Anaesthesia Machine

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CONTINUOUS FLOW ANAESTHESIA MACHINES, INCLUDING UPGRADING TO MODERN MACHINES AND SAFETY FEATURES Dr P N Viswanathan Prof and HOD, Department of Anaesthesia, Mysore Medical College, Mysore Anaesthesia machines are evolved from simple pneumatic to sophisticated, computer based fully integrated anaesthesia systems. It is essential for the anesthesiologist to achieve a thorough understanding of anaesthesia machines for the safe practice of anaesthesia. Development of the Anaesthesia Machine : The early half of 20th century saw the development of a large number of machines. Teters was the earliest gas oxygen outfit that made possible the use of rebreathing, positive pressure and addition of ether vapour to gases. Gwathmey introduced the sight feed type of flow meters. Heidbrink in USA ( 1913 ) introduced the Heidbrink apparatus. Dr. H. Edmund. G. Boyle ( 1917 ) in Britain introduced the early version of Boyles apparatus after using Gwathmeys machines, in France during the first world war, He found that Gwathmeys machine hnd several defects, mainly leakage at gas unions. He suggested changes which were put in to effect by coxeters. The early Boyles machine was designed on a wooden frame mounted on castors. It had a triple sight feed water bubble flow meter for delivering nitrous oxide, oxygen and carbon dioxide and measuring their rates of flow. There were chloroform and ether bottle mounted their outlet was connected to a two gallon rubber bag with a Barth 3 way or similar stop cock and rubber padded face piece. Breathing circuits incorporating Heidbrink expiratory valves were used much later. A metallic chain from the back bar up to the ground level was an accessory of the machine as antistatic. Modifications of the early version of Boyles machine : 1926 1931 33 1928 1937 Bypass control valves for regulating the amount of ether or chloroform were incorporated. The dry flow meter and the pressure reducing regulator were added. Dr. Brain C. Sword introduced the circle absorber. Rotameter superseded the coxeter bobbin meter after Richard salt suggested the use of rotameter. 1

1953 1955 1958

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Colour coding of the cylinders was introduced. Pin index safety system was added. The Bodok seal was added.

Other older machines : Merret machine was manufactured in 1958. The anaesthetic head was capable of being adjusted to give practically any closed or semi closed breathing circuit. A safety trilene interlock was present. Ohio machine ( 1958 ) incorporated within the breathing circuit a water manometer to measure the positive pressure in the breathing circuit, with a facility for closed circle system. Gillis machine : It was the British version of the machines of 1950s. It was a portable machine with facilities for closed circuit anaesthesia. New model of Boyles machine : It has the table and the frame of stainless steel. Cylinder yokes and pressure regulators are permanently fixed to the table frame and the tubings between the Adams regulator and the rotameter is made of metal. A permanently fixed halothane vaporizer is present instead of trichrloroethylene bottle. The take off mount is transposed from its usual place to a lower point at a corner of the front of the table.

Overview of Anaesthetic machines :

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A typical machine consists of : 1. A rigid metal box frame work on wheels, to this a surface of compressed gas either pipeline or cylinders are attached. 2. Pressure regulators to reduce the high pressure to working pressure of the machine and keep the pressure constant. 3. Secondary pressure regulators. 4. Pressure gauges to show pipeline and cylinder pressures. 5. Flow meters to measure and adjust various gas flows. 6. Vapourizers for addition of volatile anaesthetic agents. 7. Oxygen failure warning devices to warn of failure of O2 supply and to avoid hypoxic mixture delivery. 8. High flow oxygen flush, a flow meter bypass system for administration of high flow pure O2 in an emergency. 9. Common gas outlet. A single outlet delivering the gases and vapours in to an attached breathing system. The anaesthesia gas machine consists of Pneumatic system. Electric system.

