Anesthesia

Anesthesia Machine: Purpose, Design, Working Mechanism, Handling

An anesthesia machine is a medical device that supplies oxygen gas along with other inhalation anesthetics to the patient. The main purpose of an anesthesia machine is to supply inhalation anesthetic and to assist in the breathing of the patients. It is useful in any kind of anesthesia such as local, regional, or general anesthesia.  So, in this blog, we will be discussing the working principle, testing procedure, maintenance, and contact details for the purchase of the anesthesia machine in Nepal.

Besides that, we will also discuss the functions, working principles, and handling of the key components of the machine ( such as CO2 absorber, breathing circuit, bellow, vaporizer, APL valve, etc.).

Types of Anesthesia

  • Local Anesthesia:- The target is a small part of a body. The patient remains awake during the whole process. For example, while taking out the teeth by a dentist.
  • Regional Anesthesia:- The target consist of a larger area than the local anesthesia such as limbs. For example, epidural and spinal anesthesia.
  • General Anesthesia:– The patient becomes unconscious during the process. It is used during major surgeries.

Breathing Circuit

Breathing Circuit showing CO2 absorber, APL valve, vaporizer, bellow, etc

The breathing circuit for the anesthesia machine is the circular pathway(not practically circular) that allows the recycling of the inspiratory as well as expiratory gases.

From the figure, we can see some of the major components present in the breathing circuit.  They are as follows:

  1. Unidirectional valve:- It allows unidirectional flow of fresh gas flow (FGF). They are two in number. One is for inspiration control while the other one is for expiration control.
  2. Adjustable Pressure Limiting valve (APL valve):- The pressure limiting valve can be adjusted to the required value. Once the set pressure is reached, the expired gases are forced to come out of the APL valve which is then connected to the scavenging system.
  3. Reservoir Bag/ Bellow:- This acts as a reservoir for expired gases. During inhalation, the bag deflates and vice versa.
  4. CO2 Absorber:- It absorbs the expired CO2. In the absence of this absorber, there will be an overall increase in the CO2 concentration in the circuit which can be dangerous to the patient.
  5. Vaporizer:- It is for storing and supplying the inhalation anesthetics to the patient.

We will again talk about these components individually towards the end of the topic.

Working Mechanism of Anesthesia Machine

Oxygen, N2O, and air are supplied to the patient via the breathing circuit. During this process, these gases carry anesthetic gases along with them. This gas composition is then inhaled by the patient.

1) During spontaneous breathing

  • Inspiration

During inspiration, oxygen, nitrous oxide, air, and anesthetic gases go to the patient from the flowmeter and reservoir bag. The CO2 absorber absorbs the incoming carbon dioxide from the reservoir bag. Also, the bellow begins to deflate. The unidirectional valve in the inspiratory tube opens while the other valve remains closed.

  • Early expiration

During the initial stage of expiration, the one-way valve of the expiration tube opens and another valve remains closed. The unused fresh gas and expired gases go to the reservoir bag. The bag begins to inflate.

  • Late expiration

During the late expiration, the reservoir bag fills up. The excess pressure gas goes to the scavenging system via the APL valve.

2) During forced breathing

  • Inspiration

During forced breathing, positive pressure is applied to the bellow. This is possible by supplying oxygen gas to the cylinder containing bellow. This causes the bellow to squeeze. So, pressurized gas comes out of the bellow.

Also, the APL valve closes. This sets the maximum pressure which prevents gases to escape to the scavenging system.

  • Expiration

During expiration, oxygen comes out of the cylinder containing bellow. This removes the positive pressure from the bag. Now, the expired gases fill up the bellow causing it to inflate.

Also, APL vale opens.

 

1. CO2 Absorber

CO2 absorber, soda lime
Image Source:- https://en.wikipedia.org/wiki/Soda_lime

The CO2 absorber undergoes the mechanism of chemical reactions to absorb (nullify) carbon dioxide from the incoming air. Various types of chemicals act as CO2 absorbers. However, soda-lime is mostly used in anesthesia machines.

Types of CO2 Absorbents

Types of the CO2 absorber Absorptive capacity
Soda-lime 26 liters of CO2 in 100 grams of soda lime
Baralyme 26 liters/100 grams
Amsorb 10.2 liters/100 grams

Features of ideal CO2 Absorbents

  • Non- toxic
  • High efficiency
  • Non- reactive to oxygen, nitrous oxide, and anesthetic drugs
  • Less resistivity
  • The granules of CO2 absorbent should be uniform in shape (for providing uniform fresh gas flow). It should consist of a mesh number of 4-8.
  • The granules should be of appropriate size. Too large granules provide insufficient surface area for absorption. Similarly, too small granules provide high resistance for the fresh gas flow.

The life span of the CO2 Absorber

You should not use the absorber for more than 14 hours.

Factors affecting the life span

  • Rate of fresh gas flow
  • Amount of carbon dioxide present in the expired gas
  • The capacity of the canister
  • Handling of the canister

Color indications for CO2 Absorber

Different manufacturers, manufacture CO2 absorbents by adding different kinds of dyes. These dyes react to the absorbed carbon dioxide giving out a different color. This color change is the indicator of the life expectancy of the absorbent.

