Anesthesia Machine PDF

2 The Anesthesia Machine and Workstation ROBERT G. LOEB | JAMES B. EISENKRAFT CHAPTER OUTLINE vaporizer(s) (see Chapter 3), breathing system (see Chapter 4), Anesthesia Gas Delivery System...

2 The Anesthesia Machine and Workstation ROBERT G. LOEB | JAMES B. EISENKRAFT CHAPTER OUTLINE vaporizer(s) (see Chapter 3), breathing system (see Chapter 4), Anesthesia Gas Delivery System ventilator (see Chapter 6), and waste gas scavenging system (see The Anesthesia Machine as a Component of an Anesthe- Chapter 5). The basic arrangement of these elements is the same sia Workstation in all contemporary anesthesia gas delivery systems.1 While it Anesthesia Machine and Workstation may not be intuitive from looking at an anesthesia gas delivery Overview of Gas Flow Through the Anesthesia Machine system, Fig. 2.1 shows that the breathing system is the functional center of the anesthesia gas delivery system, because it connects Anesthesia Machine Components to all the other components, as well as to the patient. Gas Inlets The patient breathes in from and out to the breathing system. Gauges and Sensors If ventilation is spontaneous or manually assisted, gas passes Pressure Regulators back and forth through the breathing system to and from the Oxygen Flush reservoir bag. If the lungs are mechanically ventilated, the Pressure Sensor Shut-Off (Fail-Safe) Valves and Alarm ventilator bellows or piston acts as a counterlung, exchanging System its volume with the patient’s lungs via the breathing system. The Oxygen Ratio Proportioning Systems patient consumes oxygen and takes up anesthetic agents, which Fresh Gas Controllers and Flowmeters would be depleted from the breathing system if no fresh gas were Vaporizer Manifolds added. Adding fresh gas is the function of the anesthesia machine. Common Gas Outlets and Outlet Check Valves The anesthesia machine receives gases (oxygen and sometimes Electrical Systems air, nitrous oxide, heliox or carbon dioxide) under pressure Gas Flow Through the Anesthesia Machine from their sources of storage (see Chapter 1), safely creates a gas Oxygen mixture of known composition and flow rate, and delivers it to Nitrous Oxide a concentration-calibrated vaporizer, which adds a controlled Air concentration of potent, inhaled, volatile anesthetic agent. The Other Medical Gases resulting gas mixture of oxygen with other compressed gases and Anesthesia Workstation Obsolescence and Pre-Use vaporized anesthetics is delivered to the machine’s common gas Checks outlet (CGO). This fresh gas mixture flows continuously from the CGO into the patient breathing system, most commonly a Contemporary U.S. Anesthesia Workstations circle breathing system. Typically, more fresh gas is added than Fabius (Dräger Medical, Telford, PA) is consumed by the patient, and this excess gas must be able to Apollo (Dräger Medical, Telford, PA) leave the breathing system to prevent a progressive increase in Perseus A500 (Dräger Medical, Telford, PA) circuit (and consequently airway) pressure. Excess gas leaves the Aestiva MRI (GE Healthcare, Madison, WI) breathing system via the adjustable pressure-limiting (APL) valve Carestation 600 Series (GE Healthcare, Madison, WI) during spontaneous or manual ventilation, or via the ventilator Avance CS2 (GE Healthcare, Madison, WI) pressure relief valve during mechanical ventilation. The waste Aisys CS2 (GE Healthcare, Madison, WI) gas that exits the breathing system enters the scavenging system, FLOW-i (Maquet, Critical Care, Getinge Group, Solna, which is the safety interface to a facility system that discharges Sweden) the gas to the outside. An understanding of the structure and A-Series Advantage (Mindray North America, Mahwah, NJ) function of the anesthesia gas delivery system is essential to the Penlon Prima 400 Series (Penlon, Minnetonka, MN) safe practice of anesthesia.␣ ANESTHESIA MACHINE AND WORKSTATION Anesthesia Gas Delivery System Gas delivery systems continue to evolve as advances in THE ANESTHESIA MACHINE AS A COMPONENT technology and safety are incorporated into current designs. OF AN ANESTHESIA WORKSTATION The recent evolution can be traced through the voluntary consensus standards that have been developed with input The modern anesthesia gas delivery system is composed of from manufacturers, users, and other interested agencies. the anesthesia machine (see the following section), anesthesia The current applicable international standard is ISO/IEC 25 26 PART 1 Gases and Ventilation N2O Air O2 Compressed gases Waste gases Anesthesia Machine Vaporizers Common gas outlet Reservoir Bag Patient’s Breathing Lungs System Ventilator Adjustable Pressure Limiting Valve (APL) Scavenging Fig. 2.1 Organization of the anesthesia System From Ventilator Pressure Relief Valve delivery system (area within dashed-line box). Arrows indicate directions of gas flows. 