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

OBJECTIVE: To compare cardiac index and oxygen extraction at equivalent depths of anaesthesia between isoflurane-anaesthetized horses and horses anesthetized with isoflurane and dexmedetomidine CRI. STUDY DESIGN: Sequential, blinded, randomized, balanced, crossover study. ANIMALS: Eight horses weighing a mean ± standard deviation of 478 ± 58 kg. METHODS: Horses 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. RESULTS: Cardiac 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 RELEVANCE: Cardiovascular 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.

Antinociceptive effect of buprenorphine and evaluation of the nociceptive withdrawal reflex in foals.

OBJECTIVE: To elicit and evaluate the NWR (nociceptive withdrawal reflex) in 2 and 11 day old foals, to investigate if buprenorphine causes antinociception and determine if the NWR response changes with increasing age. The effect of buprenorphine on behaviour was also evaluated. STUDY DESIGN: Prospective, experimental cross-over trial. ANIMALS: Nine Norwegian Fjord research foals. METHODS: Buprenorphine, 10 µg kg(-1) was administered intramuscularly (IM) to the same foal at 2 days and at 11 days of age. The NWR and the effect of buprenorphine were evaluated by electromyograms recorded from the left deltoid muscle following electrical stimulation of the left lateral palmar nerve at the level of the pastern. Mentation, locomotor activity and respiratory rate were recorded before and after buprenorphine administration. RESULTS: We were able to evoke the NWR and temporal summation in foals using this model. Buprenorphine decreased the root mean square amplitude following single electrical stimulation (p < 0.001) in both age groups, and increased the NWR threshold following single electrical stimulation in 2 day old foals (p = 0.0012). Repeated electrical stimulation at 2 Hz was more effective to elicit temporal summation compared to 5 Hz (p < 0.001). No effect of age upon the NWR threshold was found (p = 0.34). Sedation when left undisturbed (11 occasions), increased locomotor activity when handled (9 occasions) and tachypnea (13 occasions) were common side-effects of buprenorphine. CONCLUSION AND CLINICAL RELEVANCE: These findings indicate that buprenorphine has antinociceptive effect in foals. Opioid side effects often recognized in adult horses also occur in foals.