Anesthesia Machine Second Medical College

三峡大学仁和医院 Jian Dao-lin

简道林

Overview The gas delivery systemⅠ Breathing systemsⅡ Anesthesia ventilatorⅢ Safety featuresⅣ ScavengingⅤ Gas analysisⅥ AccessoriesⅦ New generation anesthesia machinesⅧ

Definition Anesthesiologists define the machine which is u

sed to support the administration of anesthesia as the anesthesia machine. The original concept was invented by the British anesthetist H.E.G. Boyle in 1917. Prior to this time, anesthetists often carried all their equipment with them, but the development of heavy, bulky cylinder storage and increasingly elaborate airway equipment meant that this was no longer practical for most circumstances.

Overview

The function of the anesthesia machine is to prepare a gas mixture of precisely known but variable composition. The machine provides a controlled flow of oxygen, nitrous oxide, air, and anesthetic vapors. These are delivered to a breathing system, which provides a means to deliver positive pressure ventilation and to control alveolar carbon dioxide by minimizing rebreathing and/or by absorbing carbon dioxide. A mechanical ventilator is connected to the breathing system, freeing up the anesthetist's hands for other tasks. Several types of monitors are used to observe the function of the system, to detect equipment failures, andto provide information about the patient.

Made in China

Drager Narkomed 2A

Ohmeda Aestiva

A Datex Ohmeda British machine

The commonest type of anaesthetic machine in use in the developed world is the continuous-flow anaesthetic machine, which is designed to provide an accurate and continuous supply of medical gases ( such as oxygen and nitrous oxide ), mixed with an accurate concentration of anesthetic vapour (such as isoflurane), and deliver this to the patient at a safe pressure and flow. Modern machines incorporate a ventilator, suction unit, and patient-monitoring devices.

A schematic diagram of an anesthesia machine

Non-rebreathing Systems

rebreathing system (Vaporizer-in-the-Circle)

rebreathing system(Vaporizer-out-of-the-Circle)

Ⅰ The gas delivery system - Oxygen supplyⅠ Ⅰ - Piped gasesⅠ Ⅱ - Flow control valvesⅠ Ⅲ - FlowmetersⅠ Ⅳ - VaporizersⅠ Ⅴ - The common gas outlet Ⅰ Ⅵ - Oxygen flush valveⅠ Ⅶ

1.Central oxygen supply systemLiquid oxygen

Ⅰ- Oxygen supplyⅠ

2. Oxygen bank (smaller oxygen cylinders or tanks)

Ⅰ- Piped gasesⅡ

Wall outlets supply oxygen, nitrous oxide and air at a pressure of 50 to 55 pounds/in2 (psi). These outlets and the supply hoses ( corrugated tube ) to the machine are diameter indexed and color-coded.

Outlet of central oxygen supply system

1. A full cylinder of oxygen (in size E) has a pressure of 2,000 to 2,200 psi and contains the equivalent of 660 L of gas at atmospheric pressure and room temperature. The oxygen cylinder pressure decreases in direct proportion to the amount of oxygen in the cylinder.

1 （ psi, pound per square inch ） = 6. 895 （ kPa ） = 0.0703 （ kg/cm2 ） = 0.0689 （ bar ） =0.068 大气压（ atm ） . 1 （ mmH2O ） =9.80665 帕（ Pa ） , 1 （ mmHg ） =133.322 （ Pa ）

2. A full cylinder of nitrous oxide (in size E) has a pressure of 745 psi and contains the equivalent of 1,500 L of gas at atmospheric pressure and room temperature. The nitrous oxide in the cylinder is a liquid; the cylinder pressure does not decrease until the liquid content is exhausted, at which time one-fourth of the total volume of gas remains.

3. Air cylinders (in size E) are present on some machines. A full cylinder has a pressure of 1,800 psi and contains the equivalent of 630 L at atmospheric pressure and room temperature.

