Main components and design
The machine consists of several key units responsible for storing, dosing, and delivering anesthetics.
- Gas cylinders. Contain oxygen and nitrous oxide. Oxygen is stored in blue cylinders under high pressure (150 atm). For safe delivery, pressure regulators are installed at the outlet, reducing the pressure to 3–4 atm.
- Vaporizers. Designed to convert liquid volatile agents (e.g., halothane) into vapor. Simple models do not provide high dosing accuracy because their operation depends on ambient temperature (this is especially critical for diethyl ether). Modern vaporizers are equipped with thermal compensators, making it possible to precisely adjust the dose in volume percentages.
- Flowmeters. Control the delivery of gaseous agents. Float-type rotameters are most commonly used: the gas flow moves up through a glass tube, and the displacement of the float indicates the minute volume in liters.
- Breathing circuit. The delivery system comprising hoses, valves, breathing bag, bellows, and absorber. During inspiration, the mixture flows from the flowmeter to the patient; during expiration, it returns to the machine or is vented.
Formation of the breathing mixture
The anesthetic mixture is always prepared on an oxygen base (its proportion must be at least 20%, and the total flow rate 8–10 L/min). The mixing process depends on the state of matter of the anesthetic:
- Gaseous agents (cyclopropane, nitrous oxide) pass through their respective flowmeters and are then mixed with oxygen in a special chamber in set proportions.
- Liquid agents turn into vapor right as the oxygen flow passes through the vaporizer chamber («bowl»), after which the vapors are directed through a valve into the circuit.
Types of breathing circuits
The classification of circuits (circulation methods) is based on how the anesthetic mixture and the patient's exhaled air interact with the operating room atmosphere.
- Open circuit. The patient inhales ambient air that passes through the vaporizer and exhales directly into the environment. It is characterized by high drug consumption and severe air pollution, which is harmful to medical staff.
- Semi-open circuit. Inspiration is drawn from the machine (a mixture of oxygen and anesthetic), while expiration is vented to the atmosphere. Considered the safest option for the patient.
- Semi-closed circuit. Inspiration comes from the machine, and expiration is split: part goes into the room air, and part returns to the system. The returned portion passes through the absorber, where carbon dioxide is removed, mixes with fresh gas flow, and is delivered back to the patient.
- Closed circuit. Complete isolation from the environment. Expiration is directed back into the machine, cleaned by a chemical carbon dioxide absorbent, and mixed with a fresh gas portion. This is an economical and environmentally clean method, but it harbors the risk of hypercapnia if the absorbent quality is poor (it must be replaced every hour).
Algorithm for preparing the machine for operation
Before performing inhalational anesthesia (mask, endotracheal, or endobronchial technique), the equipment must be strictly prepared according to the following algorithm:
- Open the gas cylinder valves and check their pressure using the regulator pressure gauges.
- Assemble the breathing circuit by connecting the cylinders to the machine with hoses.
- Fill the vaporizers with liquid volatile anesthetics.
- Fill the absorber canister with fresh chemical absorbent to remove carbon dioxide.
- Perform grounding of the machine (a mandatory measure to prevent the buildup of static electricity and explosion risk).
- Check the entire assembled circuit for leaks, ensuring there are no gas leaks.
To memorize circuit types, use the «doors» rule: Open — both «doors» are open (inspiration from the atmosphere, expiration to the atmosphere). Semi-open — only the exit door is open (inspiration from the machine, expiration outwards). Closed — both «doors» are locked (complete isolation, recirculation).
Class and exam questions
Why is an absorber needed in an anesthesia machine?
The absorber is filled with a chemical absorbent and used in semi-closed and closed circuits to remove carbon dioxide from the exhaled mixture before it is re-delivered to the patient.
How does a rotameter work?
This is a float-type flowmeter: the gas flow is fed into a glass tube from bottom to top. The displacement of the float under gas pressure indicates the minute volume in liters.
Which breathing circuit is considered the safest for the patient?
The semi-closed circuit is the safest for the patient because all exhaled gas is vented to the atmosphere, completely eliminating the risk of hypercapnia due to carbon dioxide accumulation.
How often should the absorbent be replaced in a closed circuit?
The chemical absorbent in the absorber must be replaced every 40 minutes to 1 hour of operation; otherwise, there is a risk of hypercapnia.
What else to study on this topic
- Special features of using diethyl ether and halothane in modern practice.
- The difference between mask, endotracheal, and endobronchial methods of inhalational anesthesia.
- The physics of modern vaporizers equipped with thermal compensators.
- Chemical composition and properties of CO₂ absorbents for canisters.
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Other topics in the section "CHAPTER 3. ANESTHESIA"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.