Anesthesia is the use of drugs to prevent or reduce pain during a medical procedure.
There are two major classes of drugs: - Local anesthetics: these drugs block transmission
of pain signals from peripheral nerve endings to the central nervous system.
And - General anesthetics: these act on the central
nervous system itself to induce unconsciousness and total lack of sensation.
There are 3 major categories of anesthesia procedures:
- Local anesthesia: a local anesthetic is administered directly to the site of procedure
to numb a small area such as a tooth during a dental manipulation.
- Regional anesthesia: a local anesthetic is injected near a cluster of nerve roots
to prevent pain sensation from the area innervated by those nerves.
Epidural given to women in labor is an example of this type.
- General anesthesia: general anesthetics are used to suppress the entire central nervous
system, resulting in loss of consciousness.
A cocktail of several drugs are inhaled, given intravenously, or both.
This type is used for major surgical procedures.
Apart from pain management, general anesthesia has some other goals: prevent formation of
new memories, relax muscles, and suppress autonomic response to surgical injuries which
could otherwise be extreme and harmful.
General anesthetics are commonly used in combination with other drugs to achieve these end points.
An example of general anesthetic drug is Propofol.
The exact mechanism of action of Propofol remains unclear, but it is thought to inhibit
responsiveness of neurons via its binding to GABA receptor.
GABA is a major inhibitory neurotransmitter in the central nervous system.
Upon binding, it triggers GABA receptor - a ligand-gated chloride channel - to open and
allow chloride ions flow into the neuron, making the cell hyperpolarized and less likely
to fire.
In other words, GABA makes the brain cells less responsive to new stimuli.
Propofol binding has been proposed to potentiate GABA receptor, keeping the channel open for
longer time and thus exaggerating this inhibition effect.
It is believed, however, that under anesthesia the brain does not simply shut down.
Instead, the connections between different parts of the brain are lost.
Using various brain imaging techniques it’s been shown that an anesthetized brain is still
reactive to stimuli such as light and sounds, but somehow this sensory information is not
processed resulting in no further consequences.
A variety of anesthetic drugs are available, each of which may have different target molecules
in the brain.
However, if used at a high enough dosage, they can all cause unconsciousness.
This is probably because consciousness is the result of a complex network of various
brain functions, disruption of any of which could result in network dysfunction.
Emerging from unconscious state is not simply the result of drugs wearing off.
As the connections between parts of the brain were lost, the brain has to somehow find the
way to connect them back upon awakening.
This usually happens in a certain order: the most basic and essential functions, such as
respiratory and digestive reflexes, come back first, more complex brain functions return
after.
This may explains why older patients and people with pre-existing neurological conditions
may take longer to recover all cognitive brain functions.
The risk and extent of postoperative delirium – a state of mental confusion after surgery
- are also higher in these patients.
The right dose of overall anesthesia is critical.
It is usually calculated based on patient’s weight, age and medical history.
Past or current uses of recreational drugs also have to be taken into account.
Too much anesthesia results in a too deep state of unconsciousness, and consequently
greater risks of postoperative complications and long-term cognitive dysfunction.
On the other hand, a too low dose may cause the patient to wake up during the surgery,
a phenomenon known as anesthesia awareness, which might be a traumatic experience to some
patients.