refer to those breathing systems in which an inhalation anesthetic
is vaporized by the patient's breathing. These include the original
methods of administration of inhalation anesthetics, where the agent
is poured over a pad or gauze which is then applied to the patient's
face. Such methods are obsolete, with the possible exception of
Cox's mask which may still be useful for field anesthesia of horses
in rural areas and developing countries.
The most simple
form of modern drawover system consists of two reservoir tubes,
a vaporizer and a non-rebreathing valve:
(P) inspires and expires via the non-rebreathing valve (V). Air
(A) enters the system from the atmosphere and may be supplemented
with oxygen. The tubes provide reservoirs of oxygen (if it is being
used) and anesthetic-containing gas.
systems incorporate a self-inflating bag (e.g. Ambu bag) so that
ventilation can be controlled or assisted if necessary:
are most useful in field anesthesia, developing countries and in
other situations where supplies of compressed gases are not readily
available. They are particularly useful if ether is used, owing
to the lack of respiratory depression produced by this agent.