SN3 SHORT NOTE 5M

Describe Soda Lime in detail.

General Surgery · Final Professional

Definition

Soda lime is the granular carbon-dioxide absorbent used in the circle breathing system to remove CO₂ from exhaled gas, allowing rebreathing and low-flow anaesthesia. It is composed chiefly of calcium hydroxide with small amounts of sodium (and sometimes potassium) hydroxide, water, and a pH indicator dye.

Chemistry

CO₂ dissolves to form carbonic acid, which is neutralised by the hydroxides to produce calcium carbonate, water and heat. As the absorbent is used up, the pH-sensitive indicator changes colour (commonly white → violet), showing that the soda lime is exhausted and should be changed.

CLINICAL PEARL

The indicator colour can revert on resting the absorber, so an exhausted canister may look fresh again after a pause; confirm exhaustion by a rising inspired CO₂ on capnography rather than colour alone.

Practical Points

The reaction usefully warms and humidifies the inspired gas. Absorbent that is allowed to become very dry can react with some volatile agents to form toxic compounds (e.g. Carbon monoxide), so canisters are changed regularly and not left with gas flowing when unused.

Practical Use in the Circle

In practice the absorber canister is inspected before each list and changed when the indicator shows exhaustion or when a rising inspired CO₂ is seen on the capnograph during use. Two absorbers in series are often used so that fresh absorbent always lies downstream, and the anaesthetist avoids leaving fresh gas flowing through the absorber overnight, since drying it out both wastes its capacity and creates the conditions for toxic degradation-product formation with certain agents.

CLINICAL PEARL

Judge soda lime by capnography, not colour alone: the indicator can revert on resting, so a rising inspired CO₂ is the reliable sign that the absorbent is spent and must be changed.

DANGER / REMEMBER

An often-forgotten hazard is that very dry soda lime reacting with some volatile agents can generate carbon monoxide, so absorbent is kept adequately moist, fresh gas is not left running through it when the machine is idle, and canisters are changed on a regular schedule rather than only when the colour changes.

Exhaustion is shown by colour change and rising inspired CO₂.

Component	Proportion	Role
Calcium hydroxide	About 80%	Main CO₂ absorber
Sodium hydroxide	About 4%	Activator
Water	14–19%	Essential for reaction
Silica	Small amount	Hardness, prevents dust
Indicator dye	Trace	Colour change on exhaustion

KEY POINT

Key points TO remember

Soda lime = CO₂ absorbent for the circle system; mainly Ca(OH)₂ + NaOH + water + indicator.
CO₂ → carbonic acid → neutralised → CaCO₃ + water + heat.
Indicator colour change (white→violet) signals exhaustion; may revert on resting.
Warms/humidifies gas; very dry absorbent can form toxic products — change regularly.

EXAM TIP

Sources: Morgan & Mikhail’s Clinical Anesthesiology; Miller’s Anesthesia; Ajay Yadav’s Short Textbook of Anaesthesia.

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