Anesthesia Machine Working Principles


Anesthetic machine

Anesthesia is one of the most important aspects of the medical field, so much so that there is a separate branch of medicine focused on this called anesthesiology and hence the doctors trained for this specialty are called anesthesiologists. A separate branch exists because anesthesia administration requires great precision. This also became the reason for the invention of anesthetic machines that helped in the improvement of the quality of drug administration and reduce patient risk during the surgery and administration of anesthesia. There are various types of anesthesia depending upon the type of surgical procedure like local, general or regional. It enables the patient to undergo surgery without the fear of pain and helps in better management of the patient from the doctor’s side as well. Let’s look at some of the important details about the anesthetic machine that has revolutionized the domain of anesthesia.

What is an Anesthesia Machine?

An anesthesia machine is a medical device used by anaesthesiologists to provide an accurate and continuous supply of medical gases (like oxygen and nitrous oxide) mixed with the prescribed concentration of anesthetic vapor and hence support the administration of anesthesia.
In most cases these days, the machine is used in conjunction with mechanical ventilators, breathing systems, suction equipment, and patient monitoring devices; strictly speaking, the term “anesthesia machine” refers only to the component that generates gas flow, but modern machines integrate all these devices into a single unit and collectively call the whole unit an ‘Anaesthetic Machine’.
In the past few decades, anesthesia machines have evolved from simple pneumatic devices to complex mechanical, electrical, and computer-controlled devices. This change was largely motivated by a desire to improve patient safety and user convenience.

A Brief History

For a lot of years after anesthesia was invented, there was no need for the anesthetic machine. It was only after the introduction of oxygen (O2) and nitrous oxide (N2O) as compressed gases, which had to be administered along with anesthesia in the late 19th century, that a metal framework was required to support these cylinders. Even though somewhat similar continuous-flow machines had been used in France and the United States before Boyle’s anesthetic machine came about in 1917, continuous-flow machines were popularised principally by the British anesthetist Henry Boyle at St Bartholomew’s Hospital in London, United Kingdom.

His machine was based on the American Gwathmey apparatus of 1912 and was the most famous continuous-flow anesthetic machine of the early 1900s. British Oxygen Company (BOC) acquired Coxeter and Sons, which was the first manufacturer of the Boyles apparatus under Lord George Wellesly. The trade name “Boyle” was used by BOC. The name was chosen as a tribute to the then-thought inventor, Boyle. However, Boyle was not the one to invent the anesthesia machines. There had already been two great men who had done excellent work before him. It was James Taylor Gwathmey who invented the Gwathmey machine in 1912, while he was working in New York. As a follow-up to Gwathmey’s machine, Geoffrey Marshal developed a machine during the First World War (1914-1918). In 1918, Boyle presented his invention to the Royal Society of Medicine in London. He developed his machine from Gwathmey’s basic model in 1917. Even though Marshal had developed his machine a long time before Boyle, his machine was finally presented to the medical community in 1919, nine years after Boyle. Gwathmey and Marshal had developed their machines before, but they got little or no credit for the invention as Boyle’s name had become very popular by then.

Anesthetic machine
1921 – Waters to and fro absorption apparatus was introduced.
1927 – A flow meter for carbon dioxide was included, the volatile controls were of the lever type and the familiar back bar made its first appearance.
1930 – The plunger of the vaporizer appeared in the 1930 model.
1930 – The circle absorption system was introduced by Brian Sword.
1933 – Dry bobbin flow meters were introduced.
1952 – Pin index safety system (PISS) by Woodbridge.
1958 – Introduction of Bodok seal.
The advanced ventilators are the biggest difference between the new and older machines.

Parts of the Anesthesia Machine

A modern anesthesia machine includes the following components.

