Understanding Anesthesiology - The Global Regional Anesthesia ...

NITROUS OXIDE
Class
Nitrous oxide is an inhaled agent but not a volatile
agent. It is used as an adjunct to general anesthesia. It
has a weak effect and therefore cannot be used as the
sole agent for general anesthesia and is most commonly
used in combination with a volatile agent. It can
be used on its own for sedation or analgesia as can be
seen in the obstetric or dental setting.
Mechanism of Action
Uncertain
Dose
Delivered in concentrations of up to 70% in oxygen. Actual
MAC is 104%.
Onset
Immediate due to very low solubility.
Duration
Offset of effect is rapid after discontinuation.
Elimination
Pulmonary
Effects
CNS
N 2 O is a potent analgesic. It increases cerebral metabolic
rate, cerebral blood flow and intracranial pressure
and is therefore not a good choice for patients with decreased
intracranial compliance.
CVS
N 2 O has a mild sympathomimetic effect but causes direct
myocardial depression. The net effect is a modest
decrease in blood pressure and heart rate. Increased
coronary tone may exacerbate ischemia in susceptible
patients.
Respiratory
N 2 O produces mild respiratory depression which is potentiated
by opioids, hypnotics and volatile anesthetics.
It has no bronchodilatory effect. It exacerbates pulmonary
hypertension.
Misc.
N 2 O expands the volume of gas-containing spaces as
N 2 O diffuses across membranes more readily than nitrogen
can diffuse out. Thus the size of a pneumothorax,
emphysematous bleb or distended bowel loop will increase
when N 2 O is used. Bone marrow suppression
due to inhibition of methionine synthetase, can occur if
N 2 O is used for extended periods. N 2 O enhances
opioid-induced rigidity. Finally, N 2 O is an operating
room pollutant; N 2 O levels (in parts per million) in the
operating room environment are measured regularly to
comply with workplace safety regulations.
Contraindications
Raised intracranial pressure, pneumothorax or bowel
obstruction. Should be used with caution in patients
with coronary disease or emphysema.
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