Blood:gas partition coefficient

Core concept → what it predicts at the bedside

• The blood:gas partition coefficient (λB:G) is the key determinant of how quickly alveolar partial pressure (FA) rises towards inspired partial pressure (FI), and therefore how quickly the brain partial pressure rises.
  • Lower λB:G → less uptake into blood for a given alveolar partial pressure → faster rise in FA/FI → faster induction and faster recovery (all else equal).
  • Higher λB:G → more uptake into blood → slower rise in FA/FI → slower induction and slower recovery.
• λB:G does not measure potency (that is MAC); it measures solubility in blood relative to gas, which influences kinetics.

Clinical implications you can state in a viva

• Low λB:G agents (e.g. desflurane, sevoflurane, nitrous oxide) give rapid changes in depth and rapid wake-up; high λB:G agents (e.g. isoflurane, halothane) change more slowly.
• When λB:G is high, factors that increase uptake (high cardiac output, high alveolar–venous partial pressure gradient) have a larger slowing effect on FA/FI.
• In low λB:G agents, the system is less uptake-limited; changes in ventilation and delivered concentration translate to faster changes in FA.

Definition and interpretation

• Blood:gas partition coefficient (λB:G) = ratio of concentration of anaesthetic in blood to that in alveolar gas when partial pressures are equal at equilibrium.
  • It is a solubility ratio: how much dissolves in blood compared with remaining in gas phase at the same partial pressure.
  • Units: dimensionless.
• A practical way to phrase it: λB:G tells you how big the ‘blood reservoir’ is for a given alveolar partial pressure.

Typical values (approximate; order matters more than exact numbers)

• Desflurane: ~0.42
• Nitrous oxide: ~0.47
• Sevoflurane: ~0.65
• Isoflurane: ~1.4
• Halothane: ~2.4

Link to FA/FI and uptake (the standard FRCA explanation)

• Rate of rise of FA/FI is slowed by uptake of agent from alveoli into blood.
• Uptake is increased by: higher λB:G, higher cardiac output, higher alveolar–venous partial pressure gradient, and higher inspired concentration (concentration effect).
  • High λB:G increases the amount taken up for a given alveolar partial pressure, so FA rises more slowly.
• Ventilation: increasing alveolar ventilation increases delivery of agent to alveoli and tends to increase FA/FI; this effect is more pronounced for soluble agents (high λB:G) because uptake would otherwise keep FA low.

Determinants of λB:G (what changes it?)

• λB:G depends on solubility in whole blood: plasma solubility + red cell solubility (including binding to proteins/lipids).
• Temperature: increasing temperature generally decreases solubility of gases in liquids → lowers λB:G → faster FA rise (small effect clinically).
• Haematocrit: can alter blood solubility depending on agent’s affinity for RBCs; effect is agent-dependent and usually modest clinically.
• Plasma proteins/lipids: increased lipid content can increase solubility for lipophilic agents; major clinical relevance is limited compared with cardiac output/ventilation effects.

Clinical scenarios: how λB:G interacts with physiology

• High cardiac output (e.g. sepsis, pregnancy, anxiety/pain): increases uptake from alveoli → slows FA/FI rise, especially for high λB:G agents; may need higher delivered concentration to achieve target FA.
• Low cardiac output (e.g. shock): reduces uptake → FA rises faster (risk of rapid deepening); but tissue perfusion is reduced so equilibration to brain may be less predictable.
• Hyperventilation: speeds induction more for soluble agents; less impact for poorly soluble agents where FA already rises quickly.
• Recovery: low λB:G → less blood and tissue storage → faster washout; high λB:G → more storage and slower washout (particularly after long cases due to tissue compartments).

Distinguish from other partition coefficients

• Oil:gas partition coefficient correlates with potency (MAC) via Meyer–Overton; not with speed of onset.
• Brain:blood (or vessel-rich group:blood) partition influences how much agent is required to saturate brain tissue, but λB:G remains the key driver of FA rise and early kinetics.