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Abstract

Objective—To determine whether opioids with varying interactions at receptors induce a reduction in minimum alveolar concentration (MAC) of isoflurane in cats.

Animals—12 healthy, female, spayed cats.

Procedure—Cats were anesthetized with isoflurane and instrumented to allow collection of arterial blood and measurement of arterial blood pressure. Each drug was studied separately, and for each drug cats were randomly allocated to receive 2 doses. The drugs studied were morphine (0.1 or 1.0 mg/kg), butorphanol (0.08 or 0.8 mg/kg), buprenorphine (0.005 and 0.05 mg/kg), and U50488H (0.02 and 0.2 mg/kg). All drugs were diluted in 5 ml of saline (0.9% NaCl) solution and infused IV for 5 minutes. The MAC of isoflurane was determined in triplicate, the drug administered, and the MAC of isoflurane redetermined for a period of 3 hours.

Results—All drugs had a significant effect on MAC over time. With morphine only, the effect on MAC over time was different between doses. The greatest mean (± SD) reductions in MAC of isoflurane in response to morphine, butorphanol, buprenorphine, and U50488H administration were 28 ± 9, 19 ± 3, 14 ± 7, and 11 ± 7%, respectively.

Conclusions and Clinical Relevance—Morphine (1.0 mg/kg) and butorphanol (0.08 and 0.8 mg/kg) induced significant reductions in MAC of isoflurane that were considered clinically important. Although significant, reductions in MAC of isoflurane induced by morphine (0.1 mg/kg), buprenorphine (0.005 and 0.05 mg/kg), and U50488H (0.02 and 0.2 mg/kg) were not considered clinically relevant because they fell within the error of the measurement technique. Administration of morphine or butorphanol decreases the need for potent inhalant anesthetics in cats and could potentially be beneficial in combination with inhalants. (Am J Vet Res 2002;63:1198–1202)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To determine the minimum infusion rate (MIR50) for propofol alone and in combination with ketamine required to attenuate reflexes commonly used in the assessment of anesthetic depth in cats.

Animals—6 cats.

Procedure—Propofol infusion started at 0.05 to 0.1 mg/kg/min for propofol alone or 0.025 mg/kg/min for propofol and ketamine (low-dose [LD] constant rate infusion [CRI] of 23 µg/kg/min or high-dose [HD] CRI of 46 µg/kg/min), and after 15 minutes, responses of different reflexes were tested. Following a response, the propofol dose was increased by 0.05 mg/kg/min for propofol alone or 0.025 mg/kg/min for propofol and ketamine, and after 15 minutes, reflexes were retested.

Results—The MIR50 for propofol alone required to attenuate blinking in response to touching the medial canthus or eyelashes; swallowing in response to placement of a finger or laryngoscope in the pharynx; and to toe pinch, tetanus, and tail-clamp stimuli were determined. Addition of LD ketamine to propofol significantly decreased MIR50, compared with propofol alone, for medial canthus, eyelash, finger, toe pinch, and tetanus stimuli but did not change those for laryngoscope or tail-clamp stimuli. Addition of HD ketamine to propofol significantly decreased MIR50, compared with propofol alone, for medial canthus, eyelash, toe pinch, tetanus, and tail-clamp stimuli but did not change finger or laryngoscope responses.

Conclusions and Clinical Relevance—Propofol alone or combined with ketamine may be used for total IV anesthesia in healthy cats at the infusion rates determined in this study for attenuation of specific reflex activity. ( Am J Vet Res 2003;64:907–912)

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in American Journal of Veterinary Research

SUMMARY

Objective

To evaluate effect of incremental doses of alfentanil on isoflurane minimum alveolar concentration (MAC) in cats to determine whether alfentanil reduces isoflurane MAC and, if so, maximal isoflurane MAC reduction.

Animals

6 healthy spayed female cats.

Procedure

Cats were anesthetized with isoflurane and instrumented to allow collection of arterial blood for measurement of gas tensions, pH, and plasma alfentanil concentration and to measure arterial blood pressure. Isoflurane MAC was determined in triplicate, and alfentanil was administered IV, using a computer-driven syringe pump to achieve estimated plasma alfentanil concentrations of 50, 100, 250, 500, 750, and 1,000 ng/ml; isoflurane MAC was determined at each alfentanil concentration. Cats were allowed to recover, and the process was graded as poor, good, or excellent.

