Isoflurane-anaesthetized Horses Research Articles

Retrospective evaluation of the impact of atropine administration on incidence of post-operative colic in healthy, isoflurane-anaesthetised horses.

In anaesthetised horses, bradycardia secondary to high vagal tone can reduce cardiac output and blood pressure. The use of anticholinergics in horses is limited due to concerns about ileus and abdominal discomfort. This retrospective study sought to determine the prevalence of post-operative abdominal discomfort in healthy horses that received atropine under isoflurane anaesthesia. A retrospective evaluation of 222 general anaesthesia events between January 2019 and December 2019 was undertaken. One hundred and eleven horses that received atropine were identified, and 111 case match controls that did not receive atropine were also selected. Information gathered from the medical records included signalment, anaesthetic drugs, surgical procedures, duration of anaesthesia and surgery, dobutamine and atropine administration, and the occurrence of abdominal discomfort for 24 h after anaesthesia. After initial data analysis, a second cohort of records was assessed separately. The horses in this group were castrated under general anaesthesia (with or without atropine; n = 68). Logistic regression models and Fisher's exact tests were used to look for factors contributing to abdominal discomfort post-anaesthesia. The significance level was set to 5% (p < 0.05). Atropine administration was not associated with the development of post-anaesthetic abdominal discomfort (OR = 2.121, 95% CI [0.767, 5.869]; p = 0.2). Overall, 18/222 (8.1%) incidences of abdominal discomfort were identified. All incidents occurred in colts undergoing castration. In a separate analysis of only horses anaesthetised for castration, atropine was associated with developing abdominal discomfort (OR = 3.143, 95% CI [1.082, 9.132]; p = 0.04). Atropine was not associated with post-operative abdominal discomfort except in colts undergoing castration. All episodes of discomfort were mild and resolved with minimal intervention. The potential impact of insufficient analgesia in horses undergoing castration is a confounding factor and requires prospective investigation. Overall, atropine at a dose of 0.006 mg/kg IV appears to be a safe method to treat bradycardia in otherwise healthy horses anaesthetised for orthopaedic and upper airway procedures. Further work is required to determine if atropine is safe for colts undergoing castration.

Influence of acepromazine on the cardiovascular effects of dobutamine in isoflurane anaesthetised horses premedicated with romifidine

ObjectiveTo explore the influence of acepromazine on the cardiovascular effects of dobutamine in isoflurane-anaesthetised horses premedicated with romifidine. Study designProspective randomised clinical trial. AnimalsA total of 18 horses undergoing elective arthroscopy were enrolled, of which 12 horses requiring dobutamine were included. MethodsHorses were randomised to receive acepromazine 0.02 mg kg-1 (Group A+) intravenously (IV) or none (Group A-), 35 minutes before anaesthesia. Horses received xylazine 0.2 mg kg-1 concurrently to facilitate IV access. Horses were premedicated with romifidine 0.08 mg kg-1, induced with ketamine 2.2 mg kg-1 and diazepam 0.08 mg kg-1 IV, and maintained with isoflurane in oxygen. Dobutamine infusion was commenced when mean arterial pressure (MAP) was < 60 mmHg. Cardiovascular data were collected prior to dobutamine, and at a target MAP of ≥ 70 mmHg. Dobutamine start time from induction, duration and dose to reach target MAP were compared using Mann-Whitney U test. Cardiovascular variables were compared using repeated measures ANOVA and post-hoc Fisher’s least significant difference test. ResultsCardiac index (CI) and its percentage change was significantly higher at target MAP in group A+ [42.8 (17.0–68.7) %] than in A- [-4.05 (-21.2–13.0) %] (p = 0.003). Group A+ required significantly earlier dobutamine [20 (18–25) minutes] than group A- [36 (27–60) minutes] (p = 0.02). Group A+ required significantly higher dobutamine dose [1.5 (1–2.5) μg kg-1 minute-1] to reach target MAP than group A- [0.5 (0.5–1) μg kg-1 minute-1] (p = 0.009). No significant difference in infusion duration to reach target MAP was found between groups. Conclusion and clinical relevanceDobutamine significantly increased MAP and CI following pre-anaesthetic acepromazine sedation, in isoflurane-anaesthetised horses premedicated with romifidine. Without acepromazine, dobutamine increased MAP but not CI. Interactions between acepromazine, romifidine and dobutamine on the cardiovascular system should be considered.

