Medical Gas Systems: Tanks and Pipelines

Pipeline Supply, Safety Features, and Complications

• Hospitals use a pipeline system to deliver O₂, air, N₂O, and CO₂ from a central gas supply to the operating room at approximately 50 psi.¹
• Medical gas pipelines are typically made of seamless copper tubing.²
• Gas-specific outlet connections are located in the wall or ceiling of the operating room.
  • The DISS provides noninterchangeable threaded connections for each specific gas line to prevent the wrong gas from being connected to a delivery site.

• Complications of pipeline systems include excessive pipeline pressures, inadequate O₂ pressure, and pipeline crossovers.¹
• Inadequate O₂ pressure can occur when hospital construction projects cause pipeline damage.
  • A low O₂ pressure alarm will sound once the O₂ pressure falls below 30 psi.^2,4
  • If low O₂ pressure occurs, the E-cylinder should be opened, and the O₂ pipeline source should be assessed to determine the cause of the pressure loss.⁵
  • If the E-cylinder was already open when the O₂ pipeline failed, the low O₂ pressure alarm would not sound until all the gas in the E-cylinder had been used.⁵ Therefore, it is important to keep the E-cylinder closed during standard pipeline use.⁵
• A pipeline crossover can occur through the delivery of gas from an incorrect central gas tank or the connection of a gas pipeline to an incorrect gas inlet and can cause patient death if not promptly recognized.
  • If a pipeline crossover is suspected, the backup cylinder must be opened and the pipeline supply disconnected.¹

• Please see the A&A Videos in Clinical Anesthesia on Hospital Oxygen Supply for more details. Link

Medical Gas Storage: E-cylinder Tanks

• O₂, air, and N₂O are often stored in E-cylinder tanks.
  • These E-cylinders can be used if the pipeline fails.
  • While there is no universal standard for cylinder colors, in the United States, O₂ is stored in green tanks, air in yellow tanks, and N₂O in blue tanks.

• A hanger yoke supports the E-cylinder and connects it to the anesthesia machine.
  • The hanger yoke includes a washer that maintains a gas-tight seal and a check valve that ensures unidirectional gas flow into the machine.²
  • Each hanger yoke also has a pin-index safety system, an important safety feature that prevents cylinder interchangeability by providing unique pin configurations for each medical gas cylinder.¹
    • The pins for an O₂ cylinder are found at positions 2 and 5.²
    • The pins for a N₂O cylinder are found at positions 3 and 5.²

• The cylinder pressure regulator reduces the cylinder pressure to a lower level appropriate for the anesthesia machine.
  • The O₂ cylinder pressure regulator reduces O₂ pressure from up to 2200 psi to approximately 45 psi.¹
  • The N₂O cylinder pressure regulator reduces N₂O pressure from 745 psi to approximately 45 psi.¹
• As long as the pressure in the pipeline is above 45 psi, the anesthesia machine will preferentially use the medical gas from the pipeline supply because it has a higher psi than the medical gas stored in E-cylinders.

Gas-Specific Cylinder Properties and Pressure-Volume Relationships

• A full E-cylinder of O₂ has a volume of 660 L and a pressure of 2000 psi.⁴
  • O₂ exists in gaseous form at room temperature.
  • The volume of O₂ in the cylinder is directly proportional to the pressure in the cylinder.
• A full E-cylinder of N₂O has a volume of 1,590 L and a pressure of 745 psi.²
  • N₂O can exist in liquid form at room temperature because its critical temperature is 36.5°C.² A full E-cylinder of N₂O contains approximately 90 to 95% liquid N₂O and 5 to 10% gaseous nitrous oxide.²
  • Once there is approximately 253 L (16%) of N₂O left in the tank, the remaining N₂O will exist in gaseous form.²
  • Prior to this point, the pressure will read 745 psi and will not reflect the true volume of N₂O remaining in the cylinder.
  • After all the liquefied gas has been used up, the volume of N₂O in the cylinder is directly proportional to its pressure.
  • The residual volume of N₂O can be determined at any point by weighing the cylinder and subtracting the tare weight.^2,4