Results

Alfentanil had a significant dose effect on isoflurane MAC reduction. Significant regression was found for normalized isoflurane MAC versus estimated plasma alfentanil concentration. A quadratic term was necessary to fit the model and, using this curve, MAC reduction (35.0 ± 6.6%) was estimated to be maximal at a plasma alfentanil concentration of 500 ng/ml. Significant differences were evident in rectal temperature, bicarbonate concentration, base deficit, arterial carbon dioxide and oxygen tensions, and arterial pH between isoflurane alone and some plasma alfentanil concentration and the corresponding reduction in isoflurane concentration.

Conclusions

Infusion of alfentanil resulted in maximal MAC reduction midway between that reported for horses and dogs. At such plasma alfentanil concentration, adverse effects were minimal, but included increase in rectal temperature, metabolic acidosis, and decrease in Pao2 . Provided cats were not handled during the recovery period, recovery was smooth and quiet.

Clinical Relevance

Infusion of alfentanil decreases the need for potent inhalant anesthetics in cats and could potentially be a clinically useful anesthetic regimen in sick cats. (Am J Vet Res 1997;58:1274–1279)

Free access
in American Journal of Veterinary Research

SUMMARY

Objective

To determine whether administration of opioids to anesthetized cats induced less cardiovascular depression than that induced by an equivalent amount of anesthetic alone, and to measure endocrine responses to a noxious stimulus.

Animals

6 healthy female cats.

Procedure

Anesthesia was induced with isoflurane and was maintained for 60 minutes at 1.3 isoflurane MAC. Blood gas tensions, pH, and plasma alfentanil and hormone concentrations, blood pressures, and cardiac output were measured. A noxious stimulus was applied for 5 minutes, while blood acquisition and measurements were repeated. Alfentanil was administered IV to achieve estimated plasma concentration of 500 ng/ml, and end-tidal isoflurane concentration was reduced by 35%. After another 60 minutes, blood was obtained and measurements were taken, then a second 5-minute noxious stimulus was applied while blood acquisition and measurements were retaken.

Results

Alfentanil administration and reduction of isoflurane concentration significantly increased body temperature, heart rate, mean arterial pressure, mean pulmonary arterial pressure, stroke index, cardiac index, hemoglobin, oxygen delivery index, Pvo2 and Pvco2 , dopamine, epinephrine (EPI), norepinephrine (NOREPI), and cortisol values, and significantly decreased arterial and venous pH. Application of a noxious stimulus significantly increased heart rate, stroke index, cardiac index, Pao2 , oxygen delivery index, arterial and venous pH, and NOREPI values, and decreased bicarbonate, Paco2 , Pvco2 , and EPI values. Alfentanil administration blunted cardiac index, Paco2 , oxygen delivery index, arterial pH, Pao2 , and EPI, and NOREPI responses to a noxious stimulus.

Conclusions

Compared with isoflurane alone, alfentanil administration and reduction of isoflurane MAC improved cardiovascular variables, and blunted respiratory, hormonal, and most hemodynamic responses to a noxious stimulus in cats.

Clinical Relevance

Use of the balanced opioid anesthesia regimen induced some beneficial effects in healthy cats; effects were similar to, although greater in nature, than effects induced by a noxious stimulus. (Am J Vet Res 1997;58:1267–1273)

Free access
in American Journal of Veterinary Research

SUMMARY

Atracurium besylate, a nondepolarizing neuromuscular blocking agent, was administered as an infusion to 8 anesthetized cats in which neuromuscular blockade was assessed, using the train-of-four response. Once 50% depression of the first-twitch (T1) response was achieved, the infusion was held constant for 60 minutes before being discontinued and the recovery time was determined. The time for recovery was recorded as the time for the train-of-four ratio (T4 ratio) to increase from 50% to 75%. After-recovery, atracurium infusion was reinstituted and the cats were again maintained for 60 minutes at 50% depression. A single bolus of gentamicin sulfate (2.0 mg/kg of body weight) was administered iv, and the infusion was continued for another 60 minutes before it was discontinued and the time for recovery was recorded.

