69. Pre-Medication Flashcards
What are the indications for pre-medication in modern anaesthetic
practice?
This question can be answered in a list fashion in the knowledge that the
examiner will want you to elaborate on a number of your answers. The main
indications are as follows:
Think of the seven As
Anxiolysis
Amnesia
Anti-emesis
Analgesia – systemic and topical (for venepuncture)
Antacids
Antisialogogues
Additional – oxygen, nebulisers, steroids, heparin, etc.
Tell me what you would use for: ‘anxiolysis/amnesia’
It is worth mentioning that the pre-operative visit is possibly the most
important component of anxiolysis by establishing a rapport with the patient,
discussing the anaesthetic technique and answering any questions they may
have. Parental anxiety in paediatric practice can also be addressed at this stage
Benzodiazepines are probably the most commonly prescribed
pre-medicants. They act by enhancing GABA, an inhibitory neurotransmitter
that causes an influx of chloride ions thereby hyperpolarising the neurone.
They produce anxiolysis, amnesia and sedation and can be given orally,
intramuscularly or intranasally. Typical doses are:
Temazepam 10−30 mg orally in adults
0.5−1 mg/kg orally in children upto 20mg
Midazolam 0.2−0.75 mg/kg orally in children (max 20 mg)
5−10 mg i.m.
0.2−0.3 mg/kg intranasally
The α2-agonists clonidine and dexmedetomidine reduce sympathetic
outflow and have been used as pre-medicants with sedative, anxiolytic and
analgesic properties.
‘Anti-emesis’
Most anaesthetists would target specific groups of patients at high risk of
post-operative nausea and vomiting for pre-operative anti-emetics:
Previous history of PONV / motion sickness (three incidences of PONV)
Those having ‘high risk’ surgery, e.g. gynaecological, upper abdominal,
middle ear and squint surgery.
Females (2–4 that of males)
Other risk factors – use of opioids, nitrous oxide, volatile versus TIVA
Dopamine antagonists:
Histamine antagonists:
Muscarinic antagonist
5HT3 antagonists
Dopamine antagonists:
This group includes the phenothiazines
(commonly prochlorperazine),
butyrophenones (droperidol) and metoclopramide.
The evidence for the efficacy of these drugs is often variable
and they can produce extra-pyramidal side
effects, e.g. dyskinesia, tremor, dystonia and
oculogyric crisis.
Histamine antagonists:
These act directly on the vomiting centre, e.g. cyclizine
Muscarinic antagonists
This group, which includes hyoscine and atropine,
is probably used less commonly than in times
when reducing excessive secretions was an
important component of pre-medication. Side
effects include dry mouth, blurred vision,
sedation and disorientation in elderly patients.
5HT3 antagonists
: The advantage of these drugs, e.g. ondansetron
is their efficacy and side effect profile compared
to the more traditional agents. They are,
however, more expensive.
Other modalities include:
Dexamethasone The mechanism of action of dexamethasone is
poorly understood.
Acupuncture and acupressure
NK1 antagonists
Cannabinoids
It should be noted that the use of anti-emetics as part of a pre-med does not
reduce PONV any more than giving them at the end of surgery.
‘Analgesia’
Routine opioid pre-medication for elective surgery is used less frequently than
in years gone by and the concept of pre-emptive analgesia
(modulating spinal cord nociceptive transmission)
has yet to be translated into a proven clinical
entity.
Treating acute preoperative pain should be guided by the clinical situation.
EMLA
In paediatric practice, EMLATM cream is commonly used as a topical
anaesthetic before venepuncture.
This is the eutectic mixture of local anaesthetics and is a mixture of the unionised forms of lignocaine and prilocaine. It should be applied for at least 1 hour with an occlusive dressing
covering it.
‘Antacids’
The overall incidence of aspiration related to anaesthesia has been quoted as
1:3216, with a higher incidence for emergency surgery (1:895). Of these, 64%
do not develop any further symptoms and 20% require mechanical ventilation.
It is interesting to note that Warner et al. (1993) found no difference in the aspiration rate if pharmacoprophylaxis was used or not.
There are many risk factors that have been associated with peri-operative
aspiration, e.g. emergency surgery, obstetrics, obesity and hiatus hernia.
Historically, a significant residual volume of gastric juice (>0.4 ml/kg) and low
pH (below 2.5) were thought to be important factors.
This has since been
questioned.
The main drugs used to alter gastric secretions are:
H2-antagonists – these agents, e.g. ranitidine alter both the production and
pH of gastric contents.
Sodium citrate is used to neutralize the pH of gastric contents, particularly
in the obstetric setting
Prokinetic agents such as metoclopramide
