Induction of anaesthesia with desflurane and isoflurane in the rabbit

Sedation and Anesthesia in Exotic Animal Critical Care

Sedation and anesthesia of exotic animals in inherently challenging, but often facilitates the best care for patients. Critical illness or injury adds on another layer of complexity to their management for obtaining diagnostics and providing treatments. This article serves to review some of the more recent literature of sedation and anesthesia within exotics practice, bringing to light some nuances and considerations for when those patients are critically ill or injured.

Determination of minimum alveolar concentration and cardiovascular effects of desflurane in positive-pressure ventilated sheep

OBJECTIVE To determine the minimum alveolar concentration of desflurane (MAC_DES) and effects on cardiovascular variables in positive-pressure ventilated sheep. ANIMALS 13 adult female sheep. PROCEDURES Anesthesia was induced with desflurane. After a 30-minute equilibration at an end-tidal concentration of desflurane (et_DES) of 10.5%, an electrical stimulus (5 Hz/ms and 50 mA) was applied for 1 minute or until gross purposeful movement occurred. The et_DES was then changed by 0.5% (modified up-down method), depending on whether a positive motor response had been elicited, and stimulation was repeated. The MAC_DES was the et_DES midway between a positive and negative response. After MAC_DES was determined, et_DES was increased to 1.3 and 1.6 MAC_DES. Animals were allowed to equilibrate for 15 minutes, and cardiovascular, blood gas, acid-base, and hematologic variables were measured. Times to induction of anesthesia, extubation, attainment of sternal position, and standing and duration of anesthesia were recorded. RESULTS Mean ± SD MAC_DES was 9.81 ± 0.79%. Times to intubation, extubation, and standing were 4.81 ± 2.21 minutes, 14.09 ± 4.05 minutes, and 32.4 ± 12.5 minutes, respectively. Duration of anesthesia was 226 ± 22 minutes. Heart rate increased significantly at induction of anesthesia but otherwise remained at preanesthetic rates. Arterial blood pressures progressively decreased with increasing et_DES; pressures increased slightly only in response to noxious stimulation. CONCLUSIONS AND CLINICAL RELEVANCE The MAC_DES determined here compared favorably with that determined for other sheep populations and indicated similar anesthetic potency as in other species. Desflurane caused dose-dependent arterial hypotension, which indicated the need for careful blood pressure monitoring.

Different sensitivity to the suppressive effects of isoflurane anesthesia on cardiorespiratory function in SHR/Izm, WKY/Izm, and Crl:CD (SD) rats

Isoflurane is a widely used anesthetic, but its effects with increase in inspired concentration on cardiovascular function have not yet been clarified in rodents. Additionally, there are only a few studies comparing isoflurane-induced cardiorespiratory effects between rat strains. Thus, we investigated the differences in cardiorespiratory responsiveness to increasing concentration of inspired isoflurane in SHR/Izm, WKY/Izm and Crl:CD (SD) rats, by increasing the setting values of vaporizer's dial indicator. The rats were anesthetized with 1.5% isoflurane, and electrocardiograms, blood pressure, and respiratory rate were recorded simultaneously. Thereafter, the inspired concentration was increased stepwise to 2%, 3%, 4%, and 5%, and cardiorespiratory parameters were obtained at each concentration. Under anesthesia at more than 4%, although prolongation of the RR and PR intervals was observed in all strains, shortening of the QT_C interval was found only in SHR/Izm rats. From frequency domain analysis of heart rate variability, an increase in LF/HF ratio and a decrease of HF components were observed in SHR/Izm and WKY/Izm rats, respectively, with 5% isoflurane anesthesia. Blood pressure and heart rate were remarkably reduced in SHR/Izm rats at higher concentrations, whereas the reduction was smallest in WKY/Izm rats among the three strains examined. Respiratory rate was inspired concentration-dependently decreased in all strains. These results suggested that SHR/Izm rats are more sensitive to suppressive effects of isoflurane anesthesia on cardiovascular function among these rat strains.

