Cylinder Supply Source

Anesthesia machines have reserve E cylinders if a pipeline supply source is not available or if the pipeline fails. Color-coded cylinders are attached to the anesthesia machine through the hanger yoke assembly. The hanger yoke assembly orients and supports the cylinder, provides a gas-tight seal, and ensures a unidirectional flow of gases into the machine.^[8] Each hanger yoke is equipped with the Pin Index Safety System (PISS). The PISS is a safeguard introduced to eliminate cylinder interchanging and the possibility of accidentally placing the incorrect gas on a yoke designed to accommodate another gas. Two pins on the yoke are arranged so that they project into the cylinder valve. Each gas or combination of gases has a specific pin arrangement.^[31]

Gas travels from the high-pressure cylinder source to the anesthesia machine when the cylinder is turned on (see Fig. 9-3 ). A check valve is located downstream from each cylinder if a double-yoke assembly is used. The check valve has several functions. First, it minimizes gas transfer from a cylinder at high pressure to one with lower pressure. Second, it allows an empty cylinder to be exchanged for a full one while gas flow continues from the other cylinder into the machine with minimal loss of gas. Third, it minimizes leakage from an open cylinder to the atmosphere if one cylinder is absent.^{[8] [9]} A cylinder gauge indicating supply pressure is located downstream from the check valves. The gauge indicates the pressure in the cylinder having the higher pressure when two reserve cylinders of the same gas are opened at the same time.^[25]

Each cylinder supply source has a pressure-reducing valve known as the cylinder's pressure regulator. It reduces the high and variable storage pressure in a cylinder to a lower, more constant pressure suitable for use in the anesthesia machine. The oxygen cylinder's pressure regulator reduces the ylinder's oxygen pressure from a high of 2200 psig to approximately 45 psig. The nitrous oxide cylinder's pressure regulator receives pressure of up to 745 psig and reduces it to approximately 45 psig.^{[8] [9]}

The cylinders should be turned off except during the preoperative machine checking period or when a pipeline source is unavailable. If left on, the reserve cylinder supply can be silently depleted whenever the pressure inside the machine decreases to a value lower than the regulated cylinder pressure. Oxygen pressure within the machine can decrease below 45 psig with oxygen flushing or with ventilator use, particularly at high peak flow rates. The pipeline supply pressures of all gases can be less than 45 psig if problems exist in the central piping system. If the cylinders are left on, they will eventually become depleted, and no reserve supply will be available if a pipeline failure occurs.^{[9] [32]}