Handbook of Clinical Anesthesia
The Anesthesia Workstation and Delivery Systems
Modern anesthesia machines are properly referred to as anesthesia workstations (Riutort KT, Brockwell RC, et al: The anesthesia workstation and delivery systems In Clinical Anesthesia. Edited by Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC. Philadelphia: Lippincott Williams & Wilkins, 2009, pp 644–694). The anesthesia workstation is a system for administering anesthetics to patients. The workstation consists of the anesthesia gas supply device, the anesthesia ventilator, monitoring devices, and protection devices (designed to prevent hazardous output caused by incorrect delivery or barotrauma).
- Anesthesia Workstation Standards and Pre-Use Procedures
Workstations may have computer-assisted self-tests that automatically perform all or part of the pre-use machine checkout procedure. Ultimately, performing adequate pre-use testing of the anesthesia workstation is the responsibility of the individual.
- Stanadards for Anesthesia Machines and Workstations
These standards provide guidelines to manufacturers regarding minimum performance, design characteristics, and safety requirements for anesthesia machines. To comply with the American Society for Testing and Materials Standards, newly manufactured workstations must have monitors to measure specific parameters and possess a prioritized alarm system (Table 26-1).
III. Failure of Anesthesia Equipment
The most common malfunction of the medical-gas delivery system is related to the breathing circuit. Pulse oximetry is the principal monitor for alerting the anesthesia professional to an equipment problem.
Table 26-1 American Society for Testing and Materials Standards for Manufactured Workstations
- Safety Features of Newer Anesthesia Workstations
- Checkout of the Anesthesia Workstation
A complete anesthesia apparatus checkout procedure must be performed each day before the first use of the anesthesia workstation. (A “machine checklist” is most applicable to older anesthesia machines.) Newer workstations may perform an automated checkout. The three most important preoperative checks are O2 analyzer calibration, the low-pressure circuit leak test, and the circle system test (Table 26-3).
- Anesthesia Workstation Pneumatics
- The Anatomy of an Anesthesia Workstation(Fig. 26-1)
- Gases such as O2, nitrous oxide (N2O), and air are usually supplied from a central pipeline with cylinders on the machine as a backup. The pipeline source is usually at 50 psig (pounds per square inch gauge). A full O2cylinder contains only gas, and the tank pressure decreases linearly from a maximum of about 2200 psig as it is consumed. N2O is compressed to a liquid in tanks and maintains a pressure of 745 psig until all the liquid is dissipated.
Table 26-2 Comparison of Anesthesia Workstation Functions
- Oxygen failure cut-off (“fail safe”) valvesare located downstream from the N2O supply source and serve as an interface between the O2 and N2O supply sources. This value shuts off or proportionally decreases the supply of N2O if the O2 of supply decreases.
- Regulatorsdownstream from the O2 supply source adjust the pressure to about 14 psig before entering the flow meter assembly.
Table 26-3 Preoperative Anesthesia Workstation Checklist
Figure 26-1. Draeger Medical Fabius GS anesthesia workstation (A) and GE Healthcare Aisys anesthesia workstation (B).
- Flow control valvesseparate the intermediate-pressure circuit from the low-pressure circuit (the part of the machine that is downstream from the flow control valves). The operator regulates flow entering the low-pressure circuit by adjusting the flow control valves. After leaving the flow tubes, the mixture of gases travels through a common manifold and may be directed to a calibrated vaporizer.
- A one-way check valve located between the vaporizer and common gas outlet prevents backflow into the vaporizer during positive pressure ventilation.
- The O2flush connection joins the mixed-gas pipeline between the one-way check valve and the machine outlet. When the O2 flush valve is activated, the pipeline O2 pressure is reflected in the common gas outlet.
- Pipeline Supply Source.Most hospitals have a central piping system to deliver medical gases such as O2, N2O, and air to the operating room at appropriate pressures for the anesthesia workstation to function properly.
- Flow meter assembliesprecisely measure gas flow to the common gas outlet. Depending on the setting of the flow control valve, gases flow through variable orifice, tapered tubes at a rate indicated by the position of a float indicator in relation to a calibrated scale.
- At low flow rates, the viscosity of the gas is dominant in determining flow; density is dominant at high flow rates.
- Safety features include use of standardized colors for each gas, an O2flow meter dial that is distinct from the others, and positioning of the O2 flow meter immediately proximal to the common gas outlet to minimize the chance of delivery of hypoxic mixtures in the event of leaks in the flow meter assembly.
- Problems with Flow Meters
- Leaksare hazardous because flow meters are located downstream from all machine safety devices except the O2 analyzer. The use of electronic flow meters and the removal of conventional glass flow tubes helps eliminate this potential
source of leak and risk for delivery of hy-poxic gas mixtures (minimized if the O2 flow meter is located downstream from all other flow meters).
- Inaccuracyof flow measurement may occur (dirt or static electricity may cause a float to stick)
- With an ambiguous scale,the operator reads the float position beside an adjacent but erroneous scale (this is minimized by etching the scale into the tube).
