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Last Updated 8/14/03
High Pressure System

Color coding of gases
Color can be used as a means to identify gases. The color code used in the United States is: O2 - green, N2O - blue, Air - yellow. The international color code, however, has a different color coding: O2 - white, N2O - blue, Air - white & black.

This simulation uses the U.S. color coding by default with tank and outlet colors indicating oxygen and nitrous oxide. The international (ISO) color code and others may be selected by clicking on the "Gas Color Codes" link to the right of the animation.

Connection systems
Quick connectors allow the supply hoses to be connected to specific gas connection points. Insertion into an incorrect outlet is prevented by the use of different shapes for mating portions, different spacing of mating portions, or some combination of these.

The simulation shows Quick connectors for O2, N2O and vacuum, which can be connected and disconnected by clicking with the left button of a mouse.

The Pin Index Safety System is used on size E and smaller cylinders. The pins protruding from the cylinder yoke of a particular gas have a unique configuration that fits into corresponding holes in the cylinder valve. This prevents the misconnection of cylinders to the wrong yokes.

Function of Pressure Regulators
Pipeline gas supply is supplied at a nominal presure of 50 pounds per square inch gauge (psig) but may range from 40 - 60 psig depending on overall gas usage from the multiple outlets connected to the pipeline supply. The pressure in a gas cylinder varies with the temperature, the amount of gas remaining, and the state of the contents (gas or liquid). To maintain constant flow with changing supply pressures, the anesthesia machine is fitted with pressure regulators for both the pipeline and cylinder supplies. The cylinder pressure regulator reduces the cylinder pressure to a constant 45 psig. A secondary function of the pressure regulator is to close off the cylinder gas supply when pipeline gas supply exceeding 45 psig is present. This is made possible because of the small pressure differential (nominally 5 psig) between the pipeline gas supply and downregulated cylinder pressures. This prevents usage and depletion of the backup cylinder when there is still an adequate pipeline gas supply.
Note that leaving the cylinder open will mean that there will be no audible low O2 supply pressure warning if the pipeline supply fails
. When the low O2 supply pressure audible alarm eventually sounds, both the pipeline and cylinder supplies will have failed and there will be no back-up gas supply. Therefore, it is imperative to remember to close the O2 cylinder after opening it to check its contents.

This secondary function of the cylinder pressure regulator can be observed in the simulation. For example, if the oxygen cylinder is open, the higher O2 pipeline pressure closes the pressure regulator and prevents gas from flowing from the cylinder. If the O2 pipeline supply is then disconnected, oxygen can then flow through the pressure regulator.

Can the cylinder pressure gauge indicate that an empty cylinder is full?
Although pressure gauges, in general, accurately display the cylinder and pipeline pressures, the above situation can indeed occur. This situation is possible because of the check valve and pressure regulator in the cylinder manifold. When a full cylinder is first opened, the cylinder pressure gauge will indicate the cylinder pressure. When the cylinder is subsequently closed and removed, the check valve will prevent gas from flowing in a backward direction. If pipeline pressure exceeds 45 psig, the pressure regulator will be closed and will prevent the trapped gas from flowing in a forward direction. Until the pipeline is disconnected, the cylinder pressure gauge will continue to indicate a full tank pressure (2000 psig) even if an empty cylinder is now placed in the yoke and opened!

To observe this behavior of the cylinder pressure gauge in the simulator, open and close the oxygen cylinder with the O2 pipeline supply connected. The cylinder pressure will indicate a positive pressure but will not drop back to zero when the cylinder is closed again. When the O2 pipeline supply is disconnected and an O2 draw is created using the flush valve or the O2 flow control valve, the cylinder pressure gauge will display zero again.

Function of the fail-safe device
The fail-safe device shuts off the N2O supply if there is a loss of O2 supply pressure. It senses only pressure and does not check whether the supplied gas is actually oxygen. Because of the fail-safe device, a decrease in N2O flow (with the N2O flowmeter bobbins dropping to zero) may actually be the first sign of loss of oxygen supply pressure.

