To assess the antinociceptive efficacy and safety of neuraxial morphine in inland bearded dragons (Pogona vitticeps).
10 healthy adult bearded dragons.
Animals were sedated with alfaxalone (15 mg/kg) SC prior to neuraxial injections. In a randomized, blinded, placebo-controlled, crossover design, animals received preservative-free morphine (0.5 mg/kg) combined with lidocaine (2 mg/kg) or lidocaine (2 mg/kg) only (control treatment). For both treatments, saline (0.9% NaCl) solution was used for dilution to a total volume of 0.3 mL/kg. If the initial injection did not result in motor block of the pelvic limbs or cloaca relaxation within 10 minutes, a second injection was performed. Measurements consisted of bilateral mechanical stimulation of the limbs and at 25%, 50%, and 75% of the trunk’s length as well as cloacal tone to assess spread and duration of motor block. Pelvic limb withdrawal latencies in response to a thermal noxious stimulus were measured over a 48-hour period to assess antinociception.
Success rate following the first injection was 90% (18/20 injections) and increased to 100% following a second injection. Motor block occurred within 5 minutes with both treatments. Pelvic limb withdrawal latencies were significantly prolonged following neuraxial morphine versus control treatment for at least 12 hours after injection. By 24 hours, no effect of morphine on pelvic limb latencies was detectable.
These results demonstrated that neuraxial administration of morphine results in regional antinociceptive effects for at least 12 hours and has no clinically relevant adverse effects in healthy bearded dragons. This technique has potential for providing regional analgesia in this species.
Objective—To evaluate the effects of epidural administration of 3 doses of dexmedetomidine on isoflurane minimum alveolar concentration (MAC) and characterize changes in bispectral index (BIS) induced by nociceptive stimulation used for MAC determination in dogs.
Animals—6 adult dogs.
Procedures—Isoflurane-anesthetized dogs received physiologic saline (0.9% NaCl) solution (control treatment) or dexmedetomidine (1.5 [DEX1.5], 3.0 [DEX3], or 6.0 [DEX6] μg/kg) epidurally in a crossover study. Isoflurane MAC (determined by use of electrical nociceptive stimulation of the hind limb) was targeted to be accomplished at 2 and 4.5 hours. Changes in BIS attributable to nociceptive stimulation and cardiopulmonary data were recorded at each MAC determination.
Results—With the control treatment, mean ± SD MAC values did not change over time (1.57 ± 0.23% and 1.55 ± 0.25% at 2 and 4.5 hours, respectively). Compared with the control treatment, MAC was significantly lower at 2 hours (13% reduction) but not at 4.5 hours (7% reduction) in DEX1.5-treated dogs and significantly lower at 2 hours (29% reduction) and 4.5 hours (13% reduction) in DEX3-treated dogs. The DEX6 treatment yielded the greatest MAC reduction (31% and 22% at 2 and 4.5 hours, respectively). During all treatments, noxious stimulation increased BIS; but changes in BIS were correlated with increases in electromyographic activity.
Conclusions and Clinical Relevance—In dogs, epidural administration of dexmedetomidine resulted in dose-dependent decreases in isoflurane MAC and that effect decreased over time. Changes in BIS during MAC determinations may not represent increased awareness because of the possible interference of electromyographic activity.
Objective—To determine plasma concentrations and behavioral, antinociceptive, and physiologic effects of methadone administered via IV and oral transmucosal (OTM) routes in cats.
Animals—8 healthy adult cats.
Procedures—Methadone was administered via IV (0.3 mg/kg) and OTM (0.6 mg/kg) routes to each cat in a balanced crossover design. On the days of drug administration, jugular catheters were placed in all cats under anesthesia; a cephalic catheter was also placed in cats that received methadone IV. Baseline measurements were obtained ≥ 90 minutes after extubation, and methadone was administered via the predetermined route. Heart and respiratory rates were measured; sedation, behavior, and antinociception were evaluated, and blood samples were collected for methadone concentration analysis at predetermined intervals for 24 hours after methadone administration. Data were summarized and evaluated statistically.
Results—Plasma concentrations of methadone were detected rapidly after administration via either route. Peak concentration was detected 2 hours after OTM administration and 10 minutes after IV administration. Mean ± SD peak concentration was lower after OTM administration (81.2 ± 14.5 ng/mL) than after IV administration (112.9 ± 28.5 ng/mL). Sedation was greater and lasted longer after OTM administration. Antinociceptive effects were detected 10 minutes after administration in both groups; these persisted ≥ 2 hours after IV administration and ≥ 4 hours after OTM administration.
Conclusions and Clinical Relevance—Despite lower mean peak plasma concentrations, duration of antinociceptive effects of methadone was longer after OTM administration than after IV administration. Methadone administered via either route may be useful for perioperative pain management in cats.
Objective—To compare cardiovascular effects of sevoflurane alone and sevoflurane plus an IV infusion of lidocaine in horses.
Animals—8 adult horses.
Procedures—Each horse was anesthetized twice via IV administration of xylazine, diazepam, and ketamine. During 1 anesthetic episode, anesthesia was maintained by administration of sevoflurane in oxygen at 1.0 and 1.5 times the minimum alveolar concentration (MAC). During the other episode, anesthesia was maintained at the same MAC multiples via a reduced concentration of sevoflurane plus an IV infusion of lidocaine. Heart rate, arterial blood pressures, blood gas analyses, and cardiac output were measured during mechanical (controlled) ventilation at both 1.0 and 1.5 MAC for each anesthetic protocol and during spontaneous ventilation at 1 of the 2 MAC multiples.
