant‐current electrical stimuli at the skin of the lateral pastern region. Triplicate MAC estimations were obtained and averaged in each pony. Monitoring included pulse oximetry, electrocardiography, anaesthetic gas monitoring, arterial blood pressure measurement and arterial blood gases. Normocapnia was maintained by mechanical ventilation. Analysis of variance (treatment and period as fixed factors) was used to detect differences between treatments (α= 0.05). Results: An intravenous (i.v.) dexmedetomidine CRI decreased mean ± s.d. sevoflurane MAC from 2.42 ± 0.55 to 1.07 ± 0.21% (mean MAC reduction 53 ± 15%). Conclusions and potential relevance: A dexmedetomidine CRI at the reported dose significantly reduces the MAC of sevoflurane.     

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

Abstract

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.     

The effect of medetomidine premedication upon propofol induction and infusion anaesthesia in the dog

The effect of premedication with four different intramuscular doses of medetomidine (5.0,10.0, 20.0 and 40.0 μg.kg-¹) and a saline placebo were compared in a group of six adult beagle dogs anaesthetised with propofol on five separate occasions. Anaesthesia was induced 30 minutes after premedication and maintained by intravenous injection and continuous infusion of propofol. The effects of medetomidine were reversed with atipamezole 30 minutes after anaesthetic induction. The marked synergistic effects of medetomidine with propofol were demonstrated by a dose related reduction in the induction and infusion requirements for a similar degree of anaesthesia. The effect appeared exponential in nature; lower medetomidine doses produced a disproportionately greater effect.
The maintenance of anaesthesia with propofol following a saline placebo or low doses of medetomidine proved to be difficult. Higher doses of medetomidine required less propofol for induction and infusion and allowed a more stable anaesthesia to be maintained. Propofol produced no statistically significant change in heart rate during infusion. Changes in respiratory rate were markedly group specific. A significant reduction in respiratory rate was seen in dogs given either 5 μg.kg- or 10 μ-g.kg-¹ medetomidine. No change was recorded in dogs given 20 /μg.kg-¹ medetomidine and a significant increase was seen in dogs given 40 μg.kg-¹ medetomidine. Recovery was monitored following the termination of propofol infusion after the reversal of medetomidine using atipamezole at five times the medetomidine dose. Recovery was slower for dogs given lower doses of medetomidine and consequently higher doses of propofol.     

Effect of low-dose butorphanol on halothane minimum alveolar concentration in ponies

Minimum alveolar concentration (MAC) for halothane was measured before and after administration of intravenous butorphanol (0.022 and 0.044 mg/kg in bodyweight in nine yearling Shetland ponies. Arterial blood pressure, heart rate, respiratory rate, expired CO2 and rectal temperature was also measured. Even though mean MAC values decreased 10 and 9 per cent after the low and high doses respectively, they were not statistically different from those measured prior to butorphanol. Halothane MAC values increased after butorphanol in two ponies, both animals increasing locomotor activity and demonstrating apparent central nervous system stimulation. No significant differences were seen in any variable measured after butorphanol administration.     

Adrenocortical and metabolic responses to dobutamine infusion during halothane anaesthesia in ponies

The study investigated whether hypotension in halothane-anaesthetised ponies is the stimulus inducing an endocrine stress response by assessing the effect of maintenance of normotension with a dobutamine infusion. Groups of six ponies were studied. After premedication with acepromazine (0.04 mg/kg) anaesthesia was induced with thiopentone (10 mg/kg) and maintained for 120 min with halothane (group AN). Dobutamine was infused to effect (1.1–4.4 μg/kg/min) to maintain arterial pressure at pre anaesthetic levels. The conscious group (CON) were prepared as for AN and then received only dobutamine infusion 1.0 μg/kg/min for 120 min. Arterial blood pressure, pH, oxygen and carbon dioxide tension, pulse rate, haematocrit, and plasma cortisol, glucose and lactate concentrations were measured before, at 20 min intervals during anaesthesia, and 20 and 120 min after anaesthesia ceased. Blood pressure remained close to control in both groups. The AN group became hypercapnic and acidotic, pulse rate and haematocrit increased, cortisol increased more than twofold and plasma glucose and lactate did not change. All values remained at control in the CON group except for small increases in haematocrit and decreases in pulse rate. Maintenance of normotension during halothane anaesthesia did not blunt the adrenocortical response to anaesthesia nor did the same dose of dobutamine alone increase plasma cortisol. Hypotension appears not to be the sole stimulus to equine adrenocortical activity during halothane anaesthesia.     

