23 General Anaesthesia and Oscillations in Human Physiology …
363
cord there are a two-digit billion number of neurons, each one of which has hundreds
to thousands of contacts with neighbouring cells. Thus, how should these cells and
their interactions be monitored in terms of identifying general anaesthetic action, or
even more difficult: to determine degree of action in a quantitative way? The answer
throughout the soon 175 years of history of general anaesthesia has been to monitor
surrogate or secondary signs of unconsciousness and diminished reflex actions. The
very simple end logic monitoring will just be to test unconsciousness and immobility
when you make a small wound in the skin with the scalpel: Does the patient move?
Does the patient moan or show any signs of discomfort? If no—the surgeon may
go ahead, if yes—more anaesthetic drug effect is needed. While this may work
in some way, there are three problems with this approach: (1) The testing has to
be done frequently during a procedure, because different types of surgical activity
will require different level of anaesthetic drug effect. (2) Frequent testing will delay
and complicate surgery, and there is a risk for discomfort as well as vigorous and
dangerous movements if the stimuli is too strong (3) There is no good way to ensure
that anaesthesia is not overdosed: a non-moving patient may be a sign of adequate
dose level or a profound overdose.
Thus, other signs of low, medium or strong anaesthetic effect without necessarily
testing with a knife have to be looked for. Also, the term “effect” becomes problematic, as anaesthetic effect should be determined in a situation with ongoing surgery.
This has lead to use of the term “anaesthetic depth” to define quantitatively the total
impact of anaesthetic dosing in the individual. The idea is that an adequate depth
of anaesthesia should be established without testing before start of surgery, and still
ensure close to 100% guarantee of no movement or moaning during subsequent
surgery.
During the first decade of ether as a the mono-anaestetic drug, the position and
size of the pupils were used as a surrogate sign of anaesthetic effect on the brain,
together with signs of respiratory and circulatory depression. With the advent of
intravenous agents, i.e. hypnotics and opioids, often used in mixture with inhalational agents; the pupil signs became less specific and the baseline monitoring of
anaesthetic effect shifted towards simple general features of unconsciousness: low
blood-pressure, low heart rate, attenuation or cessation of spontaneous ventilation,
no gross movements upon vigorous surgical stimulation etc. Signs of sympathetic
stimulation; such as sweating [4], tear production, together with increased heart rate
and blood-pressure were taken as signs of inadequate depth of anaesthesia with the
need of either increased anaesthetic drug dosing or reduced surgical stimulation, or
both.
Because minor movements or muscle contractions may occur even if the depth
of anaesthesia is otherwise adequate (i.e. the patient is unconscious), the use of
curare drugs became popular from the mid 20
th -century in order to reduce the dose
of anaesthetic drugs, resulting in less hemodynamic depression and faster recovery
after end of anaesthesia. Curare is also needed when even a slight movement of the
patient during surgery may have serious consequences, such as during neurosurgery,
eye surgery, middle ear surgery or other areas of microsurgery. However, with the
successful advent and use of curare a new problem emerged: some patients got too
363
cord there are a two-digit billion number of neurons, each one of which has hundreds
to thousands of contacts with neighbouring cells. Thus, how should these cells and
their interactions be monitored in terms of identifying general anaesthetic action, or
even more difficult: to determine degree of action in a quantitative way? The answer
throughout the soon 175 years of history of general anaesthesia has been to monitor
surrogate or secondary signs of unconsciousness and diminished reflex actions. The
very simple end logic monitoring will just be to test unconsciousness and immobility
when you make a small wound in the skin with the scalpel: Does the patient move?
Does the patient moan or show any signs of discomfort? If no—the surgeon may
go ahead, if yes—more anaesthetic drug effect is needed. While this may work
in some way, there are three problems with this approach: (1) The testing has to
be done frequently during a procedure, because different types of surgical activity
will require different level of anaesthetic drug effect. (2) Frequent testing will delay
and complicate surgery, and there is a risk for discomfort as well as vigorous and
dangerous movements if the stimuli is too strong (3) There is no good way to ensure
that anaesthesia is not overdosed: a non-moving patient may be a sign of adequate
dose level or a profound overdose.
Thus, other signs of low, medium or strong anaesthetic effect without necessarily
testing with a knife have to be looked for. Also, the term “effect” becomes problematic, as anaesthetic effect should be determined in a situation with ongoing surgery.
This has lead to use of the term “anaesthetic depth” to define quantitatively the total
impact of anaesthetic dosing in the individual. The idea is that an adequate depth
of anaesthesia should be established without testing before start of surgery, and still
ensure close to 100% guarantee of no movement or moaning during subsequent
surgery.
During the first decade of ether as a the mono-anaestetic drug, the position and
size of the pupils were used as a surrogate sign of anaesthetic effect on the brain,
together with signs of respiratory and circulatory depression. With the advent of
intravenous agents, i.e. hypnotics and opioids, often used in mixture with inhalational agents; the pupil signs became less specific and the baseline monitoring of
anaesthetic effect shifted towards simple general features of unconsciousness: low
blood-pressure, low heart rate, attenuation or cessation of spontaneous ventilation,
no gross movements upon vigorous surgical stimulation etc. Signs of sympathetic
stimulation; such as sweating [4], tear production, together with increased heart rate
and blood-pressure were taken as signs of inadequate depth of anaesthesia with the
need of either increased anaesthetic drug dosing or reduced surgical stimulation, or
both.
Because minor movements or muscle contractions may occur even if the depth
of anaesthesia is otherwise adequate (i.e. the patient is unconscious), the use of
curare drugs became popular from the mid 20
th -century in order to reduce the dose
of anaesthetic drugs, resulting in less hemodynamic depression and faster recovery
after end of anaesthesia. Curare is also needed when even a slight movement of the
patient during surgery may have serious consequences, such as during neurosurgery,
eye surgery, middle ear surgery or other areas of microsurgery. However, with the
successful advent and use of curare a new problem emerged: some patients got too