A regulator or pressure reducing valve on anesthesia machines reduces the cylinder pressure to 50 psi. If this valve were not part of the anesthesia machine, the pressure of the gas entering the machine would be the same as the pressure in the cylinder. The industry standard for pressure within the anesthesia machine has been set at 50 psi. The regulators divide the machine into high-pressure (proximal to the regulator) and low-pressure (distal to the regulator) systems.

Ⅰ- Ⅲ A regulator or pressure reducing

valve(Flow control valves)

A needle valve controls the flow of each gas. As a safety feature, the oxygen control knob is fluted and protrudes more than the nitrous oxide and air controls. Gas pressures are reduced from 45 to 55 psi (high pressure) to near atmospheric pressure (low pressure) bythe needle valves.

A regulator or pressure reducing valve

Oxygen flow control valve

As gas flows out of the low-pressure chamber, the drop in pressure reduces the force generated by the diaphragm (D) against the spring (S), allowing the valve (V) to open and admit gas from the high-pressure chamber. The output pressure may be adjusted by a screw (A) that alters the force applied by the spring.

Figure —Cross-sectional view of a needle valve

Needle valves are similar in design and operation to the globe valve. Instead of a disk, a needle valve has a long tapered point at the end of the valve stem. When the long taper of the valve element is clock-wise turned, the valve is opened. Contrarily, the valve is closed. A cross-sectional view of a needle valve is illustrated in figure.

Ⅰ- Ⅳ flowmeters

Flowmeters. Each flowmeter is a calibrated tapered glass tube in which a bobbin or ball floats to indicate the flow of gas. In other words,

Flowmeters can be of varying styles however most anesthesia machines have flowmeters that use a bobbin that floats on a column of gas to

determine the amount of flow.

The needle valve is the most common means of regulating gas flow rate. As the valve is opened, the orifice around the needle becomes larger and flow increases. The valve cartridge itself is usually removable so it can be replaced if it is damaged. The valve must not be over-tightened--this will drill out the orifice and cause it to become incompetent. Some valves, such as are found on most medical anesthetic machines, incorporate a stop to prevent the valve being over-tightened. The valve control knob is usually color-coded. In addition, oxygen flowmeter knobs frequently have fluted edges to distinguish them from those of other gases.

                        

The inside of glass clinder becomes wider as the bobbin floats higher in the cylinder thereby allowing more gas to flow up the tube and out of the flowmeter.

The flowmeter allows the operator to control and know the flow rate of each gas; usually in liters per minute or mL per minute. Consists of a float, usually a little ball inside a tube. It is usually read from the center of the ball and usually colorcoded, like the gas tanks.

Flowmeters are used to control the flow of a gas (oxygen usually) which is being delivered to the circle.

As the indicator or float of the flowmeter rises in the tube, more gas flows around it.Some machines have two flowmeters, one for high flow and one for low flow of oxygen.

Gas (oxygen, nitrous oxide) usually enters at the bottom, travels through a tapered tube, and then exits at the top.

When the valve is closed, it should be turned only until the flow of gas ceases as further tightening may result in damage to the pin or seat.

When the machine is not used, the flow control knobs should be opened until the gas pressure is zero, then closed.

Before using a machine, the flowmeters should be checked to see if they are in the closed position. If they have be left open, when the gas supply to an open flow control valve is restored, the indicator may rise to the top of the tube where its pre-sense may not be noticed. A very high oxygen flow may result.

Ⅰ-Ⅴ Vaporizers An anesthetic vaporiser is a device generally attache

d to an anesthetic machine which delivers a given concentration of a volatile anesthetic agent.

The design of these devices takes account of varying ambient temperature fresh gas flow agent vapor pressure

Vaporizer-in-the-Circuit

Vaporizer-out-of-the-Circuit

Vaporizer-out-of-the-Circuit

The purpose of an anesthetic vaporizer is to produce a controlled and predictable concentration of anesthetic vapor in the carrier gas passing through the vaporizer.