Agreement of High-Definition Oscillometry (HDO) and Invasive Blood Pressure Measurements at a Metatarsal Artery in Isoflurane-Anaesthetised Horses.

Simple SummaryIn equine anaesthesia, valid and reliable blood pressure monitoring is crucial for adequate blood pressure management. Various non-invasive blood pressure monitors have been studied with varying results. High-definition oscillometry (HDO) promises greater accuracy than conventional oscillometric devices. With the cuff placed at the tail, the monitor gives satisfactory readings in normotensive anaesthetised horses, while for measuring low and high blood pressure, reliability was inadequate. To date, high-definition oscillometry has not been evaluated at the easily accessible equine metatarsal area. Therefore, the objective of this study was to assess agreement between HDO and invasive blood pressure, both obtained at a metatarsal artery at different blood pressure ranges in anaesthetised horses. Additionally, compliance with the American College of Veterinary Internal Medicine consensus guidelines was assessed. Agreement of HDO and invasive blood pressure was acceptable for mean arterial blood pressure during normotension only. During hypotension and hypertension, measurements were not valid. The monitor failed to meet most of the consensus validation criteria. Consequently, invasive blood pressure measurement remains preferable in haemodynamically unstable patients.High-definition oscillometry (HDO) over the metatarsal artery (MA) in anaesthetised horses has not yet been evaluated. This study aimed to assess agreement between HDO and invasive blood pressure (IBP) at the metatarsal artery, and to evaluate compliance with the American College of Veterinary Internal Medicine (ACVIM) consensus guidelines. In this experimental study, 11 horses underwent general anaesthesia for an unrelated, terminal surgical trial. Instrumentation included an IBP catheter in one and an HDO cuff placed over the contralateral MA, as well as thermodilution catheters. Systolic arterial pressure (SAP), mean arterial pressure (MAP), diastolic arterial pressure (DAP), and cardiac output were measured simultaneously. Normotension (MAP 61–119 mmHg) was maintained during the surgical study. Subsequently, hypotension (MAP ≤ 60 mmHg) and hypertension (MAP ≥ 120 mmHg) were induced pharmacologically. For MAP, the agreement between HDO and IBP was acceptable during normotension, while during hypotension and hypertension, IBP was overestimated and underestimated by HDO, respectively. The monitor failed to meet most ACVIM validation criteria. Consequently, if haemodynamic compromise or rapid blood pressure changes are anticipated, IBP remains preferable.

Effect of Fentanyl Infusion on Heart Rate Variability and Anaesthetic Requirements in Isoflurane-Anaesthetized Horses.

Simple SummaryHistorically, the use of opioids in equine anaesthesia has been limited due to many reported adverse effects. Even though more research is being conducted, there is conflicting evidence on the efficacy of opioids and their behavioural effects in horses. The aim of this study was to investigate the effect of the synthetic opioid fentanyl on various parameters measured during general anaesthesia, as well as to determine its possible sparing-effect on inhalant anaesthetic.Controversy continues to surround the use of opioids in equine anaesthesia, with variable effects reported. This blinded clinical study aimed to investigate the influence of a low-dose fentanyl continuous rate infusion (CRI) on isoflurane requirements, parasympathetic tone activity (PTA), and anaesthetic parameters in horses during general anaesthesia. All of the twenty-two horses included in the research underwent a standard anaesthetic protocol. Eleven horses in the fentanyl group (Group F) received a loading dose of fentanyl at 6 µg/kg, followed by a CRI of 0.1 µg/kg/min during anaesthesia. A further 11 horses in the control group (Group C) received equivalent volumes of normal saline. Anaesthetic parameters and PTA index were recorded during anaesthesia. The achieved mean fentanyl plasma concentration was 6.2 ± 0.83 ng/mL. No statistically significant differences between groups were found in isoflurane requirements, MAP values, and mean dobutamine requirements. However, horses in Group F required a significantly lower dose of additional ketamine to maintain a sufficient depth of anaesthesia. Significantly higher PTA values were found in the fentanyl group. Further research is warranted to determine the limitations of PTA monitoring, and the influence of various anaesthetics on its values.

