Anaesthesia Equipment and Anaesthetising Facilities: Low Flow Anaesthesia Guidelines

Definition

Classification of fresh gas flow rates

Equipment

How to do it

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The delivery of anaesthesia gases in the most efficient, economical, and environmentally friendly manner is a requirement of modern anaesthesia practice. This practice requires an understanding of the physics of the delivery system and the properties of the agents being used. It is an appropriate technique for the current administration of most volatile-based anaesthetics.

Definition

Low flow anaesthesia is anaesthetic gas delivery using a fresh gas flow less than the patient’s minute volume.

Classification of fresh gas flow rates

• High-flow: greater than 4 litres per minute
• Moderate flow: 2–4 litres per minute
• Low-flow: less than 2 litres per minute
• Basal flow: 250–500 ml per minute

250 ml/min is the minimal basal oxygen requirement for metabolic processes at rest in a normothermic patient and must therefore be administered as 100% O₂.

Equipment

Anaesthesia delivery system

Standard anaesthesia equipment as per SASA guidelines with an emphasis on:

• Anaesthesia machine capable of delivering accurate, fresh gas flow of less than 1 L per minute
• Flow meters capable of increments of less than 100 ml
• Leak of fewer than 100 ml on machine and circuit check done immediately before the case
• Carbon dioxide absorber
• Circle system with unidirectional flow valves

Monitoring

• Standard routine monitors as required for any anaesthetic (ECG, pulse oximeter, BP and other monitoring according to the case)
• Inspired oxygen monitor
• Capnography
• Agent analyser
• Ventilatory pressure and tidal volume

CO₂ absorber

• Canister containing between 450ml and 3L of carbon dioxide absorbent that undergoes a colour change when exhausted (Soda lime or Baralyme undergo a colour change when exhausted).
• When the absorber is exhausted, a rise in the inspired CO₂ (F_iCO₂) is seen on the capnograph.
• Absorber must be changed once F_iCO₂ is greater than 4mmHg (0.5 kPa)

Volatile delivery

• Vaporiser out of circuit
• Desflurane, isoflurane and sevoflurane are the agents of choice
• Older, soluble, highly metabolised agents like halothane and enflurane cannot be delivered at high enough concentrations to be easily delivered with flows less than 1L
• Poorly metabolised gases like desflurane may accumulate during lengthy procedures at very low flows
• The use of nitrous oxide is not recommended when using low flows as it increases the risk of hypoxia

Gas delivery

• Gas delivery using a standard anaesthetic machine
• Flow meters must be able to deliver flows from 250ml to 10L per minute
• Flow meters must be graduated to measure accurately at flows less than 1L, preferably in 50ml to 100ml increments

Closed circuit

• Often called a circle system as gas flow is in a single direction from machine to patient and back from patient to machine
• Circuit to be able to deliver constant gas flow at low resistance with minimal leak
• Circuit to be checked before use for leaks during spontaneous and assisted ventilation
• Do not proceed if leak is greater than 100ml/min at 30cm H₂O

Unidirectional valves

• Circuit requires two unidirectional valves that move freely with low resistance to flows down to less than 250ml
• Valves must delineate the circuit’s inspiratory and expiratory limb, allowing flow to occur through the circuit in one direction only without mixing inspiratory and expiratory gases

Oxygen delivery

• Inspiratory and expiratory oxygen concentration needs to be monitored constantly
• Hypoxia can occur due to inadequate fresh gas flow, delivery of a low F_iO₂, changes in patients’ metabolic rate, and dilution by accumulating gases such as N₂O, CO₂, CO, and methane

Set up of the circuit

• Fresh gas must be delivered into the reservoir bag or ventilator bellows and not directly into the circuit
• Gas is delivered to the patient and returned via the CO₂ absorber in a circular pattern using a “closed system”. The same gas is continuously recycled with small amounts released from the ventilator or breathing valve to allow for additional fresh gas to be introduced, with one-way valves producing unidirectional flow around the circuit.

How to do it

The basic principle is that it takes a long time for any change in gas concentration to occur at low flows. The lower the fresh gas flow, the longer this change takes.

