A practical guide to low-flow anaesthesia in veterinary practice

Low-flow anaesthesia is generally considered to be anaesthesia with a fresh gas flow (FGF) of 0.5 to 1l/min (Brattwall et al., 2012). A more recent concept, however, is to use lower flows than those currently used (Hendrickx et al., 2023).

The lowest flows can only be used safely with the appropriate monitoring of inspired and expired gases, which is not readily available in many veterinary practices. This article is aimed as a practical guide to safely deliver lower flows using affordable breathing systems and commonly available monitoring equipment.

The benefits of low-flow anaesthesia in veterinary medicine

There are three main advantages to low-flow anaesthesia:

1. Humidification and warming of inspired gases: oxygen (O₂), air and nitrous oxide (N₂O) are different “carrier” gases used for anaesthesia. These are usually stored in cylinders, making them cold and dry. During anaesthesia, an endotracheal tube bypasses normal mechanisms to warm and humidify gases in the nose and mouth. Temperature and humidity in the patient’s airway are better maintained during low-flow anaesthesia, preserving respiratory and mucociliary function (Bilgi et al., 2011)
2. Cost savings: low-flow anaesthesia reduces the amount and cost of volatile anaesthetic agents, O₂ and air used during anaesthesia (Ryu et al., 2011; Doolke et al., 2001)
3. Environmental pollution: volatile anaesthetic agents are potent greenhouse gases, most of which are vented straight into the atmosphere, constituting a large proportion of the carbon footprint of human hospitals (Hu et al., 2021). Low-flow anaesthesia to reduce the amount of anaesthetic agents used will significantly reduce the environmental impact of veterinary practices

Equipment requirements for low-flow anaesthesia

• Anaesthetic machine: if an FGF of under 1l/min is used, separate flowmeters ranging from 0 to 1l/min increase accuracy. Flows are measured from the top of the bobbins and the middle of the balls
• Breathing system: circle breathing systems are essential for low-flow anaesthesia. Several designs are available which can be used on dogs weighing over 7kg. Feline anaesthesia guidelines advocate the use of appropriate circle systems in cats over 3kg (Robertson et al., 2018). Circles contain a carbon dioxide (CO₂) absorber, usually soda lime, to prevent the rebreathing of exhaled CO₂
• No leaks: all equipment (anaesthetic machines, breathing systems, capnograph monitoring lines and endotracheal tubes) needs to be leak-free to prevent gas escaping from the breathing system. Anaesthetic machine and breathing system checks before every anaesthetic are important to identify leaks and ensure patient safety
• Endotracheal tube: an appropriately sized endotracheal tube should be placed, and the cuff inflated to seal the trachea. Digital cuff inflator syringes (eg AG Cuffill) provide a measurement of cuff pressure and are considered safer than other methods to avoid excessive cuff pressures that could result in tracheal mucosal ischaemia (Hung et al., 2020)

Monitoring requirements for low-flow anaesthesia

When it comes to pulse oximetry, the monitoring of adequate haemoglobin O₂ concentration in the patient’s arterial blood is essential, aiming for readings of 95% or more.

Capnography, or the monitoring of inspired and expired CO₂ levels, gives information about the adequacy of ventilation. Monitors continuously sample gas from the breathing system at rates ranging from 50 to 250ml/min; this loss of gas should be accounted for in the FGF.

Ideally, other inspired and expired gases are monitored – inspired O₂ (FiO₂), inspired and expired inhalational anaesthetic agent, etc – with an alarm for low FiO₂ concentration. This gas monitoring allows more accurate adjustment of the FGF and vaporiser settings, particularly when using flows of under 1l/min.

Considerations for low-flow anaesthesia

Minimal FGF and safety

Although minimum patient O₂ requirements are as low as 2 to 3ml/kg/min, in practice flows of at least 0.5 to 1l/min are recommended. This gives more control over the depth of anaesthesia and allows for capnography sampling (up to 250ml/min) and minor leaks.