Effect of temperament on recovery in isoflurane-anaesthetised horses

Background:Recovery is a crucial phase of equine anaesthesia and factors influencing recovery quality are an active area of research.Aim:To investigate the effect of temperament on recovery score after isoflurane-anaesthesia in 30 adult horses undergoing elective surgery.Methods:Two veterinarians used a numerical rating scale to score each horse's response to five tests as a gauge of temperament. Owners used a numerical rating scale to score their horse's temperament according to seven behaviour-related questions. Horses underwent elective surgery under general anaesthesia using a standardised protocol. Recovery was recorded and scored by a blinded assessor using the simple descriptive scale for scoring recovery (R1) and the Edinburgh system (R2).Findings:There was no correlation between veterinarian or owner-assessed temperament and recovery score. Veterinary-assessed temperament score was negatively correlated with pre-induction romifidine and total romifidine dose. Both recovery scores were negatively correlated with anaesthetic duration and R1 was positively correlated with time to first movement in recovery.Conclusions:Temperament did not influence recovery score in our population of horses.

The effect of dobutamine and bolus crystalloid fluids on the cardiovascular function of isoflurane-anaesthetised horses.

Cardiac output does not always increase with dobutamine administration in anaesthetised horses and information on peripheral perfusion is lacking. To determine the effect of intravenous (i.v.) dobutamine infusion with and without a concurrent 20 mL/kg bodyweight (bwt) bolus of crystalloid fluids on the cardiovascular function of acepromazine premedicated, hypotensive, isoflurane-anaesthetised horses. Randomised, cross-over experiment. A total of 6 horses aged 5-13 years, weighing 464-578 kg were premedicated with acepromazine 0.02 mg/kg bwt and then sedated with xylazine 0.8 mg/kg bwt i.v. Anaesthesia was induced with ketamine 2.2 mg/kg bwt and diazepam 0.08 mg/kg bwt i.v. and maintained with isoflurane, adjusted to achieve a target mean arterial pressure (MAP) (60 mmHg ± 5%) 60 min post-induction of anaesthesia (T0). One of 2 treatments was then given. In treatment D, dobutamine was initially infused at 0.5 μg/kg bwt/min and adjusted to achieve a target MAP (80 mmHg ± 5%) within 30 min of infusion initiation. In treatment D+F dobutamine was administered as described for treatment D, with 20 mL/kg bwt Hartmann's solution infused i.v. over 20 min. Cardiac index (CI), haemoglobin concentration ([Hb]), arterial oxygen content (CaO2 ), oxygen delivery index (DO2 I) and bilateral femoral arterial blood flow (FBF) were recorded at T0, 30 min following dobutamine initiation (T1) and 15 min following dobutamine cessation (T2). Data were analysed using a mixed-effect linear model (P<0.05 considered significant). A significant increase in DO2 I (P = 0.008, T0/T1), CaO2 (P = 0.0002, T0/T1) and [Hb] (P<0.0001, T0/T1) and in CaO2 (P = 0.0005, T1/T2) and [Hb] (P = 0.002,T1/T2) occurred during treatment D. A significant increase in FBF (P = 0.005, upper limb; P = 0.042 lower limb, T0/T1) occurred during treatment D+F. Significant differences between treatments were recorded at T1 ([Hb] P = 0.0001, CaO2 P = 0.0003) and T2 ([Hb] P = 0.013). There was no change in CI during either treatment. The increase in FBF seen with co-administration of fluids and dobutamine may provide a beneficial effect on muscle compared with the use of dobutamine alone.