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Schematic of flow arrangements of a contemporary anesthesia machine. Pneumatic System : The pneumatic part of the machine can be divided into High pressure system Intermediate pressure system Low pressure system. The high pressure system : This receives gases from a cylinder at a high variable pressure and reduces that pressure to a lower more constant pressure suitable for use in the machine. It includes a) Hanger yoke b) Cylinder pressure indicator c) Pressure reducing device. a) Hanger Yoke

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Its functions are to orient and support the cylinder, provide a gas tight seal and to ensure a unidirectional flow of gases in the machine. Body : The body of the yoke is threaded into the frame of the machine. It provides support for the cylinder. Retaining Screw : This is threaded to the distal end of the yoke. Tightening the screw presses the outlet of the cylinder valve against the washer and the nipple so that a gas tight seal is achieved. Nipple : Index pins : The cylinder is then supported by the retaining screw, the nipple and the pin index pins. It is that part of the yoke through which the gas enters the machine. If The pins of the pin index safety system are below the nipple. The it is damaged, it is impossible to obtain a tight seal with the cylinder valve. holes in to which the pins fit must have specific depth. If they extend too far in to the body of the yoke it may be impossible to insert an incorrect cylinder in to the yoke. Pins on the yoke fit into the holes on the cylinder valve. It consists two pins, 4 mm & 6mm on the yoke of the anaesthesia machine. A 9 / 16 inch circumference semi circle is made and six equidistant points are made on the arc ( 7th if entonox is to be used ). The pin index for gases that can be used are: Oxygen Nitrous oxide Cyclopropane Air Nitrogen Carbon dioxide Carbon dioxide 2, 5 3, 5 3, 6 1, 5 1, 4 7 ( < 7.5% ) 2, 6 ( > 7.5% ) 1, 6

Entonox ( 50% air + 50% nitrous oxide )

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Hanger yoke for an oxygen tank showing pin indexed safety system. Washer / Bodak Seal : It is placed around the nipple to effect a seal between the cylinder valve and the yoke. Broken washers should not used. No more than one washer should be used as they may prevent establishment of a tight seal or nullify the pin index safety system. Filter : ( 100 maximum ) This is installed between the cylinder and the reducing device to prevent particulate matter from entering the machine. Check valve assembly : The check valve assembly allows gas from a cylinder to enter the machine but prevents gas from exiting the machine when there is no cylinder in the yoke. This allows an empty cylinder to be replaced with a full one without having to turn off the in use cylinder. It also prevents the transfer of gas from one cylinder to another with a lower pressure in a double yoke. It consists of a plunger that slides away from the side of greater pressure. When the cylinder pressure exceeds the pressure of the machine, the plunger is pushed to right and gas passes around it and in to the machine. When the

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machine pressure exceeds cylinder pressure, it moves to left, blocking the flow of gases. Check valves are not permanent seals for empty yokes and may allow a small amount of gas to escape. As soon as the cylinder is exhausted it must be replaced by a full one, if not available, a yoke plug ( dummy cylinder block or plug ) should be placed in the empty yoke. b) Cylinder pressure indicator ( Gauge ) ( Bourdon pressure gauge ) This displays the pressure of the gas in the cylinder. If there is more than one yoke for a gas, one indicator may be provided for each yoke or one indicator for a group of yokes of the same gas. The indicators may be located on the front of the machine on a panel or on the cylinders. If the indicator is circular, the lowest pressure indication must be between 6 O clock and 9 O clock position on a clock face. The scale must be at least 33% more than the maximum filling pressure. It is constructed with a special heavy glass window and a safety back designed to act as a pressure fuses, so that gas is released from the back of the casing if there is a sudden rise in pressure.

Bourdon tube gauge

c)

Pressure reducing

device ( Regulator ) It is a device that reduces a high variable input gas pressure to a constant, lower out put pressure. Cylinder O2 enters the machine at pressure of upto 1900 Psig (depends on how full the cylinder is) and this is reduced to a constant output pressure of 45 Psig by the O2 cylinder pressure regulator. Reducing devices are preset at factory and located under the machines work surface. Failure of the pressure regulator can transmit excessively high pressures to machine low pressure system. To protect against this there is a pressure relief valve incorporated into the regulator, where by excess pressures are vented to atmosphere.

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Intermediate pressure system This includes a) Pipeline inlet connections b) Pipeline pressure indicators c) Piping d) Gas power outlet e) Oxygen pressure failure devices f) Oxygen flush g) Secondary pressure regulators h) Flow control valves. a) Pipeline inlet connections : Pipeline inlets for O2 & N2 O are mandatory. Each pipeline source is attached to the machine via a gas specific connection. Theses inlets are fitted with threaded non inter changeable diameter index safety system ( DISS ). Each outlet must contain a check valve to prevent the flow of gas from machine into piping system and a filter to prevent impurities from entering the machine.

Diameter Index Safety System. Connectors are of different diameters, making them gas specific.

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b) Pipeline pressu