Veterinary anaesthesia has changed in many ways over the last 30 years. In the past, the patient’s safety was measured by survival; keeping the subject asleep and immobile was the priority. However, the physiological well-being of the patient, particularly pain management and temperature regulation, has rightly gained prominence. More recently, the safety and welfare of the veterinary team, the environmental impact and sustainability have all received greater emphasis in the world of veterinary anaesthesia.
Decision making in veterinary anaesthesia
When making decisions about any complex clinical technique, it is important that you do not consider issues solely from one perspective, as this could be misleading and result in suboptimal outcomes.
A balanced approach to anaesthesia decision making has been discussed elsewhere (Wheeler, 2022), but ultimately it boils down to giving equal consideration to multiple relevant criteria. These are:
- Patient well-being
- Operational efficiency
- Environmental sustainability
- Cost-effectiveness
What guidance is available?
The Royal College of Anaesthetists provides a succinct summary for patients on the environmental impact of all aspects of their anaesthetic journey. Whereas it will be challenging to make any meaningful reduction in some of the aspects, the Royal College of Anaesthetists highlights changes in techniques that could be immediately impactful without compromising patient care or safety.
The Association of Anaesthetists has also published a “Guide to green anaesthesia” with the following suggestions for human anaesthesia:
- Avoid nitrous oxide whenever possible
- Avoid desflurane
- Use low-flow anaesthesia
- Consider swapping volatile-agent-based anaesthesia for a total intravenous anaesthesia technique
- Consider the use of central neuraxial block or regional anaesthesia
Further, a special environmental issue edition of Anaesthesia News in 2020 highlighted many issues relevant to veterinary anaesthesia, including gaseous emissions.
Veterinary low-flow anaesthesia
Low-flow anaesthesia and the ensuing benefits are established concepts (Wagner and Bednarski, 1992); however, care must be taken not to jeopardise patient well-being when seeking potential environmental benefits.
Low-flow anaesthesia and the ensuing benefits are established concepts; however, care must be taken not to jeopardise patient well-being when seeking potential environmental benefits
Traditional veterinary anaesthetic techniques in our smallest patients have used relatively high fresh gas flow rates well above the metabolic requirements of patients. This is highly wasteful of oxygen and inhalational anaesthetic agents, which, in turn, is unnecessarily costly, can expose the veterinary team and could be environmentally damaging. Low-flow anaesthesia, therefore, can and should be considered.
Advantages of low-flow anaesthesia
Some of the advantages of low-flow anaesthesia are:
- A reduction in oxygen flow and inhalant agent consumption to less than 10 percent of the high-flow non-rebreathing systems typically used in patients under 10kg
- Delivery of warm gas to the patient rather than the cold, dry gases delivered by high-flow systems. This can help reduce anaesthetic hypothermia, particularly during clip and prep phases and in tiny patients
- A reduction in the outflow of inhalation agents from the pop-off valve, enhancing team safety and reducing environmental emissions
- A reduction in the use of oxygen concentrators, which are inefficient and carry high monetary and environmental costs
- Significant cost savings of consumables
Combatting the challenges of low-flow anaesthesia
Though there are challenges when using low-flow anaesthesia in veterinary practice, as discussed below, some of the challenges of low-flow anaesthesia can be addressed using innovative modern veterinary equipment rather than re-purposed human equipment.
For example, low-flow techniques cannot be used with non-rebreathing systems which require high gas flows to eliminate expired CO2, such as Bain circuits or the Ayres T-piece. Also, low-flow should not be used when nitrous oxide is being administered unless there is a fail-safe system for oxygen supply and in-circuit oxygen concentration monitoring.
