We understand that the use of nitrous oxide in anesthesia is not common in many hospitals. For those who do use it, we have integrated safety features in the UAM that are important to prevent hypoxic mixtures.

Modern anesthesia machines have safety features that prevent the delivery of gaseous mixtures with oxygen concentrations of less than 25%. This can be achieved in many ways, including one or more of these methods:

• Mechanical cut-off: a chain links the oxygen and nitrous oxide flow control valves. Increasing the flow rate of nitrous oxide leads to a proportional increase in oxygen flow rate. Alternatively, if the oxygen levels drops below 25%, the chain prevents the nitrous oxide from turning on.
• Pressure Sensor Shut-off (Fail-Safe) Valves: When the main ON/OFF switch is in the ON position, oxygen pressurizes and holds open a pressure-sensor shut-off valve. These valves reduce or interrupt the supply of nitrous oxide and other hypoxic gases to their flowmeters if the oxygen supply pressure falls below the threshold setting.
• Solenoid Valve: An oxygen sensor continuously measures the oxygen concentration. Nitrous oxide flow is switched off automatically by an electronic solenoid valve when oxygen concentration falls under 25%.

This last method is what is used in the UAM. Its oxygen sensor continuously measures the oxygen concentration in the gas mixture entering the breathing block. If the oxygen concentration falls below 25%, the oxygen monitor produces an audible and visual alarm and a solenoid valve closes and completely cuts off the nitrous oxide supply.

The default position for the solenoid valve on the UAM is closed. It must receive an electrical current from the oxygen monitor with a reading above 25% in order to open (the oxygen monitor must be powered on and working correctly). The flow of nitrous oxide will then shut off if there is a decrease in oxygen below 25%, an exhausted oxygen sensor, or an exhausted battery because all of these will cause an interruption of the current to the valve.

This is a very important safety feature that helps prevent hypoxic mixtures. However, it is crucial to note that there is generally a lag time between the cut-off and when the monitor says the gas mixture has returned to above 25% once the nitrous oxide flow is cut off. The length of this lag is dependent on the oxygen flow set on the rotameter and the amount of air entrained through the air inlet valve in the backbar. For safety, continuously observe the oxygen monitor until the gas mixture has stabilized at an appropriate oxygen level.