The Anesthesia Machine - The Gas Delivery System (Fig. 9.1)
- Piped gases. 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 in order to safeguard against the administration of a hypoxic gas mixture.
- 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. It is important to make sure that each anesthesia machine has a cylinder wrench to enable the flow of gas when piped gases are not available.
- A full cylinder of oxygen (green) has a pressure of 2,000 to 2,200 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.
- A full cylinder of nitrous oxide (blue) has a pressure of 745 psi and contains the equivalent of 1,500 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.
- Air cylinders are present on some machines. A full cylinder (yellow) has a pressure of 1,800 psi and contains the equivalent of 630 L at atmospheric pressure and room temperature.
- 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. If both cylinders and pipelines are connected and open, gas flows preferentially from the pipeline because its pressure is slightly higher than the regulated cylinder pressure. The regulators divide the machine into high-pressure (proximal to the regulator) and low-pressure (distal to the regulator) systems. In the event that the pipeline pressure fails or is less than the cylinder pressure, the cylinder will supply the gas until it is emptied.
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