Understanding Anesthesiology - The Global Regional Anesthesia ...

Contraindications to the inhaled agents
The use of volatile agents is absolutely contraindicated
in patients who are known or suspected to have malignant
hyperthermia.
The use of nitrous oxide is contraindicated in patients
with pneumothorax or bowel obstruction. As N2O
raises intracranial pressure, its use is avoided in patients
with intracranial pathology. Caution should be
used in those patients with coronary artery disease or
emphysema.
Opioids
Opioids are used intra-operatively to provide analgesia,
and to reduce the requirement of other maintenance
agents. The commonly used intravenous agents
are the synthetic opioids fentanyl, sufentanil, remifentanil
and alfentanil. They are favoured by anesthesiologists
over the more familiar agents, such as morphine
and meperidine. Their shorter duration of action allows
finer titration to provide adequate analgesia during
the variable, but intense nature of surgical stimulation,
while still allowing for awakening at the end of
the procedure. While there are many different opioids
available for use, the discussion below is limited to the
three synthetic agents which are most commonly used
in anesthetic practice.
blunt the response to intubation, which is a very stimulating
maneuver, but it establishes a plasma level of
opioid which can then be supplemented as needed
throughout the surgical procedure. In the case of remifentanil,
which is extremely short-acting, a bolus followed
by an infusion is most practical.
Mechanism of action
There are five different opioid receptors of which the
most clinically relevant are the Mu and Kappa receptors.
Binding to different receptors produces distinct responses.
Activation of Mu receptors produces analgesia,
respiratory depression, bradycardia, euphoria and
decreased gastrointestinal motility. Binding to Kappa
receptors produces analgesia, sedation and meiosis.
The major receptors for analgesia are the Mu-1 receptor
at the periaqueductal gray area of the midbrain and the
Kappa receptor at the substantia gelatinosa of the spinal
cord. Each opioid has its own unique profile of agonism
and antagonism for each receptor. Unfortunately,
an agent which possesses agonism exclusively at the
analgesia receptors has not yet been developed.
Dose, onset, and duration
All opioids are relatively lipid soluble. The greater the
lipid solubility, the greater the potency. As a general
rule:
Usually, an opioid is administered in the form of a loading
dose, prior to induction. Not only does this help to
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