80601-2-13: Particular requirement for basic safety and Most modern workstations include hardware and software to essential performance of an anaesthetic workstation, which automate much of the pre-use check. This chapter describes was first published in 2011, and confirmed to be up-to-date the basic components and functions of a traditional anesthesia in 2017.2 It superseded the American Society for Testing machine (i.e., the gas delivery device described in ASTM and Materials (ASTM) standard F1850-00, first published standard F1850-00) referring to outdated products at times so in 2000, which introduced the term “workstation” in the reader can appreciate some of the changes that have been distinction to “anesthesia (or gas) machine.”3 The anesthesia made in the most recent models. workstation is defined as a system for the administration Dräger Medical Inc. (Telford, PA) and GE Healthcare of anesthesia to patients consisting of the gas delivery (Waukesha, WI) manufacture most of the anesthesia system, breathing system, anesthetic gas scavenging system, workstations sold in the United States. This chapter reviews anesthetic vapor delivery system, anesthesia ventilator, and the features of a basic anesthesia delivery system, referring associated monitoring and protection devices. This standard to Dräger and GE outdated products when appropriate. The superseded anesthesia machine standard ASTM F1161-88, flow of compressed gases from the point of entry into the first published in 1989.4 The original anesthesia machine machine, through the various components, and to the exit standard was American National Standards Institute (ANSI) at the CGO is described. The function of each component Z79.8, first published in 1979.5 All of these standards were is discussed so that the effects of failure of that component, cowritten by anesthesia practitioners and were voluntarily as well as the rationale for the various machine checkout adopted by anesthesia machine manufacturers of the day. procedures, can be appreciated. This approach provides While voluntary consensus standards are not mandated, it a framework from which to diagnose problems that may is highly unlikely that a manufacturer would build, or that arise with the machine. Of note, the individual workstation the Food and Drug Administration (FDA) would allow to be manufacturer’s operator and service manuals represent the marketed, a workstation that did not comply with the current most comprehensive reference for any specific model of or most recent voluntary consensus standards. machine, and the reader is strongly encouraged to review the The evolution of the anesthesia workstation and advances relevant manuals. The manufacturers also produce excellent in technology have led to many changes in design. Although educational materials,6–9 and a number of simulations are basic operations remain the same, the components are more also available on the Internet.10–13␣ technologically advanced. For example, in most new models the glass tube flowmeters (rotameters) are replaced by digital OVERVIEW OF GAS FLOW THROUGH THE flow indicators or virtual flowmeters displayed on an electronic ANESTHESIA MACHINE information screen. The gas flow–control needle valves may be replaced by electronic flow controllers. Digital pressure The gas flow arrangements of a basic two-gas anesthesia machine gauges have replaced many mechanical compressed gas gauges. are shown in Fig. 2.2. The machine receives each of the two  2 The Anesthesia Machine and Workstation 27 N2O pipeline Check supply valve Concentration-calibrated “Fail-safe” vaporizers with N 2O A valve interlock system C D Pressure relief Pressure valve N2O regulator Second-stage N2O pressure Rotameter regulator flowmeter E Anesthesia B Outlet Common gas outlet ventilator check with retaining device driving valve gas circuit Needle valve To patient Second-stage breathing circuit O2 pressure regulator O2 supply low-pressure Main alarm on/off switch Low pressure O2 system Auxiliary O2 flowmeter Auxiliary O2 DISS connector O2 Line pressure gauge O2 flush valve O2 pipeline supply Fig. 2.2 Schematic of flow arrangements of a generic contemporary anesthesia machine. (A) The “fail-safe” valve is discussed in detail in the sec- tion on Oxygen Supply Failure Devices. The valve terminates the flow of nitrous oxide when oxygen supply pressure is lost. Some contemporary anesthesia machines do not have “fail-safe” valves and perform this safety requirement in a different way. Many, but not all, contemporary anesthesia machines have second-stage oxygen (B) and nitrous oxide (C) pressure regulators. Not all contemporary anesthesia machines have a pressure relief valve (D) and/or check valve (E) upstream of the common gas outlet. DISS, Diameter index safety system. (Modified from Checkout: a guide for preoperative inspection of an anesthesia machine. Schaumburg, IL, 1987, American Society of Anesthesiologists. Reproduced by permission of the American Society of Anesthesiologists.) compressed gases, oxygen (O2) and nitrous oxide (N2O), from anesthesia machines also incorporate safety features designed to two supply sources: a cylinder source and a pipeline source. The prevent the delivery of a hypoxic mixture to the breathing system. storage and supply of these gases to the operating room (OR) These features include the oxygen supply pressure failure alarm, are described in Chapter 1. pressure sensor shut-off (“fail-safe”) system, gas flow proportioning The basic functions of any anesthesia machine are to receive systems, and usability features designed to decrease use errors. compressed gases from their supplies and to create a gas mixture The anesthesia machine gas pathways have been conveniently of known composition and flow rate at the CGO. The relation divided by some authors into three systems14: between pressure and flow is stated in Ohm’s law: 1. a high-pressure system that includes parts upstream of the first-stage regulator, where compressed gas pressures Pressure are between 45 and 2200 pounds per square inch gauge Flow = pressure (psig), Resistance 2. an intermediate pressure system that includes parts Controlling the flow of gases from high-pressure sources downstream of the first-stage pressure regulator and up- through the machine to exit the CGO at pressures approximating stream of the gas flow control valves, where pressures are atmospheric requires changes in pressure and/or resistance. Modern between 16 and 55 psig, 28 PART 1 Gases and Ventilation 3. a low-pressure system that includes all parts downstream of the gas flow control valves, where pressures are nor- mally slightly greater than atmospheric pressure. Other authors consider the high-pressure system to be simply all components upstream of the gas flow control valves and the low-pressure system to be all components downstream of the gas flow control valves. Indeed, this agrees with the system descriptions in the U.S. FDA 1993 pre-use checkout recommendations.15 Either way, most classification schemes agree on the definition of the low-pressure system.␣ Anesthesia Machine Components GAS INLETS The pipeline inlets into the anesthesia machine are gas specific Fig. 2.3 Diameter index safety system (DISS) hose connections to by a national standard (Fig. 2.3), known as the diameter index workstation pipeline supply connections. safety system (DISS), that ensures that a pipeline gas hose cannot be connected to the wrong anesthesia machine gas inlet.16 The DISS system specifies the internal bore dimensions of the anesthesia machine inlet into which only a compatible hose connector with matching outer dimensions can insert, and the oxygen connector has a different design than all of the others (Fig. 2.4). There are specific DISS connectors for oxygen, air, nitrous oxide, helium, heliox, carbon dioxide, suction, and waste anesthetic gas suction, as well as nitrogen, nitric oxide, xenon, and many nonmedical gases. All anesthesia machine pipeline inlets incorporate a filter that prevents particles greater than 100 micrometers from entering, and a check valve that prevents leakage from the machine if the pipeline is not connected and compressed gas cylinders are in use (Fig. 2.5). Failure of this valve would cause gas to leak from the machine if the pipeline hose were not connected. Upstream of the pipeline check valve is a juncture to a pressure gauge that measures the pipeline gas supply pressure (see Fig. 2.2). The upstream location, on the pipeline side of the check valve, means that the gauge reading falls to zero as soon as the pipeline hose is disconnected. Oxygen can also be supplied to the pipeline inlet from special freestanding E-size cylinders that incorporate a regulator that delivers oxygen at 50 psig to a DISS connector. Thus, if the oxygen pipeline fails, the machine’s oxygen hose could be connected to this tank outlet using a compatible fitting (Figs. 2.6 and 2.7). In remote locations, large H-cylinders with pressure regulators can supply compressed gas via the pipeline inlets (Fig. 2.8). Compressed gas can also be supplied to the machine from a back-up E-cylinder attached via a hanger yoke (Fig. 2.9). The medical gas pin index safety system ensures that only an Fig. 2.4 Cross-section schematic of diameter index safety system oxygen cylinder fits correctly into an oxygen hanger yoke (see (DISS) connectors. The DISS is a Compressed Gas Association (CGA) Chapter 1).17 standard for noninterchangeable, removable connections for use with medical gases, instrument air, vacuum for suction, and vacuum for Several considerations apply before a cylinder is hung waste anesthetic gas disposal (WAGD). Noninterchangeable indexing in a yoke. First, the plastic wrapper that surrounds the is achieved by a series of increasing and decreasing diameters in the cylinder valve must be removed, taking care to place the stems and bodies of each connector. The oxygen assembly has a dis- included plastic or rubber washer (Bodok seal) on the yoke tinctively different design from the rest. inlet. Checking that the washer is in place, the cylinder is then hung in the yoke by aligning the gas outlet hole with direction because a tightened T-handle screw might damage the strainer nipple, and aligning the two yoke pins with the the tank safety relief device which can be confused with the corresponding holes in the cylinder. Then the T-handle is cylinder gas outlet (see Chapter 1). Although changing a tightened to press the cylinder stem outlet against the washer compressed gas cylinder on an anesthesia machine may seem and the yoke inlet, creating a