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As gas is supplied from a central supply or a cylinder, it passes through a fail-safe valve while traveling toward the flowmeters located in the anesthesia machine. The meters are equipped with a proportioning system and specially designed to prohibit a hypoxic gas mixture from being delivered to the patient. After the meters, the gas enters a manifold or mixing chamber, where it passes through vaporizers and continues to the common gas outlet, and eventually to the patient. Several safety features in the anesthesia breathing system ensure that the patient receives adequate oxygen supply: (1) fail-safe valve; (2) rotameter; and (3) proportioning device.


This fail-safe device prevents hypoxic mixtures if there is a decreased oxygen supply at the flowmeters’ level. A pressure sensor shutoff valve is typically used in Ohmeda anesthetic machines (Datex-Ohmeda, Inc, Madison, WI). This device is present in gas lines supplying all flowmeters except for oxygen and is controlled by the oxygen supply pressure. It does this by interrupting the supply of the other gases if the oxygen supply is reduced to a certain level, usually below 30 psi. That level is the opening threshold pressure for use of the other gases. In a Drager anesthetic machine (Draeger Medical, Telford, PA), there is an oxygen protection device. This is similar to the Ohmeda shutoff valve. The only difference is that as the oxygen pressure is decreased, the other gases decrease proportionally with the opening oxygen threshold pressure of 12 psi. Additionally, an oxygen failure alarm system sounds if oxygen supply falls below a certain value (30 psi).

An oxygen flush valve can provide high flows of oxygen (35-75 L/min) directly to the common gas outlet, bypassing the flowmeters. It is important to note, because of the high pressure, the patient is at risk for barotrauma if oxygen flush is utilized while the breathing circuit is in continuity with the patient’s lungs.


Flowmeters control gas proportions and gas flow to the common gas outlet. One of the most common types of flowmeters are rotameters, or the variable orifice flowmeters with fixed pressure difference. It adjusts gas flow by means of flow control needle valves and flow tubes. Gas flow enters at the base of a glass flow tube. This glass tube is tapered in that its diameter increases with height. A small metal bobbin or ball rides the gas jet. As the bobbin rises, the space around it, known as the annulus, increases (variable orifice). Greater flow jets are required as the orifice widens to keep the bobbin afloat at that level. The pressure remains constant due to a force counteracting gravity and low flow resistance with a greater annulus. The top of the bobbin or the middle of the ball indicates the flow in liters per minute. Notches are made in the bobbin causing it to rotate centrally ...

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