Most vaporizers are of the plenum type, which consists of a vaporizing chamber containing the liquid anesthetic, and a bypass. Gas passing through the vaporizing chamber volatilizes the anesthetic and is then mixed with the anesthetic-free gas bypassing the chamber, the proportion of vapor-containing gas and bypass gas beingcontrolled by a tap.

Ⅰ- The common gas outletⅥ The common gas outlet is the port where gases exit the machine and is connected to the breathing system via the fresh gas hose.

Ⅰ- Oxygen flush valveⅦ This valve allows a high flow oxygen to go directly to the breathing system without going through a vaporizer (usually). One hundred percent oxygen at 45 to 55 psi comes directly from the high pressure system to the common gas outlet. Oxygen flow can be as high as 40 to 60 L/min.

Ⅱ Breathing systemsThe delivery systems which conduct anesthetic gases from an anesthetic machine to the patient are known as the breathing systems or circuits They are designed to allow either spontaneous respiration or intermittent positive pressure ventilation (IPPV) and consist of a reservoir bag, anesthetic tubing, and a pressure relief valve. A number of mechanical ventilators include a specific breathing system. Other ventilators have been designed to operate with existing breathing systems.

Anesthetic gas exits the anesthesia machine (via the common gas outlet) and then enters a breathing circuit. The function of the circuit is to deliver oxygen and anesthetic gases to the patient and to eliminate carbon dioxide. The carbon dioxide may be eliminated by gas inflow

or by soda lime absorption.

-Ⅱ ⅠClassification of breathing systems - The carbon dioxide absorberⅡ Ⅱ - Two one-way valves (Ⅱ Ⅲ inspiratory

and expiratory) - Breathing Hoses and Y-piece adapterⅡ Ⅳ - Reservoir bagⅡ Ⅴ - APL(adjustable pressure-limiting valve ) vaⅡ Ⅵ

lve or pop-off valve

Ⅱ- Classification of breathing systemsⅠ Various classification systems have been developed

to aid understanding of how  breathing systems operate. Classification of breathing systems are followed as:

1 Open, semi-open, semi-closed, and closed breathing system. 2 Non-rebreathing and rebreathing systems

1 Open, semi-open, semi-closed, and closed breathing system. This classification is made mainly

according to the concentration value of re-breathing carbon dioxide. The boundary concentration value of re-breathing carbon dioxide is 1 percent.

Unfortunately, in different parts of the world, the same terminology is used for different systems, so this classification is probably best voided.

(1)Open systems Open systems have no valves, no tubing: for

example open drop ether, or a nasal cannula. In either, the patient has access to atmospheric gases. There is not any re-breathing of carbon dioxide.

(2) Semi-open system Semi-open system refers to that in which the conc

entration of re-breathed carbon dioxide is less than 1 percent. Semi-open system should be no re-breathing and must have high fresh gas flow and higher minute ventilation.

(3) Semi-closed system Semi-closed system refers to that in which the co

ncentration of re-breathing carbon dioxide is more than 1 percent. In semi-closed system, some re-breathing occurs, FGF and pop-off settings should be at intermediate values.

(4) Closed system

Closed system is such a condition that fresh gas inflow exactly equal to patient uptake and that complete carbon dioxide is absorbed after carbon dioxide absorber takes the effects and pop-off is closed.

Mode Reservoir

(breathing bag) Rebreathing Example

Open No No Open drop

Semi-open Yes No Nonrebreathing circuit, or Circle at high FGF (> VE)

Semi-closed Yes Yes, partial Circle at low FGF

(< VE)

Closed Yes Yes, complete Circle (with pop-off valve

[APL] closed)

Remark: APL valve is the abbreviaten of adjustable pressure-limiting valve.

Comparison of Open, semi-open, semi-closed, and

closed breathing system

2. Non-rebreathing and rebreathing systems The classification of non-rebreathing versus re

breathing systems is more widely used.

