Anesthetic Machine

The anaesthetic machine is a fundamental component of modern anaesthetic practice. However, many practitioners lack a comprehensive understanding of its operational principles. It is imperative to ensure the proper functioning of this machine as it directly correlates with patient safety.

A significant number of anaesthetic-related deaths and mishaps can be attributed to a lack of familiarity with the machine and neglecting to conduct thorough functional checks before its utilization.

In simple terms, an anaesthesia machine is specifically designed to deliver a mixture of fresh gases (oxygen and nitrous oxide) and anaesthetic vapor to a patient through a breathing circuit. It also facilitates various modes of ventilation, including spontaneous, manual, or mechanical ventilation.

These machines are meticulously engineered to minimize the wastage of anaesthetic vapors and prevent the contamination of the operating room environment with anaesthetic gases.

• An ideal anaesthetic machine design would provide instantaneous control of oxygen and anaesthetic vapour delivered to the patient.
• It would have the capabilities of an intensive care ventilator.
• The machine would be easy to use and never breakdown or malfunction.
• Unfortunately, an ideal anaesthetic machine does not exist, so the informed clinician must understand the capabilities and limitations of the machine(s) they use for patient care.

• A standard anaesthetic machine has a metallic framework with one or two chest drawers.
• The commonest type of machine is the continuous flow machine of which the Boyles machine is a prototype.
• New machines have undergone a lot of evolutions and are now sophisticated computer-based machines with integrated anaesthesia systems.
• Anesthetic machines are designed to deliver predetermined gases and vapor to a patient at a safe rate, concentration, and volume.

Types of Anaesthetic Machines

A. Intermittent Flow Machines

B. Continuous Flow Machines

Intermittent Flow

• The gases flow only in response to the patient's respiratory effort, i.e., patient-powered.

Continuous Flow

• The gases flow at a regular continuous rate to the patient through a predetermined circuit from a reservoir of gases.
• The rate of flow is adjustable and determined by the anaesthetist.

1. To reduce the pressure of the gases from the pipelines or cylinders to a safe level for use on patients.
2. To allow accurate delivery of varying flows of gases to the patient.
3. To allow for the addition of anaesthetic vapours to the fresh gas flow.
4. To allow for the use of accessories and monitors.

1. A rigid box framework or trolley with wheels.
2. Source of compressed gases in cylinders or from central pipe systems.
3. Pressure regulators to reduce high pressured gases to a working pressure.
4. Pressure gauges for pipeline and cylinder pressure.
5. Flow meters with their own regulators.
6. Vaporizers to blend anaesthetic gases with vapors of volatile anaesthetic agents.
7. Oxygen failure devices.
8. Back-pressure relief valve, a safety mechanism for releasing high-pressure buildup in the system.
9. Oxygen flow bypass system for the administration of high flow of pure oxygen in an emergency.
10. Common gas outlet, a single port for delivering the final gas mixture to the breathing circuit of the patient.

Additional Features of Modern Anaesthetic Machines

• Some modern machines are equipped with a variety of other features, including:
• Ventilators
• Waste-gas scavengers
• Oxygen analyzers
• Integrated monitors (ECG, Pulse oximeters, capnographs)

Oxygen Flush System

• The oxygen flush system provides a high flow of oxygen directly to the common gas outlet, bypassing the flow meters and vaporizers.
• A flow of about 35-45 liters/min at a pressure of about 400kpa is produced.
• Note that the high pressure puts the patients at risk of barotrauma and should be used cautiously.

Flowmeters

• Flowmeters are glass tubes wider at the top, containing a lightweight bobbin.
• The gas is fed in at the lower end of the tube via the needle valve.
• This causes the bobbin to rise and rotate at the same time.
• Because of the peculiar shape of the glass tube, both laminar and turbulent flows are encountered, making viscosity and density of the carrier gas relevant.
• Readings can be wrong if the bobbin sticks to the walls of the flow meter. Causes of this include:
• Dirt, usually from contaminated gas supply
• Static electricity, which usually builds up over time
• Tubes must be vertically mounted as any bending will give abnormal readings
• Backpressure transmitted back to the bobbin, which leads to greater delivery of gas than is indicated
• The float may stick permanently to the top or inside

Vaporizers

• Volatile anaesthetic agents such as Halothane, Isoflurane, and Sevoflurane change from liquid to vapor before being delivered to the patients by the vaporizers.
• Vaporizers contain a chamber in which the carrier gas becomes saturated with the volatile agents.
• There are different models of vaporizers:
1. The simple Boyles bottle
2. The copper kettle vaporizers: the flow through this vaporizer determines the ultimate concentration of inhalational agents. It is classified as a measured flow vaporizer. The amount of vapor pressure of the anaesthetic agent, flow rate of the carrier gas to the vaporizer, and the barometric pressure.
3. Temperature-compensated vaporizers: capable of delivering a constant concentration of the agent regardless of temperature changes or flow through them.
• These vaporizers are agent-specific, and there are filling devices to prevent filling the vaporizer with the wrong anaesthetic agent.
• The vaporizers are designed to deliver set concentrations regardless of temperature changes or flow through the vaporizers. A calibrated control knob is set at the desired concentration. These vaporizers are located outside the circle system between the flow meter and the common gas outlet.

Oxygen Analyzer

It is mandatory to have an oxygen analyzer in the breathing circuit. This is to measure oxygen concentration. All oxygen analyzers should have a low-level alarm.