What to give

1. Oxygen: A minimum of 250ml of oxygen is needed to meet the basic metabolic requirements of a normothermic, awake patient at rest. The delivery of at least this amount of oxygen per minute is required in an anaesthetised patient. This replacement will be constant, provided the metabolic rate remains constant. At flows of less than 1L per minute, a F_iO₂ of at least 50% is recommended. If the monitored F_iO₂ decreases, the minimum amount of oxygen delivered will need to be increased.
2. All circuits have a small leak that will usually be detected by the preoperative machine check (most machines will allow a leak of fewer than 40ml per minute to pass this test). In addition to the gas loss from sampling lines and uncuffed tubes, this leak needs to be added to the minimal amount of gas delivered. Most people are comfortable with a minimum of 300–500ml per minute fresh gas flow.
3. Volatiles are delivered from a vaporiser out of circuit. This gas replaces metabolised and absorbed volatile agent and gas vented to the atmosphere. A variable replacement is needed as it depends on the solubility and amount of agent metabolised.
4. N₂O, when used as an adjuvant anaesthetic agent, must always be added to at least 250ml of oxygen. Saturation occurs quickly, and metabolism is minimal, so N₂O will start to accumulate over time. It is not recommended to use nitrous oxide with low flow.
5. Air is often added to flows above 250ml of oxygen to avoid complications from using 100% oxygen, provided the F_iO₂ is maintained.

Principles of altering gas concentrations

The following are the principles behind establishing or changing any gas concentration using low flow:

1. The entire circuit comprises about 6L - the tubing makes up about 1.5L, the ventilator or rebreathing bag about 2L, the carbon dioxide absorber 2L, and the patient’s tidal volume 0.5L.
2. The aim is to obtain a constant concentration of volatile agent throughout this 6L at the MAC value we require.
3. This 6L needs to be changed five times to reach a steady gas state, so a total of 30L of circuit gas needs to be changed.
4. If we introduce the volatile agent at 1L per minute, it will take 6 minutes to change all the gas in the circuit once, and 6 x 5 = 30 minutes to reach steady state. At 250ml, it takes 120 minutes to reach steady state.
5. One can also speed this up by setting a greater volatile concentration than is desired until the agent is at the required concentration on the agent analyser.

Starting a case

Use the above principles at the beginning of a case. The circuit contains no volatile agent. The aim is to saturate it with a certain MAC of volatile agent in a patient who will awaken quickly from the initial dose of the induction agent.

• Following an initial IV and opiate induction.
• Fresh gas flow of 2L/minute and desflurane at 12% or sevoflurane at 4% until the end-tidal agent concentration reaches the desired concentration (6% desflurane or 2% sevoflurane).
• Lower fresh gas flow to 250–500ml and monitor oxygen and volatile concentration. Adjusting the vaporiser as needed.

Changing gas concentration

• Using the above principles shows that changing the concentration of volatile to a higher or lower MAC while at low flow can take quite a long time.
• Rapid changes in concentration can be achieved by increasing the fresh gas flow to 2 litres.

Maintenance of anaesthesia

• Due to the metabolism and absorption of volatile agents, the concentration of volatile agent in the entire system will slowly decrease over time. While the circuit receives low fresh gas flow, the absorbed and metabolised agent is not completely replaced; the result is a further decrease in alveolar volatile concentration. This may result in the patient becoming aware.
• Therefore, a higher vaporiser setting than the desired end tidal agent concentration is needed.
• Minimally metabolised agents like desflurane, isoflurane and sevoflurane require a vaporiser setting of 10–20% higher than required to accommodate for this but may also start to accumulate over time.
• An agent analyser eliminates this guesswork.
• Nitrous oxide in a closed circuit will start to accumulate over time as it is minimally metabolised, resulting in hypoxia.

Switching off

1. The same principles apply to wakening the patient at the end of the procedure. If the volatile agent is switched off and low fresh gas flow maintained, the patient can take between 10 and 20 minutes to wake up while the volatile concentration slowly decreases.
2. A fresh gas flow of 6 L will rapidly flush the circuit of residual volatile agent.

Paediatrics

In paediatric anaesthesia, the same principles apply but use smaller bore tubing, size-appropriate filters and cuffed tubes.

Troubleshooting

1. Hypoxia – due to low delivered F_iO₂, dilution from other gases, increased metabolic rate.
2. Tachycardia – due to rapid changes in gas concentration with desflurane (the rule is fresh gas flow x volatile concentration should be less than 24, i.e., 2 L x 12% = 24).
3. Overdose – due to prolonged high fresh gas flow and high vaporiser setting.
4. Awareness – due to circuit leak or low vaporiser setting.
5. Hypercapnoea – carbon dioxide absorber exhaustion.
6. Prolonged awakening – fresh gas flow still low during emergence.
7. Low circuit volume – due to leaks in the circuit and can lead to inadequate ventilation as bellows and reservoir bag are not refilled, hypoxia and awareness. 

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South African Society of Anaesthesiologists (SASA) Practice Guidelines_2022_Reviewed by: E Welch