Introduction The anesthesia gas machine is a device which delivers a precisely-known but variable gas mixture, including anesthetizing and life-sustaining gases. Original Boyle was made by the firm COXTERS. There are several differences between newer and older anesthesia machines. Advanced ventilators are the biggest difference between newer and older gas machines.
The differences between older gas machines -such as the Ohmeda modulus, Excel, or Aestiva and the DrägerNarkomed GS, Mobile, MRI, 2B, 2C, 3 or 4- are less than their similarities. While the differences between new models from GE Healthcare (ADU, Aisys, Aespire, Avance) and Dräger (Fabius GS, Narkomed 6000/6400, Apollo) are more than their similarities.
Types of anesthesia machine Intermittent-Gas flows only during inspiration Egs: Entonox apparatus ,Mackessons apparatus Continuous-Gas flows both during inspiration and expiration. Egs : Boyle Machine Forregar Dragger
Anesthesia work station Integrates most of the components necessary for administration of anesthesia into single unit. Standard guidelines have been given to manufacturers for minimum performance, design, characteristics and safety requirements of machine. The current standard for anesthesia workstation as( promulgated by American society for testing and materials) (ASTM) is F1850. European standard is EN740
Components of anesthesia machine
Pressure units to remember 1 atm pressure = 1 bar = 760 mm Hg = 14.7 psi = 100 kilopascals
Components of pressure systems
Limitations of check valve assembly The check valves are not designed to act as permanent seals for empty yokes. Small amounts of gases can escape if the yoke is empty or an empty cylinder (or cylinder with low pressure) and valve open is present in the yoke.
In order to minimize such losses – Yokes should not be left vacant for extended periods An empty cylinder should be replaced as soon as possible , if not then, An yoke plug can be used to prevent gas leak or An empty cylinder can be left behind after closing the valve
Yoke Block(Dummy Cylinder plug) Yoke block is a solid piece of metal that has a conical depression on one side to fit into the retaining screw and a hollow area on the other side to fit over the nipple. They are pin indexed. Uses Prevents gas leak from the machine when placed in an empty yoke. Connect cylinders larger than size E to the machine. Connect pipeline supply to the machine that does not have pipeline inlet connections.
Limitations of yoke block Can be responsible for crossover of gases in the machine. Pressure regulators in the machine are designed to accept gases at cylinder pressure may not function properly when supplied with gases at reduced pressure(pipeline supply through yoke block)
Limitations of pin index safety system A wrong cylinder can be placed in yoke if- 2 washers are placed on the port. Pins on the yoke are broken. Holes on the cylinder valve are too deep.
Pin index safety system
Bourdon pressure gauge
Safety features in cylinder pressure indicator Gauge is usually color coded. Name and symbol of gas are written over dial. If bourdon tube ruptures gas is vented from back side Gauges are angled and placed in such a way that it can be easily read by anesthetist. Instructions like “use no oil’’ “open the valve slowly’’ are written on the gauge.
Electronic cylinder pressure indicator Light emitting diodes(LED’S)in electronic pressure gauge indicate Cylinder valve is close –Dark color Cylinder valve is open – Pressure adequate –Green Pressure inadequate-Red
Types of pressure regulators Adam’s pressure regulatorvalve- Used in past Working principle is same differs in internal structure from newer valves(toggle levers instead of push rod) Fins on Adam’s regulator for N2O which are not present in newer regulators. Mackesson’sregulator Medishieldvalve Endurancevalve Modernpresettypeofreducingvalve-DirectandIndirect.
Safety features on pressure regulator Pressure regulators have safety relief valves If due to any reason there is build up of pressure in pressure regulator then the safety valve blow off at a set pressure of 525 k pa(70psi)
Oxygen failure safety valve Pressure sensor shut off valve Based on threshold principle Cuts off N2O supply when O2 pressure falls below 25 psi. MOA-O2 pressurises and holds open shut off valve that interrupts the supply of N2O and other gases if O2 pressure<threshold setting. oxygen failure protection device Based on proportioning principle Gas loaded regulator When O2 pressure decreases there is a proportional decrease in N2O supply and complete cut off seen at<12psi. MOA-O2 pressure regulator(primary regulator) controls secondary(slave) regulator located in N2O line
Master switch Turning the master switch to the ‘on’ position activates both pneumatic and electrical functions of the machine as well as certain alarms and safety devices.
Electronic flow meter Prefer digital system Solenoid valves Control flow on or off valves Computer controlled .
Oxygen ratio monitor controllerSensitive oxygen ratio controller Linear resistors [3:1 ratio for N2O & O2)between O2 andN2O flow control valves. Ensure 25% O2 by limiting N2O flow. ORMC shuts off N2O if ratio of O2 flow falls below 30% S-ORC-newest hypoxic guard . installed in Fabius-GS by Drager.ensures aFiO2 of 23%.O2 flow <200ml/min. .
Limitations of proportioning system Anti hypoxic device may also deliver hypoxic mixture under following conditions- Wrong supply of gas. Defective pneumatics/mechanics. Inert gas administration(3rd gas-He,N2,CO2). Leaks downstream.