Some of the challenges of low-flow anaesthesia can be addressed using innovative modern veterinary equipment rather than re-purposed human equipment
Many veterinary and human medical vaporisers are designed to operate at gas flows between 500ml/min and 5l/min, so are imprecise and not reliable operating at 200ml/min, a typical oxygen low-flow rate for animals under 7kg. Some veterinary systems claim to be “low flow” but have drawbacks such as high resistance, inability to prevent rebreathing of CO2 at oxygen flows under 100ml/kg/min and a very slow (up to 20 minutes) rate of change of inspired anaesthetic concentration at the start of anaesthesia or if the vaporiser setting is changed (Dunlop et al., 2012a; Dunlop et al., 2012b).
What equipment should be used?
Key equipment requirements (Table 1) for low-flow anaesthesia are:
- A veterinary vaporiser capable of precise and reliable anaesthetic delivery at fresh gas flows down to 200ml/min. The vaporiser should be calibrated at low flows and tested down to 200ml/min (Dunlop and Dunlop, 2023)
- A low-volume low-resistance circle absorber with rapid response to changes in vaporiser settings (eg five breaths) at 200ml/min fresh gas flow. This can be used on all patients down to 2kg, permitting fresh gas flows 10 times lower than in currently used non-rebreathing systems. Generally, low-flow systems use 10 to 30ml/kg/min, with a minimum of 200ml/min (Brown, 2015)
- A circle absorber with one-way inspiratory/expiratory valves that shut reliably with minimum force such as would be expected from a 2kg animal with a 20ml tidal volume breathing from low-volume tubing
- A precise oxygen flow meter with an expanded scale that can easily be visualised at fresh gas flow rates down to 200ml/minute (Figure 1)
- A safety APL (waste gas “pop-off” valve) that relieves pressure at 25 to 30cmH20 if in the closed position
| Drugs | Equipment | |
|---|---|---|
| Acepromazine | ACP Injection | Darvall Stinger Ultra Veterinary Anaesthesia Machine |
| Methadone | Comfortan, 10mg/ml | |
| Propofol | PropoFlow Plus 10mg/ml | Darvall DVM Iso Vaporiser |
| Isoflurane | Isoflurane-Vet 100% | |
There are several veterinary anaesthesia systems available for low-flow anaesthesia, but some are not suitable for all patients because of high resistance, slow response to vaporiser changes and the high gas flows required to prevent rebreathing of CO2 in patients below 10kg (Dunlop et al., 2012a; Dunlop et al., 2012b). Therefore, veterinary teams should be cautious in purchasing “low flow” systems, only to find that they do not function effectively and safely in small patients at flows down to 200ml/min (Alibhai et al., 1999; Artu and Katz, 1987).
It is worth remembering that around 85 percent of veterinary anaesthetics are administered to patients from 15kg down to 0.3kg body weight
It is worth remembering that around 85 percent of veterinary anaesthetics are administered to patients from 15kg down to 0.3kg body weight. Hence, suitable equipment is required for these very small animals that make up the vast majority of our patients.
A case for low flow
A 7.5kg Dachshund (Figure 2) presented for anaesthesia for thoracolumbar spinal decompressive surgery.
A Darvall Stinger Ultra anaesthetic machine was set up with a 0.35l CO2 absorber canister, a 0.5l breathing bag and 12mm ID heated smooth wall tubing with 180ml volume in each limb of the circuit. The system was pressure tested prior to use. The dog was then premedicated with acepromazine 0.03mg/kg and methadone 0.3mg/kg SC 20 minutes before anaesthetic induction.
Anaesthesia was induced with propofol, and the patient was intubated and connected to the breathing system and the cuff inflated. The APL “pop-off” valve was set fully open.
Anaesthesia was maintained with isoflurane delivered via a DVM Iso vaporiser. Initially, the vaporiser was set to 2.5 percent with an oxygen flow of 60ml/kg/min to flush the room air out of the Stingray breathing system (Figure 3). After five minutes, the oxygen flow was lowered to 30ml/kg/min (225ml/min) and the vaporiser set to 2 percent, with the patient’s vital signs checked and monitored. No further adjustment was needed.
Surgery duration was 80 minutes, and the total anaesthesia time was 115 minutes. Recovery was uneventful.
