The Anesthetic Machine: Components, Functions, and Operation

Comprehensive vocabulary flashcards covering the components, mechanisms, safety valves, sizing formulas, and operational procedures of veterinary anesthetic machines.

Vaporizer

An anesthetic machine component that converts a liquid anesthetic agent into a gas state and adds a controlled amount of this vaporized gas to the oxygen flowing through it.

Vaporizer Dial

A calibrated control dial graduated in percent concentration ($0\text{--}5\%$) that opens an internal gate within the vaporizer, exposing precise amounts of liquid anesthetic to incoming oxygen.

Precision Vaporizer

A vaporizer calibrated to deliver the exact percentage of anesthetic gas indicated on the dial regardless of flow rate or room temperature; each precision vaporizer is adapted specifically to a single volatile anesthetic agent.

Vaporizer Outside the Circle (VOC)

An anesthetic circuit configuration in which oxygen travels through the vaporizer to pick up anesthetic vapor before entering the breathing circuit.

Key Lock System

A safety filling mechanism utilizing a color-coded and uniquely shaped key adapter for each volatile anesthetic, preventing the accidental addition of the wrong agent into a vaporizer.

Inhalant Anesthetic Color Codes

Standardized color identification used on agent bottles, vaporizers, and filler keys: Isoflurane is purple, Sevoflurane is yellow, and Desflurane is blue.

Fresh Gas

The mixture of carrier gas (oxygen) and vaporized anesthetic gas that exits the vaporizer and is directed toward the reservoir bag and inhalation breathing tube.

Reservoir Bag (Rebreathing Bag)

A collapsible bag (ranging from $0.5\,\text{L}$ to $5\,\text{L}$ in small animals) positioned directly in line with the inhalation hoses that provides a gas reservoir for inhalation, deflates during inspiration, and allows observation of respiratory rate and depth.

Tidal Volume ($V_T$)

The volume of air inhaled during each breath, which correlates to the depth of respiration and is calculated as $10\,\text{ml/kg}$ of body weight.

Reservoir Bag Size Calculation

Calculated by multiplying the patient's tidal volume by 6 ($\text{patient weight in kg} \times 60\,\text{ml/kg}$) or by multiplying patient body weight in pounds by $30\,\text{ml}$.

Intermittent Positive Pressure Ventilation (IPPV)

Manual ventilation ('bagging') performed by closing the pop-off valve and gently squeezing the reservoir bag to force oxygen and anesthetic into the alveoli, preventing atelectasis; circuit pressure must never exceed $15\text{--}20\,\text{cm H}_2\text{O}$.

Atelectasis

The collapse of lung alveoli resulting from reduced tidal volume during anesthesia, which impairs oxygen exchange and is prevented by periodic manual bagging (IPPV).

Pressure Manometer

A gauge that measures pressure within the anesthetic circuit and reservoir bag in $\text{cm H}_2\text{O}$ or $\text{mm Hg}$; pressure must remain $< 2\,\text{cm H}_2\text{O}$ during normal breathing and $< 20\,\text{cm H}_2\text{O}$ (or $< 15\,\text{mm Hg}$) during manual ventilation.

Pop-Off Valve (Exhaust / Relief Valve)

An adjustable exhaust port that vents excess carrier gas and waste anesthetic to the scavenging system to prevent pressure buildup; it must be kept fully open during spontaneous breathing and fully closed only while bagging.

Pop-Off Occlusion Valve

A specialized safety relief valve that allows temporary closure for bagging by holding down a button, which automatically reopens as soon as finger pressure is released.

Breathing Hoses (Types A, B, and C)

Corrugated tubing delivering gas to and removing exhaled gases from the patient; depicted as: A = rubber Y-breathing tube, B = plastic Y-breathing tube, and C = Universal F-circuit breathing tube.

$\text{CO}_2$ Canister

A canister filled with absorbent granules (soda lime or barium lime, mostly calcium hydroxide) that removes carbon dioxide from exhaled gases percolating through the rebreathing circuit.

$\text{CO}_2$ Absorption Reaction

The exothermic chemical reaction taking place in the soda lime canister: $2\text{CO}_2 + \text{Ca(OH)}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{CO}_3 + 2\text{H}_2\text{O} + \text{Heat}$.

Soda Lime Exhaustion Indicator

A pH indicator (ethyl violet) that turns granules from white to blue or purple as they absorb carbon dioxide and become saturated; absorbent must be replaced when $1/3$ to $1/2$ of the canister has changed color or after a maximum of $14\,\text{hours}$ of use.

Channeling (Absorbent Canister)

A failure state occurring when loosely packed soda lime allows exhaled gas to travel along a preferred pathway, exhausting that specific strip while leaving the rest white, resulting in patient rebreathing of $\text{CO}_2$.

Flutter Valves (Unidirectional Valves)

Passive, one-way inhalation and exhalation valves that lift with patient breathing to ensure forward gas circulation and allow visual monitoring of respiratory rate.

Negative Pressure Relief Valve

A safety valve mounted on top of the inhalation valve that automatically opens to admit room air into the system if a negative vacuum occurs (such as when an oxygen tank empties).

Oxygen Flush Valve

A valve that rapidly delivers pure oxygen at $35\text{--}75\,\text{L/min}$ directly into the breathing circuit while bypassing the flowmeter and vaporizer; it must never be pressed while a patient is connected.