Results—Cardiorespiratory variables did not differ significantly between anesthetic protocols. Blood pressures were highest at 1.0 MAC during spontaneous ventilation and lowest at 1.5 MAC during controlled ventilation for either anesthetic protocol. Cardiac output was significantly higher during 1.0 MAC than during 1.5 MAC for sevoflurane plus lidocaine but was not affected by anesthetic protocol or mode of ventilation. Clinically important hypotension was detected at 1.5 MAC for both anesthetic protocols.
Conclusions and Clinical Relevance—Lidocaine infusion did not alter cardiorespiratory variables during anesthesia in horses, provided anesthetic depth was maintained constant. The IV administration of lidocaine to anesthetized nonstimulated horses should be used for reasons other than to improve cardiovascular performance. Severe hypotension can be expected in nonstimulated horses at 1.5 MAC sevoflurane, regardless of whether lidocaine is administered.
Objective—To determine effects of a continuous rate infusion of lidocaine on the minimum alveolar concentration (MAC) of sevoflurane in horses.
Animals—8 healthy adult horses.
Procedures—Horses were anesthetized via IV administration of xylazine, ketamine, and diazepam; anesthesia was maintained with sevoflurane in oxygen. Approximately 1 hour after induction, sevoflurane MAC determination was initiated via standard techniques. Following sevoflurane MAC determination, lidocaine was administered as a bolus (1.3 mg/kg, IV, over 15 minutes), followed by constant rate infusion at 50 μg/kg/min. Determination of MAC for the lidocaine-sevoflurane combination was started 30 minutes after lidocaine infusion was initiated. Arterial blood samples were collected after the lidocaine bolus, at 30-minute intervals, and at the end of the infusion for measurement of plasma lidocaine concentrations.
Results—IV administration of lidocaine decreased mean ± SD sevoflurane MAC from 2.42 ± 0.24% to 1.78 ± 0.38% (mean MAC reduction, 26.7 ± 12%). Plasma lidocaine concentrations were 2,589 ± 811 ng/mL at the end of the bolus; 2,065 ± 441 ng/mL, 2,243 ± 699 ng/mL, 2,168 ± 339 ng/mL, and 2,254 ± 215 ng/mL at 30, 60, 90, and 120 minutes of infusion, respectively; and 2,206 ± 329 ng/mL at the end of the infusion. Plasma concentrations did not differ significantly among time points.
Conclusions and Clinical Relevance—Lidocaine could be useful for providing a more balanced anesthetic technique in horses. A detailed cardiovascular study on the effects of IV infusion of lidocaine during anesthesia with sevoflurane is required before this combination can be recommended.
Objective—To assess the effects of ketamine hydrochloride, propofol, or compounded thiopental sodium administration on intraocular pressure (IOP) and qualities of induction of and recovery from anesthesia in horses.
Animals—6 healthy adult horses.
Procedures—Horses were sedated with xylazine hydrochloride (0.5 mg/kg), and anesthesia was induced with guaifenesin followed by ketamine (2 mg/kg), propofol (3 mg/kg), or thiopental (4 mg/kg) in a crossover study with ≥ 1 week between treatments. For each horse, IOP in the right eye was measured with a handheld applanation tonometer before and after xylazine administration, at the time of recumbency, and every 3 minutes after induction of anesthesia until spontaneous movement was observed. Cardiorespiratory responses and venous blood measurements were recorded during anesthesia. Induction of and recovery from anesthesia were subjectively evaluated by investigators who were unaware of the anesthetic treatment of each horse. Data were analyzed via a repeated-measures ANOVA with Holm-Ŝidák post hoc comparisons.
Results—Compared with findings after xylazine administration (mean ± SD, 17 ± 3 mm Hg), thiopental decreased IOP by 4 ± 23%, whereas propofol and ketamine increased IOP by 8 ± 11% and 37 ± 16%, respectively. Compared with the effects of ketamine, propofol and thiopental resulted in significantly lower IOP at the time of recumbency and higher heart rates at 3 minutes after induction of anesthesia. No other significant differences among treatments were found.
Conclusions and Clinical Relevance—These findings support the use of thiopental or propofol in preference to ketamine for horses in which increases in IOP should be minimized.
Objective—To evaluate the effects of increasing doses of remifentanil hydrochloride administered via constant rate infusion (CRI) on the minimum alveolar concentration (MAC) of isoflurane in cats.
Animals—6 healthy adult cats.
Procedures—For each cat, 2 experiments were performed (2-week interval). On each study day, anesthesia was induced and maintained with isoflurane; a catheter was placed in a cephalic vein for the administration of lactated Ringer's solution or remifentanil CRIs, and a catheter was placed in the jugular vein for collection of blood samples for blood gas analyses. On the first study day, individual basal MAC (MACBasal) was determined for each cat. On the second study day, 3 remifentanil CRIs (0.25, 0.5, and 1.0 μg/kg/min) were administered (in ascending order); for each infusion, at least 30 minutes elapsed before determination of MAC (designated as MACR0.25, MACR0.5, and MACR1.0, respectively). A 15-minute washout period was allowed between CRIs. A control MAC (MACControl) was determined after the last remifentanil infusion.
Results—Mean ± SD MACBasal and MACControl values at sea level did not differ significantly (1.66 ± 0.08% and 1.52 ± 0.21%, respectively). The MAC values determined for each remifentanil CRI did not differ significantly. However, MACR0.25, MACR0.5, and MACR1.0 were significantly decreased, compared with MACBasal, by 23.4 ± 7.9%, 29.8 ± 8.3%, and 26.0 ± 9.4%, respectively.
Conclusions and Clinical Relevance—The 3 doses of remifentanil administered via CRI resulted in a similar degree of isoflurane MAC reduction in adult cats, indicating that a ceiling effect was achieved following administration of the lowest dose.