Medical gas supply

Published on 13/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

Print this page

This article have been viewed 3076 times

Medical gas supply

Martin L. De Ruyter, MD

Medical gases most common to anesthesia include oxygen (O₂), nitrous oxide (N₂O), and air. Historically the less frequently used medical gases include helium (He), nitrogen (N₂), and carbon dioxide (CO₂), but there has been a recent surge in the use of CO₂ secondary to the advancement of laparoscopic and robotic procedures. Several governing bodies regulate medical gases, but the containment and delivery of these gases via a medical gas cylinder system is controlled via standards set by the United States Department of Transportation. Medical gas cylinders are the foundation for central pipeline supply of gases to the operating room (OR) and hospital. Additionally, a cylinder system (typically the smaller E cylinders) exists in the OR as a backup for unanticipated failure of the central pipeline supply (Figures 1-1 to 1-3).

Figure 1-1 The index safety system is one of several features of medical gas cylinders that is in place to ensure that the correct cylinder is attached to the correct gas inlet in the back of the anesthesia machine. Cylinders are color coded; each cylinder has a label identifying which gas it contains, and the cylinders attach to the back of the anesthesia machine using the pin-index safety system. Two pins incorporated into the yoke of the machine, just below the gas inlet, line up with two holes on the gas cylinder, allowing only the correct cylinder to be connected to the correct inlet. (© Mayo Foundation for Medical Education and Research. All rights reserved.)

Figure 1-2 A close-up view of the yoke assembly in the back of the anesthesia machine. The two pins that are immediately below the gas inlet on the anesthesia machine occupy one of seven possible standardized positions. For O₂, the pins are in the 2 and 5 position and line up with the corresponding inlet on the O₂ cylinder. As the clamp on the yoke assembly is tightened, the cylinder is secured up against the gas inlet for O₂. If the pins line up correctly, when the valve on the cylinder is open, the correct gas will flow into the correct inlet. If the pins are not lined up correctly, the cylinder will not be able to be tightened into the yoke, no contact will be made between the gas inlet and cylinder, and, therefore, the gas won’t flow into the inlet. (© Mayo Foundation for Medical Education and Research. All rights reserved.)

Figure 1-3 Hoses, color coded for each gas, can be connected to the appropriate gas outlet with an adaptor that has a disc-index system unique to the gas for that outlet. The system comprises a central tube, through which the gas flows, and two small metal rectangles placed on the surface of the adaptor that are, again, unique to each gas, such that an O₂ hose can only be connected to an O₂ outlet. (© Mayo Foundation for Medical Education and Research. All rights reserved.)

Medical gas cylinders store compressed gas. Cylinder sizes vary and are designated by letters, with A being the smallest and H being the largest. H cylinders are large-capacity storage containers that typically provide the central pipeline supply of medical gas that is piped into the OR. E cylinders are smaller and are the most commonly encountered cylinders in the OR. A typical anesthesia machine will have an attachment for two (O₂ and N₂O) or three (two O₂ and one N₂O) E cylinders. E cylinders are also commonly used to supply O₂ to patients during transport. Cylinders are color coded according to the gas they contain. Unfortunately, there is no global agreement, and the colors in the United States are not the same as those accepted internationally. Table 1-1 lists the common medical gases, the cylinder capacity, the color of the cylinders, and the form under which medical gases are stored.

Table 1-1

Medical Gas Cylinders

	Cylinder Capacity (L)*		Color
Gas	E	H	Pressure (psi) at 20° C	U.S.	Non-U.S.	Form
O₂	625-700	6000-8000	1800-2200	Green	White Blue^†	Gas
Air	625-700	6000-8000	1800-2200	Yellow	White and black	Gas
N₂O	1590	15,900	745	Blue	Blue	Liquid

psi, Pounds per square inch.

^*Cylinder sizes are designated by letters, with A being the smallest and H the largest.

At ambient temperature, when gases are compressed and stored in cylinders, gases will either liquefy or remain in a gas state. When stored in medical cylinders, compressed O₂, He, and air remain as gases at ambient temperature. In contrast, N₂O, when compressed and stored in medical cylinders, becomes a liquid at ambient temperature. Knowledge of nonliquefied gases and liquefied gases allows one to estimate the amount of gas that remains in a cylinder as the gas is being consumed. As gas is consumed, the pressure gauge will decrease in a linear proportion to the cylinder’s remaining content. For example, an E cylinder filled with O₂ contains approximately 660 L of nonliquefied O₂ at a pressure of approximately 2000 pounds per square inch (psi). When the gauge reads 1000 psi, approximately 330 L of O₂ remain. One can, therefore, estimate how long before a cylinder will empty when delivering gas at a certain flow rate. An equation to estimate the time remaining in a cylinder is as follows:

< ?xml:namespace prefix = "mml" />Approximate remaining time (h) = O2 cylinder pressure (psi)200 × O2 flow rate (L/min)

The volume remaining in a cylinder of liquefied gases, such as N₂O, cannot be estimated in the same manner. The pressure gauge of the N₂O cylinder reads the pressure of the small amount of vapor above the liquid. As gas is consumed, more gas moves from the liquid phase to the gas phase, maintaining the vapor pressure and, hence, the reading of the pressure gauge. Only when nearly all of the liquid N₂O is vaporized does the pressure start to fall. For example, a full E cylinder of N₂O contains 1590 L and reads 745 psi; this pressure will remain constant until nearly all of the N₂O is vaporized, at which point the pressure starts to drop. At this point, approximately 400 L of N₂O remain in the cylinder. The only reliable way to estimate the volume of N₂O remaining in a cylinder is to weigh the cylinder. Each cylinder is stamped with a tare weight (empty weight), and the difference between the measured weight and tare weight represents the amount of liquefied gas present.

E cylinders attach directly to the anesthesia machine via a hanger-yoke assembly. This assembly orients and supports the cylinder, provides a gas-tight seal, and ensures unidirectional flow of gases into the machine. As a safety measure, to prevent connecting the wrong gas cylinder to the machine (and thus potentially delivering a hypoxic mixture), a pin index safety system is in use. Each gas cylinder has two holes in its cylinder valve that interface with corresponding pins in the yoke of the anesthesia machine. The positioning of the holes on the cylinder valve and the pins on the yoke are unique for each gas. This safety measure is designed to prevent the wrong gas cylinder from being attached to the anesthesia machine. This safety mechanism can be breached if the yoke pins are broken or missing or intentionally instrumented in some way.

Today’s ORs commonly have a pipeline supply of medical gases. Large-capacity tanks, such as liquid O₂ storage tanks or H cylinders connected in series by a manifold, use pipes to deliver O₂ throughout the hospital. In the OR, these pipes connect to one of three common systems: gas columns, hose drops, or articulating arms. Color-coded hoses with a quick-coupling mechanism connect to one of these systems, and, in turn, the hoses then interface with the anesthesia machine via a diameter index system.

Related

Fausts Anesthesiology Review Expert Consult 4e