1. Connections to piped oxygen, medical air, and inhalation anesthetic from a wall supply within the edifice, or reserve gas cylinders of oxygen, air, and nitrous oxide attached via a pin index safety system yoke with a Bodok seal
2. Pressure gauges, regulators, and ‘pop-off’ valves, to observe force per unit area throughout the system and protect the machine components and patient from excessive rises
3. Flowmeters like rotameters for oxygen, air, and nitrous oxide also called the laughing gas
4. Vaporisers to produce accurate dosage control when using volatile anesthetics
5. A high-flow oxygen flush, which bypasses the flowmeters and vaporizers to supply pure oxygen at 30-75 liters/minute
6. Gas monitoring systems, including oxygen failure warning devices, that are used for administering and exhaling gases to patients
Systems for monitoring the patient’s heart rate, ECG levels, BP, and oxygen saturation could also be incorporated, in some cases with additional options for monitoring end-tidal carbon dioxide and temperature.

Breathing systems are typically incorporated with a manual reservoir bag for ventilation in combination with an adjustable pressure-limiting valve, as well as an integrated mechanical ventilator, to accurately ventilate the patient during anesthesia. In dentistry, a relative analgesia machine is a simplified anesthetic machine without a ventilator or anesthetic vaporizer. In this manner, the dentist can maintain a conscious state while depressing the patient’s pain during mild inhalation sedation with nitrous oxide and oxygen.

Components of Gas Supply System:-

Gas Cylinders

In order to practice safe anesthesia, it is essential to know how gas supply systems work. There are many instances of medical gas supply failures and misuses that have resulted in fatalities in operating rooms. Among the medical gases utilized in anesthesia and intensive medical care are oxygen, nitrous oxide, medical air, Entonox, carbon dioxide, and heliox. Apart from the main set of gas cylinders, the second set of cylinders is located in the yokes attached to the anesthesia machine. This second set of cylinders acts as a reserve.


The yokes are very essential parts of every anesthetic machine. Most of the machines have two yokes and some have one. A yoke for the oxygen gas cylinder is absolutely necessary to have. In addition to oxygen, another gas that uses the yoke is Nitrous Oxide. A ‘Pin Index Safety System’ ensures that the correct tanks are placed in the correct yokes since the design of each pin to the corresponding holes to fit is unique.

Pressure Regulator

The pressure regulators allow an easy and continuous flow of gases when the reserve cylinder is turned on without interrupting the supply. Whenever there is a surge in the pressure, the regulators bring it down to 275 kPa (40 psi). The regulator comprises two low-pressure outlets, one high-pressure outlet, and one high-pressure inlet, and the latter is connected through a one-way valve. The valve prevents the backflow of air from the empty cylinder into the piping system.

Pressure Guage

Pressure gauges help to determine the contents of the cylinders. It also helps to determine the right pressure for the cylinders. Usually, if the pressure is above 150 Km/cm square, the cylinder should be disconnected immediately and replaced. For oxygen, the operating range of the gauge is 0 to 150 Kg/ cm square and the indicator is always a little above this mark when a new cylinder is installed. With the gradual use of the gas, the reading keeps coming down.

Other Important Components:


The filter is usually installed between the cylinder and pressure regulator. It prevents the entry of particulate material into the machine. There are two main types of filters: mechanical and electrostatic. Mechanical filters physically stop particles, while electrostatic filters attract and capture charged particles. Filters made from resin-bonded, hydrophobic glass fibers (e.g. HEPA) provide high resistance to gas flow. In order to decrease this resistance, the sheet is pleated in order to maintain a large surface area in a more compact package. Inertial impaction and interception filter large particles (>0.3 m), while Brownian diffusion captures smaller particles. These filters may be modified to perform additional functions like the conservation of heat and moisture content of inhaled respiratory and anesthetic gases. They should not be used in the presence of active humidification, nebulized drugs, copious secretions, or pulmonary edema and should be visible to detect contamination, obstruction, or disconnection.


In most anesthesia machines, vaporizers are used between the flow meter and the common gas outlet. A modern vapouriser compensates both flow and temperature, calibrates concentration, can be controlled by a dial or direct reading, and is unaffected by positive-pressure ventilation. Standard 22-mm fittings, or screw-threaded, weight-bearing fittings with female inlet and male outlet, are required for use in breathing systems. It is necessary to mark the vapouriser “for breathing systems” along with the direction of gas flow.