(1)Mapleson (Non-rebreathing breathing system) (circuits)

One kind of breathing system was introduced by Professor W.W. Mapleson is most commonly used in the world (Figure 1). Mapleson described five different arrangements of breathing circuits. He classifed these circuits and they are now known as the Mapleson systems, termed A-E. This classification does not include systems with carbon dioxide absorption.

①The original Mapleson A system The Mapleson A (Magill) system was designed by Sir Ivan

Magill in the 1930's and remains an excellent system for spontaneous ventilation (Figure 2). Fresh gas enters the system at the fresh gas outlet of the anesthesia machine.

This is an efficient system for spontaneously breathing patients if carbon dioxide absorption is not available.

During controlled ventilation the Magill circuit works in a different way and becomes wasteful and inefficient, requiring high fresh gas flows to prevent rebreathing.

The expiratory valve (Heidbrink valve) is very clos

e to the patient to reduce the dead space. The respiratory cycle has three phases during spontaneous breathing; inspiration, expiration and the expiratory pause. During inspiration gas is inhaled from the 2 litre reservoir (breathing) bag which partially collapses giving a visual confirmation that breathing is occurring.

During expiration the bag and tubing are initially refilled with a combination of exhaled dead space gas (containing no carbon dioxide) and fresh gas flowing from the anaesthetic machine. Once the bag is full, the pressure within the breathing system rises and the expiratory valve near the patient opens allowing the alveolar gas (containing carbon dioxide) to be vented( 排出 )from the system. During the expiratory pause, more fresh gas enters the system driving any remaining alveolar gas back along the corrugated (anesthetic) tubing and out through the valve. If the fresh gas flow is sufficiently high, all the alveolar gas is vented from the circuit before the next inspiration and no rebreathing will take place.

With careful adjustment the fresh gas flow can be reduced until there is only fresh gas and dead space gas in the breathing system at the start of inspiration. When the system is functioning correctly, without any leaks, a fresh gas flow (FGF) equal to the patient's alveolar minute ventilation is sufficient to prevent rebreathing. In practice however, a FGF closer to the patients total minute ventilation (including dead space) is usually selected to provide a margin of safety. An adult's minute volume is approximately 80mls/kg /min and thus for a 75kg man a FGF of 6 litres per minute will prevent rebreathing.

During controlled ventilation, The inspiratory force is provided by the anesthesiologist with squeezing the reservoir bag after partly or completely closing the expiratory valve next to the patient. During lung inflation some of the gas is vented from the circuit and at the end of inspiration the reservoir bag is less than half full. During expiration, dead space and alveolar gas pass down the corrugated tubing and may reach the bag, which will then contain some carbondioxide.

During the next inspiration when the bag is compressed alveolar gas re-enters the patients lungs followed by a mixture of fresh, dead space and alveolar gas. A FGF of two and a half times the patient's minute volume is required to vent enough alveolar gas to minimise rebreathing (FGF of about 12-15 litres /min) which is obviously very inefficient. In practice the Magill circuit should not be used for positive pressure ventilation except for short periods of a few minutes at a time.

②Modification of Mapleson A system

The Lack circuit and the Bain system A disadvantage of the Magill system is that the expiratory

valve is attached close to the patient making it awkward to use (particularly when a scavenging circuit is added). A simple modification of the Mapleson A became co-axial system in which the exhaled gases travel down a central tube located within an outer corrugated tube towards the expiratory valve. The Lack circuit, upper one in Figure 3, still is a Mapleson A system.

However, lower one in Figure 3, the Bain system, became Mapleson D system. In those modified system, the inner tubing is wide enough to prevent an increase in the work of breathing and the expiratory valve is placed next to the reservoir bag, by the common gas outlet. The fresh gas flows required for both spontaneous and controlled ventilation are as described for the standard Mapleson A system.

