Gas Supplies
The machine’s primary function is to reduce supply-line pressures, mix a number of gases
(most typically oxygen, nitrous oxide, and air), and deliver a controlled output to the
breathing circuit. Primary gas supplies feed the machine 50 psi. A pressure-relief valve
opens above 75 psi in case of infrastructure system failure. Technicians and engineers
need to be familiar with a number of pressure-measurement units. The most common are
pounds per square inch, millimeter of mercury, and centimeters of water (psi, mmHg,
and cmH2O, respectively). A rough equivalent is that one psi is about 50 mmHg and about
70 cmH2O. In the same units of measure, an anesthesia machine needs to safely reduce
and control gases fed at 3500 cmH2O and to supply them to the breathing circuit normally
operating at about 35 cmH2O. In other words, the supply pressures are 100 times that of
the breathing circuit. Machines are constructed with a high-pressure side and a lowpressure
side. The high-pressure side is primarily the supply, and the low-pressure side is
any part operating near breathing-circuit pressure.