Desflurane anaesthesia in the horse: minimum alveolar concentration following induction of anaesthesia with xylazine and ketamine

The minimum alveolar concentration (MAC) of desflurane was measured in 6 ponies (aged 1 or 2 years) following induction of anaesthesia with iv xylazine (1.1 mgkg bwt) followed by ketamine (2.2 mgkg bwt). The stimulus utilised was electrical; pulses of 50 volts, 10 dsec duration delivered at 5 Hz, applied for 60 s across needles inserted 1 cm apart into the buccal mucosa (n = 2) and/or the coronary band (n = 6). Stimulation was not commenced until at least 1 h following anaesthetic induction, and the time of the final MAC measurement ranged from 2.5–3.5 h. The mean (± sd) MAC of desflurane under these circumstances was 7.0% (± 0.85) with a range from 5.8%-8.3%. One pony behaved aberrantly following a positive response to the stimulus and the MAC measurement was repeated 10 weeks later. At the end of the anaesthetic period xylazine (0.2 mgkg bwt) was administered iv. Mean time to standing was 13.3 (± 2.9) min, range 9–17 min, and the quality of recovery was excellent.     

Minimum infusion rate of alfaxalone for total intravenous anaesthesia after sedation with acepromazine or medetomidine in cats undergoing ovariohysterectomy

ObjectiveTo determine the induction doses, then minimum infusion rates of alfaxalone for total intravenous anaesthesia (TIVA), and subsequent, cardiopulmonary effects, recovery characteristics and alfaxalone plasma concentrations in cats undergoing ovariohysterectomy after premedication with butorphanol-acepromazine or butorphanol-medetomidine.Study designProspective randomized blinded clinical study.AnimalsTwenty-eight healthy cats.MethodsCats undergoing ovariohysterectomy were assigned into two groups: together with butorphanol [0.2 mg kg^?1 intramuscularly (IM)], group AA (n = 14) received acepromazine (0.1 mg kg^?1 IM) and group MA (n = 14) medetomidine (20 μg kg^?1 IM). Anaesthesia was induced with alfaxalone to effect [0.2 mg kg^?1 intravenously (IV) every 20 seconds], initially maintained with 8 mg kg^?1 hour^?1 alfaxalone IV and infusion adjusted (±0.5 mg kg^?1 hour^?1) every five minutes according to alterations in heart rate (HR), respiratory rate (f_R), Doppler blood pressure (DBP) and presence of palpebral reflex. Additional alfaxalone boli were administered IV if cats moved/swallowed (0.5 mg kg^?1) or if f_R >40 breaths minute^?1 (0.25 mg kg^?1). Venous blood samples were obtained to determine plasma alfaxalone concentrations. Meloxicam (0.2 mg kg^?1 IV) was administered postoperatively. Data were analysed using linear mixed models, Chi-squared, Fishers exact and t-tests.ResultsAlfaxalone anaesthesia induction dose (mean ± SD), was lower in group MA (1.87 ± 0.5; group AA: 2.57 ± 0.41 mg kg^?1). No cats became apnoeic. Intraoperative bolus requirements and TIVA rates (group AA: 11.62 ± 1.37, group MA: 10.76 ± 0.96 mg kg^?1 hour^?1) did not differ significantly between groups. Plasma concentrations ranged between 0.69 and 10.76 μg mL^?1. In group MA, f_R, end-tidal carbon dioxide, temperature and DBP were significantly higher and HR lower.Conclusion and clinical relevanceAlfaxalone TIVA in cats after medetomidine or acepromazine sedation provided suitable anaesthesia with no need for ventilatory support. After these premedications, the authors recommend initial alfaxalone TIVA rates of 10 mg kg^?1 hour^?1.     

Cardiopulmonary effects of medetomidine in sheep and in ponies

Medetomidine was administered intravenously to six sheep at 5, 10 and 20 μg kg⁻¹ and to one horse and four ponies at 5 and 10 μg kg⁻¹. In both species medetomidine resulted in significant decreases in heart rate and cardiac output and, initially, in an increase in arterial blood pressure. In the ponies this increase in blood pressure was followed by a significant and prolonged decrease, but in the sheep the secondary decrease in blood pressure was not statistically significant. In the sheep, the three doses of medetomidine resulted in profound and significant decreases in arterial oxygen tensions, which were significantly dose related, but in the ponies the arterial blood oxygen tensions were not significantly decreased. In both species medetomidine caused a small but significant increase in arterial blood carbon dioxide tensions.