Carrier gases

N₂O, largely withdrawn from veterinary anaesthesia due to its environmental impact, is not taken up by the patient, whereas O₂ is used for metabolism. As a result, at low flows, O₂ (and volatile inhalational agent) concentration within the breathing system gradually reduces. Fresh gases should consist of at least 50 percent O₂ (FiO₂ over 45 percent where measured), with an FGF of at least 0.5l/min O₂ to prevent a hypoxic mix being inspired.

Minimum alveolar concentration

Minimum alveolar concentration (MAC) is the lowest alveolar concentration of the volatile anaesthetic agent that prevents a purposeful movement in response to surgical stimulation in 50 percent of patients. MAC varies in cats and dogs (Table 1) and can be reduced by drugs and regional anaesthesia techniques. As MAC is the alveolar concentration, this closely reflects the end tidal concentration, which can be very different to the vaporiser setting at low flows.  

Inspired anaesthetic concentration

In higher-flow anaesthesia, the inspired anaesthetic concentration is closer to the vaporiser concentration setting because a relatively high proportion of the breathing system and patient’s lung volume is replaced by the FGF containing the anaesthetic. In low flows, a smaller volume of fresh gas and anaesthetic agent enters the breathing system, so the inspired concentration may be very different (usually lower) than the vaporiser setting. This is particularly significant at the start of anaesthesia and in larger patients.

A four-phase technique for low-flow anaesthesia

Four phases can be considered for low-flow anaesthesia in veterinary practice:

1) Denitrogenation and wash-in

The breathing system and patient lungs contain room air with 21 percent O₂. After the induction of anaesthesia and connection to the breathing system, a higher FGF and anaesthetic vaporiser setting is used to rapidly replace the breathing system air with O₂ and the anaesthetic agent. Monitoring of the end tidal anaesthetic agent can help guide adjustments of the vaporiser, aiming for a reading closer to the MAC.

2) Maintenance

Once the breathing system gases have been largely replaced by O₂ and the anaesthetic agent, the FGF can be reduced to 1l/min (or an FGF of 0.5l/min with gas monitoring). As the FGF is lower than the patient’s minute volume, delivered gases will be diluted by the gases in the breathing system. Vaporiser concentrations need to be set higher than the desired end tidal anaesthetic concentration.

3) Wash-out

FGF is increased to 2l/min with the vaporiser switched off to replace the breathing system gas with O₂.

4) Change of depth of anaesthesia

Increasing the FGF (2l/min or above) with a higher or lower concentration of inhalational agent for a few minutes can be used to increase or decrease the depth of anaesthesia. The reservoir bag contents can also be dumped to change the breathing system gases more rapidly.

An even more rapid increase in anaesthetic depth can be achieved using appropriate boluses of the induction agent.

The potential problems of low-flow anaesthesia

Hypoventilation

Circle breathing systems contain components that increase the resistance to breathing and can result in hypoventilation (evident by increased end tidal CO₂ levels or increased respiratory effort), particularly in smaller patients.

Manufacturer’s instructions for these systems should be consulted before use in small dogs and cats. Hypoventilation can be minimised by reducing dead space and using smooth internal bore tubing.

Light plane of anaesthesia

Inspired concentrations of the inhalational anaesthetic agent need to be higher with low flows compared to higher flows. This is because the amount of the anaesthetic agent entering the breathing system will be lower and diluted by gases already in the system.

Hypoxic gas mixture

Fresh gases should consist of at least 50 percent O₂ with an FGF of at least 0.5l/min O₂ to minimise the risk of inspiring a hypoxic gas mixture.

Soda lime

Soda lime contains a dye which changes colour as the granules become exhausted from absorbing CO₂. The colour can revert with time, so it is important to replace the absorbent as soon as practicable.

Compound A

When used with low flows, sevoflurane has been associated with the production of compound A due to a reaction with CO₂ absorbents containing potassium or sodium hydroxide. Compound A has been shown to be nephrotoxic in rats, but there is no evidence of toxicity in people (Kennedy et al., 2019).

Datasheets advise against long-duration low-flow sevoflurane anaesthesia, but a recent editorial argues that there is no rationale to support the avoidance of low-flow anaesthesia with sevoflurane (Kennedy et al., 2019).