gas-tight pressure fitting. The straightforward, one study showed that a significant number cylinder should never be mounted 180 degrees in the wrong of senior residents in a simulator could not perform this task  2 The Anesthesia Machine and Workstation 29 Checking to see that a backup tank contains sufficient oxygen is an important part of the pre-use checkout and also ensures that a tank wrench is available for opening and closing the cylinder valve.19 However, different locations of the cylinder gauge or the presence of two oxygen hanger yokes can complicate this process. On some anesthesia machines with one oxygen hanger yoke the oxygen cylinder pressure gauge is located upstream of the check valve, on the cylinder side of the check valve, so that the gauge indicates the pressure in the cylinder. In this situation, the gauge reads zero when the cylinder is closed and immediately indicates the pressure in the cylinder when it is open. On other anesthesia machines, and all those with two oxygen hanger yokes (Fig. 2.11), the oxygen cylinder pressure gauge is located downstream of the check valve, and indicates the pressure in the high-pressure system of the anesthesia machine.20 In this case, the gauge may read a higher-than-zero pressure when the tank is closed and stay at that pressure when the tank is open. If so, the only way to be sure of the pressure in the tank is to (1) close the oxygen cylinder(s), (2) disconnect the oxygen pipeline, (3) turn on oxygen flow to bleed off the high oxygen pressure line, and (4) open the oxygen cylinder. The gauge indicates the true pressure in the oxygen cylinder only if the gauge reading increases when the oxygen cylinder is opened. To check a second oxygen cylinder on a different yoke, close the first cylinder and begin from step 3. Then reconnect to the oxygen pipeline. The check valve in each hanger yoke is designed to prevent gas from flowing out of the machine through an unoccupied yoke, but it is best practice to insert a yoke plug (Fig. 2.12) into an unoccupied yoke to keep dust out of the inlet and as a backup measure. In the situation of two oxygen yokes (see Fig. 2.11), the check valves also prevent transfilling of one oxygen cylinder to the other when both are open, since without a check valve, oxygen would tend to flow from the full tank to the empty one. Without check valves, the transfilling and sudden compression of oxygen into the empty cylinder could cause a rapid temperature rise in the pipes, gauge, and tank with an associated risk of fire. This is known as an adiabatic change, in which the state of a gas is altered without the gas being permitted to exchange heat energy with its surroundings.21␣ GAUGES AND SENSORS Pipeline and cylinder supply pressure gauges (Fig. 2.13) in Fig. 2.5 Machine pipeline inlet check valve. Top: The location in the traditional machines are of the Bourdon tube design. In anesthesia machine pipeline is circled. Bottom: The flow of oxygen from principle, the Bourdon tube is a coiled metal tube sealed at the wall supply opens the pipeline inlet valve. If the wall supply hose was its inner end and open to the gas pressure at its outer end (see disconnected with the tank oxygen in use, the pressure of oxygen in the machine would force the check valve to its seated position, preventing Chapter 9). As gas pressure increases, the coiled tube tends to loss of oxygen via this connector. DISS, Diameter index safety system. straighten. A pointer attached to the inner-sealed end thereby (From Bowie E, Huffman LM: The anesthesia machine: essentials for un- moves across a scale calibrated in units of pressure. If the derstanding. Madison, WI, 1985, GE-Datex-Ohmeda.) Bourdon tube were to burst, the inside of the gauge could be exposed to high pressure. The gauge is therefore constructed with a special heavy glass window and a mechanism designed satisfactorily, possibly because it is generally performed by to act as a pressure fuse so that gas is released from the back technical staff.18 of the casing if the pressure were to suddenly increase. The The compressed gas enters the machine through a sintered cylinder and pipeline pressure gauges for the gases supplied metal strainer incorporated into yoke assembly (Fig. 2.10) to the machine are generally situated in a panel on the front designed to prevent dirt or other particles greater than 100 of the anesthesia machine (see Fig. 2.13). They are designed microns from entering the machine. The oxygen then flows past to be easy to read, typically have colored and lettered labels a hanger yoke (“floating”) check valve to enter the anesthesia to indicate the gas, and icons to indicate whether cylinder machine at high pressure. or pipeline pressures are being displayed (see Chapter 18). 