The cardiovascular status of isoflurane-anaesthetized horses with and without dexmedetomidine constant rate infusion evaluated at equivalent depths of anaesthesia

ObjectiveTo compare cardiac index and oxygen extraction at equivalent depths of anaesthesia between isoflurane-anaesthetized horses and horses anesthetized with isoflurane and dexmedetomidine CRI. Study designSequential, blinded, randomized, balanced, crossover study. AnimalsEight horses weighing a mean ± standard deviation of 478 ± 58 kg. MethodsHorses were premedicated with 0.03 mg kg−1 acepromazine intramuscularly (IM) and 8 μg kg−1 dexmedetomidine intravenously (IV). Anaesthesia was induced with 2.5 mg kg−1 ketamine and 0.1 mg kg−1 midazolam IV and maintained with isoflurane in oxygen and air. Horses were mechanically ventilated. Fractional concentration of end-tidal isoflurane (Fe’Iso) was stabilized at 1.7% with a CRI of 0.9% NaCl (IsoNaCl), or at 1.1% with 1.75 μg kg−1 hour−1 dexmedetomidine (IsoDex). Mean arterial blood pressure was maintained above 60 mmHg by dobutamine infusion. Following nociceptive electrical stimulation, Fe’Iso was stabilized at a 0.1% lower concentration and nociceptive stimulation was repeated. This procedure was continued until the horse moved. Fe’Iso values prior to the concentration at which movement occurred were considered to indicate equivalent depths of anaesthesia between treatments. Cardiac index and oxygen extraction were compared at equivalent depths of anaesthesia using a paired Student’s t-test. ResultsCardiac index differed between IsoNaCl at 61 ± 12 mL kg−1 minute−1 and IsoDex at 48 ± 10 mL kg−1 minute−1 (p = 0.047). In addition, oxygen extraction differed between IsoNaCl at 3.4 ± 0.8 mL kg−1 minute−1 and IsoDex at 4.5 ± 0.5 mL kg−1 minute−1 (p = 0.0042). Two horses receiving IsoNaCl were administered dobutamine at equivalent depths of anaesthesia (7.0 and 28.8 μg kg−1 hour−1, respectively). Conclusions and clinical relevanceCardiovascular function in horses receiving isoflurane and 1.75 μg kg−1 minute−1 dexmedetomidine is more compromised than in horses receiving a higher concentration of isoflurane and 0.9% NaCl CRI.

A study of measurement of noninvasive blood pressure with the oscillometric device, Sentinel, in isoflurane-anaesthetized horses

ObjectiveTo assess accuracy of noninvasive blood pressure (NIBP) measured by oscillometric device Sentinel compared to invasive blood pressure (IBP) in anaesthetized horses undergoing surgery. To assess if differences between the NIBP measured by the Sentinel and IBP are associated with recumbency, cuff placement, weight of the horse or acepromazine premedication and to describe usefulness of the Sentinel. Study designProspective study examining replicates of simultaneous NIBP and IBP measurements. AnimalsTwenty-nine horses. MethodsInvasive blood pressure was measured via a catheter in the facial artery, transverse facial artery or metatarsal artery. NIBP was measured using appropriate size cuffs placed on one of two metacarpal or metatarsal bones or the tail in random order. With both techniques systolic (SAP), mean (MAP), and diastolic (DAP) arterial blood pressures and heart rates (HR) were recorded. A mixed effects model compared the IBP to the NIBP values and assessed potential effects of catheter placement, localisation of the cuffs in combination with recumbency, weight of the horse or acepromazine premedication. ResultsNoninvasive blood pressure yielded higher measurements than IBP. Agreement varied with recumbency and cuff position. Estimated mean differences between the two methods decreased from SAP (lateral recumbency: range -5.3 to -56.0 mmHg; dorsal recumbency: range 0.8 to -20.7 mmHg), to MAP (lateral recumbency: range -1.8 to -19.0 mmHg; dorsal recumbency: range 13.9 to -16.4 mmHg) to DAP (lateral recumbency: range 0.5 to -6.6 mmHg; dorsal recumbency: range 21.0 to -15.5 mmHg). NIBP measurement was approximately two times more variable than IBP measurement. No significant difference between IBP and NIBP due to horse's weight or acepromazine premedication was found. In 227 of 1047 (21.7%) measurements the Sentinel did not deliver a result. Conclusion and clinical relevanceAccording to the high variability of NIBP compared to IBP, NIBP measurements as measured by the Sentinel in the manner described here are not considered as an appropriate alternative to IBP to measure blood pressure in anaesthetized horses.