The vaporizers function by two main methods which help us distinguish them:

  1. By splitting the fresh gas flow so that only a portion passes through the vapourising chamber and the rest bypasses it – variable bypass vaporizers.
  2. By injecting the vapor directly into the total fresh gas flow without using split-measured flow vaporizers.

Anaesthesia Machine Working Principle


  1. The pressure drop across the constriction is constant for all positions in the tube and is equal to the weight of the float divided by its cross-sectional area.
  2. The size of the annular opening – the larger the size of the annular opening around the bobbin, the higher will be the flow.
  3. Physical characteristics of the gas – because the annular space is tubular at low flow rates, flow is laminar and viscosity determines the gas flow rate and hence follows Poiseuille’s law. The annular space simulates an orifice at high flow rates, and turbulent gas flow then depends predominantly on the density of the gas and follows Graham’s law.

Pressure regulators

The pressure regulators work on the basic principle “force = pressure × area”. When force is kept constant with spring and the area inside the regulator is increased using a diaphragm, then automatically pressure of the gas decreases. By keeping the force exerted by the spring high, changes in the cylinder pressure due to its usage will not affect the output pressure. The output pressure is fixed by the manufacturing company and hence these are called ‘fixed pressure regulators.

What Happens During the Anesthesia Administration Procedure?

During general anesthesia, the nerve signals between your brain and body are interrupted. The patient breathes through a mask that helps them fall asleep. Once they’re asleep, the doctor might put a tube through their mouth into their windpipe. This tube ensures that the patient just gets enough oxygen during surgery. The doctor will first provide them with medicine to relax the muscles in their throat. The patient won’t feel anything when the tube is inserted. Finally, after all the mandatory sterilization checks have taken place, the surgery begins, and the anesthesia machine takes care of the subsequent things:

1. Vaporizers add precise amounts of volatile anesthetics to the fresh gas flow. Many agent-specific vaporizers are now electronically controlled. On most machines, the whole of the fresh gas flows and enters variable bypass vaporizers and splits into carrier gas — which flows over the liquid anesthetic — and bypass gas — which doesn’t enter the vaporizer’s chamber — and links up again at the vaporizer’s outlet for delivery into the patient breathing circuit. Providers control the splitting ratio to increase or decrease the gas flow and concentration, and eventually, what proportion of the anesthetic the patient receives.

2. Flowmeters let providers set levels of nitrous oxide (also known as the laughing gas), oxygen, air delivery, the required percentage of inspired oxygen, and total fresh gas flow (in liters per minute). On slightly older models, flowmeters on newer machines have minimum oxygen flow settings of 50ml/minute compared to 200 to 300 ml/minute. Low-flow settings let providers use less volatile anesthetics, which reduces the price and environmental impact of anesthesia care. Additionally, the slow and steady delivery of anesthetics helps to maintain the patient’s body temperature. These days, the electronic flowmeters let computerized anesthesia record and chart fresh gas flow, and are said to be more accurate in gas flow delivery than the previous ones.

3. Ventilators help maintain a near standard respiratory rate and normal blood chemistry in a wide variety range of patients. They are the features of anesthesia machines that have changed the most in recent years. Older devices had ventilators with 2 settings: on or off. The ventilators were set to breathe for deeply anesthetized patients. Newer machines boast up to 7 different modes of ventilation so as to match the pliability needed for keeping older, sicker, and heavier patients breathing spontaneously within the OR (operating room).

4. The Breathing circuits deliver oxygen and anesthetic gases to patients while also removing the CO2 that they exhale. Current machines offer breathing circuits adaptable to numerous patients, from newborns weighing less than 10 lbs. to morbidly obese patients weighing 300 lbs. or more. There are 2 main varieties of breathing circuits: non-rebreathing and circle circuits. Circle breathing circuits are the most popular and widely used systems today. The circuits cleanse CO2 expelled from the patient, allowing the rebreathing of exhaled anesthetic gases.

