Cardiopulmonary effects of a two hour medetomidine infusion and its antagonism by atipamezole in horses and ponies

Comparison of Xylazine and Lidocaine Infusion versus Medetomidine Continuous Rate Infusion during General Anesthesia with Isoflurane in Horses Undergoing Emergency Laparotomy

(1) The main goals of general anesthesia include pain management and a safe anesthetic protocol for smooth recovery. In this retrospective study, we compared two anesthetic protocols for general anesthesia with isoflurane during emergency laparotomy: sedation with xylazine and the intraoperative infusion of lidocaine (X group) versus medetomidine as a preoperative sedation and intraoperative infusion (M group). (2) The medical records of horses who underwent emergency laparotomies between 2016 and 2023 were reviewed. According to the anesthetic protocol, patients were allocated to the X or M groups. Data about the horse, signalment, history, and anesthetic variables were analyzed. (3) Group X had a significantly higher heart rate (HR), lower respiratory rate (RR) and mean and diastolic arterial pressure (MAP/DAP). A progressive increase in HR and RR was observed in both groups. Group X underwent a decrease in RR and an increase in DAP. In Group M, a decrease in MAP and DAP was observed. Group M exhibited a longer recovery time with similar recovery scores. Both protocols provided safe anesthesia for emergency laparotomy, with minor cardiovascular and respiratory depression. Minor respiratory depression was detected when xylazine was used, while recovery was longer with medetomidine.

Comparison of Recovery Quality Following Medetomidine versus Xylazine Balanced Isoflurane Anaesthesia in Horses: A Retrospective Analysis

Simple Summary

Recovery from general anaesthesia poses the most critical phase of equine anaesthesia and is the main cause for the relatively high anaesthetic mortality rate compared to other species. It is, therefore, essential to identify anaesthetic protocols that promote safe recoveries. This retrospective study compared the quality of 470 recoveries following general anaesthesia with the anaesthetic gas isoflurane combined with a constant rate infusion of two different alpha-2 adrenergic agonists (xylazine or medetomidine). On the basis of video recordings, recovery quality was scored by two observers unaware of animal details, procedure, or drugs used. Additionally, factors that may affect recovery (e.g., breed, age, procedure, duration of anaesthesia, and intraoperative complications) were taken into consideration. Horses needing higher doses of xylazine to sedate prior to anaesthesia, the intraoperative use of tetrastarch for cardiovascular support, and the use of salbutamol to improve inadequate blood oxygenation during general anaesthesia were related to poorer recovery scores. Whilst recoveries of horses treated with medetomidine took significantly longer compared to xylazine, the attempts to stand and the overall quality of recovery were similar for both groups, indicating that both anaesthetic protocols promote similarly safe recoveries.

Abstract

Medetomidine partial intravenous anaesthesia (PIVA) has not been compared to xylazine PIVA regarding quality of recovery. This clinical retrospective study compared recoveries following isoflurane anaesthesia balanced with medetomidine or xylazine. The following standard protocol was used: sedation with 7 µg·kg⁻¹ medetomidine or 1.1 mg·kg⁻¹ xylazine, anaesthesia induction with ketamine/diazepam, maintenance with isoflurane and 3.5 µg·kg⁻¹·h⁻¹ medetomidine or 0.7 mg·kg⁻¹·h⁻¹ xylazine, and sedation after anaesthesia with 2 µg·kg⁻¹ medetomidine or 0.3 mg·kg⁻¹ xylazine. Recovery was timed and, using video recordings, numerically scored by two blinded observers. Influence of demographics, procedure, peri-anaesthetic drugs, and intraoperative complications (hypotension, hypoxemia, and tachycardia) on recovery were analysed using regression analysis (p < 0.05). A total of 470 recoveries (medetomidine 279, xylazine 191) were finally included. Following medetomidine, recoveries were significantly longer (median (interquartile range): 57 (43–71) min) than xylazine (43 (32–59) min) (p < 0.001). However, the number of attempts to stand was similar (medetomidine and xylazine: 2 (1–3)). Poorer scores were seen with increased pre-anaesthetic dose of xylazine, intraoperative tetrastarch, or salbutamol. However, use of medetomidine or xylazine did not influence recovery score, concluding that, following medetomidine–isoflurane PIVA, recovery is longer, but of similar quality compared to xylazine.

Clinical Randomized Comparison of Medetomidine and Xylazine for Isoflurane Balanced Anesthesia in Horses

Introduction: To assess drug plasma levels, preanesthetic sedation, cardiopulmonary effects during anesthesia and recovery in horses anesthetized with isoflurane combined with medetomidine or xylazine.

Study design: Prospective blinded randomized clinical study.

Animals: Sixty horses undergoing elective surgery.

Methods: Thirty minutes after administration of antibiotics, flunixine meglumine or phenylbutazone and acepromazine horses received medetomidine 7 μg kg⁻¹ (group MED) or xylazine 1.1 mg kg⁻¹ (group XYL) slowly intravenously (IV) and sedation was assessed 3 min later. Anesthesia was induced with ketamine/diazepam and maintained with isoflurane in oxygen/air and medetomidine 3.5 μg kg⁻¹ h⁻¹ or xylazine 0.69 mg kg⁻¹ h⁻¹. Ringer's acetate 10 mL kg⁻¹ h⁻¹ and dobutamine were administered to maintain normotension. All horses were mechanically ventilated to maintain end-tidal carbon dioxide pressures at 45 ± 5 mmHg (5.3–6.7 kPa). Heart rate (HR), invasive arterial blood pressures, inspired and expired gas compositions, pH, arterial blood gases, electrolytes, lactate and glucose were measured. For recovery all horses received intramuscular morphine 0.1 mg kg⁻¹ and medetomidine 2 μg kg⁻¹ or xylazine 0.3 mg kg⁻¹ IV. Recovery was timed and scored using three different scoring systems. Plasma samples to measure medetomidine and xylazine concentrations were collected at predetermined timepoints. Repeatedly measured parameters were analyzed using a two-way repeated-measures analysis of variance for differences between groups and over time; p < 0.05 was considered statistically significant.