30 PART 1 Gases and Ventilation Fig. 2.6 Left, The E-cylinder valve incorporates a regulator with a diameter index safety system (DISS) oxygen connector that supplies oxygen at 50 psig. The anesthesia machine oxygen hose is shown connected to a DISS wall outlet for oxygen. Right, If the central pipeline oxygen supply fails, the machine’s oxygen hose can be disconnected from the wall and reconnected to the 50-psig DISS connector on the oxygen cylinder. Fig. 2.8 Large capacity H-cylinders can supply compressed oxygen and compressed air to the anesthesia machine pipeline inlets in remote locations within the facility that lack central compressed gas outlets. Fig. 2.7 An adaptor that converts from a diameter index safety system Valve stem oxygen connector to a proprietary noninterchangeable quick-connector, in this case a Chemetron quick-connect oxygen outlet, which allows the T-handle Cylinder valve machine’s oxygen hose to be disconnected from a wall Chemetron oxy- gen outlet and reconnected to the oxygen cylinder without tools in the Gasket event of a central pipeline oxygen failure. To machine Newer workstations typically use electronic transducers to measure cylinder and pipeline pressures, with the readings displayed on an electronic information screen (Fig. 2.14). Gas outlet Hanger yoke check valve (inside) Electronic pressure transducers measure the voltage through a Wheatstone bridge that is connected to a strain gauge, the Pin index resistance of which changes as it bends when force is applied configuration (Fig. 2.15). An advantage of electronic pressure transducers is that they can be monitored by the workstation’s alarm system. A disadvantage is that the cylinder and pipeline pressures are not always displayed in a constant location, and Fig. 2.9 Hanger yoke for an oxygen tank showing the PISS. PISS, Pin- may even be hidden during some workstation operational indexed safety system. (From Bowie E, Huffman LM: The anesthesia machine: modes.␣ essentials for understanding. Madison, WI, 1985, GE-Datex-Ohmeda.)  2 The Anesthesia Machine and Workstation 31 Fig. 2.10 Top: The location of a hanger yoke in the anesthesia machine is circled. Bottom: Cross-section of hanger yoke showing flow of oxygen from the tank through the strainer nipple into the machine. (From Bowie E, Huffman LM: The anesthesia machine: essentials for understanding. Madi- son, WI, 1985, GE-Datex-Ohmeda.) PRESSURE REGULATORS whether coming from the cylinder or the pipeline (see Fig. 2.2), A pressure regulator is a device that converts a variable, high- resulting in sudden failure of oxygen delivery.␣ input gas pressure to a constant, lower output pressure. Pressure regulators are used in a number of places within anesthesia OXYGEN FLUSH machines. All anesthesia machines have cylinder pressure regulators, sometimes termed the first-stage regulators (see The oxygen flush button activates a simple valve that opens Fig. 2.2). These reduce the pressures of compressed gases flow from the oxygen pipeline or oxygen first-stage regulator coming from cylinders at variable pressures of up to 2200 to the CGO. Pressing the oxygen flush button results in a psig, depending on the gas composition and volume within flow of 35–75 L/min of pure oxygen, bypassing any ON/OFF the cylinder, to a constant lower output pressure, typically 45 switches, flowmeters, and vaporizers.3 The pressure at the psig. As seen in Fig. 2.11, on anesthesia machines fitted for two CGO could increase up to the oxygen supply pressure unless oxygen cylinders, oxygen from both yokes flows via a common some pressure relief mechanism is present. The oxygen flush pathway to a single first-stage regulator. The principles of action can be used for a number of reasons. It can be used to fill the of a pressure regulator are shown in Fig. 2.16A and B, and breathing system with oxygen in order to manually ventilate described in the legend.21 a patient’s lungs in an emergency when the anesthesia Failure of the pressure reduction function of a regulator workstation is off. It can be used to fill the breathing system can transmit excessively high pressure downstream. To protect when mask ventilating a patient, but it dilutes any inhaled against such occurrences, the regulator incorporates a pressure anesthetics in the breathing system. To avoid barotrauma relief valve in the low-pressure chamber through which excess in a patient, caution is necessary when the oxygen flush pressures are vented to the atmosphere. If the diaphragm were is activated, particularly during mechanical ventilation. to rupture or develop a hole, the regulator would fail and gas Contemporary machines incorporate a pressure-limiting would escape around the adjustment screw and spring. The high device in the ventilator to prevent potentially harmful flow of escaping gas would make a loud sound, suggesting the pressures, but the inspiratory pressure could reach this limit possibility of a regulator failure. Failure of an oxygen first-stage if the oxygen flush is activated during the inspiratory phase regulator would cause a significant loss of oxygen pressure, of positive-pressure ventilation (see Chapter 6). The oxygen 32 PART 1 Gases and Ventilation Fig. 2.11 Top: The location of a double-hanger yoke in the anesthesia machine is circled. Bottom: Double-hanger yoke assembly with oxygen tank hanging in yoke A. Gas flows into the machine via the floating check valve. Gas cannot escape via yoke B because the oxygen pressure closes the check valve. If gas should leak past the check valve, its flow is