Gases arrive from a central hospital supply via a pipeline system that connects to the anesthesia machine. The pipeline and the hoses are both color coded: green for oxygen, yellow for air, and blue for nitrous oxide. The hoses connect to the machine using a diameter-index safety system (DISS) . The DISS (Fig. 10.2) is a non interchangeable threaded connection that makes it physically impossible to attach an oxygen hose to any port other than an oxygen outlet – because the size and diameter of the wall connections and hose adapters are gas specific.

Gas cylinders use the pin index safety system (PISS) to prevent connection errors. On the back of the anesthesia machine, one can find places for at least one back-up gas cylinder (oxygen). As with the pipeline and the hoses, every cylinder is color coded to prevent errors. These cylinders are generally reserved for back-up use in case of a pipeline or central gas supply failure (Fig. 10.3).

Cylinders come in a variety of standard sizes. The most commonly used in the operating room are E-cylinders (Table 10.2). By understanding the physical properties of the gases stored in a cylinder, one can calculate the amount of gas (and or time) left in a cylinder.

For example: if the pressure gauge on an oxygen cylinder reads 1100 psi and you plan to deliver oxygen at a rate of 6 L/min → one could estimate that there are 312 l remaining in the tank (1100 psi/2200 psi x 625 l). At 6 L/min, one could deliver oxygen for approximately 52 min.

Gases coming from the pipeline or central hospital supply have a wall pressure of 50–60 psi. A full oxygen tank delivers gas at 2200 psi and a full nitrous oxide tank delivers gas at 745 psi. In order to ensure a consistent and acceptable pressure is delivered to the patient, machines have pressure regulators incorporated into the gas flow. These regulators will drop cylinder gas pressures to 45–47 psi and pipeline pressures to no more than 50–60 psi. This allows gas to be preferentially taken from the central pipeline supply, rather than the cylinders, to ensure that cylinders are not unnecessarily drained. Finally, there is also a high-pressure relief valve for each individual gas that opens when pressure in the machine exceeds (95–110 psi).

In the event of a failure in the oxygen supply, if the oxygen pressure (not flow) drops below a critical point, the supply of other gases will be interrupted and an alarm will sound. This is known as the fail-safe system. This system does not prevent against delivering hypoxic gas mixtures because if the oxygen pressure is normal, other gases still be delivered. This is why inclusion of a working oxygen analyzer (see below) in the inspiratory limb of the breathing circuit and a proportioning system in the machine are critical.

Flowmeters are the division line between the high pressure and the low pressure systems. The pipeline, cylinders and gas lines before the flow meters are considered part of the high-pressure circuit whereas those after them are considered part of the low-pressure system. There are three types of flow meters: variable-orifice, electronic and constant-pressure.

Gas flow increases when the flow valve control is turned counterclockwise – delivering the amount gas desired. It is worth mentioning that flow meters are calibrated for the gas they deliver and are not interchangeable. The oxygen flowmeter is always downstream from all other flowmeters (far right in the U.S.) to reduce the chances of delivering a hypoxic mixture should a leak occur within a flowmeters.

In order to prevent delivery of a hypoxic mixture of oxygen and nitrous oxide all machines include an oxygen/nitrous oxide ratio controller that links the nitrous oxide flow to the oxygen gas flow. This guarantees a minimum oxygen concentration of 21–25 %.

The main function of the vaporizer is to vaporize the volatile anesthetics before they reach the patient. All vaporizers are agent specific and have concentration-calibrated dials that tightly control the amount of anesthetic gas deliver to the patient.