Dyes Color change
Phenolphthalein Red to white
Ethyl violet White to purple
Clayton yellow Pink to cream
Titan yellow Pink to cream
Mimosa Z Red to white

If the absorber is not used for quite some time, the color might change to its initial stage. Thus this is not an effective method. Thus doctors should analyze the situation of the patient to know the condition of the absorber. An increase in FiCO2, fluctuation in the heart rate, blood pressure, etc. can be some of the symptoms.

Handling of CO2 Absorber

  • Soda-lime is quite corrosive in nature. So, wear gloves and safety goggles while replacing this chemical.
  • Refill the canister properly. Else there can be leakage.
  • You can also replace it with a new one.
  • The operator should always keep an eye on the canister.
  • Set the low FGF rate as far as possible. This will keep the granules moist.
  • Turn off the oxygen flow as soon as the case is complete. Else, there can be a chance of the formation of carbon monoxide with the anesthetic agents.

 

2. APL valve

An Adjustable Pressure Limiting Valve (APL valve) is a type of flow control valve that can limit the flow of air. The other names include an expiratory valve, relief valve, etc.

Applications of APL valve

  • Anesthesia Machine
  • Suction Machine, etc.

Design of Adjustable Pressure Limiting valve

The valve consists of a knob at the top. This knob is in contact with the spring. Spring is also in contact with the disc that covers the orifice within the valve. From this orifice, air flows out.

Generally, when you rotate the knob in a clockwise direction, the knob moves downward compressing the spring and vice versa. This deforming force is directly proportional to the nature of the material and the change in length of the spring. So, if you compress a spring more, more force will exert onto the disc.

APL valve in Anesthesia Machine

Breathing Circuit showing CO2 absorber, APL valve, vaporizer, bellow, etc
Adjustable Pressure Limiting valve

 

Inspiration

During the inspiration process, high pressure should be applied to the valve

  • to prevent the escape of gases to the scavenger system
  • to supply enough gases to the patient

For this, you need to tighten the knob to compress the spring.

Expiration

During the expiration process, low pressure should be applied to the valve

  • to release excess gasses to the scavenger system
  • to protect the reservoir bag/bellow from exploding

For this, you need to loosen the knob to relax the spring.

If the positive pressure is applied to the reservoir bag by squeezing it, then the valve setting should be constant. In that case, the valve should be set in the middle range.

  • Low-pressure settings during inhalation can cause excess loss of gases to the scavenger system causing scarcity to the patient.
  • High pressure during expiration can cause an explosion of the reservoir bag.

In practice, the anesthesia machine has got its own APL valve that follows the rapid ‘open-close method. This helps in the preservation of anesthetic gases.

 

3. Vaporizer

Vaporizer used in Anesthesia Machine
Image Source:- https://www.mindraynorthamerica.com/anesthesia-systems/

Vaporizers are the storage devices that store volatile inhalation anesthetics and deliver them to a patient at a certain rate. It is generally found in the anesthesia machine near the flowmeter and away from the breathing circuit.

Anesthetic agents are highly potent and toxic to the patient. Due to this reason very small and precise amount of drugs should be delivered continuously during anesthesia. This is not possible manually. So, there is a need for a vaporizer.

Ideal Vaporizer

  • The output of the vaporizer is independent of the total gas flow (TGF), the volume of anesthetic liquid present in it, temperature, vapor pressure, and respiration rate of the patient.
  • High corrosion resistance.
  • Low purchase and maintenance cost.
  • Light and portable to handle.
  • Deliver minimum dosages of the inhalation anesthetics.
  • Low resistance to the gas flow.

Classification of Vaporizers

Maintenance of Anesthesia Machine

  • Check the condition of the CO2 absorber. Once it expires, replace it with a new one.
  • Check the condition of one-way directional valves. and flowmeter(rotameter).
  • Check the amount of oxygen in the oxygen cylinder. The pressure should not drop below 1000 psi (half-filled), before the use.
  • Fill the vaporizer to an optimum level.  Do not tilt the vaporizer.
  • Connect the pipeline from the machine to the gas (N2O, O2) source properly. Check the pressure of the pipeline.
  • Every time you start a new test or replace new consumables, it is better to carry on the leakage test of the anesthesia machine. This will help you to know if there is any leakage in the components of the breathing circuit such as the CO2 absorber, APL valve, unidirectional valve, etc. The leakage test can either be manual or automatic. Also, the method of leakage test will be given to you in the machine.

There are many more tests that you may need to do for the maintenance of the anesthesia machine. For this, you need to go through your user manual.

Anesthesia Machine in Nepal

One of the suppliers that deal with the anesthesia machine in Nepal is Capital Enterprises (+977 984-3421237), which is located in Jawalakhel, Kathmandu.  It is an authorized dealer of Midray Company.  You can make a phone call and discuss the brochure, price, and other services for the machine.

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