prevented by the yoke plug, which has been tightened into yoke B, occluding the yoke nipple. (From Bowie E, Huffman LM: The anesthesia machine: essentials for understanding. Madison, WI, 1985, GE-Datex-Ohmeda.) PRESSURE SENSOR SHUT-OFF (FAIL-SAFE) flush can also be used to provide high pressure oxygen for VALVES AND ALARM SYSTEM emergency jet ventilation (but see Common Gas Outlets and Outlet Check Valves, later, for further discussion).22 Pressure sensor shut-off valves and oxygen failure protection The workstation standard requires that the oxygen flush devices (OFPDs), often referred to as fail-safe valves, were valve be self-closing and designed to minimize unintended an early safety feature of anesthesia machines. They were operation by equipment or personnel.3 A modern design for an introduced at a time when oxygen and nitrous oxide were oxygen flush button is shown in Fig. 2.17; note that the button is commonly used, often supplied from cylinders, and before the recessed in a housing to prevent accidental depression and that development of pulse oximetry or the common measurement the valve is self-closing.␣ of inspired oxygen concentration. A recognized hazard was  2 The Anesthesia Machine and Workstation 33 Fig. 2.12 Yoke plugs in unused tank hanger yokes. Fig. 2.14 Pipeline and cylinder gas supply pressures in this worksta- tion are measured by pressure transducers and are displayed digitally on the workstation screen, in this case, during checkout. The facility can configure the screen to display pressures in different units, such as kilopascals. Fig. 2.13 Gas supply pressure gauges on the front panel of an anes- thesia workstation. The three on the left display pipeline supply pres- sures; those on the right display cylinder supply pressures (note cylinder symbols). that the oxygen cylinder could be empty and the nitrous oxide would continue to flow. This was sometimes not recognized and the patient would suffer a hypoxic injury or death. Pressure sensor shut-off valves reduce or interrupt the flow of a second gas when the pressure from the oxygen pipeline or oxygen first-stage regulator falls below a set threshold. Pressure sensor shut-off valves stop the flow of nitrous oxide and other hypoxic gases (e.g., carbon dioxide) to their flowmeters if the oxygen supply pressure falls below the threshold setting. In the event of a catastrophic loss of oxygen to the anesthesia machine, pressure sensor shut-off valves ensure that oxygen is the last gas that flows to the patient. On some older anesthesia machines, even the air channel has a pressure sensor shut-off valve. Pressure sensor shut-off valves can be constructed to either Fig. 2.15 Schematic of an electronic pressure transducer. Compressed stop or to proportionally decrease the flow of a second gas as the oxygen, in this case, distorts a diaphragm that is attached to a strain oxygen pressure decreases below a preset threshold. Fig. 2.18 gauge. The electronic resistance (Rs) of the strain gauge increases as it shows an all-or-nothing type valve from an older GE-Datex- bends. Voltage measured across the Wheatstone bridge is proportional Ohmeda machine. When the oxygen pressure is higher than to oxygen pressure. 25 psig, it presses on the diaphragm with enough force to hold the valve open and allow the flow of nitrous oxide. The valve oxygen to nitrous oxide flows is coming from the flowmeters. closes and totally interrupts the flow of nitrous oxide when Thus, a normally functioning fail-safe system would permit flow oxygen pressure decreases below 25 psig.23,24 Fig. 2.19 shows a of 100% nitrous oxide, provided the machine has an adequate proportional type valve, called an OFPD, from an older Dräger oxygen supply pressure. The term “fail-safe” therefore represents machine. As the oxygen supply pressure falls and the flow of something of a misnomer because it does not ensure adequate oxygen from the machine’s flowmeter decreases, the OFPDs oxygen flow (Fig. 2.20). proportionately reduce the supply pressure of other gases to Since the fail-safe valve only senses pressure in the oxygen their flowmeters, synchronously decreasing those flows. The supply line, it would not be able to detect if a gas other than oxygen supply of nitrous oxide and other gases is completely interrupted was in the oxygen supply line. For example, the oxygen and when the oxygen supply pressure falls below 12 ± 4 psig.25 nitrous oxide supply lines could be crossed during construction The fail-safe valves only ensure that nitrous oxide cannot flow of a new OR. Similarly, nitrogen could be introduced into the when oxygen supply pressure is lost, and not that a safe ratio of oxygen supply line during the pressure testing of a new pipeline. 