SN6 SHORT NOTE 5M

Describe Concentration Effect in detail.

General Surgery · Final Professional

Definition

The concentration effect describes how the higher the inspired concentration of an inhalational agent, the faster its alveolar concentration rises towards the inspired value (F_A/F_I rises more quickly). It is most relevant to nitrous oxide, which is given in high concentrations.

Mechanism

When a gas is present at high concentration and is rapidly taken up from the alveoli, two things speed the rise of its alveolar concentration: the concentrating effect of the shrinking alveolar gas volume as the agent is absorbed, and an augmented inflow of fresh gas drawn in to replace the absorbed volume. The same rapid uptake of a high-concentration first gas underlies the second gas effect on a co-administered agent.

CLINICAL PEARL

The concentration and second-gas effects are two faces of the same phenomenon — the rapid uptake of a high concentration of nitrous oxide speeds the rise of both its own and a co-administered agent’s alveolar concentration.

Relation to Nitrous Oxide

The concentration effect is chiefly of practical importance for nitrous oxide because only nitrous oxide is administered in the very high concentrations needed to make the effect significant; the potent volatile agents are given in concentrations of only a few per cent, at which the effect is negligible. This is why nitrous oxide, despite being a weak anaesthetic, contributes so usefully to the speed of a gaseous induction, both through its own rapidly-rising concentration and through the second gas effect it exerts on the accompanying volatile agent.

CLINICAL PEARL

Remember that the concentration effect matters only for nitrous oxide, because only it is given in high enough concentrations to make the effect appreciable — the potent volatiles are delivered at a few per cent, where it is negligible.

DANGER / REMEMBER

Keep it linked to the second gas effect in your answer: both are consequences of the rapid uptake of a high concentration of nitrous oxide, one accelerating nitrous oxide’s own rise and the other that of a co-administered agent.

In Brief

So although the effect is a general principle of uptake, in the clinic it is essentially a nitrous-oxide phenomenon.

Only clinically relevant for nitrous oxide, given in high concentration.

KEY POINT

Key points TO remember

Concentration effect: higher inspired concentration → faster rise of alveolar concentration (F_A/F_I).
Due to concentration of shrinking alveolar gas + augmented inflow as agent is absorbed.
Most relevant to high-concentration nitrous oxide.
Closely related to the second gas effect.

EXAM TIP

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

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