Plane 3: total analgesia
This plane is usually obtained with concentrations of 50-70% nitrous oxide (50-30%
oxygen). It has been claimed, that analgesia is so complete that extraction of teeth
may be carried out in this plane. This has not been our experience. In this plane there
is an increased tendency to dream. It is important to recognize that in a small number
of patients as little as 50% nitrous oxide may bring about loss of consciousness. It is
for this reason that dentists must exercise considerable caution if the concentration of
gas coming from the machine rises above 40% nitrous oxide.
If the patient does become too deeply sedated and enters this third plane of total
analgesia, he or she begins to lose the ability to independently maintain an open
mouth and will be unable to co-operate or respond to the dentist's requests. If this
'open mouth' sign is lost, the operator can be sure that the patient is too deep in the
plane of total analgesia and within a few minutes is likely to enter the plane of light
anaesthesia. It is for this reason that a mouth prop must never be used, for if a prop is
used the open mouth sign would not function.
If sedation is too deep and the patient shows signs of failing to co-operate, then the
dentist should reduce the concentration of nitrous oxide by 10 or 15% for a couple of
minutes. If it is considered necessary to lighten the sedation even more rapidly, the
nasal hood should be removed and the patient allowed to breathe ambient air. The
patient will return to a lighter plane within 15-20 s.
This plane of total analgesia is regarded as a buffer zone between the clinically useful
planes of moderate and dissociation sedation and analgesia, and the potentially
hazardous plane of light anaesthesia.
4.11.8 Clinical application
The technique of nitrous oxide sedation can be used for a wide range of procedures
involving the cutting of hard or soft tissue where local anaesthesia will usually be
needed to supplement the general analgesia from the nitrous oxide. The major
disadvantage (or minor if handled properly) is the inconvenience of the nasal hood
restricting access to the upper incisor area if an apicectomy is required. This problem
can be overcome by careful retraction of the upper lip and counterpressure from the
thumb held on the bridge of the nose.
4.11.9 Scavenging
Repeated exposure to environmental pollution with nitrous oxide can cause
megaloblastic anaemia and problems with both conception and pregnancy. The
Control of Substances Hazardous to Health (COSHH) advises that over a time-
weighted average (TWA) of 8 h, the exposure should not exceed 100 p.p.m. This is
five times lower than the safest dose found in animal studies.
Surgery contamination is affected by the modus operandi of the dental surgeon.
Considerable care needs to be taken to discourage the patients from mouth breathing,
to use rubber dam whenever possible, and ensure that full recovery is carried out with
the nasal hood in place. Effective scavenging equipment is extremely simple in design