Medical gas supply

Medical gas supply

1. Pipeline gas system. Wall outlets deliver oxygen, nitrous oxide, and air, from a central supply source, at a pressure of 50 to 55 pounds/in² (psi). These outlets and the supply hoses to the machine are diameter indexed and color coded by gas to safeguard against the administration of a hypoxic gas mixture. This schema is referred to as the Diameter Index Safety System.
2. Gas cylinders. Gas cylinders are used as backup sources when wall delivery fails or in locations where piped anesthesia gases are not available. Anesthesia machines use the size E cylinder. In a similar fashion to the gas supply hoses, cylinder colors are specific for each gas and pin indexed to prevent connection to the wrong regulator. This schema is referred to as the Pin Index Safety System.
   1. A full cylinder of oxygen (green, USA; white, international) has a pressure of 2000 to 2200 psi and contains the equivalent of 660 L of gas at atmospheric pressure and room temperature. The oxygen cylinder pressure decreases in direct proportion to the amount of oxygen remaining in the cylinder.
   2. A full cylinder of nitrous oxide (blue) has a pressure of 745 psi and contains the equivalent of 1500 L of gas at atmospheric pressure and room temperature. The nitrous oxide in the full cylinder is primarily in the liquid phase; the cylinder pressure does not decrease until the liquid content is exhausted, at which time approximately one-fourth of the total volume of gas remains.
   3. Air cylinders are present on some machines. A full cylinder (yellow, USA; black/white, international) has a pressure of 1800 psi and contains the equivalent of 630 L at atmospheric pressure and room temperature.
   4. Pressure regulators reduce the high pressure from the cylinders to about 45 psi (just below pipeline pressure) so that, when cylinder gases are used, adjustments at the flow valves (rotameters) are not needed to compensate for the changing pressure that occurs as the cylinders empty. The regulators divide the machine into high-pressure (proximal to the regulator) and low-pressure (distal to the regulator) systems. It is important to keep the gas cylinder closed when relying on pipeline gas supply of the same gas. If both a cylinder and the pipeline are connected and open, gas should flow preferentially from the pipeline because its pressure is slightly higher than the regulated cylinder pressure. If the pipeline pressure fails or is less than the cylinder pressure, the cylinder will supply the gas until it is emptied. It is important to ensure that each anesthesia machine has a cylinder wrench to enable the user to open the cylinder valves when piped gases are not available.