Lynette Bester

Sedative-hypnotic effects of midazolam in goats after intravenous and intramuscular administration

OBJECTIVE To examine the effect of dose and route of administration on the sedative-hypnotic effects of midazolam. DESIGN Prospective randomized controlled study ANIMALS: Six indigenous, African bred goats. METHODS Pilot studies indicated that the optimum dose of midazolam for producing sedation was 0.6 mg kg-1 for intramuscular (IM) injection, while the optimum intravenous (IV) doses causing hypnosis without, and with loss of palpebral reflexes were 0.6 mg kg-1 and 1.2 mg kg-1, respectively. These doses and routes of administration were compared with a saline placebo in a randomized block design in the main experiment, and the sedative-hypnotic effects evaluated according to pre-determined scales. RESULTS Intramuscular midazolam produced sedation with or without sternal recumbency in all animals with the peak effect occurring 20 minutes after administration. The scores for IM sedation with midazolam were significantly different (p < 0.05) from placebo. Intravenous midazolam at 0.6 mg kg-1 resulted in hypnosis, and at 1.2 mg kg-1 increased reflex suppression was observed. The maximum scores for hypnosis at both doses were obtained 5 minutes after IV injection. The mean (± SD) duration of lateral recumbency was 10.8 (± 3.8) minutes after IV midazolam (0.6 mg kg-1) compared to 20 (± 5.2) minutes after midazolam at 1.2 mg kg-1. Compared to baseline, the heart rate increased significantly (p < 0.05) after high dose IV midazolam. CONCLUSION Intramuscular midazolam (0.6 mg kg-1) produced maximum sedation 20 minutes after injection. Intravenous injection produced maximum hypnosis within 5 minutes. Increasing the IV dose from 0.6 to 1.2 mg kg-1 resulted in increased reflex suppression and duration of hypnosis. CLINICAL RELEVANCE For a profound effect with rapid onset midazolam should be given IV in doses between 0.6 and 1.2 mg kg-1.

Preliminary investigation into the ventilatory effects of midazolam in isoflurane-anaesthetised goats

The ventilatory effects of intravenous midazolam (MDZ) were evaluated in isoflurane- anaesthetised goats. Eight female goats aged 2-3 years were fasted from food and water for 12 h. Anaesthesia was then induced using a face mask with isoflurane in oxygen, whilst the trachea was intubated with a cuffed tracheal tube and anaesthesia maintained with isoflurane at 1.5% end-tidal concentration. Ventilation was spontaneous. The goats were treated with either a saline placebo (PLC) or MDZ intravenously at 0.2 mg/kg. Analysis of variance for repeated measures was used for the analysis of data. Significance was taken at the 0.05 level. Differences between treatments were not statistically significant (p > 0.05) for tidal volume, ventilation rate, tidal volume/kg (VT/kg) and end-tidal carbon dioxide partial pressure. Within treatments, VT and VT/kg differed 5 min after MDZ administration; this was statistically significant (p < 0.05). The occurrence of apnoea in the MDZ-treated goats was statistically significant (p = 0.04) compared with the PLC treated goats. Intravenous MDZ at 0.2 mg/kg administered to isoflurane-anaesthetised goats may result in transient apnoea and a mild decrease in VT and VT/kg.

Anaesthetic induction and recovery characteristics of a diazepam-ketamine combination compared with propofol in dogs

Induction of anaesthesia occasionally has been associated with undesirable behaviour in dogs. High quality of induction of anaesthesia with propofol has been well described while in contrast variable induction and recovery quality has been associated with diazepam-ketamine. In this study, anaesthetic induction and recovery characteristics of diazepam-ketamine combination with propofol alone were compared in dogs undergoing elective orchidectomy. Thirty-six healthy adult male dogs were used. After habitus scoring (simple descriptive scale [SDS]), the dogs were sedated with morphine and acepromazine. Forty minutes later a premedication score (SDS) was allocated and general anaesthesia was induced using a combination of diazepam-ketamine (Group D/K) or propofol (Group P) and maintained with isoflurane. Scores for the quality of induction, intubation and degree of myoclonus were allocated (SDS). Orchidectomy was performed after which recovery from anaesthesia was scored (SDS) and times to extubation and standing were recorded. Data were analysed using descriptive statistics and Kappa Reliability and Kendall Tau B tests. Both groups were associated with acceptable quality of induction and recovery from anaesthesia. Group P, however, was associated with a poorer quality of induction (p = 0.014), prolonged induction period (p = 0.0018) and more pronounced myoclonus (p = 0.003), but had better quality of recovery (p = 0.000002) and shorter recovery times (p = 0.035) compared with Group D/K. Diazepam-ketamine and propofol are associated with acceptable induction and recovery from anaesthesia. Propofol had inferior anaesthetic induction characteristics, but superior and quicker recovery from anaesthesia compared with diazepam-ketamine.

