The discovery of the first inhalational anaesthetics in 1846
changed how surgery was carried out. Instead of needing to complete a procedure
in the fastest time possible amidst the protests of a fully aware patient, a
surgeon was able to attempt much more ambitious techniques while the patient
was held in a painless state of unconsciousness. Today, anaesthetic drugs can
be administered intravenously (e.g. Thiopental) or by inhalation (e.g.
Isoflurane). There are many levels of anaesthesia ranging from full
consciousness through sedation down to the loss of consciousness, loss of
reflexes and analgesia. Additionally, cardiac and respiratory muscle
contraction are weakened at surgical doses and for this reason, there is often
a narrow margin between surgical anaesthesia and respiratory death.
Anaesthetists have the fiddly task of monitoring and adjusting the level of
anaesthesia throughout surgery.
Anaesthesia of a patient occurs in three main phases:
induction, maintenance and recovery. A common procedure in major surgery is to
induce unconsciousness rapidly with an intravenous agent, maintain
unconsciousness using inhalational anaesthetics and withdraw them in the
recovery stage. Additional application of painkilling drugs such as opioids and
neuromuscular blockers (relaxants) is often made. The speed of recovery needs
to be maximised so that the chance of respiratory failure is kept low.
How do these drugs actually work? Inhalational and intravenous
agents are all soluble in lipids to varying degrees and this is an important
factor determining their characteristics. This solubility means that they
easily cross lipid-based cell membranes and alter cellular function. It is
thought that anaesthetics accumulate in cell membranes and influence the
excitability of the cells. This can be through the increase of inhibitory
transmission or the decrease of excitatory transmission or both. By doing this,
anaesthetics are able to depress the nervous system and induce a loss of
function.
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This lipid solubility also affects other aspects of
anaesthesia. Inhalational anaesthetics must cross from the lungs into the
bloodstream and often take longer to exert their effect than injected
anaesthetics. In addition to higher potency, high lipid solubility in these
agents means that the onset and recovery from anaesthesia is slower. If a
patient has higher amounts of fatty tissue such as in obesity, the extra fat
sponges up large amounts of the drug and as a result, obese patients are much
harder to anaesthetise.
Though anaesthesia always carries risks, these are often
short-term. One long-term risk that has been hypothesised is the effect of
anaesthetics on child development. Epidemiological studies and preclinical studies
on rodents have suggested that the depressant effect of anaesthesia may have a
permanent impact in the maturation of infant nervous systems that are very
sensitive to environmental factors. In reality, however, this risk is not
proven since the anaesthesia in these studies is difficult to separate from
diseases that the child had at the time and translation of rodent studies to
human is also difficult e.g. differences in development rates.
In summary, general anaesthetics are an essential part of
surgery due to inducing a loss of consciousness that makes procedures much
easier for all parties. Their non-specific mechanism of action also means that
there are many risks, especially in the respiratory system.
