Chapter 3: The Anesthesia Workstation

GAS SUPPLY

In its most basic form, the anesthesia machine receives medical gases from a gas supply, controls the flow and reduces the pressure of desired gases to a safe level, vaporizes volatile anesthetics into the final gas mixture, and delivers the gases at the common gas outlet to the breathing circuit connected to the patient’s airway. A mechanical ventilator attaches to the breathing circuit but can be excluded with a switch during spontaneous or manual (bag) ventilation. An auxiliary oxygen supply and suction regulator are also usually built into the workstation.

Pipeline Inlets

Oxygen and nitrous oxide (and often air) are delivered from their central supply source to the operating room through a piping network. The tubing is color-coded and connects to the anesthesia machine through a noninterchangeable diameter-index safety system (DISS) fitting that prevents incorrect hose attachment. Interchangeability is prevented by making the bore diameter of the body and that of the connection nipple specific for each supplied gas.

Cylinder Inlets

Cylinders attach to the machine via hanger-yoke assemblies that use a pin index safety system to prevent accidental connection of a wrong gas cylinder. The yoke assembly includes index pins, a washer, a gas filter, and a check valve that prevents retrograde gas flow. The gas cylinders are also color-coded for specific gases to allow for easy identification. In North America, the following color-coding scheme is used: oxygen = green; nitrous oxide = blue; CO₂ = gray; air = yellow; helium = brown; nitrogen = black.

FLOW CONTROL CIRCUITS

Pressure Regulators

Unlike the relatively constant pressure of the pipeline gas supply, the high and variable gas pressure in cylinders makes flow control difficult and potentially dangerous. To enhance safety and ensure optimal use of cylinder gases, machines use a pressure regulator to reduce the cylinder gas pressure to 45–47 psig.¹ This pressure, which is slightly lower than the pipeline supply, allows preferential use of the pipeline supply if a cylinder is left open (unless pipeline pressure drops below 45 psig).

Oxygen Supply Failure Protection Devices

Whereas the oxygen supply can pass directly to its flow control valve, nitrous oxide, air (in some machines), and other gases must first pass through safety devices before reaching their respective flow control valves. In other machines, air passes directly to its flow control valve; this allows the administration of air even in the absence of oxygen. These devices permit the flow of other gases only if there is sufficient oxygen pressure in the safety device and help prevent accidental delivery of a hypoxic mixture in the event of oxygen supply failure.

Most modern machines use a proportioning safety device instead of a threshold shut-off valve. These devices, called either an oxygen ...