Vital signs monitoring during injectable and inhalant anesthesia in mice

Selecting the appropriate anesthetic protocol for the individual animal is an essential part of laboratory animal experimentation. The present study compared the characteristics of four anesthetic protocols in mice, focusing on the vital signs. Thirty-two male ddY mice were divided into four groups and administered anesthesia as follows: pentobarbital sodium monoanaesthesia; ketamine and xylazine combined (K/X); medetomidine, midazolam, and butorphanol combined (M/M/B); and isoflurane. In each group, rectal temperature, heart rate, respiratory rate, and O₂ saturation (SPO₂) were measured, and the changes over time and instability in these signs were compared. The anesthetic depth was also evaluated in each mouse, and the percentage of mice achieving surgical anesthesia was calculated. K/X anesthesia caused remarkable bradycardia, while the respiratory rate and SPO₂ were higher than with the others, suggesting a relatively strong cardiac influence and less respiratory depression. The M/M/B group showed a relatively lower heart rate and SPO₂, but these abnormalities were rapidly reversed by atipamezole administration. The pentobarbital group showed a lower SPO₂, and 62.5% of mice did not reach a surgical anesthetic depth. The isoflurane group showed a marked decrease in respiratory rate compared with the injectable anesthetic groups. However, it had the most stable SPO₂ among the groups, suggesting a higher tidal volume. The isoflurane group also showed the highest heart rate during anesthesia. In conclusion, the present study showed the cardiorespiratory characteristics of various anesthetic protocols, providing basic information for selecting an appropriate anesthetic for individual animals during experimentation.

Pre-clinical functional Magnetic Resonance Imaging Part II: The heart

One third of all deaths worldwide in 2008 were caused by cardiovascular diseases (CVD), and the incidence of CVD related deaths rises ever more. Thus, improved imaging techniques and modalities are needed for the evaluation of cardiac morphology and function. Cardiac magnetic resonance imaging (CMRI) is a minimally invasive technique that is increasingly important due to its high spatial and temporal resolution, its high soft tissue contrast and its ability of functional and quantitative imaging. It is widely accepted as the gold standard of cardiac functional analysis. In the short period of small animal MRI, remarkable progress has been achieved concerning new, fast imaging schemes as well as purpose-built equipment. Dedicated small animal scanners allow for tapping the full potential of recently developed animal models of cardiac disease. In this paper, we review state-of-the-art cardiac magnetic resonance imaging techniques and applications in small animals at ultra-high fields (UHF).

Continuous intravenous anaesthesia with sufentanil and midazolam in medetomidine premedicated New Zealand White rabbits

BACKGROUND

Anaesthesia in rabbits is associated with a high mortality rate, compared to that in cats and dogs. Total intravenous anaesthesia (TIVA) with drugs that provide cardiovascular stability and are rapidly metabolised could be of benefit for use in rabbits. The aim was to evaluate cardiorespiratory effects of TIVA with sufentanil-midazolam in eight New Zealand White rabbits. Subcutaneous premedication with medetomidine (0.1 mg/kg BW) was followed by IV administration of a mixture of 2.5 μg/mL sufentanil and 0.45 mg/mL midazolam at a rate of 0.3 mL/kg BW/h for anaesthetic induction. Additionally, intravenous boluses of 0.1 mL of the mixture were administered every 20 s until the righting reflex was lost. Following endotracheal intubation, anaesthesia was maintained for 60 min with an infusion rate adjusted to supress the pedal withdrawal reflex. Air and oxygen (1:2) were delivered at 3 L/min. Physiological variables were recorded before induction and at predefined time points during and after anaesthesia.

RESULTS

Righting and pedal withdrawal reflexes were lost within 3 and 5 min, respectively. Doses of sufentanil and midazolam were 0.48 μg/kg BW and 0.09 mg/kg BW for induction, and 0.72 μg/kg BW/h and 0.13 mg/kg BW/h for maintenance. Apnoea occurred in two rabbits. Induction of anaesthesia caused a significant increase in heart rate, cardiac output and arterial CO2 partial pressure and a decrease in mean arterial pressure, respiratory rate and pH. Mean time from stopping the infusion to endotracheal extubation was 5 min, and to return of the righting reflex 7 min. Anaesthesia was characterized by induction and recovery without excitation, with muscle relaxation, and absence of the pedal withdrawal reflex.

