Understanding Anesthesiology - The Global Regional Anesthesia ...

and trauma surgery pose a higher risk of awareness because
of the nature of the anesthetic given for those procedures.
It may be prudent to warn such patients of the
risk pre-operatively.
Intra-operatively, care should be taken to ensure delivery
of adequate amounts of hypnotic drugs such as inhaled
agents, propofol, benzodiazepines or ketamine.
Opioids alone provide very little hypnosis and muscle
relaxants provide none whatsoever! Signs of awareness
should be sought. In an un-paralyzed (or partially paralyzed)
paralyzed patient, this includes movement.
However, a fully paralyzed patient is only able to communicate
through the autonomic nervous system with
signs of sympathetic hyperactivity, such as hypertension,
tachycardia, sweating and tearing. Not surprisingly,
the overwhelming majority of cases of awareness
have been reported in paralyzed patients.
Positioning
The patient is positioned to facilitate surgical access. Depending
on the procedure, the patient may be placed in
the supine, prone, lateral, lithotomy, jack-knife, kidney
or even the sitting position to name but a few. Most of
the consequences of positioning involve the cardiovascular,
respiratory and peripheral nervous systems.
Kinking of, or pressure on major vessels leads to decreased
venous return, decreased cardiac output and
hypotension. This is particularly relevant when the
prone or kidney position is used. In the semi-sitting position,
venous pooling in the legs has a similar effect.
Very occasionally, surgery is performed in the sitting
position which is associated with the risk of venous air
embolism.
The airway may become obstructed or dislodged while
the patient is in the prone position. The prone, trendelenburg
and lithotomy positions may cause an upward
displacement of the diaphragm due to an increase in
intra-abdominal pressure. This leads to ventilation/
perfusion mismatching and decreased lung compliance
which may manifest as hypoxemia, hypercarbia or increased
airway pressure.
Nerve injury results from compression on pressure
points or stretching. Other factors such as prolonged
surgery, hypothermia, hypotension, obesity and diabetes
may play a role in increasing the risk of a postoperative
neuropathy. The ulnar nerve, because of its
superficiality, is at risk of compression in almost any position.
Padding is commonly used but has not been
shown convincingly to be helpful. Careful positioning
is probably most important in this regard. The brachial
plexus is at risk of stretch injury when arms are abducted
in the supine position. The angle of abduction
should be kept below 90 degrees and the head should
be turned slightly toward the abducted arm. Many
nerves including the sciatic, lateral femoral cutaneous
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