Within 1 minute of gentamicin administration, the mean ± SD T1 response decreased from 49 ± 5% to 33 ± 8% of baseline and the T4 ratio decreased from 28 ± 19% to 14 ± 11%. Peak effect occurred at 5 minutes, with a T1 response of 29 ± 6% of baseline and a T4 ratio of 13 ± 12%. By 60 minutes after gentamicin administration, the T1 response had increased to 38 ± 7% of baseline and the T4 ratio had increased to 21 ± 13%. The time for recovery significantly (P < 0.03) increased from 9.9 ± 3.4 minutes during the control study to 18.1 ± 10.7 minutes during the gentamicin study.

In this study, gentamicin potentiated the neuromuscular blockade induced by atracurium and increased the recovery time. Residual blockade, observed after gentamicin administration was reversed with edrophonium.

Free access
in American Journal of Veterinary Research

SUMMARY

The central arterial pharmacokinetics of alfentanil, a short-acting opioid agonist, were studied in rabbits, sheep, and dogs after short-duration infusion of the drug. Alfentanil was infused until a set end point (high-amplitude, slow-wave activity on the eeg) was reached. This required a larger alfentanil dose and a higher alfentanil arterial concentration in sheep, compared with rabbits and dogs. The plasma concentration-time data for each animal were fitted, using nonlinear regression, and in all animals, were best described by use of a triexponential function. In this study, differences in the disposition kinetics of alfentanil among the 3 species were found for only distribution clearance and initial distribution halflife. In dogs, compared with rabbits and sheep, the first distribution half-life was longer, probably because of pronounced drug-induced bradycardia (mean ± SD, 48 ± 21 beats/min). Distribution clearance was faster in sheep, compared with dogs, also probably because of better blood flow in sheep. Elimination half-life was similar in all species (rabbits, 62.4 ± 11.3 minutes; sheep, 65.1 ± 27.1 minutes; dogs, 58.3 ± 10.3 minutes). This rapid half-life resulted from a small steady-state volume of distribution (rabbits, 908.3 ± 269.0 ml/kg; sheep, 720.0 ± 306.7 ml/ kg; dogs, 597.7 ± 290.2 ml/kg) and rapid systemic clearance (rabbits, 19.4 ± 5.3 ml/min/kg; sheep, 13.3 ± 3.0 ml/min/kg; dogs, 18.7 ± 7.5 ml/min/kg). On the basis of these pharmacokinetic variables, alfentanil should have short duration of action in rabbits, sheep, and dogs. This may be beneficial in veterinary practice where rapid recovery would be expected after bolus administration for short procedures or after infusion for longer procedures.

Free access
in American Journal of Veterinary Research

Abstract

Objective—To characterize the effects of ketamine administration on the cardiovascular and respiratory systems and on acid-base balance and to record adverse effects of ketamine in isoflurane-anesthetized dogs.

Animals—6 healthy adult mongrel dogs.

Procedure—Dogs were anesthetized with isoflurane (1.25 times the individual minimum alveolar concentration) in oxygen, and ketamine was administered IV to target pseudo–steady-state plasma concentrations of 0, 0.5, 1, 2, 5, 8, and 11 µg/mL. Isoflurane concentration was reduced to an equipotent concentration. Cardiovascular, respiratory, and acid-base variables; body temperature; urine production; and adverse effects were recorded before and during noxious stimulation. Cardiac index, stroke index, rate-pressure product, systemic vascular resistance index, pulmonary vascular resistance index, left ventricular stroke work index, right ventricular stroke work index, arterial oxygen concentration, mixed-venous oxygen concentration, oxygen delivery, oxygen consumption, oxygen extraction ratio, alveolar-arterial oxygen partial pressure gradient, and venous admixture were calculated. Plasma ketamine and norketamine concentrations were measured.

Results—Overall, ketamine administration improved ventilation, oxygenation, hemodynamics, and oxygen delivery in isoflurane-anesthetized dogs in a dosedependent manner. With the addition of ketamine, core body temperature was maintained or increased and urine production was maintained at an acceptable amount. However, at the higher plasma ketamine concentrations, adverse effects such as spontaneous movement and profuse salivation were observed. Myoclonus and dysphoria were observed during recovery in most dogs.

Conclusions and Clinical Relevance—Infusion of ketamine appears to be a suitable technique for balanced anesthesia with isoflurane in dogs. Plasma ketamine concentrations between 2 to 3 µg/mL elicited the most benefits with minimal adverse effects. (Am J Vet Res 2005;66:2122–2129)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To determine the thermal antinociceptive effect of oral administration of tramadol hydrochloride at doses between 0.5 and 4 mg/kg in cats.

