TIVA with sufentanil-midazolam was evaluated in medetomidine pre-medicated NZW rabbits. Induction was without excitation, apnoea or muscle rigidity, providing good conditions for endotracheal intubation. Anaesthesia maintenance was characterised by absence of a pedal withdrawal reflex and muscle relaxation, although marked respiratory depression and hypotension were produced.
We successfully intubated the airways of 6 rabbits after 2–4 blind attempts. The conditions for intubation were good, as judged by lack of jaw muscle tone and reflex responses to intubation, and when an intubation attempt failed, the endotracheal tube ended up in the oesophagus. No laryngospasm, swelling of the larynx or bleeding was encountered. The difficulty of endotracheal intubation in rabbits has been described in numerous studies [25–27]. Rabbits have a narrow oral cavity which makes visual intubation difficult and successful blind intubation requires both practice and routine. Using a specially designed laryngoscope such as the Flecknell™ small animal laryngoscope (Alstoe Ltd. Animal Health, UK) may be helpful.
Apnoea during induction must be avoided, since it complicates blind intubation and rapidly leads to hypoxemia in rabbits, due to their high metabolic rate. To avoid apnoea, anaesthesia must be induced slowly. Midazolam alone can cause apnoea in humans if infused too quickly IV . Slow induction also prevents muscle rigidity which occurs if sufentanil is administered too fast, which has been shown in pigs  and humans. In the currents study, catalepsy was not observed.
A similar IV drug combination (xylazine-alfentanyl-midazolam) has been evaluated in NZW rabbits , although not as a continuous infusion. The effects on the respiratory and cardiovascular systems were similar to those seen in the present study, but muscle relaxation was poor and seizures were induced. In comparison, TIVA with sufentanil-midazolam seems to be a better alternative.
During development of the current protocol, premedication with fentanyl-fluanisone was abandoned due to occurrence of apnoea during induction of anaesthesia. This was not the case after premedication with medetomidine at 0.1 mg/kg BW. In combination with local anaesthesia cream, medetomidine sedation produced satisfactory conditions for placement of vessel catheters. During TIVA maintenance however, apnoea occurred occasionally in two rabbits.
Medetomidine is known for its adverse cardiovascular effects , even in very low doses . In human anaesthesiology, alpha-2-adrenergic agonists are increasingly used in low doses to improve cardiovascular stability and prevent tachycardia .
Sufentanil-midazolam anaesthesia caused a marked decrease in respiratory rate and marked hypercapnia. The level of hypercapnia produced was similar to that produced by 2 MAC isoflurane . Opioids are known respiratory depressants, and midazolam has been shown to slightly add to this depression in rabbits . Despite marked respiratory depression, we chose to ventilate the rabbits minimally in order to evaluate the extent of respiratory depression. It is likely that the respiratory depression had an effect on other variables, such as the HR. However, experience shows that during short term anaesthesia, some degree of hypercapnia (6–7 kPa) is well tolerated in healthy cats and dogs , and may even be beneficial since it stimulates respiration and increases blood flow to the brain . The occurrence of apnoea during maintenance of anaesthesia necessitates the use of mechanical ventilation, which is the main disadvantage of the protocol since it requires successful endotracheal intubation as well as the knowledge and skills of using a ventilator.
The heart rate increased after induction with TIVA, although the values did not reach the levels reported in awake unrestrained rabbits . A possible explanation for the increase is the sympathomimetic stimulation caused by endotracheal intubation . Alpha-2-agonists are able to block cardiovascular responses caused by intubation and surgery in humans  and opioids can block the sympathetic response to surgery in dogs [23, 24]. If intubation caused the increased HR in the present study, the medetomidine and sufentanil doses used must have been insufficient to prevent the response. The simultaneous hypercapnia and hypotension that developed may also have contributed to the increase in HR. Isoflurane increases heart rate by 20% because of depression of the vagal tone rather than an increase in the sympathetic tone .
Rather than an increase in HR we had anticipated a decrease, since opioids are known to cause bradycardia through medullary vagal stimulation . Sufentanil-medetomidine anaesthesia, albeit in a higher doses than in the present study and without the concurrent use of an alpha-2-adrenergic agonist, causes bradycardia in dogs  as well as humans .
