Recently licensed anaesthetic agents are constantly arriving on the veterinary market. They are often heralded as having many advantages over older agents making them worthy of their price tag. But how does the veterinary surgeon decide whether they offer any real clinical advantage. As experience grows with modern anaesthetic drugs we learn to use these drugs at doses which may vary from those originally recommended. The combination of different agents leads to a reduction in the amount of other drugs required, balancing cost as well as achieving balanced anaesthesia.

When assessing the cost effectiveness of anaesthesia we must look at the evidence to support the use of the chosen anaesthetic agents. There is much to be said for staying loyal to one technique whilst adapting it with the new advances. Familiarity is of paramount importance during anaesthesia: having the experience to predict how a patient will respond to a drug, how to titrate that drug to effect and how to monitor depth with that technique. But for new graduates, drug choice can be a minefield. They are faced with many agents, having been taught that drug A has better qualities than drug B, but they are not sufficiently equipped to argue its benefits against cost to their new boss. Many human studies have tried to evaluate the cost effectiveness of different anaesthetic regimens; however, their end points are much clearer with patient satisfaction and hospital stay often assessed. Outpatient surgery is more common in veterinary practice so we must set ourselves different targets to consider such as:

 Morbidity and mortality

 Patient comfort

 Owner satisfaction

 Veterinary and nursing team satisfaction

 Overall cost, including oxygen consumption and capital equipment costs

Morbidity and Mortality

Mortality risks were recently investigated in the Confidential Enquiry into Perioperative Small Animal Fatalities. The overall risk of death was high in our species compared to humans (0.17% in dogs, 0.24% in cats, 1.39% in rabbits compared to 0.002-0.01% in humans). Anaesthetic drugs per se did not appear to have a dramatic effect although halothane maintenance and mask inductions had increased odds of anaesthetic-related death in dogs. In rabbits in particular familiarity with the anaesthetic used was associated with reduced odds, highlighting the importance of knowing your drugs. Familiarity with the way a healthy patient responds to an anaesthetic protocol allows prediction of how a sick patient will respond and how drug dosages should be altered. In sick patients it is vital that a familiar technique is used and it is certainly not the time to experiment with newer drugs, no matter how much 'safer' the drug company's adverts claim it to be.

Owner Satisfaction and Patient Comfort

Owner satisfaction and patient comfort go together, with behavioural changes such as decreased appetite, decreased playfulness and contact seeking being associated with perceived pain by owners.

Overall Cost, Including Oxygen Consumption and Capital Equipment Costs

Cost comparisons have been investigated for different anaesthetic techniques in dogs and cats. Premedication affected cost the most due to its own expense and its effect on the amount of induction and maintenance agents required.

Analgesia

Interestingly, the choice of analgesic drug does not seem to affect cost too much, with morphine being comparable to buprenorphine in price as well as analgesia for mild to moderate pain. Again the NSAIDs licensed for perioperative use are comparable in price on a £/kg scale and choice may be down to personal preference and owner compliance. The use of analgesics preemptively may reduce postoperative pain and therefore analgesia requirements but their influence during anaesthesia should not be forgotten. The opioids can markedly reduce minimum alveolar concentration (MAC) of inhalational agents as well as having sparing effects on induction agents.

Ketamine is another analgesic agent that is more commonly used as part of the induction technique or as an infusion during anaesthesia. Infusions of ketamine can substantially decrease MAC in dogs and cats as well as reducing postoperative analgesia requirements. Postoperative opioid use is reduced in humans following ketamine administration, and work is ongoing in dogs. A dose of 10 μg/kg/hour equates to a cost of £0.005/kg/hour and does not require expensive syringe drivers as it can be administered in intravenous fluids, although ideally a fluid pump would be used.

Potent opioids are often advocated for reducing MAC during anaesthesia but are associated with marked respiratory depression. Use of a ventilator or having a nurse performing manual ventilation, offsets some of these disadvantages.

Local anaesthetic techniques are cheap and effective reducing inhalational and postoperative analgesia requirements along with improving demeanour postoperatively.