Accuracy of different oxygenation indices in estimating intrapulmonary shunting at increasing infusion rates of dobutamine in horses under general anaesthesia

The aim of this study was to evaluate the correlation of commonly used oxygenation indices with venous admixture (Qs/Qt) in anaesthetised horses under different infusion rates of dobutamine. Six female horses were anaesthetised with acepromazine, xylazine, diazepam, ketamine, and isoflurane, and then intubated and mechanically ventilated with 100% O2. A Swan–Ganz catheter was introduced into the left jugular vein and its tip advanced into the pulmonary artery. Horses received different standardised rates of dobutamine.For each horse, eight samples of arterial and mixed venous blood were simultaneously obtained at fixed times. Arterial and venous haemoglobin (Hb) concentration and O2 saturation, arterial oxygen partial pressure (PaO2), venous oxygen partial pressure (PvO2), and barometric pressure were measured. Arterial (CaO2), mixed venous (CvO2), and capillary (Cc′O2) oxygen contents were calculated using standard formulae. The correlations between F-shunt, arterial oxygen tension to fraction of inspired oxygen ratio (PaO2/FiO2), arterial to alveolar oxygen tension ratio (PaO2/PAO2), alveolar to arterial oxygen tension difference (P[A − a]O2), and respiratory index (P[A − a]O2/PaO2) were tested with linear regression analysis. The goodness-of-fit for each calculated formula was evaluated by means of the coefficient of determination (r2). The agreement between Qs/Qt and F-shunt was analysed with the Bland–Altman test.All tested oxygen tension-based indices were weakly correlated (r2 < 0.2) with the Qs/Qt, whereas F-shunt showed a stronger correlation (r2 = 0.73). F-shunt also showed substantial agreement with Qs/Qt independent of the dobutamine infusion rate. F-shunt better correlated with Qs/Qt than other oxygen indices in isoflurane-anaesthetised horses under different infusion rates of dobutamine.

Comparison of cardiovascular function and quality of recovery in isoflurane‐anaesthetised horses administered a constant rate infusion of lidocaine or lidocaine and medetomidine during elective surgery

The effects of lidocaine combined with medetomidine or lidocaine alone on cardiovascular function during anaesthesia and their effects on recovery have not been thoroughly investigated in isoflurane-anaesthetised horses. To determine the effects of an intraoperative i.v. constant rate infusion of lidocaine combined with medetomidine (Group 1) or lidocaine (Group 2) alone on cardiovascular function and on the quality of recovery in 12 isoflurane-anaesthetised horses undergoing arthroscopy. The combination would depress cardiovascular function but improve the quality of recovery when compared to lidocaine alone in isoflurane-anaesthetised horses. Lidocaine (2 mg/kg bwt i.v. bolus followed by 50 microg/kg bwt/min i.v.) or lidocaine (same dose) and medetomidine (5 microg/kg bwt/h i.v.) was started 30 min after induction of anaesthesia. Lidocaine administration was discontinued 30 min before the end of surgery in both groups, whereas medetomidine administration was continued until the end of surgery. Cardiovascular function and quality of recovery were assessed. Horses in Group 1 had longer recoveries, which were of better quality due to better strength and overall attitude during the recovery phase than those in Group 2. Arterial blood pressure was significantly higher in Group 1 than in Group 2 and this effect was associated with medetomidine. No significant differences in cardiac output, arterial blood gases, electrolytes and acid-base status were detected between the 2 groups. The combination of an intraoperative constant rate infusion of lidocaine and medetomidine did not adversely affect cardiovascular function in isoflurane-anaesthetised horses and improved the quality of recovery when compared to an intraoperative infusion of lidocaine alone.