Results: Mean arterial blood pressures (MAP) stayed within normal ranges but were higher (p = 0.011) in group XYL despite significant lower dobutamine doses (p = 0.0003). Other measured parameters were within clinically acceptable ranges. Plasma levels were at steady state during anesthesia (MED 2.194 ± 0.073; XYL 708 ± 18.791 ng mL⁻¹). During recovery lateral recumbency (MED 42.7 ± 2.51; XYL 34.3 ± 2.63 min; p = 0.027) and time to standing (MED 62.0 ± 2.86; XYL 48.8 ± 3.01 min; p = 0.002) were significantly shorter in group XYL compared to group MED. Recovery scores did not differ significantly between groups.

Conclusion and Clinical Relevance: In horses anesthetized with isoflurane and medetomidine or xylazine, xylazine maintained higher MAP, reduced the dobutamine consumption and recovery time, whilst overall recovery quality was unaffected.

A retrospective comparison of induction with thiopental/guaifenesin and propofol/ketamine in Thoroughbred racehorses anesthetized with sevoflurane and medetomidine during arthroscopic surgery

This study compares clinical characteristics between induction with thiopental/guaifenesin and propofol/ketamine in Thoroughbred racehorses anesthetized with sevoflurane and medetomidine. Clinical records of 214 horses that underwent arthroscopic surgery between 2015 and 2016 were retrospectively retrieved. Horses were premedicated with medetomidine and midazolam to sedate at the adequate level for smooth induction, and then induced with either thiopental (4.0 mg/kg) and guaifenesin (100 mg/kg) in Group TG (n=91) or propofol (1.0 mg/kg) and ketamine (1.0 mg/kg) in Group PK (n=123). Anesthesia was maintained using sevoflurane with constant rate infusion of medetomidine. Quality of induction/recovery, sevoflurane requirement, cardiovascular function and recovery characteristics were evaluated. Anesthetic induction scores (median, range) for Group TG (5, 2-5) and Group PK (5, 2-5) were not significantly different. There were no significant differences in end-tidal sevoflurane concentration (mean ± standard deviation) between Group TG and Group PK (both 2.4 ± 0.2%). Dobutamine infusion rate (µg/kg/min) required for keeping mean arterial blood pressure (MAP) above 70 mmHg in Group PK (0.43, 0.10-1.40) was significantly lower than in Group TG (0.67, 0.08-1.56). Recovery score in Group PK (5, 2-5) was significantly higher than in Group TG (4, 2-5). Both propofol/ketamine and thiopental/guaifenesin provided a smooth induction of anesthesia. Moreover, induction with propofol/ketamine resulted in lower dobutamine requirements for keeping MAP above 70 mmHg during maintenance, and better quality of recovery. Induction with propofol/ketamine would be preferable to thiopental/guaifenesin in Thoroughbred racehorses anesthetized with sevoflurane and medetomidine during arthroscopic surgery.

Effects of vatinoxan on cardiorespiratory function and gastrointestinal motility during constant-rate medetomidine infusion in standing horses

Background

Medetomidine suppresses cardiovascular function and reduces gastrointestinal motility in horses mainly through peripheral α₂‐adrenoceptors. Vatinoxan, a peripheral α₂‐antagonist, has been shown experimentally to alleviate the adverse effects of some α₂‐agonists in horses. However, vatinoxan has not been investigated during constant‐rate infusion (CRI) of medetomidine in standing horses.

Objectives

To evaluate effects of vatinoxan on cardiovascular function, gastrointestinal motility and on sedation level during CRI of medetomidine.

Study design

Experimental, randomised, blinded, cross‐over study.

Methods

Six healthy horses were given medetomidine hydrochloride, 7 μg/kg i.v., without (MED) and with (MED+V) vatinoxan hydrochloride, 140 μg/kg i.v., followed by CRI of medetomidine at 3.5 μg/kg/h for 60 min. Cardiorespiratory variables were recorded and borborygmi and sedation levels were scored for 120 min. Plasma drug concentrations were measured. The data were analysed using repeated measures ANCOVA and paired t‐tests as appropriate.

Results

Initially heart rate (HR) was significantly lower and mean arterial blood pressure (MAP) significantly higher with MED compared with MED+V. For example at 10 min HR (mean ± s.d.) was 26 ± 2 and 31 ± 5 beats/minute (P = 0.04) and MAP 129 ± 15 and 103 ± 13 mmHg (P<0.001) respectively. At 10 min, cardiac index was lower (P = 0.02) and systemic vascular resistance higher (P = 0.001) with MED than with MED+V. Borborygmi were reduced after MED; this effect was attenuated by vatinoxan (P<0.001). All horses were sedated with medetomidine, but the mean sedation scores were reduced with MED+V until 20 min (6.8 ± 0.8 and 4.5 ± 1.5 with MED and MED+V, respectively, at 10 min, P = 0.001). Plasma concentration of dexmedetomidine was significantly lower in the presence of vatinoxan (P = 0.01).

Main limitations

Experimental study with healthy, unstimulated animals.

