Low-Flow Anaesthesia

Low-flow anaesthesia:

Requires a FGF less than the patients MV
Therefore requires a rebreathing system and a CO₂ absorber.
Typically uses FGF of 1L/min
In absence of leak, flow can be reduced to the gas volume that a patient is absorbing and metabolising
This has lead to:
- Minimal-flow anaesthesia
  Uses an even lower FGF of 0.5L/min; which in normal circumstances, this gives a rebreathing fraction of ~75-80%.
- Metabolic-flow anaesthesia
  Uses pure oxygen as a carrier gas, and replacing only the gas that is consumed by patient metabolism.

Advantages of Low-Flow Anaesthesia

Benefits include:

Clinical
- Measurement of oxygen consumption
  Can be estimated by comparing FGF requirements to circuit volume.
- ↑ humidification
  Rebreathing ↑ humidity, ↓ mucociliary impairment.
- Temperature
  Rebreathing increaeses temperature of inspired gases, ↓ heat loss under anaesthesia. This can be substantial - up to 1°C in core body temperature after 1 hour of anaesthesia.
Environmental
Inhalational anaesthetic agents are potent greenhouse gases. ↓ FGF results in ↓ volatile consumption, and therefore emission of inhalational agents.
Economic
↓ use of inhalational agents ↓ the cost of delivering anesthesia.

Disadvantages include:

Circuit gas composition does not correspond to FGF composition
Change in circuit gas composition will depend on the time-constant of the circuit.
- Remember that for an anaesthetic circuit \[\tau = {Circuit Volume \over FGF - Patient \ Gas \ Uptake}\]
  A process will be 95% complete at the end of three time-constants.
  - At low flows, the time-constant for equilibration between the circuit and the FGF gas ↑
    Results in very slow equilibration between the circuit/patient and the delivered gas.
  - E.g., for a 5L circuit + FRC volume, ↑ the FGF to 5L/min will see circuit gas composition approximate FGF composition in ~3 minutes
Risk of hypoxia
If oxygen consumption exceeds oxygen delivery:
- Circuit volume will ↓
  As the overall volume of gas has ↓.
- Circuit mixture will become relatively hypoxic
  Oxygen is consumed, however nitrogen and inhalational agent amounts remain constant; this results in a ↓ FiO₂.
↑ soda-lime concentration
As less CO₂ will be scavenged, soda-lime consumption will ↑.
Loss of circuit volume
Indicated by ↓ in minute ventilation, peak airway pressure, and collapse of the bellows or resevoir bag.
- ↑ FGF to overcome leak
- Search for source of leak

Contraindications include:

Toxic gases
Adequate FGF must be used to ensure washout of toxic gases.
Smoke inhalation
- Carbon monoxide
Cyanide toxicity
Malignant hyperthermia
As volatile agents will be ceased, there is no benefit to low-flow techniques. Additionally, high FGF is required to ensure adequate washout of CO₂.
Alcohol intoxication
Prevents pulmonary clearance of gaseous alcohol.
Acetone poisoning
- Prolongs anaesthesia
- ↑ PONV
Ketoacidosis
In severe cases, may lead to accumulation of gaseous acetone in the circuit.

Technical requirements
- End-tidal gas measurement
  As end-tidal agent concentration will differ markedly from delivered agent concentration, closed-loop monitoring must be used.
- Free of circuit leaks
Estimate VO₂
- VO₂ can be crudely measured using the Brody Formula: \[ VO_2 = 3.5 \times kg\]
  This typically overestimates consumption by 10-20%, which introduces a margin of safety.
Set an FiO2 alarm
e.g. 30%.
Deliver a volume of gas greater than the patients oxygen consumption
At minimal-flows, it easier (and safter with respect to hypoxia) to use 100% oxygen as the carrier gas to prevent buildup of nitrogen and delivery of a hypoxic gas mixture; although over time this will cause FiO2 to steadily ↑.
Use an insoluble agent if possible
Desflurane and sevoflurane are better agents for use with low-flow techniques due to their shorter time constants.

Standard induction
Initial Phase
- FGF of 4.4L/min with 32% FiO₂
  1.4L/min O₂ and 3L/min N₂O; will lead to an FiO₂ of 30-40%.
- Vaporiser:
  - Sevoflurane 2-2.5%
  - Isoflurane 1-1.5%
  - Desflurane 4-6%
At target MAC (0.8-1)
- Reduce FGF to 0.5L/min (0.3L/min O₂ and 0.2L/min N₂O)
- ↑ vaporiser:
  - Sevoflurane 3-3.5%
  - Isoflurane 2.5%
  - Desflurane 5-7.5%
Adjusting
- Rapid change in agent concentration:
  - Set vaporiser to 0.5% above target concentration
  - ↑ FGF to 4.4L/min
  - At target, reduce the FGF to 0.5 L/min
- Slow ↑ in agent concentration
  - Continue FGF at 0.5L/min
  - ↑ vaporiser by 1-2% or higher
  - At target, set vaporiser to 0.5-1% higher than target
- Slow ↓ in agent concentration
  - Continue FGF at 0.5L/min
  - ↓ vaporiser by 1-3.5%
  - At target, ↑ vaporiser to its previous setting
Reversal
- Set vaporiser to 0% ~10-15 minutes prior to end of operation
- Continue FGF at 0.5L/min
- Begin spontaneous ventilation
- At the last suture, purge the system with 100% O₂ at 6L/min