CONCLUSIONS

TIVA with sufentanil-midazolam provided smooth induction and recovery of anaesthesia in rabbits but with marked hypotension and respiratory depression, requiring mechanical ventilation. Further evaluation is needed to establish if the protocol is useful for rabbits undergoing surgery.

The use of propofol and sevoflurane for surgical anaesthesia in New Zealand White rabbits

New Zealand White (NZW) rabbits ( n = 34) received intravenous propofol (16 ± 5 mg/kg) for induction of anaesthesia followed by maintenance with sevoflurane (4.0 ± 0.5%) in oxygen. All animals underwent ovariohysterectomy. Heart rate, respiratory rate, haemoglobin oxygen saturation, end-tidal carbon dioxide concentration, end-tidal sevoflurane concentration and oesophageal temperature were monitored every 5 min. Time from induction of anaesthesia to tracheal extubation and sternal recumbency were recorded as was the quality of recovery. Direct arterial blood pressure values (mmHg) were recorded every 5 min from 19 rabbits and 22 arterial blood gases analyses were performed (11 postintubation and 11 at the time of recovery). Propofol produced smooth induction of anaesthesia without production of apnoea. Intubation was successfully performed in all but one rabbit in an average of 4 ± 3 min from the beginning of propofol administration. No ventilatory support was required during the anaesthetic period. Respiratory rate averaged 51 ± 8 bpm and end-tidal CO2 (kPa) was 4.0 ± 0.5 mmHg during anaesthesia. Blood gas values were maintained within normal limits and average mean arterial blood pressure was 73.4 ± 7.9 mmHg. Time to regain the swallowing reflex following discontinuation of sevoflurane was 2 ± 1 min and time to sternal recumbency was 8 ± 0.3 min. No anaesthetic-related mortality occurred and all animals recovered uneventfully. Propofol–sevoflurane anaesthesia produced a good quality of surgical anaesthesia for ovariohysterectomy and stable cardiopulmonary conditions. Propofol–sevoflurane anaesthesia in young healthy NZW rabbits appears to be an effective and practically useful method of anaesthesia.

Anaesthesia with a combination of ketamine and medetomidine in the rabbit: effect of premedication with buprenorphine

OBJECTIVE

To assess the effects of premedication with buprenorphine on the characteristics of anaesthesia induced with ketamine/medetomidine.

STUDY DESIGN

Prospective crossover laboratory study.

ANIMALS

Six female New Zealand White rabbits.

METHODS

Rabbits received, on occasions separated by 7 days, either buprenorphine (0.03 mg kg(-1)) or saline subcutaneously (SC) as premedication, followed 1 hour later by SC ketamine (15 mg kg(-1)) and medetomidine (0.25 mg kg(-1)) (K/M). At pre-determined time points reflex responses and cardiopulmonary parameters were recorded and arterial blood samples taken for analysis. Total sleep time was the duration of loss of the righting reflex. Duration of surgical anaesthesia was the time of suppression of the ear pinch and pedal withdrawal reflexes. Wilcoxon signed-ranks tests were used to compare data before (T(0)) and 10 minutes after (T(10)) injection with K/M.

RESULTS

All animals lost all three reflex responses within 10 minutes of injection of K/M. The duration of loss of these reflexes significantly increased in animals that received buprenorphine. At induction, animals that had received buprenorphine tended to have a lower respiration rate but there were no significant differences in arterial PCO(2), PO(2) or pH between treatments. Hypoxaemia [median PaO(2) < 6.0 kPa (45 mmHg)] developed in both treatments at T(10) but there was no significant difference between treatments. Mean arterial pressure (MAP) was lower at T(10) in animals that had received buprenorphine.

CONCLUSION AND CLINICAL RELEVANCE

Premedication with buprenorphine significantly increased the duration of anaesthesia induced by K/M, with no significant depression of respiration further to the control treatment within the first 10 minutes of anaesthesia. The MAP decreased but this was not reflected in a difference in other physiological parameters. These data show that premedication with buprenorphine, before K/M anaesthesia in the rabbit, has few negative effects and may provide beneficial analgesia.

Anesthetic protocol for videolaparoscopic surgery in rabbits

PURPOSE

To describe the anesthetic protocol and the intubation technique without visualizing the trachea in rabbits, in order to enable the videolaparoscopic surgical procedure.