Conclusions

Vatinoxan administered i.v. with a loading dose of medetomidine improved cardiovascular function and gastrointestinal motility during medetomidine CRI in healthy horses. Sedation was slightly yet significantly reduced during the first 20 min..

The Summary is available in Portuguese – see Supporting Information

Sedative and cardiopulmonary effects of dexmedetomidine infusions randomly receiving, or not, butorphanol in standing horses

Dexmedetomidine (DEX) alone, or combined with butorphanol (BUT), may be administered by constant rate infusions (CRIs) in standing horses. This blinded, randomised, crossover study in six healthy adult horses aimed to determine the sedative and cardiopulmonary effects of DEX (dexmedetomidine (3.5 µg/kg+5 µg/kg/hour CRI) and DEX/BUT (dexmedetomidine (3.5 µg/kg+3.5 µg/kg/hour CRI) and butorphanol (20 µg/kg+24 µg/kg/hour CRI)). Head height above ground (HHAG), ataxia, responses to tactile/auditory stimuli and cardiopulmonary variables were recorded before, at 5/15/30/60/90 minutes and after CRIs terminated (15/30/60 minutes). Repeated measures analysis of variance with Tukey-Kramer test were used for cardiopulmonary values (mean±SD) and HHAG reduction (per cent), and Friedman's and Dunn's for non-parametric data (P<0.05). Maximum HHAG reductions of 54 per cent (DEX) and 58 per cent (DEX/BUT) occurred at 15 minutes, with ataxia for 15 minutes in both treatments. Responses to stimuli were reduced for 30 minutes in both treatments, and auditory up to 60 minutes in DEX. Cardiopulmonary effects typical of α₂-agonists were observed, with no differences between treatments. At the doses and rates reported here, both regimens provided clinically sufficient sedation for only 30 minutes.

Clinical comparison of dexmedetomidine and medetomidine for isoflurane balanced anaesthesia in horses

OBJECTIVE

To compare the effects of two balanced anaesthetic protocols (isoflurane-dexmedetomidine versus medetomidine) on sedation, cardiopulmonary function and recovery in horses.

STUDY DESIGN

Prospective, blinded, randomized clinical study.

ANIMALS

Sixty healthy adult warm blood horses undergoing elective surgery.

METHODS

Thirty horses each were sedated with dexmedetomidine 3.5 μg kg^-1 (group DEX) or medetomidine 7 μg kg^-1 (group MED) intravenously. After assessing and supplementing sedation if necessary, anaesthesia was induced with ketamine/diazepam and maintained with isoflurane in oxygen/air and dexmedetomidine 1.75 μg kg^-1 hour^-1 or medetomidine 3.5 μg kg^-1 hour^-1. Ringer's lactate (7-10 mL kg^-1 hour^-1) and dobutamine were administered to maintain normotension. Controlled mechanical ventilation maintained end-tidal expired carbon dioxide pressures at 40-50 mmHg (5.3-6.7 kPa). Heart rate, invasive arterial blood pressure, inspired and expired gas composition and arterial blood gases were measured. Dexmedetomidine 1 μg kg^-1 or medetomidine 2 μg kg^-1 was administered for timed and scored recovery phase. Data were analysed using two-way repeated-measures analysis of variance and chi-square test. Significance was considered when p≤0.05.

RESULTS

In group DEX, significantly more horses (n=18) did not fulfil the sedation criteria prior to induction and received one or more supplemental doses, whereas in group MED only two horses needed one additional bolus. Median (range) total sedation doses were dexmedetomidine 4 (4-9) μg kg^-1 or medetomidine 7 (7-9) μg kg^-1. During general anaesthesia, cardiopulmonary parameters did not differ significantly between groups. Recovery scores in group DEX were significantly better than in group MED.

CONCLUSIONS AND CLINICAL RELEVANCE

Horses administered dexmedetomidine required more than 50% of the medetomidine dose to reach equivalent sedation. During isoflurane anaesthesia, cardiopulmonary function was comparable between the two groups. Recovery scores following dexmedetomidine were better compared to medetomidine.

Effect of low inspired oxygen fraction on respiratory indices in mechanically ventilated horses anaesthetised with isoflurane and medetomidine constant rate infusion

Horses may become hypoxaemic during anaesthesia despite a high inspired oxygen fraction (FiO2). A lower FiO2 is used commonly in human beings to minimise atelectasis and to improve lung function, and previously has been shown to be of potential benefit in horses in experimental conditions. Other studies suggest no benefit to using a FiO2 of 0.5 during clinically relevant conditions; however, low FiO2 (0.65) is commonly used in practice and in a large number of studies. The present study was performed to compare the effect of a commonly used FiO2 of 0.65 versus 0.90 on calculated respiratory indices in anaesthetised mechanically ventilated horses in a clinical setting. Eighteen healthy Thoroughbred horses anaesthetised for experimental laryngeal surgery were recruited into a prospective, non-blinded, randomised clinical study. Before anaesthesia, the horses were randomly allocated into either low (0.65) or high (0.90) FiO2 groups and arterial blood gas (ABG) analysis was performed every 30 min during anaesthesia to allow for statistical analysis of respiratory indices. As expected, PaO2 was significantly lower in horses anaesthetised with a low FiO2, but was sufficient to fully saturate haemoglobin. There were no significant improvements in any of the other respiratory indices. There is no obvious benefit to be gained from the use of a FiO2 of 0.65 compared to 0.90 for mechanically ventilated Thoroughbred horses anaesthetised in lateral recumbency with isoflurane and a medetomidine constant rate infusion.