(2) The Mapleson B and C The Mapleson B and C breathing systems are similar in construction, with the fresh gas flow entry and the expiratory valves located at the patient end of the circuit. They are not commonly used in anaesthetic practice, although the C system is used on intensive care units. High flows of gases are needed to prevent rebreathing of CO2 and C system was at one time combined with a canister of soda lime to absorb CO2 (Waters' "To and Fro" Circuit). However the cannister proved too bulky for practical use and there was a risk of the patient inhaling soda lime dust. There is different length of the corrugated tubing between the both

systems.

(3) The Mapleson D, E and F systems

The Mapleson D, E and F systems are all functionally similar . The Mapleson D, E, and F act as T pieces with the FGF delivered to the patient end of the circuit and differ only in the presence of valves or breathing bags at the expiratory end of the circuit.

A.for spontaneous respiration

The Mapleson D, E, and F act as T pieces with the FGF delivered to the patient end of the circuit and differ only in the presence of valves or breathing bags at the expiratory end of the circuit. These systems are all inefficient for spontaneous respiration (Figure 4).

Figure 4 Mode of action of Mapleson D breathing system during spontaneous ventilation

The spontaneous respiration (inefficient): During expiration exhaled gas and fresh gas mix in the corrugated tubing and travel towards the reservoir bag. When the bag is full, the pressure in the system rises and the expiratory valve opens venting a mixture of fresh and exhaled gas to the atmosphere. During the expiratory pause fresh gas continues to push exhaled alveolar gas down the tubing towards the valve. However, unless the FGF is at least twice the patient's minute volume, rebreathing of alveolar gas will occur. A FGF of at least 8-10 litres/min (150mls/kg/min) is required to prevent rebreathing inan adult.

The controlled ventilation: When used for controlled ventilation the Mapleson D system functions more efficiently. During expiration the corrugated tubing and reservoir bag fill with a mixture of fresh and exhaled gas. Fresh gas fills the distal part of the corrugated tube during the expiratory pause prior to inspiration. When the bag is compressed this fresh gas enters the lungs and when the expiratory valve opens a mixture of fresh and exhaled gas is vented. The degree of rebreathing that occurs depends on the FGF. A FGF of 70ml/kg/min is usually adequate for controlled ventilation; 100mls/kg/min will result in a degree of hypocapnia

(lowered CO2 level in the blood).

B. For the controlled ventilation:

C. Modifications of the Mapleson D system (the Bain Circuit):

The Bain Circuit is the most commonly used form of the Mapleson D system. It is a co -axial circuit that was introduced in 1972 by Bain and Spoerel. Unlike the Lack co-axial circuit described above, fresh gas flows down the central narrow bore tubing (7mm i.d.) to the patient and exhaled gases travel in the outer corrugated tubing (22mm i.d.). The degree of rebreathing that occurs during IPPV will depend on the FGF. In an adult, fresh gas flows of 70-80mls/kg/min (6-7litres/min) will maintain a normal arterial carbon dioxide tension (normocapnia) and a flow of 100mls/kg/min will result in mild hypocapnia.

D. The Mapleson E system

The Mapleson E system performs in a similar way to the Mapleson D, but because there are no valves and there is very little resistance to breathing it has proved very suitable for use with children. It was originally introduced in 1937 by P Ayre and is knownas the Ayre's T-piece.

E. The Mapleson F system The version most commonly used is the Jackson-Rees modi

fication which has an open bag attached to the expiratory limb (classified as a Mapleson F system although it was not included in the original description by Professor Mapleson). Movement of the bag can be seen during spontaneous breathing, and the bag can be compressed to provide manual ventilation. As in the Bain circuit, the bag may be replaced by a mechanical ventilator designed for use with children. This system is suitable for children under 20kg. Fresh gas flows of 2 - 3 times minute volume should be used to prevent rebreathing during spontaneous ventilation. During controlled ventilation in children normocapnia can be maintained with a fresh gas flow of 1000mls + 100mls/kg.

(2) rebreathing systems(Circle Breathing Circuit)

The term rebreathing implies that expired alveolar gas containing 5% carbon dioxide is inspired as part of the next tidal volume. Anaesthetic circuits are designed to minimise this, as it may lead to serious elevations in blood CO2 levels.