Vaporizer

A machine component that converts liquid anesthetic into a vaporized gas state and adds a controlled concentration of this gas to the oxygen flowing through the breathing circuit.

Precision Vaporizer

A calibrated device designed to deliver an exact graduated percent concentration ($0\text{--}5\%$) of anesthetic gas regardless of temperature, flow rate, or duration of use, specifically tailored to a single agent.

Vaporizer Outside the Circle (VOC)

A breathing circuit configuration where the carrier gas (oxygen) passes through the vaporizer before entering the breathing circuit.

Anesthetic Agent Color Coding

A standard safety identification system where each volatile anesthetic agent, its corresponding vaporizer, and fill adaptors are matched by color: purple for Isoflurane, yellow for Sevoflurane, and blue for Desflurane.

Key Lock System

A safety filling mechanism utilizing uniquely shaped and color-coded keyed adaptors to prevent the accidental filling of a vaporizer with the incorrect liquid anesthetic.

Fresh Gas

The specific mixture of carrier gas (oxygen) and vaporized inhalant anesthetic that exits the vaporizer outlet port and travels to the breathing circuit.

Reservoir Bag (Rebreathing Bag)

A flexible storage bag ranging in volume from $0.5$ to $5\,\text{L}$ in small animal machines that holds a reserve of fresh gas, allows visual monitoring of respiration rate and depth, and facilitates manual ventilation.

Intermittent Positive Pressure Ventilation (IPPV)

The technique of manual ventilation (also known as "bagging") performed by closing the pop-off valve and gently squeezing the reservoir bag to inflate the patient's lungs with gas and anesthetic, not exceeding $15\text{--}20\,\text{cm}\,\text{H}_2\text{O}$ of pressure.

Atelectasis

The collapse of pulmonary alveoli that commonly results from reduced tidal volume under anesthesia, which is prevented by periodic manual bagging to re-expand alveoli and maintain adequate gas exchange.

Tidal Volume ($\text{V}_{\text{T}}$)

The volume of gas inhaled and exhaled during each normal respiratory cycle, correlating to depth of respiration, standardly estimated as $10\,\text{ml/kg}$ of body weight.

Reservoir Bag Sizing Calculation

The mathematical determination of minimum reservoir bag volume calculated as patient tidal volume multiplied by $6$ ($60\,\text{ml/kg}$ of body weight) or body weight in pounds multiplied by $30\,\text{ml}$.

Pressure Manometer

A gauge measuring the internal gas pressure within the breathing circuit and the patient's respiratory tract, which should remain below $20\,\text{cm}\,\text{H}_2\text{O}$ during bagging and below $2\,\text{cm}\,\text{H}_2\text{O}$ during unassisted spontaneous breathing.

Pop-Off Valve (Relief / Exhaust Valve)

An adjustable exhaust valve that vents excess carrier gas and anesthetic gas out of the circuit into the scavenging system, preventing excessive pressure accumulation within the system.

Pop-Off Occlusion Valve

A specialized spring-loaded valve assembly allowing the user to temporarily occlude the exhaust port by pressing a button to bag the patient, which automatically reopens upon release to prevent accidental barotrauma.

Breathing Hoses

Corrugated conduits—available as rubber Y-tubes, plastic Y-tubes, or coaxial Universal F-circuits—that transport fresh gas to the patient and return exhaled gases containing $\text{CO}_2$ back to the machine.

$\text{CO}_2$ Absorbent Canister

A clear canister containing soda lime or barium lime granules (primarily calcium hydroxide) that chemically scrub exhaled carbon dioxide from recirculating gases.

Soda Lime Reaction

The chemical reaction ($2\,\text{CO}_2 + \text{Ca(OH)}_2 + 2\,\text{NaOH} \rightarrow \text{Na}_2\text{CO}_3 + 2\,\text{H}_2\text{O} + \text{Heat}$) occurring in the canister that neutralizes acidic carbon dioxide while generating water and warmth.

Ethyl Violet Indicator

A pH-sensitive chemical indicator inside soda lime granules that turns from white to blue or purple as granules become chemically saturated with carbon dioxide.

Channeling

The preferential flow of exhaled gas along paths of low resistance through loosely packed absorbent granules, which rapidly exhausts only that specific pathway and causes the patient to rebreathe $\text{CO}_2$ while surrounding granules remain deceptively white.

Flutter Valves (Unidirectional Valves)

Passive, one-way wafer valves that open and close in synchronization with inhalation and exhalation, ensuring the unidirectional flow of gas through the rebreathing circuit.

Negative Pressure Relief Valve

A safety valve mounted on the inspiratory assembly that automatically draws in ambient room air if negative pressure develops within the circuit (such as when oxygen supply fails or the reservoir bag collapses).

Oxygen Flush Valve

A control that delivers a high flow rate ($35\text{--}75\,\text{L/min}$) of unvaporized, pure oxygen directly into the breathing circuit, bypassing the flowmeter and vaporizer; it must never be pressed while a patient is connected to the circuit.

Purging (Flushing) the System

The practice of using the oxygen flush valve at the conclusion of an anesthetic event to evacuate residual anesthetic gas from the circuit hoses and reservoir bag before disconnecting the patient.