Continuous positive airway pressure (CPAP) decreases pulmonary shunt in anaesthetized horses

OBJECTIVE

To evaluate the effects of continuous positive airway pressure (CPAP) on intrapulmonary shunt, cardiac output and oxygen delivery in horses subjected to a 6 hour period of general anaesthesia.

STUDY DESIGN

Randomized, experimental, crossover study.

ANIMALS

Ten healthy adult horses.

METHODS

Following medetomidine, diazepam and ketamine administration, orotracheal intubation was performed and horses positioned in dorsal recumbency. Anaesthesia was maintained with isoflurane carried in an oxygen and air mix (FiO₂ 0.5) combined with a medetomidine infusion. Horses were anaesthetized twice and either CPAP (8 cmH₂ O) or physiologic airway pressure (NO CPAP) was applied to the lungs for 6 hours; the order of treatments was randomly assigned. Following induction of anaesthesia, cardiovascular and respiratory variables (including arterial blood gas analysis) were recorded every 30 minutes, cardiac output was measured every 60 minutes using the lithium dilution technique and oxygen delivery calculated. If PaCO₂ exceeded 100 mmHg (13.3 kPa), controlled ventilation was initiated and horses excluded from further data collection. Groups were compared using a general linear model.

RESULTS

Data from eight horses were analysed. PaO₂ was 15-56 mmHg (2.00-7.45 kPa) higher (p < 0.001) and shunt fraction 6-14% lower (p < 0.001) in the CPAP group. No differences were seen for cardiac output and oxygen delivery. The lack of difference in oxygen delivery was attributed to lower haemoglobin levels in the CPAP group than in the NO CPAP group.

CONCLUSIONS AND CLINICAL RELEVANCE

CPAP of 8 cmH₂ O can be used in dorsally recumbent horses to decrease pulmonary shunt fraction without causing a decrease in cardiac output during longterm anaesthesia.

Effects of a constant rate infusion of medetomidine-propofol on isoflurane minimum alveolar concentrations in horses

The aim of this investigation was to determine the isoflurane-sparing effect and impact on arterial blood pressure and anaesthetic recovery of a constant rate infusion of medetomidine-propofol in horses. In a prospective, crossover, randomised study, six healthy horses (mean ± SD age, 13.7 ± 7.7 years; weight, 433 ± 51 kg) were anaesthetised twice with isoflurane and were randomly assigned to receive one of two treatments on each occasion, at least 2 weeks apart. The first treatment was saline (CTL group) and the second a medetomidine-propofol infusion (MP group; 1.25 µg/kg/h medetomidine and 3 mg/kg/h propofol). The isoflurane minimum alveolar concentration (MAC) was determined and the reduction in anaesthetic requirements was calculated. Cardiopulmonary data were recorded at different intervals during the procedure and anaesthetic recovery was blindly assessed using three independent scales. The MAC in the MP group (0.43 ± 0.08%) was 65% lower than in the CTL group (1.23 ± 0.10%). The MP group had a higher mean arterial blood pressure and required less dobutamine than the CTL group. The recovery quality in both groups was considered fair or good and an improvement was observed using the Donaldson scale in the MP group. The administration of a medetomidine-propofol constant rate infusion reduced anaesthetic isoflurane requirements to a clinically significant extent and improved stability of arterial blood pressure together with a good quality recovery. This regime could be useful for providing balanced anaesthesia in horses.

Pharmacokinetics and pharmacodynamics of intravenous dexmedetomidine in the horse

The aim of the study was to describe the pharmacokinetics and selected pharmacodynamics of intravenous dexmedetomidine in horses. Eight adult horses received 5 μg/kg dexmedetomidine IV. Blood samples were collected before and for 10 h after drug administration to determine dexmedetomidine plasma concentrations. Pharmacokinetic parameters were calculated using noncompartmental analysis. Data from one outlier were excluded from the statistical summary. Behavioral and physiological responses were recorded before and for 6 h after dexmedetomidine administration. Dexmedetomidine concentrations decreased rapidly (elimination half-life of 8.03 ± 0.84 min). Time of last detection varied from 30 to 60 min. Bradycardia was noted at 4 and 10 min after drug administration (26 ± 8 and 29 ± 8 beats/min respectively). Head height decreased by 70% at 4 and 10 min and gradually returned to baseline. Ability to ambulate was decreased for 60 min following drug administration, and mechanical nociceptive threshold was increased during 30 min. Blood glucose peaked at 30 min (134 ± 24 mg/dL) and borborygmi were decreased for the first hour after dexmedetomidine administration. Dexmedetomidine was quickly eliminated as indicated by the rapid decrease in plasma concentrations. Physiological, behavioral, and analgesic effects observed after dexmedetomidine administration were of short duration.

Evaluation of a romifidine constant rate infusion protocol with or without butorphanol for dentistry and ophthalmologic procedures in standing horses

OBJECTIVE

To compare the clinical usefulness of constant rate infusion (CRI) protocols of romifidine with or without butorphanol for sedation of horses.

STUDY DESIGN

Prospective 'blinded' controlled trial using block randomization.

ANIMALS

Forty healthy Freiberger stallions.