An alternative to using high flow circuits is to absorb CO2 from the expired gases which are then recirculated to the patient. These circuits are known as circle systems, were first devised by Brian Sword in 1926 and require smaller amounts of fresh gas each minute.

The amount of rebreathing that occurs with any particular anaesthetic breathing system depends on four factors: ①the design of the individual breathing circuit,

the mode of ventilation (spontaneous or controlled ),② the fresh gas flow rate and the patient's respira③tory pattern. Circuits may eliminate rebreathing either by ensuring an adequate flow of fresh gas which flushes the circuit clear of alveolar gas, or, in the case of a circle system, by the use of soda lime, which absorbs the CO2 so that low fresh gas flows may be used.

Circle system advantages and disadvantages Circle advantages:①constant inspired concentrations ②conserve respiratory heat and humidity ③useful for all ages (may use down to 10 kg, about

one year of age, or less with a pediatric disposable circuit)

④useful for closed system or low-flow ⑤low resistance (less than tracheal tube, but more

than a NRB circuit)

Circle disadvantages: ①increased dead space ②malfunctions of unidirectional valves

Schematic diagram of closed anesthesia machine

A representative circle breathing system with ventilator

Ⅱ- The Ⅱ carbon dioxide absorber Sodalime (CaOH2 + NaOH + KOH + sil

ica) or Baralyme (Ba[OH]2 + Ca[OH]2) contained in the absorber combines with carbon dioxide, forming CaCO2 and liberating heat and moisture (H2O). A pH-sensitive dye changes to a blue-violet color, indicating exhaustion of the absorbing capacity. The canister should be changed when 25% to 50% of the contents has changed color, although it should continue to absorb satisfactorily until at least the contents of the top canister have changed color.

Ⅱ- Two one-way valves (inspiratoⅢry and expiratory)

There are 2 unidirectional valves: expiratory and inspiratory. These valves insure that the flow of gas in the circle is in one direction only, and that the expiratory gases go through the carbon dioxide absorber.

Unidirectional Valves

Ⅱ- Breathing Hoses and Y-piece aⅣdapter

The Y-piece connects the inspiratory and expiratory hoses to a mask or endo-tracheal tube. The equip-ment dead space is some portion of the volume of the Y-piece.

The breathing hoses connect the inspiratory and expiratory valves to the Y-piece.

Ⅱ-Ⅴ　 Reservoir bag

The reservoir bag are located on the expiratory limb. The reservoir bag accumulates gas between inspirations. It is used to visualize spontaneous ventilation and to assist ventilation manually. Adults require a 3-L bag and children a 2-L bag. Most new machines have a valve used to switch between the reservoir bag and the ventilator. Older machines may require that the bag be removed and a hose to the ventilator be connected.

Ⅱ- APL(adjustable pressure-limiting Ⅵvalve ) valve or pop-off valve

The APL valve is used to control the pressure in the breathing system and allows excess gas to escape. The valve can be adjusted from fully open ( for spon-taneous ventilation, minimal peak pressure 1 to 3 cm H2O) to fully closed ( maximum pressure 75 cm H2O or greater ). Dangerously high pressures that can produce barotrauma and hemodynamic compromise may occur if the valve is left unattended in the fully or partially closed position.

Pop-Off Valve

Ⅲ Anesthesia ventilators Most modern anesthesia machines are fitted with a

mechanical ventilator that uses a collapsible bellows within a closed chamber. The bellows is compressed intermittently when oxygen or air is directed into the chamber, thereby pressurizing it. The ventilators are time cycled flow (as opposed to pressure) generators, controlled both mechanically and electronically, and pneumatically driven (requiring 10 to 20 L of driving gas per minute ). Ventilator controls vary among makes and models.