Pulmonary gas exchange in anaesthetised horses mechanically ventilated with oxygen or a helium/oxygen mixture

It is unknown whether administration of gas-mixtures high in inspired fraction of oxygen (FiO2) under general anaesthesia may increase formation of pulmonary atelectasis and impair gas exchange. To evaluate the effects of different FiO2 on pulmonary gas exchange in isoflurane-anaesthetised horses breathing a helium/oxygen (He/O2) mixture. Thirty healthy mature horses were sedated with i.v. acepromazine (0.02 mg/kg bwt), detomidine (0.002 mg/kg bwt) and xylazine (02-0.4 mg/kg bwt). General anaesthesia was induced with i.v. 5% guaifenesin to effect, diazepam (0.1 mg/kg bwt) and ketamine (2 mg/kg bwt), and maintained with isoflurane. Fifteen horses (Group HX) were ventilated mechanically with gas mixtures of successively increasing FiO2 (0.25-030, 0.50-0.55, > 0.90), obtained by blending 02 with Heliox (70% He/30% O2). The other 15 horses (Group O) were ventilated immediately with 100% O2 (FiO2 > 0.90). After 20 min of ventilation at the different FiO2 levels in Group HX and after 60 min in Group O, PaO2 and PaCO2 were measured and the alveolar to arterial PO2 gradient (P(A-a)O2) was calculated. Data analysis included robust categorical regression with clustering on horse (P < 0.05). Inhalation of a He/O2 mixture with FiO2 as low as 0.25-030 ensured adequate arterial oxygenation and was associated with a smaller P(A-a)O2 gradient than inhalation of pure O2 (P < 0.05). In Group HX, PaO2 increased with each rise in FiO2 and so did P(A-a)O2 (P < 0.05). The PaO2 was significantly lower and the P(A-a)O2 higher in Group O compared to Group HX at a FiOz >0.90 (P < 0.05). Administration of a He/O2 gas mixture low in FiO2 can better preserve lung function than ventilation with pure oxygen. A step-wise increase of FiO2 using a He/O2 gas mixture might offer advantages with respect to pulmonary gas exchange over an immediate exposure to 100% 2O2.

Effects of dobutamine on cardiac index and arterial blood pressure in isoflurane‐anaesthetized horses under clinical conditions

Volatile agent-induced hypotension may contribute to anaesthetic-related morbidity and mortality in horses. Dobutamine is commonly used to support arterial blood pressure (ABP) but little is known about its cardiovascular effects under clinical conditions. The aim of this clinical study was to elucidate the relationship between cardiovascular function and dobutamine infusion in isoflurane-anaesthetized horses. Forty-four horses anaesthetized for a variety of surgical procedures were studied. Premedication with acepromazine, methadone and detomidine was followed by induction of anaesthesia with ketamine and midazolam. Anaesthesia was maintained with isoflurane vaporized in oxygen. Routine anaesthetic monitoring was applied and cardiac output was measured by lithium dilution. Dobutamine was infused to maintain mean ABP above 70 mmHg. The relationship between dobutamine infusion rate, heart rate (HR), ABP and cardiac index was investigated immediately prior to (T(0)) and 15 min (T(1)) after dobutamine infusion started, followed at 30 min intervals (T(2), etc.). Arterial blood pressure increased significantly after dobutamine infusion started, HR and cardiac index increased significantly only with dobutamine infusion in combination with surgical stimulus. Although isoflurane decreases blood pressure mainly by vasodilation, dobutamine is an effective treatment for hypotension under clinical conditions in isoflurane-anaesthetized horses. The effect of dobutamine is not directly proportional to dose and surgical stimulus probably contributes to the cardiovascular improvement.