METHODS

The horses received either intravenous (IV) romifidine (loading dose: 80 μg kg(-1) ; infusion: 30 μg kg(-1)  hour(-1) ) (treatment R, n = 20) or romifidine combined with butorphanol (romifidine loading: 80 μg kg(-1) ; infusion: 29 μg kg(-1)  hour(-1) , and butorphanol loading: 18 μg kg(-1) ; infusion: 25 μg kg(-1)  hour(-1) ) (treatment RB, n = 20). Twenty-one horses underwent dentistry and ophthalmic procedures, while 19 horses underwent only ophthalmologic procedure and buccal examination. During the procedure, physiologic parameters and occurrence of head/muzzle shaking or twitching and forward movement were recorded. Whenever sedation was insufficient, additional romifidine (20 μg kg(-1) ) was administered IV. Recovery time was evaluated by assessing head height above ground. At the end of the procedure, overall quality of sedation for the procedure was scored by the dentist and anaesthetist using a visual analogue scale. Statistical analyses used two-way anova or linear mixed models as relevant.

RESULTS

Sedation quality scores as assessed by the anaesthetist were R: median 7.55, range: 4.9-9.0 cm, RB: 8.8, 4.7-10.0 cm, and by the dentist R: 6.6, 3.0-8.2 cm, RB: 7.9, 6.6-8.8 cm. Horses receiving RB showed clinically more effective sedation as demonstrated by fewer poor scores and a tendency to reduced additional drug requirements. More horses showed forward movement and head shaking in treatment RB than treatment R. Three horses (two RB, one R) had symptoms of colic following sedation.

CONCLUSIONS AND CLINICAL RELEVANCE

The described protocols provide effective sedation under clinical conditions but for dentistry procedures, the addition of butorphanol is advantageous.

The effects of a loading dose followed by constant rate infusion of xylazine compared with romifidine on sedation, ataxia and response to stimuli in horses

OBJECTIVE

To compare xylazine and romifidine constant rate infusion (CRI) protocols regarding degree of sedation, and effects on postural instability (PI), ataxia during motion (A) and reaction to different stimuli.

STUDY DESIGN

Blinded randomized experimental cross-over study.

ANIMALS

Ten adult horses.

METHODS

Degree of sedation was assessed by head height above ground (HHAG). Effects on PI, A and reaction to visual, tactile and acoustic stimulation were assessed by numerical rating scale (NRS) and by visual analogue scale (VAS). After baseline measurements, horses were sedated by intravenous loading doses of xylazine (1 mg kg(-1) ) or romifidine (80 μg kg(-1) ) administered over 3 minutes, immediately followed by a CRI of xylazine (0.69 mg kg(-1 ) hour(-1) ) or romifidine (30 μg kg(-1 ) hour(-1) ) which was administered for 120 minutes. Degree of sedation, PI, A and reaction to the different stimuli were measured at different time points before, during and for one hour after discontinuing drug administration. Data were analysed using two-way repeated measures anova, a Generalized Linear Model and a Wilcoxon Signed Rank Test (p < 0.05).

RESULTS

Significant changes over time were seen for all variables. With xylazine HHAG was significantly lower 10 minutes after the loading dose, and higher at 150 and 180 minutes (i.e. after CRI cessation) compared to romifidine. Reaction to acoustic stimulation was significantly more pronounced with xylazine. Reaction to visual stimulation was greater with xylazine at 145 and 175 minutes. PI was consistently but not significantly greater with xylazine during the first 30 minutes. Reaction to touch and A did not differ between treatments. Compared to romifidine, horses were more responsive to metallic noise with xylazine.

CONCLUSIONS

Time to maximal sedation and to recovery were longer with romifidine than with xylazine.

CLINICAL RELEVANCE

With romifidine sufficient time should be allowed for complete sedation before manipulation.

Comparison of the effects of xylazine bolus versus medetomidine constant rate infusion on cardiopulmonary function and depth of anesthesia in horses anesthetized with isoflurane

OBJECTIVE

To compare the effects of xylazine bolus versus medetomidine constant rate infusion (MCRI) on cardiopulmonary function and depth of anesthesia in dorsally recumbent, spontaneously breathing, isoflurane-anesthetized horses.

DESIGN

Prospective, randomized crossover study.

ANIMALS

10 healthy adult Standardbreds.

PROCEDURES

Horses were premedicated with xylazine or medetomidine IV. Anesthesia was induced with diazepam and ketamine and maintained with isoflurane for 150 minutes. For the xylazine treatment, end-tidal isoflurane concentration was maintained at 1.7%, and xylazine (0.2 mg/kg [0.09 mg/lb], IV) was administered as a bolus at the end of anesthesia. For the MCRI treatment, end-tidal isoflurane concentration was maintained at 1.4%, and medetomidine (0.005 mg/kg/h [0.0023 mg/lb/h], IV) was infused throughout anesthesia. Physiologic data (ie, heart rate, respiratory rate, rectal temperature, bispectral index, and electromyographic values) were compared between treatments with xylazine bolus versus MCRI.

RESULTS

Heart rate was lower, but mean arterial blood pressure was higher from 20 to 40 minutes with MCRI treatment, compared with conventional treatment with xylazine. Respiratory rate and rectal temperature were greater with MCRI treatment. Bispectral index was lower with MCRI treatment from 80 to 150 minutes, and electromyographic values were lower with MCRI treatment from 30 to 150 minutes.

CONCLUSIONS AND CLINICAL RELEVANCE

In isoflurane-anesthetized horses, premedication with medetomidine followed by administration of medetomidine as a constant rate infusion resulted in decreased heart rate, higher arterial blood pressure from 20 through 40 minutes after induction of anesthesia, and better preserved body temperature, compared with conventional treatment with xylazine. Greater depth of anesthesia and muscle relaxation were seen with MCRI treatment, despite the lower isoflurane concentration.