34 PART 1 Gases and Ventilation These potentially lethal mistakes can only be detected by the the oxygen supply pressure to the machine decreases. It does not workstation’s oxygen analyzer. prevent delivery of a hypoxic mixture to the CGO. All anesthesia machines have an oxygen supply pressure In anesthesia machines with mechanical flow controls, failure alarm that notifies the user if the oxygen supply oxygen and nitrous oxide flow controls are physically interlinked pressure is below a preset threshold, typically 30 psig. On either mechanically (older GE-Datex-Ohmeda machines) or contemporary machines, this is usually a simple electronic mechanically and pneumatically (Dräger machines) so that a sensor on the oxygen piping downstream of the pipeline fresh gas mixture containing at least 25% oxygen is created at the inlet and oxygen first-stage regulator (Fig. 2.21). Sometimes, level of the rotameters when nitrous oxide and oxygen are being the oxygen electronic pipeline and cylinder pressure gauges delivered.6,8 In anesthesia machines with electronic flow controls, serve as the sensors for the oxygen supply pressure alarm. proportioning is achieved by the computer interposed between If the oxygen pressure falls below the set point, an electrical the human user and the electronically controlled gas flow valves. switch closes which activates both an audible and visual alarm, Datex-Ohmeda and GE anesthesia machines with mechanical and alarm message (Fig. 2.22). The workstation standard flow controls use the Link-25 Proportion-Limiting Control requires that whenever oxygen supply pressure falls below System. In this system, a chain linkage between the oxygen and the manufacturer-specified threshold, a high-priority alarm is nitrous oxide flow-control knobs engages whenever the oxygen activated. The alarm remains activated until the oxygen supply flow is set less than 25% of the nitrous oxide flow (Figs. 2.23 and pressure becomes adequate, and for safety reasons the audio 2.24). When engaged, turning the oxygen down further causes the cannot be paused for more than 120 seconds.3␣ nitrous oxide knob to turn down as well, and turning the nitrous oxide up causes the oxygen knob to turn up too. However, the flow through a flowmeter assembly depends on both the control OXYGEN RATIO PROPORTIONING SYSTEMS knob position and the pressure differential across the flow control A major consideration in the design of contemporary anesthesia orifice (see Mechanical Flow-Control Valves later). So, for this machines is the prevention of hypoxic gas mixture delivery system to work, second-stage regulators located just upstream to the patient. The fail-safe system described previously only of the flowmeter assemblies tightly control the inlet pressures of interrupts, or proportionately reduces and ultimately interrupts, oxygen (14 psig) and nitrous oxide (26 psig) to their respective the supply of nitrous oxide and other gases to their flowmeters if flowmeters (see Figs. 2.2 and 2.24). Fig. 2.16 (A) Schematic of a direct-acting oxygen pressure regulator. Inset shows location of the oxygen pressure regulator within the anesthesia machine.  2 The Anesthesia Machine and Workstation 35 Fig. 2.16, cont’d (B) An inside look at the regulator as oxygen pressure is reduced. In principle, the regulator functions by balancing forces acting on the diaphragm. Gas under high pressure (P) enters the regulator and is applied to the valve over the area of the seat (a). Because Force = Pressure × Area, the force resulting from high-pressure gas is P × a. Valve opening is initially opposed by the force of the return spring (FRS). Because of the small area of the valve orifice, gas flowing through it enters the next chamber at a lower pressure (p). This lower pressure is applied over the large area of the diaphragm (A) at a force of p × A. Upward movement of the diaphragm is opposed by the force of the adjustment spring (FAS). The valve and diaphragm are connected by a thrust pin and move as one unit according to the forces applied in either direction. In equilibrium, the forces acting on the diaphragm are equal: (P × a) + FAS = (p × A) + FRS. The reduced pressure p = [(FAS − FRS) + (P × a)]/A. The regulator is designed such that p is fairly constant despite changes in P. (From Bowie E, Huffman LM: The anesthesia machine: essentials for understanding. Madison, WI, 1985, GE-Datex-Ohmeda.) Dräger anesthesia machines use different types of flow of nitrous oxide flow to oxygen flow so that it does not exceed 3:1 proportioning systems that regulate the oxygen flows to be (i.e., an oxygen concentration lower than 25% in the mixture). greater than 25% of the nitrous oxide flows, irrespective of The user has different experiences with the Dräger ORC or where the flow-control knobs are set. With these systems, S-ORC versus the GE-Datex-Ohmeda Link-25 Proportioning when the oxygen flow is below 25% of the nitrous oxide flow, Limiting System. In the Link-25 system, once the oxygen flow decreasing the oxygen flow further causes the nitrous oxide flow has been increased via the link chain and gears, the oxygen flow to decrease without its flow-control knob moving, and turning remains at the increased setting even if the nitrous oxide flow is the nitrous oxide knob up does not increase the nitrous oxide deliberately decreased. With the ORC and S-ORC systems, if the flow. Dräger anesthesia machines with mechanical flow controls system is acting to decrease the flow of nitrous oxide because the and glass flowmeters (no longer manufactured or supported user has decreased the