The Anesthetic Machine - Components and Operation

Vocabulary flashcards covering the components, functions, safety guidelines, and pressure calculations of veterinary anesthetic machines.

Carrier gas

The gas (typically pure oxygen) used to transport a volatile liquid anesthetic to and from a patient via a circuit of tubing.

Four main functions of the anesthetic machine

1. Deliver oxygen at a controlled flow rate; 2. Vaporize a designated concentration of liquid anesthetic, mix it with oxygen, and deliver it to the patient; 3. Remove exhaled gases away from the patient by disposing of or recirculating them while scrubbing carbon dioxide; 4. Deliver pure oxygen to critically ill patients.

Four main components of an anesthetic machine

1. Gas supply (compressed cylinders or concentrator); 2. Anesthetic precision vaporizer; 3. Breathing circuit (rebreathing or non-rebreathing); 4. Scavenging system.

E cylinder

A small compressed gas cylinder that mounts directly to the yoke of the anesthetic machine.

G and H cylinders

Large freestanding gas cylinders chained to a wall or transport cart that supply gas to the anesthetic machine through high-pressure gas lines.

Oxygen tank color coding

Standard cylinder coloring used for identification: green in the United States, and white in Canada and Europe.

Oxygen cylinder valve stem

The valve on top of the compressed gas cylinder opened in a counter-clockwise direction to initiate the flow of oxygen.

Oxygen tank regulator assembly

A: Threaded outlet port (gas leaves tank); B: Pressure reducing valve; C: Tank pressure gauge; D: Line pressure gauge; E: Valve stem.

Pressure reducing valve

A regulator valve that reduces incoming cylinder pressure to a safe operating level of approximately $50\,\text{psi}$ ($345\,\text{kPa}$) before the gas enters the machine lines.

$O_2$ tank pressure gauge

A pressure indicator attached to the machine yoke or tank regulator that reads the remaining cylinder pressure when the tank valve stem is open.

Full oxygen tank pressure

The internal pressure of a completely full oxygen cylinder, measuring $2200\,\text{psi}$ or $15{,}000\,\text{kPa}$.

Oxygen cylinder change threshold

The safety threshold where an oxygen tank must be replaced, defined as pressure dropping below $500\,\text{psi}$ ($3400\,\text{kPa}$).

Full nitrous oxide tank pressure

The pressure reading of a completely full nitrous oxide cylinder, measuring $745\,\text{psi}$ or $5140\,\text{kPa}$.

E tank oxygen volume calculation

A formula where pressure in psi is multiplied by the factor $0.3$ (e.g., a full tank at $2200\,\text{psi} \times 0.3 = 660\,\text{L}$ of oxygen).

H tank oxygen volume calculation

A formula where pressure in psi is multiplied by the factor $3$ (e.g., a full tank at $2200\,\text{psi} \times 3 = 6600\,\text{L}$ of oxygen).

Line pressure

Residual gas pressure remaining in the machine's supply lines after the main oxygen cylinder valve has been turned off.

Purging (bleeding/flushing) the system

The procedure of venting residual line gas by depressing the oxygen flush valve or opening the flowmeter to high flow until all pressure reads zero.

Oxygen concentrator

A electrically powered device that filters room air ($20\%$ oxygen) to generate and compress $95\%$ oxygen without requiring gas cylinders, requiring a minimum of $15\,\text{psi}$.

Flowmeter

A vertical glass tube and control valve that further steps gas pressure down to $15\,\text{psi}$ ($100\,\text{kPa}$) and sets the flow rate of oxygen delivered to the breathing circuit.

Flowmeter indicator reading

The gas delivery rate is read at the center when a ball indicator is used (1), or at the top horizontal surface when a bobbin or skirted float is used (2, 3, 4).

Oxygen flow rate

The rate at which oxygen enters the circuit, measured in liters per minute ($\text{L/min}$) and determined by the patient's body size and the anesthetic system used.

Oxygen therapy under anesthesia

The delivery of $100\%$ pure oxygen to an anesthetized animal to compensate for reduced tidal volume (shallow breathing) and elevated metabolic requirement, compared to $20\%$ found in room air.

Anesthetic Machine

A device designed to deliver volatile gas anesthetic to and from a patient through a circuit of tubing using a carrier gas, remove exhaled gases, and deliver pure oxygen to critically ill patients.

Carrier Gas

The gas—most commonly oxygen—used to vaporize, dilute, and deliver inhalant anesthetic agent to the patient.

E Cylinder

A small compressed gas cylinder attached directly to the yoke of the anesthetic machine.

G or H Cylinders

Large freestanding compressed gas cylinders chained to a wall or a cart that connect to the anesthetic machine via high-pressure gas lines.

Medical Oxygen Cylinder Color Codes

The standardized tank colors representing oxygen: green in the United States, and white in Canada and Europe.

Full Oxygen Tank Pressure

The pressure reading of a completely full oxygen cylinder, which is $2200\,\text{psi}$ or $15{,}000\,\text{kPa}$.

Oxygen Tank Replacement Threshold

The pressure level at which an oxygen tank must be switched out, specifically when pressure drops below $500\,\text{psi}$ ($3400\,\text{kPa}$).

Nitrous Oxide Full Tank Pressure

The pressure inside a full cylinder of nitrous oxide, measuring $745\,\text{psi}$ or $5140\,\text{kPa}$.

E Tank Oxygen Volume Calculation

The mathematical formula to determine available litres of oxygen in an E cylinder by multiplying tank pressure in $\text{psi}$ by $0.3$ ($\text{Volume in Litres} = \text{psi} \times 0.3$).

H Tank Oxygen Volume Calculation

The mathematical formula to determine available litres of oxygen in an H cylinder by multiplying tank pressure in $\text{psi}$ by $3$ ($\text{Volume in Litres} = \text{psi} \times 3$).

Pressure Reducing Valve

A valve that decreases the high gas pressure coming from the tank down to approximately $50\,\text{psi}$ ($345\,\text{kPa}$) before it enters intermediate-pressure lines.

Line Pressure

The residual gas pressure that remains in the delivery lines after the flow from the oxygen cylinder has been closed.

Purging (Bleeding / Flushing)

The procedure of venting line pressure after closing the cylinder by depressing the oxygen flush valve or opening the flowmeter until all gas escapes.

Oxygen Concentrator (Generator)

A medical device that filters room air ($20\%$ oxygen) and compresses it to deliver $95\%$ oxygen at a minimum of $15\,\text{psi}$, eliminating the need for compressed cylinders.

Flowmeter

A vertical glass tube that reduces oxygen pressure to $15\,\text{psi}$ ($100\,\text{kPa}$) and regulates the delivery rate of carrier gas to the patient in litres per minute ($\text{L/min}$).

Flowmeter Indicator Reading Rules

The measurement convention for flowmeter floats: read at the center of the indicator when using a spherical ball, and read at the top when using a cylindrical bobbin.

Tidal Volume

The volume of gas inhaled and exhaled with each breath, which decreases during anesthesia (shallow breathing) and causes patients to have a higher supplemental oxygen requirement.

Oxygen Tank Pressure Gauge

A gauge attached to the anesthetic machine yoke (for E cylinders) or regulator assembly (for H cylinders) that indicates remaining gas pressure in the cylinder when open.