Effects on cardiopulmonary function and oxygen delivery of doses of romifidine and xylazine followed by constant rate infusions in standing horses

The objective of this study was to compare the cardiopulmonary effects of a xylazine or romifidine loading-dose, followed by a constant rate infusion (CRI) of the same α(2)-agonist. Nine research horses were treated in a randomized, blinded, crossover design with xylazine or romifidine. After instrumentation, a loading dose of intravenous xylazine (1mg/kg) or romifidine (80μg/kg) was administered, immediately followed by a CRI of xylazine (0.69mg/kg/h) or romifidine (30μg/kg/h) for a duration of 2h. Cardiopulmonary variables were recorded before bolus administration, during CRI, and for 1h after discontinuing drug administration. A significant decrease in haemoglobin concentration (tHb), arterial oxygen content (CaO(2)), oxygen delivery (D˙O(2)), mixed venous partial pressure of oxygen, heart rate, and cardiac output (Q˙t) followed the loading dose with both treatments. Carotid arterial blood pressure (ABP), systemic vascular resistance, and right atrial pressure (RAP) increased significantly. The increased ABP was followed by a significant decrease compared to baseline. Mean pulmonary arterial pressure increased significantly with romifidine only. No significant changes in stroke volume, arterial partial pressure of oxygen, and oxygen consumption were observed. Changes in Q˙t and RAP were more pronounced with romifidine. During CRI, tHb, and CaO(2) were significantly higher with romifidine, whereas D˙O(2) did not differ between treatments. Overall, cardiopulmonary effects were more pronounced and lasted longer with romifidine compared to xylazine. However, during CRI, there was no difference in D˙O(2) between drugs. With both α(2)-agonists, cardiovascular effects were most pronounced after loading dose administration and tended to stabilize during CRI.

Cardiopulmonary effects and pharmacokinetics of i.v. dexmedetomidine in ponies

REASONS FOR PERFORMING STUDY

Currently available sedatives depress cardiopulmonary function considerably; therefore, it is important to search for new, less depressive sedatives. The study was performed to assess duration and intensity of cardiopulmonary side effects of a new sedative, dexmedetomidine (DEX), in horses.

OBJECTIVES

To study pharmacokinetics and cardiopulmonary effects of i.v. DEX.

METHODS

Pharmacokinetics of 3.5 microg/kg bwt i.v. DEX were studied in a group of 8 mature (mean age 4.4 years) and 6 old ponies (mean age 20 years). Cardiopulmonary data were recorded in mature ponies before and 5, 10, 20, 30, 45 and 60 mins after administration of DEX 3.5 microg/kg bwt i.v. Data were analysed using ANOVA for repeated measures, and where appropriate Dunnett's t test was used to detect differences from resting values (P < 0.05).

RESULTS

Within 2 h after DEX administration, plasma levels were beyond limits of quantification (0.05 ng/ml). Mean values for kinetic parameters for mature and old ponies were: Cmax (ng/ml) 4.6 and 3.8, t 1/2 (min) 19.8 and 28.9 and AUC (ng.min/ml) 34.5 and 44.3, respectively. Heart rate, central venous pressure, pulmonary artery pressure and pulmonary capillary wedge pressure did not change significantly compared to presedation values throughout the 60 min observation period. Compared to presedation values, stroke volume and mixed venous PO2 were reduced for the first 5 mins, paralleled by an increase in systemic and pulmonary vascular resistance. Cardiac index was reduced for the first 10 mins, arterial blood pressures at 20, 30 and 45 mins and respiratory rate throughout the 60 min observation period, but no change in arterial PO2 or PCO2 occurred.

CONCLUSIONS

DEX administration i.v. causes similar cardiopulmonary changes to those caused by other alpha-2 adrenoceptor agonists, but of very short duration. DEX is redistributed particularly rapidly.

POTENTIAL RELEVANCE

DEX might be safer for sedation of horses because of its very short-lasting cardiopulmonary side effects. For long duration sedation, its kinetics favour its use as a continuous infusion.

A clinical comparison of two anaesthetic protocols using lidocaine or medetomidine in horses

OBJECTIVE

To compare the effects of two balanced anaesthetic protocols on end-tidal isoflurane (Fe'ISO), cardiopulmonary performance and quality of recovery in horses.

DESIGN

Prospective blinded randomized clinical study.

ANIMALS

Sixty-nine client-owned horses, American Society of Anesthesiologists category I and II, undergoing elective surgery.

METHODS

The horses were premedicated with acepromazine (0.03 mg kg(-1)) IM 30-60 minutes before induction of anaesthesia and were randomly assigned to one of two treatments: in group L (37 horses) xylazine (1 mg kg(-1)) and in group M (31 horses) medetomidine (7 microg kg(-1)) was administered IV for sedation. Anaesthesia was induced 5 minutes later with ketamine (2.2 mg kg(-1)) and diazepam (0.02 mg kg(-1)) IV and maintained with isoflurane in oxygen/air (initial FIO2 0.40-0.50) and a constant rate infusion (CRI) of either lidocaine (2 mg kg(-1)/15 minutes loading dose followed by 50 microg kg(-1) minute(-1)) (group L) or medetomidine (3.5 microg kg(-1) hour(-1)) (group M). If horses showed movement or nystagmus, additional thiopental or ketamine was administered. Heart rate, mean arterial pressure (MAP), Fe'ISO and arterial blood gases were measured. Cardiac output was measured with the lithium dilution method in 10 (group L) and 11 (group M) horses every 45 minutes. Recovery was scored.

