Despite the recent interest and advances in intravenous anaesthetics, opioids, and infusion techniques, the vast majority of general anaesthetics continue to involve primarily inhalational agents. Low or minimal flow anaesthesia allows the anaesthetist to supply to the patient only the actual amount of drugs and oxygen being taken up, avoiding waste. The technique has been in use since the early days of general anaesthesia, with popularity varying widely over the years. There has been a resurgence of interest in low flow anaesthesia over the last few years because of:
This presentation will focus on the practical application of the
technique of low flow anaesthesia, and the new agents recently released or on the horizon
Metabolic flow = 250 ml/min
Minimal flow = 250-500 ml/min
Low flow = 500-1000 ml/min
Medium flow = 1-2 l/min
High flow = 2-4 l/min
Very high flow = > 4 l/min
Vaporiser in circuit (VIC):
Vaporiser out of circuit (VOC):
Liquid injection techniques:
Early workers developed the circle system and Water's canister, with
both VIC and VOC being popular. The popularity of low flow was enhanced by the use of
explosive cyclopropane, but later diminished with the introduction of neuromuscular
blockers and the introduction of accurate vaporisers such as the Copper Kettle and Fluotec
series. Expansionist health care economics diminished economic concerns, and the
publications of complexity of flows, uptake, and concentrations in circle also contributed
to waning popularity. Enthusiasts later described injection techniques, nomograms, etc.
Economics: The cost of volatile anaesthetics per case
decreased from $19.20 to $15.16 (US) after promotion of low flow anaesthesia as cost
containment measure at University of Michigan (NB incremental savings diminish as one
approaches closed circuit flows).
Environmental: Chlorinated anaesthetics are broken down by UV radiation and may deplete ozone.
Uptake: Uptake is relatively large in the first few minutes of anaesthesia, and then diminishes as the gradient between the patient and the circuit shrinks. The time course of this depends on the solubility of the anaesthetic - insoluble drugs like nitrous oxide, desflurane, and sevoflurane have a rapid rise in concentration and reduction in uptake, while the uptake of more soluble isoflurane, enflurane, and halothane continues with larger amounts absorbed for a long time.
Initial high flows are needed to flush the circuit, replace the FRC and to provide gases for uptake into patient. Thereafter the basic principle of low flow is to maintain constant anaesthetic state by adding gases and vapours to the circuit at the same rate that the patient consumes them.
As for high flow or i/v anaesthesia, plus:
- Halogenation only with F-, cf isoflurane.
- Lower blood solubility allows more rapid adjustment of blood level, more rapid induction & recovery.
There is little or no reason not to use low/minimal flow techniques with presently available agents, and economic & environ- mental advantages to doing so. With present understanding of anaesthetic uptake, and modern technology, there should be no increased safety concerns.
The new agents, sevoflurane and desflurane have several advantages over the present generation, agents but are not ideal and their cost-effectiveness has to be established.
1. Which of the following circuit contaminants have caused clinical
problems in man?
c) Carbon Monoxide (carboxy-haemoglobin levels of up to 30% have been reported in patients)
e) Compound A
2. Low/minimal flow anaesthesia is inherently dangerous and requires
enhanced patient monitoring because of :
a) Rapid and unpredictable fluctuations in inspired oxygen concentrations during anaesthesia
b) Difficulty with monitoring depth of anaesthesia c) Over-humidification of circuit contents
d) Exposion hazard
e) None of the above
3. Match the volatile drug with the relevant property (an agent may be
the answer to more than one question):
a) Lowest solubility
b) Best for inhalational induction
c) Has caused renal failure from fluride ion production
d) Least biodegradation
e) Least stable in vitro
Answers: a-4, b-1, c-5, d-4, e-1.
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