Cardiovascular effects of equipotent doses of isoflurane alone and isoflurane plus fentanyl in New Zealand White rabbits (Oryctolagus cuniculus)

Total intravenous anesthesia with Ketofol in rabbits: a comparison of the effects of constant rate infusion of midazolam, fentanyl or dexmedetomidine

BACKGROUND

When inhalant anesthetic equipment is not available or during upper airway surgery, intravenous infusion of one or more drugs are commonly used to induce and/or maintain general anesthesia. Total intravenous anesthesia (TIVA) does not require endotracheal intubation, which may be more difficult to achieve in rabbits. A range of different injectable drug combinations have been used as continuous infusion rate in animals. Recently, a combination of ketamine and propofol (ketofol) has been used for TIVA in both human patients and animals. The purpose of this prospective, blinded, randomized, crossover study was to evaluate anesthetic and cardiopulmonary effects of ketofol total intravenous anesthesia (TIVA) in combination with constant rate infusion (CRI) of midazolam, fentanyl or dexmedetomidine in eight New Zealand White rabbits. Following IV induction with ketofol and endotracheal intubation, anesthesia was maintained with ketofol infusion in combination with CRIs of midazolam (loading dose [LD]: 0.3 mg/kg; CRI: 0.3 mg/kg/hr; KPM), fentanyl (LD: 6 µg/kg; CRI: 6 µg/kg/hr; KPF) or dexmedetomidine (LD: 3 µg/kg; CRI: 3 µg/kg/hr; KPD). Rabbits in the control treatment (KPS) were administered the same volume of saline for LD and CRI. Ketofol infusion rate (initially 0.6 mg kg- 1 minute- 1 [0.3 mg kg- 1 minute- 1 of each drug]) was adjusted to suppress the pedal withdrawal reflex. Ketofol dose and physiologic variables were recorded every 5 min.

RESULTS

Ketofol induction doses were 14.9 ± 1.8 (KPM), 15.0 ± 1.9 (KPF), 15.5 ± 2.4 (KPD) and 14.7 ± 3.4 (KPS) mg kg- 1 and did not differ among treatments (p > 0.05). Ketofol infusion rate decreased significantly in rabbits in treatments KPM and KPD as compared with saline. Ketofol maintenance dose in rabbits in treatments KPM (1.0 ± 0.1 mg/kg/min) and KPD (1.0 ± 0.1 mg/kg/min) was significantly lower as compared to KPS (1.3 ± 0.1 mg/kg/min) treatment (p < 0.05). Ketofol maintenance dose did not differ significantly between treatments KPF (1.1 ± 0.3 mg/kg/min) and KPS (1.3 ± 0.1 mg/kg/min). Cardiovascular variables remained at clinically acceptable values but ketofol infusion in combination with fentanyl CRI was associated with severe respiratory depression.

CONCLUSIONS

At the studied doses, CRIs of midazolam and dexmedetomidine, but not fentanyl, produced ketofol-sparing effect in rabbits. Mechanical ventilation should be considered during ketofol anesthesia, particularly when fentanyl CRI is used.

Treatment of Pain in Rabbits

Rabbits occupy facets of veterinary medicine spanning from companion mammals, wildlife medicine, zoologic species, and research models. Therefore, analgesia is required for a variety of conditions in rabbits and is a critical component of patient care. Considerations when selecting an analgesic protocol in rabbits include timing of administration, route of administration, degree or anticipated pain, ability to access or use controlled drugs, systemic health, and any potential side effects. This review focuses on pharmacologic and locoregional management of pain in rabbits and emphasizes the need for further studies on pain management in this species.

Rabbit Sedation and Anesthesia

With the increasing frequency of rabbits as veterinary patients, the expectation for high-quality, intensive veterinary care, and resultantly an understanding of anesthesia has been increasing. Sedation and general anesthesia are commonly required for many routine and emergency procedures in rabbits, and this results in the need for a strong awareness of anesthetic principles, knowledge of limitations of anesthesia, and maintenance of high standards of anesthesia.

Super-Resolution Ultrasound Localization Microscopy on a Rabbit Liver VX2 Tumor Model: An Initial Feasibility Study

Ultrasound localization microscopy can image microvasculature in vivo without sacrificing imaging penetration depth. However, the reliance on super-resolution inference limits the applicability of the technique because subpixel tissue motion can corrupt microvascular reconstruction. Consequently, the majority of previous pre-clinical research on this super-resolution procedure has been restricted to low-motion experimental models with ample motion correction or data rejection, which precludes the imaging of organ sites that exhibit a high degree of respiratory and other motion. In this article, we present a novel anesthesia protocol in rabbits that induces safe, controllable periods of apnea to enable the long image-acquisition times required for ultrasound localization microscopy. We apply this protocol to a VX2 liver tumor model undergoing sorafenib therapy and compare the results to super-resolution images from conventional high-dose isoflurane anesthesia. We find that the apneic protocol was necessary to correctly identify the poorly vascularized tumor cores, as verified by immunohistochemistry, and to reveal the tumoral microvascular architecture.

Cardiovascular effects of increasing dosages of norepinephrine in healthy isoflurane-anesthetized New Zealand White rabbits

OBJECTIVE

To characterize the cardiovascular effects of increasing dosages of norepinephrine (NE) in healthy isoflurane-anesthetized rabbits.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A total of nine female ovariohysterectomized New Zealand White rabbits weighing 3.4 ± 0.2 kg (mean ± standard deviation).