METHODS

The experiment was performed on 33 female rabbits (Oryctolagus cuniculus), aged from 5 to 7 months. It consisted of general anesthesia and endotracheal intubation by manual palpation of the trachea of the rabbits, without using the laryngoscope, orally, for later videolaparoscopic surgical access to the abdominal cavity.

RESULTS

The mean values and standard deviation of vital parameters of the animals were 223.8 + or - 15.61 beats per minute for heart rate; 35 + or - 9 movements per minute for respiratory rate; 96.94 + or - 0.99% of oxymetry and 42.82 + or - 4.02 mmHg for capnometry; 16.7 + or - 4.3 minutes for pneumoperitoneum (duration of surgery) and 1 hour and 14 + or - 8.52 minutes for time of observation (from induction to recovery from anesthesia). All animals were intubated in at most three attempts. No animals were lost after the introduction of this anesthetic technique.

CONCLUSION

This protocol proved adequate, safe and easy to perform, on rabbits submitted to videolaparoscopic surgery.

M-mode and Doppler echocardiographic reference values for male New Zealand white rabbits

OBJECTIVE

To determine M-mode and Doppler echocardiographic reference values in healthy New Zealand white rabbits.

ANIMALS

52 healthy male rabbits.

PROCEDURES

The rabbits were anesthetized and M-mode measurements of the left ventricle, left atrium, and aorta and Doppler measurements of pulmonary and aortic outflow and mitral inflow were recorded.

RESULTS

Mean +/- SD heart rate during echocardiographic examination was 155 +/- 29 beats/min. Mean +/- SD measurements in diastole and systole for the interventricular septum thickness, left ventricular internal diameter, and left ventricular free wall thickness were 2.03 +/- 0.37 mm and 3.05 +/- 0.45 mm; 14.37 +/- 1.49 mm and 10.25 +/- 1.22 mm; and 2.16 +/- 0.25 and 3.48 +/- 0.55 mm, respectively. Mean +/- SD left atrial-to-aortic diameter ratio was 1.17 +/- 0.14, and mean +/- SD mitral valve E-point-to-septal separation interval was 1.71 +/- 0.29 mm. Mean +/- SD for fractional shortening and ejection fraction were 30.13 +/- 2.98% and 61.29 +/- 4.66%, respectively. Mean +/- SD maximal aortic and pulmonary artery outflow velocities were 0.85 +/- 0.11 m/s and 0.59 +/- 0.10 m/s, respectively, and the peak E-to-peak A wave velocity ratio of the mitral valve was 2.19 +/- 0.46.

CONCLUSIONS AND CLINICAL RELEVANCE

Results provide echocardiographic reference values for young adult male New Zealand white rabbits anesthetized with ketamine and medetomidine. Values obtained from unanesthetized rabbits, rabbits sedated with other agents, or rabbits of different size may differ from those reported here.

Comparison of the arterial blood gas, arterial oxyhaemoglobin saturation and end-tidal carbon dioxide tension during sevoflurane or isoflurane anaesthesia in rabbits

The effects of sevoflurane or isoflurane on arterial blood gas, arterial oxyhaemoglobin saturation and end-tidal CO₂ tension were monitored during induction and maintenance of anaesthesia in 10 premedicated New Zealand White (NZW) rabbits.

For induction, the anaesthetic agents were delivered via a face-mask. After induction was completed, an endotracheal tube was introduced for maintenance of anaesthesia for a period of 90 minutes. Changes in heart rate, respiratory rate, arterial blood gas, arterial oxyhaemoglobin saturation, blood pH and end-tidal CO₂ tension were recorded. Although sevoflurane and isoflurane produce similar cardiopulmonary effects in premedicated rabbits, sevoflurane provides a smoother and faster induction because of its lower blood/gas partition coefficient. Thus sevoflurane is probably a more suitable agent than isoflurane for mask induction and maintenance. Its lower blood solubility also makes sevoflurane more satisfactory than isoflurane for maintenance of anaesthesia because it allows the anaesthetist to change the depth of anaesthesia more rapidly.

Assessment of ketamine and medetomidine anaesthesia in the domestic rabbit

OBJECTIVE

To study the effects of ketamine and two doses of medetomidine administered by two routes of injection in a genetically diverse population of rabbits.