RESULTS

Heart rate and the cardiac index (CI) were significantly higher in group L with changes over time. In group M, MAP was significantly higher during the first 50 minutes. Group L needed more additional ketamine and thiopental to maintain a surgical plane of anaesthesia and Fe'ISO was significantly higher from 70 minutes. Recovery was longer in group M and of better quality. The significance level was set at p < 0.05.

CONCLUSIONS AND CLINICAL RELEVANCE

In group M, maintenance of stable anaesthetic depth was easier and lower Fe'ISO was required to maintain a surgical plane of anaesthesia. Recoveries were longer but of better quality. The CI was higher in group L but cardiovascular function was generally well maintained in both groups.

Effects of a muscarinic type-2 antagonist on cardiorespiratory function and intestinal transit in horses anesthetized with halothane and xylazine

OBJECTIVE

To evaluate the cardiorespiratory and intestinal effects of the muscarinic type-2 (M2) antagonist, methoctramine, in anesthetized horses.

ANIMALS

6 horses.

PROCEDURE

Horses were allocated to 2 treatments in a randomized complete block design. Anesthesia was maintained with halothane (1% end-tidal concentration) combined with a constant-rate infusion of xylazine hydrochloride (1 mg/kg/h, i.v.) and mechanical ventilation. Hemodynamic variables were monitored after induction of anesthesia and for 120 minutes after administration of methoctramine or saline (0.9% NaCl) solution (control treatment). Methoctramine was given at 10-minute intervals (10 microg/kg, i.v.) until heart rate (HR) increased at least 30% above baseline values or until a maximum cumulative dose of 30 microg/kg had been administered. Recovery characteristics, intestinal auscultation scores, and intestinal transit determined by use of chromium oxide were assessed during the postanesthetic period.

RESULTS

Methoctramine was given at a total cumulative dose of 30 microg/kg to 4 horses, whereas 2 horses received 10 microg/kg. Administration of methoctramine resulted in increases in HR, cardiac output, arterial blood pressure, and tissue oxygen delivery. Intestinal auscultation scores and intestinal transit time (interval to first and last detection of chromium oxide in the feces) did not differ between treatment groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Methoctramine improved hemodynamic function in horses anesthetized by use of halothane and xylazine without causing a clinically detectable delay in the return to normal intestinal motility during the postanesthetic period. Because of their selective positive chronotropic effects, M2 antagonists may represent a safe alternative for treatment of horses with intraoperative bradycardia.

Effects of glycopyrrolate on cardiorespiratory function in horses anesthetized with halothane and xylazine

OBJECTIVE

To evaluate cardiopulmonary effects of glycopyrrolate in horses anesthetized with halothane and xylazine.

ANIMALS

6 horses.

PROCEDURE

Horses were allocated to 2 treatment groups in a randomized complete block design. Anesthesia was maintained in mechanically ventilated horses by administration of halothane (1% end-tidal concentration) combined with a constant-rate infusion of xylazine hydrochloride (1 mg/kg/h, i.v.). Hemodynamic variables were monitored after induction of anesthesia and for 120 minutes after administration of glycopyrrolate or saline (0.9% NaCl) solution. Glycopyrrolate (2.5 microg/kg, i.v.) was administered at 10-minute intervals until heart rate (HR) increased at least 30% above baseline or a maximum cumulative dose of 7.5 microg/kg had been injected. Recovery characteristics and intestinal auscultation scores were evaluated for 24 hours after the end of anesthesia.

RESULTS

Cumulative dose of glycopyrrolate administered to 5 horses was 5 microg/kg, whereas 1 horse received 7.5 microg/kg. The positive chronotropic effects of glycopyrrolate were accompanied by an increase in cardiac output, arterial blood pressure, and tissue oxygen delivery. Whereas HR increased by 53% above baseline values at 20 minutes after the last glycopyrrolate injection, cardiac output and mean arterial pressure increased by 38% and 31%, respectively. Glycopyrrolate administration was associated with impaction of the large colon in 1 horse and low intestinal auscultation scores lasting 24 hours in 3 horses.

CONCLUSIONS AND CLINICAL RELEVANCE

The positive chronotropic effects of glycopyrrolate resulted in improvement of hemodynamic function in horses anesthetized with halothane and xylazine. However, prolonged intestinal stasis and colic may limit its use during anesthesia.

Medetomidine-ketamine anaesthesia induction followed by medetomidine-propofol in ponies: infusion rates and cardiopulmonary side effects

REASONS FOR PERFORMING STUDY

To search for long-term total i.v. anaesthesia techniques as a potential alternative to inhalation anaesthesia.

OBJECTIVES

To determine cardiopulmonary effects and anaesthesia quality of medetomidine-ketamine anaesthesia induction followed by 4 h of medetomidine-propofol anaesthesia in 6 ponies.

METHODS

Sedation consisted of 7 microg/kg bwt medetomidine i.v. followed after 10 min by 2 mg/kg bwt i.v. ketamine. Anaesthesia was maintained for 4 h with 3.5 microg/kg bwt/h medetomidine and propofol at minimum infusion dose rates determined by application of supramaximal electrical pain stimuli. Ventilation was spontaneous (F(I)O2 > 0.9). Cardiopulmonary measurements were always taken before electrical stimulation, 15 mins after anaesthesia induction and at 25 min intervals.

RESULTS

Anaesthesia induction was excellent and movements after pain stimuli were subsequently gentle. Mean propofol infusion rates were 0.89-0.1 mg/kg bwt/min. No changes in cardiopulmonary variables occured over time. Range of mean values recorded was: respiratory rate 13.0-15.8 breaths/min; PaO2 29.1-37.9 kPa; PaCO2 6.2-6.9 kPa; heart rate 31.2-40.8 beats/min; mean arterial pressure 90.0-120.8 mmHg; cardiac index 44.1-59.8 ml/kg bwt/min; mean pulmonary arterial pressure 11.8-16.4 mmHg. Recovery to standing was an average of 31.1 mins and ponies stood within one or 2 attempts.