oxygen flow, when the flow of oxygen is by Dräger) use the oxygen ratio controller (ORC; Fig. 2.25).8,26 increased again, the nitrous oxide flow will increase to its original Dräger anesthesia machines with mechanical flow controls and setting. Although of elegant designs, the ORC, S-ORC, and Link- electronic flowmeters use the sensitive oxygen ratio controller (S- 25 systems are subject to mechanical and/or pneumatic failure (see ORC) proportioning system (Fig. 2.26). However, the general Chapter 23) and should be tested according to the manufacturer’s principle of the ORC and S-ORC is otherwise the same: Two instructions during the pre-use machine checkout.27 connected diaphragms control a slave nitrous oxide flow control All of the aforementioned proportioning systems function valve that is in series with the manual nitrous oxide flow control only between nitrous oxide and oxygen and there is no valve. This slave nitrous oxide flow control valve limits the ratio interlinking of oxygen with other gases, such as air or helium, 36 PART 1 Gases and Ventilation B Fig. 2.17 (A) Top: The location of the oxygen flush valve in the anesthesia machine is circled. Bottom: Schematic of oxygen flush valve in closed position. Note that it is recessed to prevent accidental activation. When depressed, oxygen flows from the common gas outlet at a rate of 35–75 L/min. Depending on the pressure relief arrangements, the oxygen may be delivered at pipeline supply pressure. (B) Examples of oxygen flush valve buttons. Note that they are flush with the front of the workstation to avoid accidental activation. psig, Pounds per square inch gauge. (From Bowie E, Huffman LM: The anesthesia machine: essentials for understanding. Madison, WI, 1985, GE-Datex-Ohmeda.)  2 The Anesthesia Machine and Workstation 37 Fig. 2.18 (A) GE Healthcare Datex-Ohmeda pressure sensor shut-off (“fail-safe”) valve in a traditional machine. If oxygen supply pressure on the diaphragm exceeds the threshold, in this case 25 psig, the valve is lifted from its seat and nitrous oxide can flow to its flowmeter. (B) If the oxygen supply pressure falls below the threshold setting for the valve return spring pressure, the valve is no longer held off its seat and interrupts the flow of nitrous oxide to its flowmeter. All contemporary machines have a pressure sensor shut-off mechanism for each non–oxygen-containing gas (e.g., nitrous oxide, carbon dioxide, helium) supplied to the machine. In some machines, a proportional-type valve replaces the pressure sensor shut-off valve. This is a variable valve, rather than an open or shut valve. Other machines have no pressure sensor shut-off valves, and the flows of hypoxic gases are terminated in a different way when oxygen supply pressure decreases below a set threshold. DISS, Diameter index safety system; psig, pounds per square inch gauge. Inset shows location of a pressure sensor shut-off valve in the anesthesia machine. (From Bowie E, Huffman LM: The anesthesia machine: essentials for understanding. Madison, WI, 1985, GE-Datex-Ohmeda.) that might be delivered by the machine (Figs. 2.27 and 2.28). the breathing system oxygen concentration to decrease below Thus, when a third or fourth gas is in use, these proportioning 21%.28 Once again, an oxygen analyzer in the inspiratory side of systems afford no protection against a hypoxic mixture at the the patient circuit is essential if a potentially hypoxic mixture is CGO. Because of this, it is inherently safer to supply helium to be detected and thereby prevented.␣ from tanks that contain a mixture of helium and oxygen (heliox) in a 75:25 ratio. FRESH GAS CONTROLLERS AND FLOWMETERS Many manufacturers now offer anesthesia machines with electronically-controlled flow-controllers and electronic Two separate components deliver the intended flow of each gas flowmeters. In these, hypoxic gas mixtures are prevented at to the CGO: a variable resistor device controls the flow, and the level of the user interface, where the gas flows are set on another device measures the flow (Fig. 2.29A and B). a controlling computer. The user is prevented from setting an oxygen flow that is less than 25% of the nitrous oxide flow. The Mechanical Flow-Control Valves computer continuously adjusts the flow controllers to achieve In many anesthesia machines, total gas flow delivered to the the set flows, rapidly responding to any changes in supply CGO, as well as the proportions of oxygen, nitrous oxide, air, pressures. and sometimes other medical gases, are adjusted by turning Even if the fresh gas proportioning system is functioning the knobs on flow control needle valves (Fig. 2.29B). Because correctly, an oxygen leak downstream of the flowmeter, they are mechanical valves, they are turned left to loosen (i.e., or the addition of high concentrations of a potent volatile counterclockwise to open) and right to tighten (i.e., clockwise inhaled anesthetic (e.g., 18% desflurane) downstream of the to close). Needle valves are simple variable resistors, and the proportioning systems, could result in a hypoxic mixture (i.e., resultant flow depends on the size of the orifice and the pressure