Observations on the muscle relaxant rocuronium bromide in the horse – a dose–response study

ObjectiveTo investigate the onset and duration of neuromuscular blockade of rocuronium bromide and its associated haemodynamic effects at three doses in healthy horses. Study designProspective, randomized experimental study. AnimalsSeven adult horses aged 3–20 (mean 10.3) years and weighing 466 ± 44 (mean ± SD) kg. MethodsHorses were anaesthetized three times with at least 2 weeks between. They were pre-medicated with 0.6 mg kg−1 xylazine and 0.01 mg kg−1 butorphanol IV. Anaesthesia was induced with 2.2 mg kg−1 ketamine and 0.1 mg kg−1 diazepam IV. Following orotracheal intubation anaesthesia was maintained with isoflurane in 100% oxygen. Intermittent positive pressure ventilation was initiated and the horses were ventilated at a respiratory rate (fr) of 4–8 breaths minute−1. Neuromuscular function was monitored with an acceleromyograph. The peroneal nerve was stimulated with train-of-four (TOF) mode at 2 Hz every 15 seconds. Each horse received, in randomly assigned order, one of the three doses of rocuronium: 0.2 mg kg−1 (D02), 0.4 mg kg−1 (D04) or 0.6 mg kg−1 (D06) IV. Lag time, onset time, time of no response, duration of action and the TOF ratio 0.7 and 0.9 were measured. Recovery time (T125−75) was calculated. Vital parameters were recorded at 5-minute intervals on a standard anaesthetic record form. ResultsRocuronium produced a dose-dependent duration of action in isoflurane-anaesthetized horses. 100% block was observed in D04 and D06 but not in D02, in which the maximum decrease of the first twitch of TOF attained was 91.5 ± 16.5%. Time to T125 was 13.1 ± 5.5 minutes, 38.6 ± 10.1 minutes and 55 ± 9.8 minutes in D02, D04 and D06 respectively. There was a significantly shorter time for TOFR 0.9 with 0.2 mg kg−1 compared with 0.4 and 0.6 mg kg−1 rocuronium. T125−75 in D04 and D6 was not statistically significantly different. Heart rate, systolic, diastolic and mean arterial blood pressure increased slightly during the observation period. ConclusionRocuronium is an effective nondepolarizing muscle relaxant in horses under isoflurane anaesthesia. It had a dose-dependent onset and duration of action. Rocuronium did not produce significant changes in the measured cardiovascular parameters.

Influence of general anaesthesia on the pharmacokinetics of intravenous fentanyl and its primary metabolite in horses

In order to evaluate its potential as an adjunct to inhalant anaesthesia in horses, the pharmacokinetics of fentanyl must first be determined. To describe the pharmacokinetics of fentanyl and its metabolite, N-[1-(2-phenethyl-4-piperidinyl)maloanilinic acid (PMA), after i.v. administration of a single dose to horses that were awake in Treatment 1 and anaesthetised with isoflurane in Treatment 2. A balanced crossover design was used (n = 4/group). During Treatment 1, horses received a single dose of fentanyl (4 microg/kg bwt, i.v.) and during Treatment 2, they were anaesthetised with isoflurane and maintained at 1.2 x minimum alveolar anaesthetic concentration. After a 30 min equilibration period, a single dose of fentanyl (4 microg/kg bwt, i.v.) was administered to each horse. Plasma fentanyl and PMA concentrations were measured at various time points using liquid chromatography-mass spectrometry. Anaesthesia with isoflurane significantly decreased mean fentanyl clearance (P < 0.05). The fentanyl elimination half-life, in awake and anaesthetised horses, was 1 h and volume of distribution at steady state was 0.37 and 0.26 l/kg bwt, respectively. Anaesthesia with isoflurane also significantly decreased PMA apparent clearance and volume of distribution. The elimination half-life of PMA was 2 and 1.5 h in awake and anaesthetised horses, respectively. Pharmacokinetics of fentanyl and PMA in horses were substantially altered in horses anaesthetised with isoflurane. These pharmacokinetic parameters provide information necessary for determination of suitable fentanyl loading and infusion doses in awake and isoflurane-anaesthetised horses.