Some ventilators require setting of minute ventilation, rate, and inspiratory-expiratory (I:E) ratio to produce the desired tidal volume; other ventilators allow direct adjustment of tidal volume, with I : E ratio being dependent on the inspiratory flow rate, which is set independently. A portion of the fresh gas flow delivered by the machine adds to the set tidal volume during the inhalation phase. For example, an increase in total fresh gas flow from 3 to 6 L/min will increase delivered minute ventilation by an additional 1 L/min at an I:E ratio of 1:2 or by 1.5 L/min at an I:E ratio of 1:1 (more inspiratory time in the latter).

Although gas-driven ventilators can be safely driven with either oxygen or air, most often oxygen is chosen and is supplied by pipeline. Whether or not cylinder gases are used to drive the ventilator in the event of pipeline failure is usually determined by the user. If the machine is set up to drive the ventilator using cylinder oxygen, mechanical ventilation should be discontinued in the event of pipeline failure to conserve oxygen supplies.

Ⅲ- Flow generatorsⅠ Flow generators deliver a set tidal volume r

egardless of changes in patients' compliance ( unlike pressure generators ) but will not compensate for system leaks and may produce barotrauma because high pressures can be generated. They reliably deliver the preset tidal volume (even in the presence of a small leak). The risk o f barotrauma is minimal because most patients presenting to the operating room have healthy normally compliant lungs.

Ⅲ- Pressure generatorsⅡ For infants and patients with diseased lungs, the ma

intenance of preset tidal volumes may produce unacceptably high airway pressures and increased risk of barotrauma. Pressure generators are more appropriate in these situations, because airway pressure is controlled and barotrauma risk minimized.

Ⅲ- New “next generation” anesthesiaⅢventilator

The lack of flexibility of most current anesthesia machine ventilators severely limits their use in setting of abnormal lung mechanics. In these situations sometimes the use of manual ventilation or a critical care ventilator is necessary. New “next generation” anesthesia machines have versatile microprocessor-controlled ventilators that can safely and effectively ventilate patients with lung disease. These ventilators have sophisticated interfaces and controls, additional monitoring of airway pressures and flow rates, and arecapable of many ventilatory modes.

Ⅳ Safety features

It is the most important task of anesthesiologist that assure patient safety in the surgery. Some safety features are installed for anesthesiologists discovering abnormal matter as early as possible.

Ⅳ-Ⅰ audible oxygen alarm An audible oxygen alarm is fitted in the

oxygen supply line of the high pressure system. It consists of a pressure regulator and a reed or whistle that will sound when the pressure in the supply line is greater than 0 and less than about 25 psi.

Ⅳ- A pressure-operated “fail-safe” valveⅡ Oxygen failure safety (“fail-safe”) device Rationale: One of the most serious mishaps t

hat occurred with anesthesia machines in the past was depletion of the oxygen supply (usually from an exhausted cylinder) without the user being aware. The result was delivery of a hypoxic mixture. This mishap can occur even with piped gas supplies. An oxygen failure safety device prevents this hazard by stopping the flow of nitrous oxide when there is a loss of oxygen supply pressure.

A pressure-operated “fail-safe” valve in the high pressure system of the nitrous oxide supply line opens only when oxygen pressure in the high pressure system is above 25 psi. If the oxygen pressure falls below that, nitrous oxide will cease to flow. Because both the audible oxygen alarm and the fail-safe valve respond specifically to low pressure in the oxygen supply line of the high pressure system, neither protects against the delivery of an hypoxic mixture downstream in the low pressure system (e.g., if the oxygen flow control valve is accidentally shut off).

Ⅳ- Oxygen ratio controlⅢ Oxygen ratio control. All new anesthetic machines a

re fitted with a device to control the proportion of oxygen delivered. This may take the form of a mechanical link between the oxygen and nitrous oxide flow control knobs that will not allow a fraction of inspired oxygen (FIO2) of less than 25% to be set. Alternatively, some machines incorporate an oxygen ratio monitor that sounds an alarm if a low FIO2 is set. Older machines may lack any mechanism to control oxygen ratio.

