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London 2003 |
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Getting Your Department to Use Low Flows
Dr Mike Logan
Royal Infirmary of Edinburgh
Edinburgh, Scotland
Any attempt to alter the way that anaesthetists perform their work is often met with automatic resistance. Each anaesthetist has refined his technique over many years and knows what works best in his or her hands. When introducing low flow techniques to a department one is instantly faced with the problem that all the anaesthetists already give ‘perfect inhalational anaesthesia’. Therefore why should they change? The advantages of low flow have to be so convincing that everyone can be easily persuaded to adopt the technique. Fortunately, low flow anaesthesia is a remarkably simple technique that everyone can use with ease. All that is needed is a little understanding of how the circle system acts as a buffer between the fresh gas delivery from the anaesthetic machine and the patient. The circle thus slows down changes made in the concentration of the gases and volatile agents. Unfortunately, there is often a tendency for enthusiasts to totally immerse the highly predictable concentration changes within a series of uptake dynamic formulae, graphs and time constants. This provides a level of complexity which is not required to use a circle system safely at low flows. Try asking a low flow enthusiast what the time constant of his system is at 1 l/min flows. He won’t be able to tell you. And that proves that he did not need to know it to use the technique.
Not only is the theory scary to the non‑mathematically minded but there is a tendency for protagonists to show off when teaching someone low flow techniques for the first time. Closed circle in‑circuit vaporiser techniques take even more understanding and can terrify those who are not accustomed to the circle system. There is also the tendency for teachers of low flow anaesthesia to instantly launch into a list of problems just to totally banish any hope of the novice feeling comfortable with circle systems. Early hints about hypoxic mixtures being possible despite flow control interlocks and the production of carbon monoxide within the system will rapidly leave the enquirer wondering why circle system anaesthesia has not been banned. Equally, those new to the technique will be unimpressed if maintenance flows are at 4l/min during the in‑theatre tutorial on low flow anaesthesia.
The significant benefits of low flow anaesthesia for the budget of the department and for the global environment are enormous. However, neither of these aspects are as instantly demonstrable as changes in nitrous oxide uptake or volatile agent concentration differences between the vaporizer and that inspired. Financial savings and environmental factors can not be measured on our multicoloured anaesthetic monitors. Thus the prime reasons for using low flow are often eclipsed by the theoretical aspects of performing the techniques.
In most countries at the present time, financial aspects of medical practice appear to command a far higher priority than effects on the global environment. For a Department to be enthused into adopting low flow anaesthesia significant financial savings should be demonstrable. However, these savings must be significant to warrant the change of practice. In addition, the money saved must be made available to the Department for other advances. This must be obvious and self‑evident to all the anaesthetists so that there is the incentive to change their practice.
Teaching Low Flow Anaesthesia
Professor Emeritus W W (Bill) Mapleson
University of Wales College of Medicine, Cardiff
This will be a largely personal, historical, exploration of
the methods of teaching low-flow anaesthesia over the last 40 years. Methods include:
1
Mathematical modelling (in an
appendix!) with the results displayed as graphs
2
The water analogue
3
Animated overhead projection
4
Videos
In method 1, the mathematical model has no teaching value of
itself for those who do not wish to wrestle with the maths; but the graphs have
been of value in guiding clinical practice—from the early days of halothane to
the “Ideal fresh-gas flow sequence at the start of anaesthesia” that I
presented to the inaugural ALFA meeting in 1996. But the graphs do not convey real
understanding of the processes that lead to them.
The water analogue is usually restricted to representing what
happens in the patient, not the breathing system. It has been very successful
in giving understanding of the mechanisms of pharmacokinetics within the
patient, and can be used by the student to think through his own “what if”
scenarios. Although it can be extended
to include a representation of the breathing system that is limited to a
single-compartment (plus vaporizer and fresh-gas flow) with all gas in the
circle at the same, say inspired, concentration. Therefore a spill pipe connected to the
breathing-system compartment will represent spill only of inspired gas. However, a spill pipe emanating from the lung
compartment will represent spill of alveolar mixture; and one emanating from
the vaporizer compartment will represent spill of fresh gas. If necessary all three can be present—but
there is no means of working out which type of spill is correct.
Animated overhead transparencies are excellent for
demonstrating the multitude of different possible circle systems and can be
used to deduce approximately what the composition of the spill gas will be for
any particular configuration.
Finally a video can show a simulation of the gases
circulating in a breathing system: gases “inspired” and “expired” by a
pulsating lung, taken up into the pulmonary blood flow, and spilt through the
overflow valve. In the process, the way
in which concentrations in different parts of the system rise and fall with
time can be seen. It is hoped that this
will enable the student to carry away a visual image of what is happening in
the circle and perhaps make deductions about different circumstances.