- Dilution of Inspired Oxygen Concentration by Volatile Inhaled Anesthetic.When added to the inhaled gases downstream from flow meters and proportioning system, concentrations of less potent volatile anesthetics (maximum desflurane dial setting, 18%) may result in a gas–vapor mixture that contains an inspired O2 concentration below 21%.
VII. Web-Based Anesthesia Software Simulation: The Virtual Anesthesia Machine
Figure 26-2. The Virtual Anesthesia Machine Simulator, an interactive model of an anesthesia machine.
Table 26-4 Vaporizer Models and Characteristics
VIII. Vaporizers (Table 26-4)
- Variable Bypass Vaporizers
- The Datex-Ohmeda Tec 4, 5, and 7 and the North American Draeger Vapor 19n and 20n are classified as variable bypass (method for regulating output concentration), flow-over, temperature-compensated, agent-specific (keyed filling devices), out-of-breathing circuit vaporizers.
- Basic Operating Principles
- As gas flow enters the vaporizer's inlet, the setting of the concentration dial determines the ratio of flow that goes through the bypass chamber and through the vaporizing chamber. The gas diverted to the vaporizing chamber flows over the liquid anesthetic and becomes saturated with vapor.
- The amount of gas diverted into the vaporizing chamber is primarily a function of the anesthetic
vapor pressure. A temperature-compensating device helps maintain a constant vaporizer output over a wide range of temperatures.
Table 26-5 Safety Features of Variable Bypass Vaporizers
- Safety Features(Table 26-5)
- Hazards(Table 26-6)
- The Datex-Ohmeda Tec 6 vaporizer for desfluraneis an electrically heated, pressurized device specifically designed to deliver desflurane.
- Desflurane boils at 22.8°C, and its vapor pressure is three to four times that of other contemporary inhaled anesthetics.
- Desflurane's high volatility and moderate potency preclude its use with contemporary variable bypass vaporizers.
- Factors that Influence Vaporizer Output
- Varied altitudesinfluence the output of this vaporizer that is unlike contemporary variable
bypass vaporizers, which deliver a constant partial pressure of anesthetic. At a given concentration dial setting, the Tec 6 provides a lower partial pressure of the anesthetic as altitude increases. For example, at 2000-m elevation, the concentration dial setting of the Tec 6 must be increased from 10% to 12.5% to deliver the same partial pressure as at sea level.
Table 26-6 Hazards Associated with Variable Bypass Vaporizers
- Carrier Gas Composition.The vaporizer output approximates the dial setting when O2 is the carrier gas. At low flow rates using N2O as the car-rier gas (decreased viscosity compared with O2), the vaporizer output is approximately 20% less than with the dial setting.
- Safety Features.The agent-specific filler cap of the desflurane bottle prevents its use with traditional vaporizers.
- The Datex-Ohmeda Aladin Cassette vaporizeris a unique, single, electronically controlled vaporizer designed to deliver five different volatile drugs (halothane, isoflurane, enflurane, desflurane, sevoflurane).
- The MAQUET 950 series injector vaporizeris designed to be used with the MAQUET Servo Ventilator.
- Anesthetia Breathing Circuits
- An anesthetic circuit is interposed between the anesthesia machine, the common gas outlet, and the patient. The function of the circuit is to deliver anesthetic gases and O2to the patient and to remove exhaled carbon dioxide (CO2).
- Mapleson Systems
- In 1954, Mapleson described and analyzed five different semiclosed anesthetic systems (Mapleson Systems A–E) in which the amount of CO2rebreathing associated with each system is multifactorial (Table 26-7 and Fig. 26-3).
- The Bain circuitis a modification of the Mapleson D circuit, in which fresh gas flow is delivered at the end nearest the patient through a small inner tube located within the larger corrugated tubing (Fig. 26-2).
Table 26-7 Variables That Determine the Amount of Carbon Dioxide Rebreathing Associated with Mapleson Systems
- The advantages of all these systems are that they are lightweight and convenient. The main disadvantage is that high fresh gas flows are required.
- Circle Breathing Systems
- Technological changes in the traditional circle breathing system include application of single-circuit piston-type ventilators and use of new spirometry devices that are located at the Y-connector instead of at the traditional location on the expiratory circuit limb.
- The Traditional Circle Breathing System.The circle system (fresh gas inflow, inspiratory and expiratory unidirectional valves, inspiratory and expiratory corrugated tubing, Y-piece connector, overflow or pop-off valve, reservoir bag, and a canister containing a CO2 absorbent) is the most popular breathing system in the United States (Table 26-8 andFig. 26-4).
- The unidirectional valves are placed so that gases flow in only one direction and through the CO2absorber (Fig. 26-4).
- If the valves are functioning properly, the only dead space in the system is between the Y-piece and the patient.
