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Compressed Gas Supply

Gases used in anesthesia are usually supplied under high pressure in spun steel or aluminum cylinders of varying capacity. The most commonly used sizes are:

  • H - free-standing, attached to the anesthesia machine by a flexible hose. These are most economical, but reduce the mobility of the anesthesia machine. The hose to the anesthetic machine should be at a regulated pressure of 50 psi (i.e the regulator should be attached to the cylinder and not to the anesthetic machine). This is very much safer than having the hose running from the cylinder to the machine at cylinder pressure (up to 2000 psi).
  • E - attached directly to the anesthesia machine via a yoke. These allow the anesthesia machine to be more mobile, but are more expensive to operate.
Size Capacity (liters)
Oxygen Nitrous oxide
E 660 1590
H 6900 15800

The best solution is often to have an H cylinder connected to the machine when it is in the operating room, but to have E cylinders attached which can be used when the machine has to be moved.

Gases found on anesthesia machines

Gases commonly used in veterinary anesthesia include:

  • Oxygen - essential for respiration.
  • Nitrous oxide - not sufficiently potent to produce anesthesia alone in veterinary patients, but used to provide analgesia and reduce the requirement of other more potent agents.

Other gases that may be encountered (mainly on medical machines) include:

  • Carbon dioxide - used to stimulate breathing after intermittent positive pressure ventilation.
  • Helium - used in laser surgery aound the airway to reduce the flammibility of the breathing mixture.
  • Cyclopropane - an obsolete (explosive) anesthetic found on some older medical anesthesia machines.
  • Medical Air - commonly found on modern medical machines to dilute oxygen when other gases are not desired.

Other gas sources
A central supply of liquid oxygen may also be provided in large hospitals. Other sources of oxygen include oxygen generators (which chemically generate oxygen from, for example, hydrogen peroxide) and oxygen extractors (which extract oxygen from atmospheric air). Although these are not widely used in developed countries, they may be useful in areas where supplies of compressed gas are hard to obtain.

Correct identification of the gas being supplied to the anesthesia machine is clearly vital if potentially lethal accidents are to be avoided. A number of different measures are taken to ensure this:

  • Color-coding of cylinders provides a rapid means of identifying their contents. The definitive indicator of the contents is, however, always the label.
  • A gas-specific pin-index system is provided on small cylinders: pins on the yoke of the anesthetic machine mate with holes drilled in specific positions on the valve of the cylinder to provide a mechanical means of preventing incorrect connection.


  • Gas-specific connectors are used on large (G and H) cylinders that make it impossible to attach a regulator or fitting to the wrong cylinder:

CGA Fittings

Cylinder contents
A pressure gauge indicates the pressure within each cylinder. In the case of oxygen (below), and other gases which do not liquify at normal temperatures, this enables estimation of the contents since the amount of gas contained in the cylinder is proportional to the pressure.

Cylinder contents

This is not so for vapors, such as nitrous oxide, which are liquified by the high pressure within the cylinder (above). Here, the pressure remains relatively constant until all the liquid is evaporated, after which the pressure drops rapidly as the remaining gas is removed. The contents of these cylinders can only be estimated by weighing the cylinder.

The use of cylinders containing gas at high pressures presents a number of hazards:

  • Falling of unsecured cylinders may lead to:
    • Direct injury of personnel.
    • Explosive decompression if the valve breaks off, which will turn the cylinder into a lethal projectile.
    • These hazards mandate that all cylinders be secured to a wall or placed on special carts.
  • Fires and explosions may be caused by the oxidizing ability of oxygen and nitrous oxide: no grease must used in the threads of the valves or connectors.
  • Incorrect identification of the contents of cylinders is a potential hazard.


Comments on this article should be addressed to Dr Guy Watney
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