The action of the fail-safe device can be observed in the simulation when the O2 supply (pipeline or cylinder) is shut off with the N2O supply (pipeline or cylinder) still present.

Function of the low pressure alarm
The low pressure or oxygen supply failure alarm will go off when there is a significant increase or decrease of the O2 supply pressure. This occurs when there is a sudden loss of cylinder or pipeline pressure or when the anesthesia machine is turned on or off. A commonly used mechanism utilizes a pressurized canister that is filled with oxygen when the anesthesia machine is turned on. The stream of oxygen that goes into the canister passes through a whistle, and a sound can be heard when the machine is turned on. If the oxygen pressure then falls below a certain value, this canister will empty and direct a reverse stream of oxygen through the whistle. This alarm may not be heard if the O2 supply pressure drops very gradually over a long time.

The low pressure alarm can be heard in the simulation by disconnecting and then reconnecting the O2 pipeline supply (with the O2 cylinder closed).

Ventilator drive gas circuit dependence on pressurized O2 supply
The ventilator that is modeled is a "bellows in a box" design, e.g., the Ohmeda 7800. The bellows is housed in a transparent chamber and the inside of the bellows is connected to the breathing system. During inspiration, drive gas is delivered into the space between the bellows and its housing. This causes pressure to be exerted on the bellows, compressing the bellows. The drive gas in the 7800 anesthesia ventilator is O2, regulated down to 26 psig, from the same oxygen source that supplies the patient. A loss of pressurized oxygen might therefore also prevent the ventilator from functioning properly. If O2 is being supplied by the back-up cylinder, part of the finite cylinder contents will be used to drive the ventilator bellows. The rate of consumption of drive gas is approximately the same as the minute volume. Switching to manual ventilation will maximize the time before exhaustion of the O2 cylinder by eliminating the consumption of O2 as the drive gas.

The flow of oxygen can be traced from its source in the simulation. Part of the oxygen flow can be seen entering the drive gas circuit and compressing the bellows.

Using the oxygen flush
The oxygen flush can be used even when the anesthesia machine is not turned on and will operate in its regular fashion. It delivers oxygen straight from the pipeline or cylinder regulator at 45-50 psig. The flowrate will be between 35-75 l/min. For an O2 flush flowrate of 60 l/min, 1 liter of O2 flows into the breathing circuit for every second that the O2 flush button is held down.

O2 flush button pressed during mechanical ventilation and inspiration
During mechanical inspiration, drive gas pressurizes the ventilator pressure relief valve closed such that there is momentarily no outlet for excess gases. If the O2 flush button is pressed during the inspiratory phase of mechanical ventilation, pressure inside the breathing circuit will increase rapidly because approximately 1 liter of O2 is being introduced per second into a momentarily closed circuit. Dangerously high pressures could potentially be transmitted to the lungs.

The lungs will overinflate and flash red in the simulation when the oxygen flush in continuously pressed during the inspiratory phase of mechanical ventilation.

O2 flush button pressed during mechanical ventilation and expiration
If the oxygen flush is pressed during the expiratory phase of mechanical ventilation, the bellows will initially fill rapidly to its maximum capacity. After reaching maximum capacity, any pressure in excess of 2-4 cm H2O will be vented through the pressure relief valve.

The ventilator pressure relief valve underneath the bellows can be seen to open during the expiratory phase of mechanical ventilation.

Inability to use O2 flush for jet ventilation
Despite the high flow rate that can be obtained by pressing the O2 flush, it is actually not the best choice for jet ventilation during cricothyrotomy. A significant pressure drop occurs across the O2 flush valve, which significantly diminishes the tidal volumes delivered to the patient during jet ventilation through the narrow bore (high flow resistance) of the needle. An auxiliary ball-in-tube oxygen flowmeter mounted on the anesthesia machine and supplied with 50 psig O2 is the recommended mode of administering O2 during jet ventilation. Even though the flowrate through the ball-in-tube flowmeter typically does not exceed 15 L/min, the pressure drop is lower across the ball-in-tube flowmeter when it is wide open. The higher outlet pressure from the ball-in-tube flowmeter makes it more suitable for use in jet ventilation.