In either the portable or central systems, the O2 cylinder will become depleted before the cylinder of N2O. Approximately 2.5 O2 cylinders will be used for each N2O cylinder of comparable size. It is obvious that a potentially dangerous situation exists because if the O2 cylinder becomes depleted during a procedure, the patient could conceivably receive 100% N2O.
The O2 fail-safe system is designed to prevent this from happening by automatically terminating the flow of N2O whenever the delivery pressure of O2 falls below a predetermined level. For example, both N2O and O2 are delivered to the patient at a pressure of approximately 50 psig. When the pressure in the O2–compressed-gas cylinder nears zero (but is not quite at zero), the delivery pressure of O2 through the reducing valve can no longer be maintained at 50 psig. As this pressure falls (e.g., to 40 psig), the O2 fail-safe mechanism is activated and the flow of N2O gas (from a cylinder that may be almost full) is terminated. The patient at no time receives 100% N2O. Several other safety devices are activated once the O2 fail safe is brought into use. These are discussed in the following paragraphs.