METHODS

Rabbits were premedicated intramuscularly with buprenorphine (0.05 mg kg^-1) and midazolam (0.5 mg kg^-1). Anesthesia was induced with intravenous propofol and maintained with a 1.1 × minimum alveolar concentration of isoflurane for this species to induce hypotension. Rabbits were administered NE infusions at three doses: low, 0.1 μg kg^-1 minute^-1; medium, 0.5 μg kg^-1 minute^-1; and high doses, 1 μg kg^-1 minute^-1 for 10 minutes each in that order. Cardiovascular variables including heart rate (HR), cardiac output (CO) by lithium dilution technique and systolic (SAP), mean (MAP) and diastolic (DAP) invasive arterial blood pressures measured in the auricular artery were recorded at baseline, 10 minutes after the start of the infusion of each NE treatment and 10 minutes after NE was discontinued. A linear mixed model and a type III anova with Tukey's post hoc comparison was performed (p < 0.05).

RESULTS

Significant increases in SAP (28% and 90%), MAP (27% and 90%) and DAP (33% and 97%) were measured with medium and high dose treatments, respectively (p < 0.001), with no changes in CO. HR decreased and stroke volume increased significantly with high dose treatment (by 17% and 15%, respectively; p < 0.05). No arrhythmias were noticed with NE treatments.

CONCLUSIONS AND CLINICAL RELEVANCE

The infusion of NE at 0.5-1.0 μg kg^-1 minute^-1 is a potentially effective treatment for hypotension in healthy isoflurane-anesthetized New Zealand White rabbits.

Pharmacokinetics of ketamine following a short intravenous infusion to isoflurane-anesthetized New Zealand White rabbits (Oryctolagus cuniculus)

OBJECTIVE

To describe the pharmacokinetics of ketamine following a short intravenous (IV) infusion to isoflurane-anesthetized rabbits.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A total of six adult healthy female New Zealand White rabbits.

METHODS

Anesthesia was induced with isoflurane in oxygen. Following determination of isoflurane minimum alveolar concentration (MAC), the isoflurane concentration was reduced to 0.75 MAC and ketamine hydrochloride (5 mg kg^-1) was administered IV over 5 minutes. Blood samples were collected before and at 2, 5, 6, 7, 8, 9, 13, 17, 21, 35, 65, 125, 215 and 305 minutes after initiating the ketamine infusion. Samples were processed immediately and the plasma separated and stored at -80 °C until analyzed for ketamine and norketamine concentrations using liquid chromatography-mass spectrometry. Compartment models were fitted to the concentration-time data for ketamine and for ketamine plus norketamine using nonlinear mixed-effects (population) modeling.

RESULTS

A three- and five-compartment model best fitted the plasma concentration-time data for ketamine and for ketamine plus norketamine, respectively. For the ketamine only model, the volume of distribution at steady state (Vss) was 3217 mL kg^-1, metabolic clearance was 88 mL minute^-1 kg^-1 and the terminal half-life was 59 minutes. For the model including both ketamine and norketamine, Vss were 3224 and 2073 mL kg^-1, total metabolic clearance was 107 and 52 mL minute^-1 kg^-1 and terminal half-lives were 52 and 55 minutes for the parent drug and its metabolite, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

This study characterized the pharmacokinetics of ketamine and norketamine in isoflurane-anesthetized New Zealand White rabbits following short IV infusion. The results obtained herein will be useful to determine ketamine infusion regimens in isoflurane-anesthetized rabbits.

Plasma dopamine concentrations following dopamine infusion to isoflurane-anesthetized New Zealand White rabbits

OBJECTIVE

To determine the pharmacokinetics of dopamine following a short infusion in isoflurane-anesthetized rabbits.

STUDY DESIGN

Prospective, descriptive pharmacokinetic study.

ANIMALS

A group of six adult female New Zealand White rabbits weighing 4.4 ± 0.2 kg.

METHODS

Rabbits were anesthetized with isoflurane in oxygen and maintained at 1.2 × minimum alveolar concentration of isoflurane (2.3% atmosphere). Dopamine (30 μg kg^-1 minute^-1) was infused for 10 minutes. Arterial blood was sampled prior, during and following the infusion at various intervals for 1 hour.

RESULTS

A one-compartment model with baseline concentration best fitted the time-plasma dopamine concentration data. Estimated typical population value (interindividual variability) for volume of distribution and clearance were 10.3 (232%) L kg^-1 and 9.9 (508%) L minute^-1 kg^-1, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

There was a large degree of interindividual variation in the disposition of dopamine. The large volume of distribution and high metabolic clearance rate reported for dopamine in this study likely explains the lack of clinical efficacy of dopamine in rabbits at doses up to 30 μg kg^-1 minute^-1.

Advancements in Evidence-Based Anesthesia of Exotic Animals

Anesthesia and sedation of pet nondomestic species are often necessary for both invasive and noninvasive procedures. Even minimally invasive procedures can be stressful for small prey species that are not domesticated or acclimated to human contact and restraint. Recent advancements in evidence-based practice will continue to improve the field based on scientifically sound best practices and rely less on anecdotal recommendations. This article focuses on new scientific literature that has been published in the past 5 years. For ease of reading, the authors divide the article to highlight advances in anesthetic pharmacology and discoveries in anesthetic physiology and monitoring.

Assessment and Care of the Critically Ill Rabbit

Rabbits have the ability to hide their signs and often present in a state of decompensatory shock. Handling can increase susceptibility to stress-induced cardiomyopathy and specific hemodynamic changes. Careful monitoring with a specific reference range is important to detect early decompensation, change the therapeutic plan in a timely manner, and assess prognostic indicators. Fluid requirements are higher in rabbits than in other small domestic mammals and can be corrected both enterally and parenterally. Critical care in rabbits can be extrapolated to many hindgut fermenters, but a specific reference range and dosage regimen need to be determined.