CONCLUSIONS

In this paper, ketamine anaesthesia induction avoided the problems encountered previously with propofol. Cardiovascular function was remarkably stable. Hypoxaemia did not occur but, despite F(I)O2 of > 0.9, minimal PaO2 in one pony after 4 h anaesthesia was 8.5 kPa.

POTENTIAL RELEVANCE

The described regime might offer a good, practicable alternative to inhalation anaesthesia and has potential for reducing the fatality rate in horses.

Cardiopulmonary effects of prolonged anesthesia via propofol-medetomidine infusion in ponies

OBJECTIVE

To determine cardiopulmonary effects of total IV anesthesia with propofol and medetomidine in ponies and effect of atipamezole on recovery.

ANIMALS

10 ponies.

PROCEDURE

After sedation was induced by IV administration of medetomidine (7 microg/kg of body weight), anesthesia was induced by IV administration of propofol 12 mg/kg) and maintained for 4 hours with infusions of medetomidine (3.5 microg/kg per hour) and propofol 10.07 to 0.11 mg/kg per minute). Spontaneous respiration was supplemented with oxygen. Cardiopulmonary measurements and blood concentrations of propofol were determined during anesthesia. Five ponies received atipamezole (60 microg/kg) during recovery.

RESULTS

During anesthesia, mean cardiac index and heart rate increased significantly until 150 minutes, then decreased until cessation of anesthesia. Mean arterial pressure and systemic vascular resistance index increased significantly between 150 minutes and 4 hours. In 4 ponies, PaO2 decreased to < 60 mm Hg. Mean blood propofol concentrations from 20 minutes after induction onwards ranged from 2.3 to 3.5 microg/ml. Recoveries were without complications and were complete within 28 minutes with atipamezole administration and 39 minutes without atipamezole administration.

CONCLUSIONS AND CLINICAL RELEVANCE

During total IV anesthesia of long duration with medetomidine-propofol, cardiovascular function is comparable to or better than under inhalation anesthesia. This technique may prove suitable in equids in which prompt recovery is essential; however, in some animals severe hypoxia may develop and oxygen supplementation may be necessary.

Comparison of medetomidine and dexmedetomidine as premedication in isoflurane anaesthesia for orthopaedic surgery in domestic sheep

The objective of the present study was to determine the potency of dexmedetomidine in relation to medetomidine in sheep undergoing orthopaedic surgery by comparing the anaesthetic requirements and cardiovascular changes at a dose relationship that represented equipotency in vitro. Twenty-four non-pregnant, female sheep were used. The study was carried out as a blind, randomized, experimental trial. Group 1 received 5 micrograms/kg bodyweight (BW) dexmedetomidine and group 2 received 10 micrograms/kg BW medetomidine intravenously 5 min prior to induction of anaesthesia. Anaesthesia was induced with ketamine (2.0 mg/kg BW intravenously) and maintained with isoflurane in 100% oxygen. End expired anaesthetic concentration (FEIso), end expired carbon dioxide concentration (FECO2), respiratory frequency (fR), direct arterial blood pressures, heart rates (HR) and arterial blood gases were monitored. Data were averaged over time and tested for differences between groups by independent t-tests, and analysis of variance for repeated measures. Average FEIso concentrations required to maintain a surgical plane of anaesthesia were not different between groups (1: 1.02 +/- 0.04%; 2: 0.99 +/- 0.07%). There was no difference in HR, arterial blood pressures, fR, FECO2 and arterial blood gases between groups. Average mean PaO2 were 279.54 +/- 113.37 mmHg and 220.21 +/- 102.15 mmHg with individual minimum values of 27.2 mmHg and 58.5 mmHg in groups 1 and 2, respectively. In conclusion, intravenous dexmedetomidine at 5 micrograms/kg BW and medetomidine at 10 micrograms/kg BW have the same effects on isoflurane requirements and cardiopulmonary parameters in sheep, indicating an equipotent dose relationship. Both preparations induced moderate to severe hypoxaemia in individual sheep.

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.

Minimal alveolar concentration of desflurane in combination with an infusion of medetomidine for the anaesthesia of ponies

The minimum alveolar concentration of desflurane when combined with a continuous infusion of medetomidine at 3.5 microg/kg/hour was measured in seven ponies. Anaesthesia was induced with medetomidine (7 microg/kg intravenously) followed by ketamine (2 mg/kg intravenously) and maintained with desflurane in oxygen. The infusion of medetomidine was started 20 minutes after the induction of anaesthesia. The electrical test stimulus was applied at the coronary band (50 V, 10 ms bursts at 5 Hz for one minute), and heart rates and rhythms, arterial blood pressures, and arterial blood gas tensions were measured at intervals, just before the application of the stimulus. The mean (sd) minimum alveolar concentration of desflurane was 5.3 (1.04) per cent (range 3.2 to 6.4 per cent), 28 per cent less than the previously published value for desflurane alone after the induction of anaesthesia with xylazine and ketamine. The cardiopulmonary parameters remained stable throughout the period of anaesthesia. The mean (sd) time taken by the ponies to stand after the administration of desflurane ceased was 16.5 (6.17) (range 5.8 to 26) minutes, and the quality of recovery was good or excellent. However, one pony died shortly after standing; a postmortem examination revealed that it had chronic left atrial dilatation.