5. Scavenging systems (in the back) collect and take out the expelled anesthetic gases from the operating room. Systems are active (suction is applied to get rid of the gases) or passive (gas leaves through tubing to an area of ventilation exhaust grill). When active systems are used, the patient’s airway must be shielded from the suction device and positive-pressure buildup of the waste gases; passive systems require only the monitoring of positive pressure. Most new scavenging systems are receptive to the atmosphere, which is safer for the patient than closed systems that released waste gas into the atmosphere through valves.

Types of Anesthesia Machine

There are a lot of different classifications and according to the one that is most commonly used, there are mainly three types of machines as follows-

(1) Air anesthesia machine: A semi-open anesthesia machine uses air as a source of anesthesia. In essence, it has a tank for liquid medicines, a regulating switch for ether, folding bellows, a one-way valve for suction and exhalation, and a bellows. To assist in breathing and control breathing, the device uses air or oxygen as a carrier gas.

(2) DC anesthesia machine: High-pressure oxygen, a pressure reducer, a flow meter, and an evaporator are the components of a direct current anesthesia machine. Apart from providing oxygen, the device adjusts the concentration of anesthetic gas inhaled. A series of devices must be connected to the output site in order for anesthesia to be effective.

(3) Closed Anesthesia Machine: Anesthetic is administered to the patient through an outgassing flap (door) that supplies a low-flow anesthetic mixture. Through the exhalation flap, the exhaled gas is recycled into the CO2 absorber.

According to another classification, the two types along with the examples are:

  1. Intermittent type- Entonox Apparatus, Mackessons apparatus
  2. Continuous type- Boyle’s machine, Forregar, Dragger

Advantages of an Anaesthesia Machine

-The anesthesia machine is very easy to operate, and the color LCD screen allows the surgeon and the medical staff to have a detailed understanding of the patient’s situation. Moreover, the combination of the buttons and the jog dial makes it easier to perform the operation.

-The design of the intelligent alarm also makes the entire operation safer and more secure.

-The monitoring function of the anesthesia machine is particularly powerful, which makes sure that the medical workers can understand completely the changes that occur in the patient’s condition, according to which then subsequently a more suitable plan for the patient’s condition can be derived.

-In addition, the equipment adopts pneumatic and electrical control methods to incorporate a variety of ventilation modes, which reduces the workload for medical staff a little.

Limitations of the Anaesthesia Machine

Check Valve Assembly

-They do not provide permanent seals for empty yokes

-There is always some amount of gas escape in case of an empty cylinder or yoke.

Yoke Block

-Although rare, but can lead to the crossover of gases sometimes

-Pressure regulators may not work in sync with the pipeline supply through the yoke block

Pin Index System

-The pink can break easily and hence needs intense care

-Holes in the cylinder valve can be too deep leading to wrong cylinder placement in the yoke

Flow Control Valves

-Leakage of gases due to open flow control valves leading to wastage

-Failure to allow adequate gas flow


-Damage due to sudden projection can lead to indicator problems

-Can lead to leaks or inaccurate readings if control valves are open

Safety Features of Modern Machines

Safety features of a modern anesthetic machine to ensure the delivery of a safe gas mixture should include the following-

•Colour-coded pressure gauges.

•Colour-coded flowmeters.

•An oxygen flowmeter controlled by a single touch-coded knob.

•Oxygen is the last gas to be added to the mixture.

•Oxygen concentration monitor or analyzer.

•Nitrous oxide is cut off when the oxygen pressure is low.

•Oxygen: nitrous oxide ratio monitor and controller.

•Pin index safety system for cylinders and non-interchangeable screw thread (NIST) for pipelines.

•Alarm for the failure of oxygen supply.

•Ventilator disconnection alarm.

•At least one reserve oxygen cylinder should be available on machines that use pipeline supply.

Tanu Neb
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