Determination of the minimum infusion rate of alfaxalone during its co-administration with midazolam in goats.

Introduction The minimum infusion rate (MIR) of alfaxalone when co-administered with midazolam in goats was evaluated. Materials and methods Eight goats (four does and four wethers) were anaesthetised, on separate occasions, with alfaxalone at an initial dose of 9.6 mg/kg/hour combined with one of three midazolam treatments: a bolus of 0.1 mg/kg followed by constant rate infusion (CRI) of 0.1 mg/kg/hour (treatment LMID), 0.3 mg/kg followed by CRI of 0.3 mg/kg/hour (MMID), 0.9 mg/kg followed by CRI of 0.9 mg/kg/hour (HMID), intravenously. Responses to stimulation (clamping on the proximal part of one digit of the hoof with Vulsellum forceps for 60 seconds) were tested every 30 minutes. In the absence or presence of a response to stimulation, the infusion rate was reduced or increased by 1.9 mg/kg/hour. Alfaxalone MIR was calculated as the mean of the infusion rates that allowed and abolished movement. Cardiopulmonary parameters were measured. Results Alfaxalone MIR was 6.7 (6.7–8.6) mg/kg/hour, 6.7 (4.8–6.7) mg/kg/hour and 2.9 (1.0–4.8) mg/kg/hour for LMID, MMID and HMID respectively. Cardiopulmonary function was minimally affected, with hypoxaemia observed two minutes into anaesthesia during all treatments. Recovery from anaesthesia was excitement-free. Conclusions Midazolam causes a dose-dependent reduction of alfaxalone MIR in goats. Oxygen supplementation is recommended during anaesthesia with alfaxalone and midazolam in goats.

Use of near-infrared spectroscopy to identify trends in regional cerebral oxygen saturation in horses

REASONS FOR PERFORMING STUDY Alterations in cerebral haemodynamics may contribute to perianaesthetic complications in horses. Near-infrared spectroscopy (NIRS) is frequently used intraoperatively in man to provide information regarding cerebral perfusion. OBJECTIVES To determine whether NIRS can identify trends in regional cerebral oxygen saturation (rSO2) in horses and whether there is a correlation between rSO2 and venous oxygen tensions. METHODS A cerebral oximeter sensor recorded rSO2 from the dorsal sagittal sinus of 6 healthy horses. Values for rSO2, arterial and venous oxygen and carbon dioxide tensions (PaO2, PvO2, PaCO2 and PvCO2 respectively), along with arteriovenous oxygen saturations (SavO2) were recorded in unsedated (recording period [RP] 1), sedated (RP2) and anaesthetised horses (RP3-5) and during recovery (RP6-8). During anaesthesia, horses were ventilated to achieve states of normo- (RP3), hyper- (RP4) and hypocapnoea (RP5). Data were evaluated descriptively and analysed using linear mixed-effects models and Pearsons correlation coefficient. RESULTS Overall mean ± s.d. values for rSO2, PaO2, PvO2, PaCO2, SavO2 and mean arterial pressure varied significantly by RP (P<0.001). Significant decreases in rSO2 were identified between RP1 and the post anaesthetic periods (P<0.001). No significant differences in rSO2 values were identified between RP1 and the intra-anaesthesia periods or between RP3, RP4 and RP5. Significant correlations were identified between rSO2 and PaO2 (r = 0.448, P<0.001), rSO2 and PvO2 (r = 0.512, P<0.001) and rSO2 and SavO2 (r = 0.469, P<0.001). CONCLUSIONS This is the first study to identify trends in rSO2 in horses using NIRS. A positive correlation was identified between rSO2 and PvO2, suggesting that alterations in cerebral oxygenation may be reflected in PvO2 . POTENTIAL RELEVANCE Near-infrared spectroscopy may be used to monitor trends in rSO2 during equine anaesthesia. Decreasing rSO2 values may act as an early warning signal, alerting clinicians to potential cerebral desaturation events and indicating a need for intervention.