Section 2: Scientific Principles
Part A: Inhaled Anesthetics
Chapter 7: Inhaled Anesthetic Delivery Systems

Proportioning Systems

Manufacturers have equipped newer machines with proportioning systems in an attempt to prevent delivery of a hypoxic mixture. Nitrous oxide and oxygen are interfaced either mechanically or pneumatically so that the minimum oxygen concentration at the common outlet is 25 percent.

Ohmeda Link-25 Proportion Limiting Control System

Contemporary Ohmeda machines use the Link-25 System. The heart of the system is the mechanical integration of the nitrous oxide and oxygen flow control valves. It allows independent adjustment of either valve, yet automatically intercedes to maintain a minimum 25 percent oxygen concentration with a maximum nitrous oxide–oxygen flow ratio of 3:1. The Link-25 automatically increases oxygen flow to prevent delivery of a hypoxic mixture. 11, 12, 13, 14, 15, 16, 17 

Figure 7–11 shows the Ohmeda Modulus II Link-25 System. The nitrous oxide and oxygen flow control valves are identical. A 14-tooth sprocket is attached to the nitrous oxide flow control valve, and a 28-tooth sprocket is attached to the oxygen flow control valve. A chain physically links the sprockets. When the nitrous oxide flow control valve is turned through two revolutions, or 28 teeth, the oxygen flow control valve revolves once because of the 2:1 gear ratio. The final 3:1 flow ratio results because the nitrous oxide flow control valve is supplied by approximately 26 psig, whereas the oxygen flow control valve is supplied by 14 psig. Thus, the combination of the mechanical and pneumatic aspects of the system yields the final oxygen concentration. 12, 13

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FIGURE 7–11 Ohmeda Link-25 proportion limiting control system (see text for details). (Modified from Andrews JJ: Delivery systems for inhaled anesthetics. In Barash PG, Cullen BF, Stoelting RK [eds]: Clinical Anesthesia, 3rd ed. Philadelphia, New York, Lippincott-Raven, 1997, pp 535–572.)

North American Dräger Oxygen Ratio Monitor Controller

North American Dräger‘s proportioning system, the oxygen ratio monitor controller (ORMC), is used on the North American Dräger Narkomed 2A, 2B, 3, and 4. 18, 19, 20, 21, 22, 23  It is a pneumatic oxygen–nitrous oxide interlock system designed to maintain a fresh gas oxygen concentration of at least 25 ± 3 percent. The device controls the fresh gas oxygen concentration to levels substantially higher than 25 percent at oxygen flow rates of less than 1 L/min. The ORMC limits nitrous oxide flow to prevent delivery of a hypoxic mixture. 18, 19, 20, 21, 22  This is unlike the Ohmeda Link-25, which actively increases oxygen flow.

A schematic of the ORMC is shown in Figure 7–12 . It is composed of an oxygen chamber, a nitrous oxide chamber, and a nitrous oxide slave control valve; all are interconnected by a mobile horizontal shaft. The pneumatic input into the device is from the oxygen and the nitrous oxide flowmeters. These flowmeters are unique because they have specific resistors located downstream from the flow control valves. These resistors create back pressures directed to the oxygen and nitrous oxide chambers. The value of the oxygen flowtube resistor is three to four times that of the nitrous oxide flowtube resistor, and the relative value of these resistors determines the value of the controlled fresh gas oxygen concentration. The back pressure in the oxygen and nitrous oxide chambers pushes against rubber diaphragms attached to the mobile horizontal shaft. Movement of the shaft regulates the nitrous oxide slave control valve, which feeds the nitrous oxide flow control valve. 1, 19, 23 

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FIGURE 7–12 North American Dräger oxygen ratio monitor controller (see text for details). (Modified from Schreiber P: Safety Guidelines for Anesthesia Systems. Telford, Pennsylvania, North American Dräger, 1984.)

If the oxygen pressure is proportionally higher than the nitrous oxide pressure, the nitrous oxide slave control valve opens more widely, allowing more nitrous oxide to flow. As the nitrous oxide flow is increased manually, the nitrous oxide pressure forces the shaft toward the oxygen chamber. The valve opening becomes more restrictive and limits the nitrous oxide flow to the flowmeter.

Figure 7–12 illustrates the action of a single ORMC under different sets of circumstances. The back pressure exerted on the oxygen diaphragm, in the upper configuration, is greater than that exerted on the nitrous oxide diaphragm. This causes the horizontal shaft to move to the left, opening the nitrous oxide slave control valve. Nitrous oxide is then able to proceed to its flow control valve and out through the flowmeter. In the bottom configuration, the nitrous oxide slave control valve is closed because of inadequate oxygen back pressure. 1, 18, 23 

Limitations

Proportioning systems are not foolproof. Machines equipped with proportioning systems can still deliver a hypoxic mixture under the following conditions.

Wrong Supply Gas.

Both the Link-25 and the ORMC are fooled if a gas other than oxygen is present in the oxygen pipeline. In the Link-25 System, the nitrous oxide and oxygen flow control valves continue to be mechanically linked, and a hypoxic mixture proceeds to the common outlet. The oxygen rubber diaphragm of the ORMC recognizes adequate “oxygen” pressure, and flow of both the wrong gas plus nitrous oxide results. The oxygen analyzer is the only machine monitor that detects this condition in both systems.

Defective Pneumatics or Mechanics.

Normal operation of the Ohmeda Link-25 and the North American Dräger ORMC is contingent on pneumatic and mechanical integrity. 33  Pneumatic integrity in the Ohmeda System depends on properly functioning second-stage regulators. A nitrous oxide–oxygen ratio other than 3:1 results if the regulators are not precise. The chain connecting the two sprockets must be intact. A 97-percent nitrous oxide concentration can occur if the chain is cut or broken. 34  In the North American Dräger System, a functional OFPD is necessary to supply appropriate pressure to the ORMC. The mechanical aspects of the ORMC, such as the rubber diaphragms, the flowtube resistors, and the nitrous oxide slave control valve, must likewise be intact.

Leaks Downstream.

The ORMC and the Link-25 function at the level of the flow control valves. A leak down-stream from these devices, such as from a broken oxygen flowtube (see Fig. 7–10 ), can cause delivery of a hypoxic mixture. Oxygen escapes through the leak, and the predominant gas delivered at the common outlet is nitrous oxide. The oxygen analyzer is the only machine safety device that can detect the problem. 1  North American Dräger recommends a preoperative positive-pressure leak test to detect such a leak. 18, 19, 20, 21, 22  Ohmeda recommends a preoperative negative-pressure leak test because of the check valve located at the common outlet (see Checking Anesthesia Machines). 9, 10, 11, 12, 13, 15 

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FIGURE 7–10 Oxygen flowtube leak. An oxygen flowtube leak can produce a hypoxic mixture regardless of the flowtube arrangement.

Inert Gas Administration.

Administration of a third inert gas, such as helium, nitrogen, and CO2, can cause a hypoxic mixture because contemporary proportioning systems link only nitrous oxide and oxygen. 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 35  Use of an oxygen analyzer is mandatory if the operator uses a third inert gas.