Ⅳ- Pressure alarmsⅣ 1. A low pressure alarm is triggered by a

period of no pressure in the system or by a sustained pressure drop below atmospheric pressure. Low pressure may be caused by a disconnection or large leak in the system. Negative pressure usually indicates a scavenging system malfunction or that the patient is inhaling against an obstruction.

2. A high pressure alarm may have a variable or a preset (e.g., 65 cm H2O) limit. A high-pressure alarm may indicate obstruction in the tubing or endotracheal tube or a change in pulmonary compliance (e.g., bronchospasm or pneumothorax).

3. Continuing pressure alarm alerts the user in the event of high pressure being sustained for more than a few seconds. A blocked or closed pop-off valve, a malfunctioning ventilator pressure relief valve, or an obstruction in the scavenging system could create this condition.

Ⅴ Scavenging A scavenging system channels waste gases away fro

m the operating room to a location outside the hospital building or a location where the gases can be discharged safely (e.g., to a nonrecirculating exhaust ventilating system). The ambient concentration of anesthetic gases in the operating room should not exceed 25 ppm for nitrous oxide and 2 ppm for halogenated agents. Specific anesthetic gas-scavenging systems should be used routinely. These systems consist of a collecting system, a transfer system, a receiving system, and adisposal system.

The collecting system delivers waste gases to the transfer system and operates from the APL valve and from the expiratory valve of the ventilator. In addition, waste gases may be collected from the gas analyzers.

Ⅴ- collecting systemⅠ

Ⅴ- transfer systemⅡ

The transfer system consists of tubing that connects the collecting and receiving systems.

Ⅴ- receiving systemⅢ

The receiving system ensures that neither positive nor negative pressure builds at the patient end of the system. The system may be open or closed. An open system consists of a reservoir canister opened to atmosphere at one end. Suction usually is applied to the canister, exhausting the waste gas. A closed system consists of a reservoir bag with positive and negative pressure relief valves to maintain the pressure in the bag within an acceptable range.

Ⅴ- disposal systemⅣ The disposal system may be passive or active, alt

hough passive systems are inadequate for modern hospitals. A passive system consists of wide-bore tubing that carries gases directly to the exterior or into the exhaust ventilation ducts. Active systems can be powered by vacuum systems, fans, pumps, or Venturi systems.

Ⅵ Gas analysis Gas analysis. Several methods are used to monitor con

centrations of oxygen, carbon dioxide, and anesthetic gases in the breathing system. The oxygen analyzer is the single most important monitor for detection of a hypoxic gas mixture. Capnometry, the measurement of carbon dioxide, has many uses, including monitoring the adequacy of ventilation and detection of breathing system faults. Breath-to-breath monitoring of anesthetic concentrations provides tracking of anesthetic uptake and distribution. Most gas analyzers incorporate alarms. Among the techniques for measurement are the following:Mass spectrometry, infrared analysis and oxygen

concentrations analysis.

Ⅶ Accessories

A backup means of positive-pressure ventilation ( self-inflating bag, e.g., Ambu) should be available for any anesthetic procedure.

A humidifier may be used and is indicated especially for infants and small children and during high flow anesthesia. Two types generally are used during anesthesia: water bath and condenser humidifiers. Water bath humidifiers are associated with a risk of overheating with consequent injury to the patient and with risk of infection. Condenser humidifiers increase the resistance of the breathing system but are simpler to use than water bath humidifiers. High resistance makes them unsuitable for use with mall children.

A positive end-expiratory pressure (PEEP) valve can be connected to t he expiratory limb of the breathing system. Many new machines have built-in PEEP capability.

A flashlight should be available in case of power failure.

Ⅷ New generation anesthesia machines

The present day anesthesia machine works well and meets almost all needs. Current machines are at the end of their evolutionary cycle, however, and production of this generation of machines will end soon. New generation anesthesia machines are likely to present many challenges to anesthetists in terms of their increased complexity, changed layout and function, and integration of new technologies.

Good by !