Marked hypotension developed during anaesthesia (MAP 46 mm Hg), yet all rabbits recovered uneventfully. A high prevalence of hypotension, defined as a MAP < 60 mm Hg, has been reported in rabbits anaesthetized with e.g. isoflurane, haltothane or ketamine-medetomidine [8, 33, 43]. Maintenance of anaesthesia with isoflurane in NZW rabbits will give rise to MAP in the range of 33–82 mmHg . The pressure measured in the auricular artery has been shown to be approximately 8 mm Hg lower than in the carotid artery . In the present study MAP in the sedated rabbits (~70 mmHg) was lower than reported in non-sedated unrestrained resting rabbits (~80 mmHg),  and similar to MAP in rabbits after IV administration of 10 μg/kg BW dexmedetomidine (~75 mmHg), . Midazolam alone  or in combination with sufentanil , has been shown to decrease vascular resistance and blood pressure in dogs, however MAP did not fall below 65 mmHg . In humans, sufentanil-midazolam also decreases MAP (~75 mm Hg), [46, 47]. In contrast to the present study, the human and dog studies included major surgery, which may have affected MAP. Uncorrected hypotension may contribute to the high mortality levels in rabbit anaesthesia  and cardiovascular support, which is routine in other animal species during anaesthesia, may possibly decrease mortality rates.
MAP and measurement of lactate concentrations  are indirect measures of tissue perfusion. The serum lactate levels in the present study indicate that tissue perfusion was adequate despite the low MAP. The levels were low before and during anaesthesia (0.7-1.9 mmol/L), similar to those seen in rabbits lightly anaesthetised with propofol (1.3 mmol/L),  and considerably lower compared to non-sedated, restrained rabbits (7.3 mmol/L), . Increased lactate levels are a sign of tissue hypoxia and anaerobic metabolism and should be treated with therapy directed at improving tissue oxygen delivery. The acidosis that quickly developed after induction of anaesthesia was most likely of respiratory origin.
Tissue perfusion is likely to be compromised if vasoconstriction and reduced cardiac output are present. In the current study, however, CO increased after anaesthesia induction, which was likely a consequence of the increased HR and decreased MAP. Midazolam has been shown to increase cardiac output in dogs at doses as low as 1 mg/kg .
CO was measured with the lithium-dilution technique, which has not yet been validated in rabbits. It has been validated in human babies with a similar BW (~2 kg) and found to be accurate in comparison to transpulmonary thermodilution . It has also been validated in dogs  and cats . The benefit of the technique is that a peripheral instead of a central vein may be used, which reduces the risk for complications. No adverse effects were seen due to the CO measurement in the present study. Values recorded in medetomidine sedated rabbits were similar to levels measured with a transit time Doppler in NZW rabbits sedated with dexmedetomidine 0.01 mg/kg BWIV .
The glucose levels in pre-medicated rabbits were high (7–14 mmol/L) compared to those reported in non-sedated rabbits (4–8 mmol/L), . A further increase in glucose was seen after anaesthesia induction, reaching a maximum of 8–18 mmol/L at approximately 50 min after medetomidine administration. Medetomidine is known to decrease insulin levels  and has been shown to increase blood glucose levels in rabbits . Midazolam on the other hand, has been shown to decrease glucose levels during surgery in humans  and, furthermore opioids inhibit the entire sympathetic nervous system . If the sufentanil-midazolam doses were not high enough to fully inhibit sympathetic activity, the stimulation caused by endotracheal intubation may have contributed to the increase in glucose levels.
Serum concentrations of sufentanil (0.60 ng/mL) and midazolam (140 ng/mL) remained constant throughout anaesthesia, with no tendency to accumulate. Plasma concentrations have been measured in humans undergoing heart surgery during sufentanil-midazolam anaesthesia . Compared to the present study, the infusion level was higher for sufentanil and lower for midazolam and accordingly, the plasma levels higher for sufentanil (2–4 ng/mL) and lower for midazolam (75–100 ng/mL).
To further evaluate this protocol, experimental work needs to be undertaken in both healthy and sick rabbits during surgery and in comparison with inhalation anaesthesia.