Animals—6 healthy adult domestic shorthair cats.

Procedures—Baseline (before drug administration; time 0) thermal threshold was determined by applying a thermal probe to the thorax of each cat. Tramadol (0.5, 1, 2, 3, or 4 mg/kg) or a placebo was then administered orally in accordance with a Latin square design. Thermal threshold was determined by an observer who was unaware of treatment at various times until thermal threshold returned to baseline values or 6 hours had elapsed. Plasma tramadol and O-desmethyl-tramadol concentrations were measured prior to drug administration and at 1-hour intervals thereafter. Effect-concentration data were fitted to effect maximum models.

Results—Highest plasma tramadol and O-desmethyl-tramadol concentrations increased with increasing tramadol dose. Significant effects of dose and time on thermal threshold were detected. Thermal threshold was significantly higher than the baseline value at 80 and 120 minutes for the 0.5 mg/kg dose, at 80 and from 120 to 360 minutes for the 2 mg/kg dose, from 40 to 360 minutes for the 3 mg/kg dose, and from 60 to 360 minutes for the 4 mg/kg dose.

Conclusions and Clinical Relevance—Tramadol induced thermal antinociception in cats. Doses of 2 to 4 mg/kg appeared necessary for induction of significant and sustained analgesic effects. Simulations predicted that 4 mg/kg every 6 hours would maintain analgesia close to the maximum effect of tramadol.

Full access
in American Journal of Veterinary Research

Abstract

Objective—To determine the antinociceptive effects of epidural administration of morphine or buprenorphine in cats by use of a thermal threshold model.

Animals—6 healthy adult cats.

Procedures—Baseline thermal threshold was determined in duplicate. Cats were anesthetized with isoflurane in oxygen. Morphine (100 μg/kg diluted with saline [0.9% NaCl] solution to a total volume of 0.3 mL/kg), buprenorphine (12.5 μg/kg diluted with saline solution to a total volume of 0.3 mL/kg), or saline solution (0.3 mL/kg) was administered into the epidural space according to a Latin square design. Thermal threshold was determined at various times up to 24 hours after epidural injection.

Results—Epidural administration of saline solution did not affect thermal threshold. Thermal threshold was significantly higher after epidural administration of morphine and buprenorphine, compared with the effect of saline solution, from 1 to 16 hours and 1 to 10 hours, respectively. Maximum (cutout) temperature was reached without the cat reacting in 0, 74, and 11 occasions in the saline solution, morphine, and buprenorphine groups, respectively.

Conclusions and Clinical Relevance—Epidural administration of morphine and buprenorphine induced thermal antinociception in cats. At the doses used in this study, the effect of morphine lasted longer and was more intense than that of buprenorphine.

Full access
in American Journal of Veterinary Research

Abstract

Objective—To evaluate the use of a transesophageal echo-Doppler ultrasonography (TED) technique for measurement of aortic blood flow (ABF) in relation to cardiac output (CO) measured by use of a thermodilution technique in anesthetized cats.

Animals—6 adult cats (mean ± SD body weight, 5 ± 0.7 kg).

Procedures—Anesthesia was induced and maintained in cats by administration of isoflurane. A thermodilution catheter was placed in a pulmonary artery. The TED probe was positioned in the esophagus in the region where the aorta and esophagus are almost parallel. Five baseline values for ABF and CO were concurrently recorded. Cats were randomly assigned to a high or low CO state (increase or decrease in CO by at least 25% from baseline, respectively). Baseline conditions were restored, and the other CO state was induced, after which baseline conditions were again restored. For each CO state, ABF and CO were measured 5 times at 5-minute intervals. Correlation and agreement between the techniques were determined by use of the Pearson product-moment correlation and Bland-Altman method.

Results—CO ranged from 0.16 to 0.75 L/min and ABF from 0.05 to 0.48 L/min. Overall data analysis revealed a high correlation (r = 0.884) between techniques but poor agreement (limits of agreement, −0.277 to 0.028 L/min). During the low CO state, correlation between techniques was low (r = 0.413).

Conclusions and Clinical Relevance—TED did not accurately measure CO. However, it allowed evaluation of CO patterns and may be useful clinically in anesthetized cats.

Full access
in American Journal of Veterinary Research