Delivery of Medical Gases and Connecting Gases to Anesthesia Machine
Delivery of Medical Gases and Connecting Gases to Anesthesia Machine
Medical gases are delivered through a piping network from their central supply source to the operating room or from gas cylinder located near to anesthesia machine.
Pipeline
Oxygen, nitrous oxide, and often air are delivered from their central supply source to the operating room through a piping network. Pipes are sized such that the pressure drop across the whole system never exceeds 5 psig. Gas pipes are usually constructed of seamless copper tubing using a special welding technique. Internal contamination of the pipelines with dust, grease, or water must be avoided. The hospital's gas delivery system appears in the operating room as hose drops, gas columns, or elaborate articulating arms.
The tubing is color coded and connects to the anesthesia machine through a non interchangeable diameter-index safety system (DISS) fitting that prevents incorrect hose attachment. A filter helps trap debris from the wall supply and a one-way check valve prevents retrograde flow of gases into the pipeline supplies. It should be noted that some machines have an oxygen (pneumatic) power outlet that may be used to drive the ventilator or provide an auxiliary oxygen flow meter. The DISS fittings for the oxygen inlet and the oxygen power outlet are identical and should not be mistakenly interchanged. It is essential to know which gases are being supplied by pipeline from a central supply, and to confirm that the connections between the anesthetic machine and the pipelines are secure and correct.
The anesthetist should visually check the connections and then perform a 'tug test' on each pipeline connection to confirm that the connections are correctly and securely made.
After checking that the connections are correct at both ends of the flexible supply hoses, the anesthetist should check that the pipeline pressure displayed on the anesthetic machine should indicate 400 kPa.
Cylinder Connections
Similarly cylinders attach to the machine via hanger-yoke assemblies that utilize a pin index safety system to prevent errors. The yoke assembly includes index pins, a gas filter, and a check valve that prevents retrograde gas flow. The E cylinders attached to the anesthesia machine are a high-pressure source of medical gases and are generally used as a back-up supply in case of pipeline failure. H cylinder larger than E cylinder can be used for constant supply of oxygen in a place where pipe line system is not available. Some machines have two oxygen cylinders so that one cylinder can be used while the other is changed. Cylinder pressure is usually measured by a Bourdon pressure gauge. A flexible tube within this gauge straightens when exposed to gas pressure, causing a gear mechanism to move a needle pointer.
The anesthetist should ensure that cylinders are installed in the correct position. Cylinders should be seated securely and turned off after checking their contents. An adequate reserve supply of oxygen should be available from a spare cylinder and the pressure in the cylinder should be checked.
E-cylinders of oxygen, nitrous oxide, and air attach directly to the anesthesia machine. To discourage incorrect cylinder attachments, a pin index safety system has been adopted by cylinder manufacturers. Each gas cylinder sizes (A-E) has two holes in its cylinder valve that mate with corresponding pins in the yoke of the anesthesia machine. The relative positioning of the pins and holes is unique for each gas. The pin index safety system is also ineffective if yoke pins are damaged or the cylinder is filled with the wrong gas.
Sources of Gasses to Anesthesia Machine
Anesthesia machines are equipped with cylinders of oxygen and nitrous oxide for use should the central gas supply fail or if there is no central supply. Color-coded cylinders are attached to the anesthesia machine by a hanger yoke assembly, which consists of two metal pins that correspond to holes in the valve casing of the gas cylinder (pin indexed safety system [PISS]). This design makes it impossible to attach an oxygen cylinder to any yoke on the anesthesia machine other than that designed for oxygen. Otherwise, a cylinder containing nitrous oxide could be attached to the oxygen yoke, which would result in the delivery of nitrous oxide when the oxygen flow meter was activated.
Anesthesia machines must have an oxygen supply low-pressure sensor that activates a gas whistle or electric alarm sounds when inlet gas pressure drops below a threshold value (usually 20-35 psig). It must be stressed that these safety devices do not protect against other possible causes of hypoxic accidents.