- A closed systemexists when the fresh gas flow equals that being consumed by the patient (about 300 mL/min of O2 plus uptake of anesthetic gases)
and the overflow (pop-off) valve is closed. If high fresh gas flows are used, the system is semi-closed or semi-open.
Figure 26-3. Schematic diagram of the Mapleson systems and the Bain system. FGF = fresh gas flow.
Table 26-8 Characteristics of a Circle System
- Carbon Dioxide Absorbents
- Undesirable chemical reactions between desiccated Baralyme (no longer commercially available in the
United States) include exothermic reactions with sevoflurane (fires in the breathing system) and production of carbon monoxide (desflurane) and compound A (sevoflurane).
Figure 26-4. Components of the circle system. APL = adjustable pressure-limiting valve or “pop-off” valve.
Table 26-9 Chemical Reactions of carbon dioxide with Soda Lime
- Chemistry of Absorbents
- Available formulations of CO2absorbents are soda lime and calcium hydroxide lime (Amsorb).
- Advantages of calcium hydroxide are the lack of strong bases (sodium and potassium hydroxide) and the absence of undesirable chemical reactions and formation of heat, compound A, and carbon monoxide.
- Absorption of CO2is accomplished in a circle system by a chemical reaction that results in water and heat as byproducts (Table 26-9).
- Absorptive Capacity.The maximum amount of CO2 that can be absorbed by soda lime is 26 L of CO2 per 100 g of absorbent. The absorptive capacity of calcium hydroxide is lower (10.2 L of CO2 per 100 g of absorbent).
- Ethyl violet is the pH indicator added to soda lime that changes from colorless to violet in color when the pH of the absorbent decreases as a result of CO2absorption.
- Prolonged exposure of ethyl violet to fluorescent lights can produce photodeactivation of the dye (the absorbent appears white even though it may have a reduced pH and its absorptive capacity has been exhausted).
- Clinical signs that the CO2absorbent is exhausted may occur even in the absence of color changes (Table 26-10).
Table 26-10 Clinical Signs that the Carbon Dioxide Absorbent is Exhausted
- Interactions of Inhaled Anesthetics With Absorbents (Table 26-11)
The likelihood of adverse chemical reactions between CO2 absorbents and volatile anesthetics is minimized by avoiding the use of desiccated CO2 absorbents.
- Anesthesia Ventilators
- Ventilators can be classified according to their power source (electricity or compressed gas), drive mechanism (pneumatically driven), and cycling mechanism (time cycled, pressure cycled).
- Bellows Classification
- The direction of the bellows movement during the expiratory phase determines the bellows classification.
Table 26-11 Interactions of Inhaled Anesthetics with Carbon Dioxide Absorbents
Table 26-12 Hazards Associated with Ventilators
- Ascending (standing) bellows ascend during the expiratory phase, and descending (hanging) bellows descend during the expiratory phase.
- Ascending bellows will not fill if a total disconnection occurs, and a descending bellows ventilator will continue its up and down movement despite a patient disconnection (driving gas pushes bellows up during the inspiratory phase).
- An essential safety feature of any anesthesia workstation that uses a descending bellows is an integrated CO2apnea alarm that cannot be disabled while the ventilator is in use.
- Problems and Hazards(Table 26-12)
XII. Anesthesia Workstation Variations
- The Datex-Ohmeda S/5 ADU and Aisyseliminates gas flow tubes and conventional anesthesia vaporizes in exchange for a computer screen with digital fresh gas flow scales and a built-in Aladin Cassette vaporizer system.
- Entry of the fresh gas inflow on the patient side of the inspiratory valve is more efficient in delivering fresh gas flow to the patient and preferentially eliminating exhaled gases.
- This arrangement is also less likely to result in desiccation of the CO2absorbent.
- The Draeger Medical Narkomed 6000 Series, Fabius GS, and Appolo Workstationsare characterized by the absence of flow tubes and glowing electronic fresh gas flow indicators.
- Fresh gas decoupling decreases the risk of barotrauma (fresh gas coming into the system from the patient is isolated while the ventilator exhaust valve is closed) and volutrauma.
Table 26-13 Hazards Introduced by Scavenging Systems
- A disadvantage of the anesthesia circle systems that use fresh gas decoupling is the possibility of entraining room air into the patient gas circuit. High-priority audible and visual alarms are needed to notify the user that fresh gas flow is inadequate and room air is being entrained.
XIII. Scavenging Systems
- Scavenging systems are designed to collect and subsequently vent gases from operating rooms.
- These systems minimize operating room pollution but increase the complexity of the anesthesia system (Table 26-13).
- The two major causes of waste gas contamination in the operating room are incorrect anesthetic technique (failure to discontinue gas delivery from the anesthesia machine at the conclusion of anesthesia, poorly fitting face mask, flushing the breathing circuit) and equipment failure (leaks).
Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine
Title: Handbook of Clinical Anesthesia, 6th Edition
Copyright ©2009 Lippincott Williams & Wilkins
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