Electroencephalographic and cardiovascular variables as nociceptive indicators in isoflurane-anaesthetized horses

ObjectiveTo evaluate Fourier-transformed electroencephalographic (EEG) variables, mean arterial blood pressure (MAP) and pulse rate as nociceptive indicators in isoflurane-anaesthetized horses. AnimalsFive standardbred and three Norwegian cold-blooded trotter stallions undergoing castration, aged 2–4 years, mass 378–538 kg. Materials and methodsAll horses received intravenous (IV) detomidine (10 μg kg−1 IV) and butorphanol (0.01 mg kg−1 IV). Additional detomidine (4 μg kg−1 IV) was administered in the induction area. Anaesthesia was induced with ketamine (2.5 mg kg−1 IV) and diazepam (40 μg kg−1 IV), and maintained for 30 minutes with isoflurane (end-tidal concentration of 1.4%) vaporized in oxygen. The electroencephalogram, MAP and pulse rate were recorded for 15 minutes, beginning 5 minutes before skin incision. Differences between the mean values of recordings taken before, and during surgery were calculated and tested for significant differences using a two-sided Student's t-test. ResultsA significant rise in MAP and a fall in pulse rate were found. No significant change was found in any EEG variable. Conclusion/clinical relevanceOf the variables evaluated, MAP seems to be the most sensitive and reliable indicator of nociception in isoflurane-anaesthetized horses.

Muscle Metabolic Changes Associated with Long-term Inhalation Anaesthesia in the Horse Analysed by Muscle Biopsy and Microdialysis Techniques

During anaesthesia in the horse, muscle blood flow has been found to be reduced, possibly leading to hypoxia or ischaemia in the muscle. The aim of this study was to use the muscle biopsy and microdialysis techniques to determine whether long-term inhalation anaesthesia in laterally recumbent horses induces metabolic changes in gluteal muscle indicative of anaerobic metabolism. Muscle biopsies and plasma samples were taken from seven horses at the start and end of halothane anaesthesia. In six isoflurane-anaesthetised horses, given three pharmacological provocations (dobutamine, detomidine, acepromazine), repeated blood samples and microdialysis was performed during anaesthesia and muscle biopsies were taken before and at the end of anaesthesia. Adenosine triphosphate (ATP), adenosine diphosphate, adenosine monophosphate, inosine monophosphate (IMP) creatine phosphate and lactate concentrations did not differ between dependent and non-dependent muscles at either sampling time. Creatine phosphate decreased in both the halothane (-38%) and isoflurane (-28%) group. In the halothane group, ATP was decreased (-15%) at the end of anaesthesia, while IMP was increased (+32%). Lactate in muscle and plasma increased in both groups. Lactate in dialysate increased after induction and remained elevated above plasma concentrations. These results show that long-term inhalation anaesthesia in horses is associated with an anaerobic metabolic response within the muscle and that microdialysis can be used to detect metabolic changes within the muscle during equine anaesthesia.

The effects of ventilation on intercranial and cerebral perfusion pressures in isoflurane-anaesthetized horses

The effects of ventilation on intercranial and cerebral perfusion pressures in isoflurane-anaesthetized horses