Anaesthetic Machine
Anaesthetic Machine is a machine that have basic components and Breathing systems to produce anaesthesia by use of inhalent anaesthetic agents.
Basic components of anaesthetic machine prepares a precise but variable gas mixture for delivery to a breathing system and Breathing systems supplies oxygen and anaesthetic gas/vapour to the patient and eliminates carbon-di-oxide from exhaled gases.
Basic components are-
- Sources gases
- Pressure regulator
- Flowmenters
- Vaporizer
Breathing systems includes-
- Y piece
- Breathing tubes (inspiratory & expiratory)
- One way/unidirectional valve
- Fresh gas inlet
- Pop off valve or APL valve
- Reservoir bag
- Manometer
- CO2 absorber
These all components will be explained below-
Sources of Gases
Oxygen is commonly used as carrier gas. It is supplied in pressurised cylinders at 2000-2200 Psi. These cylinder colour code in India is black with white shoulders.
1 aim pressure = 14,693 Psi (2200Psi = 150 atm)
The pressure with in O2 cylinder will be as high as 150 times the atmospheric pressure. The pressure with in the pneumatic tube of a bus tyre is 3-4 times the Atmospheric pressure.
O2 cylinder break a concrete wall and move like a rocket if accidental breakage of valve occurs.
“E” type O2 cylinder are designed to be fitted with anaesthetic machine. The volume of O2 (liters) in the ‘E’ type cylinder can be calculated as pressure with in cylinder (in Psi) X 0.3
“G” or “H” type cylinders are bigger in size and are commonly used for centralized O2 supply. The volume of O2 in litres in bulk cylinders can be calculated as pressure with in cylinder (in Psi) X 3
Pressure Regulators
The pressure regulators reduces the cylinder pressure and supplies Oxygen at 50-60 Psi. (the working pressure of most anaesthetic machines).
Flow meters
Flow meters regulate the volume of O2 or nitrous oxide delivered to the breathing system.
The float in the flow meters are of two types- Rotor type and ball type. For Rotor type, the top reading of the float should be used and For Ball type, float the centre reading should be used.
Vaporizers
Vaporizers are the apparatus which vaporizes the volatile anaesthetic agents and a O2 anaesthetic vapour mixture is supplies to the patient.
Vaporizers are of two types- Precision vaporizor and Non precision vaporizor.
Precision vaporizers are the one which delivers precise concentration of anaesthetic vapour irrespective of the flow, temperature and atmospheric pressure.
Based on the location of the vaporizer, they can be called as vaporizer out of circle and vaporizer in the circle.
Vaporizer located out of breathing system are called Vaporizer out of circle and they offer least resistance to the respiratory effort of the patient.
Flush Valve
The flush valve delivers a high but unmetered flow of oxygen (35 to 75 lit/min) to the common gas outlet or to the breathing system.
“Y” piece
The ‘Y’ piece unites the endotracheal tube connector and the inspiratory and expiratory breathing tubes. The ‘Y’ piece contributes to the system’s mechanical dead space.
Breathing tubes
Breathing tubes are usually made of rubber, plastic or silicone and serve as flexible, low resistance conducts between the ‘Y’ piece and the one way valves.
The breathing tubes are corrugated to reduce the likelihood of kinking and breakage.
One way (uni-directional) valves
These paired valves direct gas flow away from the patients on expiration and towards the patients on inspiration, preventing rebreathing of exhaled gases before they pass through the CO2 absorbent canister.
Fresh gas inlet
The fresh gas inlet is the location at which gases from the common gas outlet of the anaesthesia machine enter the circle system.
Pop-off valves (Adustable pressure limiting valve)
The pop off valve vents gases to the scavenger system to prevent the build up of excessive pressure with in the circle and it allows rapid elimination of anaesthetic gases from the circle when 100% O2 is indicated.
Reservoir Bag
The reservoir bag meets the patients peak inspiratory flow demands and provides compliance in the system during exhalation.
The bag can be used for providing associated ventilation.
Excursions of the bag during spontaneous ventilation allow the anaesthetic to assess respiratory rate and to roughly estimate the ideal volume.
The minimum sisze of the reservoir bag should be six times the patients tidal volume.
Manometer
A manometer is a pressure gauze attached to the absorber assembly to assess pressures during assited or controlled ventilation.
CO2 Absorber
The absorber assembly contains the canister for the chemical absorbent ( soda lime or Baralime) for carbon-di-oxide. It is located between the two unidirectional valves.
The size of the CO2 absorber consider should be twice the patients tidal volume.
Examples of CO2 Absorber are Soda lime (Ca(OH) 2 – 94%, NaOH – 5% and KOH – 1%) and Bara lime (Ca(OH) 2 – 80% & Ba(OH) 2 – 20%).