Pre-Anaesthetic Medication

Acepromazine and medetomidine are the two most commonly used agents to provide sedation prior to anaesthesia. Obviously they differ considerably in price (£0.01/kg versus £0.07/kg respectively according to manufacturers' recommendations) (Figure 1) although with new veterinary medetomidine products ready for market launch, the cost should fall.

Figure 1. Costs of pre-anaesthetic agents.

When used appropriately, acepromazine will reduce the amount of induction and maintenance agents required. It should be combined with an opioid to maximise sedation and should be administered 30 minutes prior to anaesthesia to actualise its drug-sparing potential. Despite their reliable sedative, analgesic and anaesthetic sparing properties, α₂ agonists are still not widely used in premedication.

Medetomidine is marketed at high doses to produce effective sedation for minor procedures and radiography, necessitating the use of reversal agent (atipamezole) adding cost. Lower doses (1-4 μg/kg) still produce good sedative and analgesic effects, whilst cardiovascular effects are less severe and short-lived. Drug cost is halved and atipamezole is not required. The induction dose is dramatically reduced (propofol requirements <2 mg/kg whilst alfaxalone appears to be reduced to 0.5 mg/kg from initial studies). The cost of expensive induction agents is therefore halved and their cardiovascular effects also reduced. The maintenance period is usually extremely smooth as are recoveries, which are complete without reversal making its use ideal for day case surgeries.

Induction

There is little to choose between thiopental and propofol with regard to effects on the cardiorespiratory systems in the clinical setting. Propofol has the advantage of being non-cumulative and so incremental doses can be used during anaesthesia. Alfaxalone is a new induction agent that is dramatically more expensive (Figure 2). It is suggested that 2-3 mg/kg is administered following premedication, working out at approximately twice the price of propofol. However, if the drug is titrated slowly to effect the dose is lower (1-2 mg/kg).

Figure 2. Costs of induction agents.

Alfaxalone appears to maintain blood pressure well although comparisons with propofol need to be performed. Co-induction agents can be used, such as diazepam and midazolam, to reduce the amount of induction agent, offsetting costs as well as side effects.

Maintenance

Newer anaesthetic agents such as isoflurane and sevoflurane do offer advantages over halothane in practice. There is little to choose between these two agents in the clinical setting, with recovery times being similar for relatively short procedures. Recovery quality may be somewhat better following sevoflurane although there remains great individual variability. Many factors influence the amount of inhalational agent needed, not least the breathing systems used. Obviously the carrier gas is also an expensive component of the anaesthetic. Rebreathing circles allow low fresh gas flows to be used (5-10 ml/kg/min) if the correct equipment is available thus reducing costs dramatically. The correct use of ADEs, mini-Lacks and T-pieces can allow low flows for small patients although the use of a capnograph is invaluable in allowing maximal reduction in gas flows. The amount of inhalational agent is affected by their relative MAC values and the prices shown in Figure 3 below demonstrate approximate costs at a vaporiser setting close to

Figure 3. Costs of anaesthetic gases.

1 x MAC and a fresh gas flow rate of 2 l/min. It appears that despite having a high experimental MAC value, sevoflurane vaporisers can typically be set lower than the equipotent MAC value for isoflurane following premedication, although the exact reasons are unclear. Sevoflurane is particularly suited to using with low-flow anaesthesia, and depth of anaesthesia can be changed rapidly. If used at low fresh gas flows the cost of sevoflurane would reduce to £0.86 per hour for a 20 kg dog, similar to the cost of isoflurane.

When choosing an anaesthetic protocol, familiarity should be more important than reported advantages and cost. It must be remembered that some drugs have a short shelf life and this may also influence the cost benefit in the practice environment. It is not the anaesthetic drugs used that make a difference; it is the way that we use them!

*All prices are based on 2007 distributor prices and were correct at the time of writing. Where two or more veterinary licensed products were available, the cost has been averaged. Standard dose rates have been used according to drug manufacturers.

References

1.  Bernardski RM, Bernardski LS, Muir WW 3rd. Cost comparison of anesthetic regimens in the dog and cat. Journal of American Veterinary Medicine Association 1984; 185: 869-872.

2.  Brodbelt (2006) The Confidential Enquiry into Perioperative Small Animal Fatalities (PhD thesis).