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Manufacturers have equipped newer machines with proportioning systems in an attempt to prevent delivery of a hypoxic mixture. Nitrous oxide and oxygen are interfaced mechanically or pneumatically so that the minimum oxygen concentration at the common gas outlet is between 23% and 25%, depending on manufacturer.[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [34] [35]
Conventional Datex-Ohmeda machines use the Link-25 system. The heart of the system is the mechanical integration of the flow control valves for nitrous oxide and oxygen. It allows independent adjustment of either valve, but automatically intercedes to maintain a minimum 25% 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.[12] [13] [14] [15] [16] [17] [18] [19]
Figure 9-11 shows the Datex-Ohmeda 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. The combination of the mechanical and pneumatic aspects of the system yields the final oxygen concentration.[13] [14] The Datex-Ohmeda Link-25 proportioning system can be thought of as a system that increases oxygen flow when necessary to prevent delivery of a fresh gas mixture with an oxygen concentration of less than 25%.
A few failures of the Datex-Ohmeda Link-25 system have been described. The authors of these reports describe failures that resulted in an inability to administer oxygen without nitrous oxide or allowed creation of a hypoxic mixture. These reported failures of the Link-25 system on the Datex-Ohmeda Excel series of anesthesia machines should raise the awareness of clinicians to the possibility of a failure of this type of proportioning system.[41] [42] [43] [44]
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.[19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Figure 9-11
In the Ohmeda Link-25 Proportion-Limiting Control 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 links the sprockets. The combination
of the mechanical and pneumatic aspects of the system yields the final oxygen concentration.
The Datex-Ohmeda Link-25 proportioning system can be thought of as a system that
increases oxygen flow when necessary to prevent delivery of a fresh gas mixture with
an oxygen concentration of less than 25%. (Adapted from Andrews JJ, Brockwell
RC: Delivery systems for inhaled anesthetics. In
Barash PG, Cullen BF, Stoelting RK [eds]: Clinical Anesthesia, 4th ed. New York,
Lippincott-Raven, 2000, pp 567–594.)
A schematic diagram of the ORMC is shown in Figure 9-12 . The system 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 flow meters. These flow meters are unique because they have specific resistors located downstream from the flow control valves. The resistors create backpressures directed toward the oxygen and nitrous oxide chambers. The value of the oxygen flow tube's resistor is three to four times that of the nitrous oxide flow tube's resistor, and the relative value of these resistors determines the value of the controlled fresh gas concentration of oxygen. The backpressure 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
Figure 9-12
A schematic of the North American Dräger Oxygen
Ratio Monitor Controller (ORMC) is shown. The ORMC is composed of an oxygen chamber,
a nitrous oxide chamber, and a nitrous oxide slave control valve, all of which are
interconnected by a mobile horizontal shaft. The pneumatic input into the device
is from the oxygen and the nitrous oxide flow meters. These flow meters have resistors
located downstream from the flow control valves that create backpressures directed
to the oxygen and nitrous oxide chambers. The value of the oxygen flow tube's resistor
is three to four times that of the nitrous oxide flow tube's resistor, and the relative
value of these resistors determines the value of the controlled fresh gas concentration
of oxygen. The backpressure 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. (Adapted from Schreiber P: Safety Guidelines for Anesthesia
Systems. Telford, PA, 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 flow meter.
Figure 9-12 illustrates the action of a single ORMC under different sets of circumstances. The backpressure 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 flow meter. In the bottom configuration, the nitrous oxide slave control valve is closed because of inadequate oxygen backpressure.[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [24] [28] To summarize, in contrast to the Datex-Ohmeda Link-25 system, which actively increases oxygen flow to maintain a fresh gas-oxygen concentration greater than 25%, the Dräger ORMC and S-ORC systems limit nitrous oxide flow to prevent delivery of a fresh gas mixture with an oxygen concentration of less than 25%.
Proportioning systems are not foolproof. Machines equipped with proportioning systems still can deliver a hypoxic mixture under certain conditions.
The Datex-Ohmeda Link-25 and the Dräger ORMC/S-ORC can be 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. Nevertheless, a hypoxic mixture can proceed to the common outlet. The oxygen chamber's rubber diaphragm of the ORMC or S-ORC recognizes adequate supply pressure on the oxygen side, and flow of the wrong gas plus nitrous oxide results. The oxygen analyzer is the only machine monitor besides an integrated multigas analyzer that can detect this condition in either system.
Normal operation of the Datex-Ohmeda Link-25 and the North American Dräger ORMC/S-ORC is contingent on pneumatic and mechanical integrity.[45] Pneumatic integrity in the Datex-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% nitrous oxide concentration can occur if the chain is cut or broken.[46] In the North American Dräger system, a functional OFPD is necessary to supply appropriate pressure to the ORMC. The mechanical aspects of the ORMC/S-ORC, such as the rubber diaphragms, the flow tube's resistors, and the nitrous oxide slave control valve, also must be intact.
The ORMC/S-ORC and the Link-25 function at the level of the flow control valves. A leak downstream from these devices, such as a broken oxygen flow tube (see Fig. 9-10 ), can result in delivery of a hypoxic mixture to the common gas outlet. Oxygen escapes through the leak, and the predominant gas delivered is nitrous oxide. The oxygen monitor and integrated multigas analyzer are the only machine safety devices that can detect the problem.[1] For most of its products, North American Dräger recommends a preoperative positive-pressure leak test to detect such a leak.[19] [20] [21] [22] [23] [24] [25] [26] [27] Datex-Ohmeda almost universally recommends a preoperative negative-pressure leak test for its workstations because of the check valve located at the common gas outlet[10] [11] [12] [13] [14] [16] (see "Checking Anesthesia Machines").
Administration of a third inert gas, such as helium, nitrogen, or carbon dioxide, can cause a hypoxic mixture because contemporary proportioning systems link only nitrous oxide and oxygen.[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [47] Use of an oxygen analyzer is mandatory if the operator uses a third inert gas.
Volatile inhaled anesthetics, like inert gases, are added to the mixed gases downstream from the flow meters and the proportioning system. Concentrations of less potent inhaled anesthetics such as desflurane may account for a larger percentage of the total fresh gas than more potent agents. This can be seen when the maximum vaporizer dial settings of the various volatile agents are examined (e.g., desflurane's maximum dial setting of 18% versus isoflurane's maximum dial setting of 5%). Because significant percentages of these inhaled anesthetics may be added downstream of the proportioning system, the resulting gas-vapor mixture may contain an inspired oxygen concentration that is less than 21% despite a functional proportioning system. The anesthesiologist must be aware of this possibility, particularly when high concentrations of lower-potency, inhaled volatile anesthetics are used.
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