STUDY DESIGN

Prospective, randomized, clinical trial.

ANIMALS

One hundred and five domestic rabbits of mixed breed, sex and age.

MATERIALS AND METHODS

Rabbits undergoing orchiectomy or ovariohysterectomy received ketamine (15 mg kg(-1)) combined with medetomidine at 0.25 or 0.5 mg kg(-1), by subcutaneous (SC) or intramuscular (IM) injection. Anaesthesia was supplemented with 1.5-2% isoflurane when signs of regular jaw movements and/or slight limb twitching indicated inadequate anaesthesia. Heart and respiratory rate, blood oxygen saturation, end-tidal carbon dioxide concentration and rectal temperature were monitored at several time points. Duration of surgical anaesthesia and anaesthesia time were measured. At completion of surgery, atipamezole (1.0 or 0.5 mg kg(-1), IM or SC) was administered.

STATISTICAL ANALYSES

MANOVA was used to compare variables over time between males and females, anaesthetic doses and routes of drug administration.

RESULTS

All reflexes were lost significantly more rapidly after IM drug administration (p < 0.05). The times (in minutes) from drug injection to loss of reflexes for the respective groups were: righting reflex: 6.3 (15.0 + 0.25, SC), 5.5 (15.0 + 0.5, SC), 2.9 (15.0 + 0.25, IM) and 2.3 (15.0 + 0.5, IM); ear pinch: 9.2, 8.5, 4.8, 3.6; pedal withdrawal: 12.8, 10.4, 6.6, 5.2. Heart and respiratory rates during surgery did not differ between groups, however the highest end-tidal CO(2) concentration during surgery was significantly affected by dose, with the highest concentration occurring in group 15.0 + 0.5 IM. The number of animals requiring isoflurane tended to decrease with increasing dose of anaesthetic and significantly more females required supplementation than males (p < 0.05). Recovery from anaesthesia (return of righting reflex) was not significantly different between dose groups (p > 0.1) but was more rapid in animals given IM atipamezole (13.6 +/- 13 versus 21 +/- 17, p = 0.037). No anaesthetic-related mortality occurred and all but three animals recovered uneventfully. Five animals were killed whilst under anaesthesia because of unrelated disease.

CONCLUSION AND CLINICAL RELEVANCE

Ketamine-medetomidine combinations reliably produced surgical anaesthesia in domestic rabbits that could easily be deepened for brief periods with low concentrations of isoflurane. Subcutaneous administration was better tolerated, but the speed of induction was slower compared with IM injection. Atipamezole was an effective antagonist and produced most rapid effects when administered IM.

Measurement of aversion to determine humane methods of anaesthesia and euthanasia

The distress experienced by animals during the induction of unconsciousness remains one of the most important and yet overlooked aspects of effective methods of anaesthesia and euthanasia. Here we show that considerable differences exist in the aversive responses elicited by 12 common methods of inhalational anaesthesia and euthanasia in laboratory rats and mice. Carbon dioxide, either alone or in combination with oxygen or argon, was found to be highly aversive to both species. The least aversive agents were halothane in rats and enflurane in mice. Exposing these animals to carbon dioxide in any form, either for anaesthesia or for euthanasia, is likely to cause considerable pain and distress and is therefore unacceptable when efficient and more humane alternatives are readily available.

Degrees of aversion shown by rats and mice to different concentrations of inhalational anaesthetics

The distress associated with the induction of anaesthesia with halothane, isoflurane, enflurane and carbon dioxide was investigated in rats and mice by measuring the level of aversion they displayed on exposure to low, medium and high concentrations of these agents. The animals were exposed to each agent in a test chamber containing air or gas mixtures, which they were able to enter and leave at will, and the level of aversion was assessed in terms of the initial withdrawal and total dwelling times in the chamber. Comparisons between the anaesthetic and air-control treatments indicated that concentrations of the agents recommended for the rapid and efficient induction of anaesthesia were associated with some degree of aversion. Carbon dioxide was by far the most aversive gas for both rats and mice, with the least aversive being halothane for rats, and halothane and enflurane for mice. With all the anaesthetics, the level of aversion increased as the concentration increased.