Vapour pressure and saturated vapour pressure

Clinical relevance in anaesthesia

• Volatile agent delivery depends on vapour pressure: vaporizers add agent vapour to fresh gas; the maximum possible vapour concentration is limited by the agent’s saturated vapour pressure (SVP) at that temperature.
  • At a given temperature, the agent’s vapour in equilibrium with its liquid exerts SVP; this sets the upper limit of partial pressure of agent in the gas phase.
• Temperature changes alter SVP and therefore delivered concentration if not compensated (e.g., cooling from vaporization reduces SVP and output).
  • Modern variable-bypass vaporizers use temperature compensation (bimetallic strip/expansion element) and high thermal mass to maintain output.
• SVP is independent of atmospheric pressure, but the delivered volume % from a variable-bypass vaporizer is approximately independent of barometric pressure; however, partial pressure delivered falls with altitude.
  • At altitude: same dialled % → lower agent partial pressure (PA = F × PB).
• SVP explains why desflurane requires a heated, pressurised vaporizer (very high SVP near room temperature).
• Water vapour: humidification adds water vapour partial pressure (PH2O) which reduces the partial pressures of other gases in the mixture (Dalton’s law).
  • At 37°C, saturated water vapour pressure is ~6.3 kPa (47 mmHg).

Key definitions

• Vapour: gaseous phase of a substance that is liquid/solid at room temperature and can be condensed by compression at constant temperature.
• Vapour pressure: pressure exerted by molecules of a vapour above its liquid (or solid) phase.
  • In a closed container, vapour pressure rises as molecules evaporate until dynamic equilibrium is reached (rate of evaporation = rate of condensation).
• Saturated vapour pressure (SVP): vapour pressure when the vapour is in equilibrium with its liquid at a given temperature (i.e., maximum vapour pressure possible at that temperature).
• Boiling point: temperature at which SVP equals ambient (external) pressure; bubbles form throughout the liquid.
  • Lower ambient pressure → lower boiling point.

Determinants and temperature dependence

• SVP depends on temperature and the substance’s intermolecular forces (stronger forces → lower SVP).
• As temperature increases, more molecules have sufficient kinetic energy to escape the liquid → SVP increases (non-linear, approximately exponential).
  • Clausius–Clapeyron relationship (concept): ln(P) varies approximately linearly with 1/T; higher latent heat of vaporization → steeper temperature dependence.
• Evaporation causes cooling (latent heat absorbed) → reduces temperature of remaining liquid → reduces SVP unless heat is supplied.

SVP, partial pressure and maximum vapour concentration

• At equilibrium, the vapour’s partial pressure equals SVP (for that temperature).
• Maximum volume fraction (or %) of agent vapour in a carrier gas at total pressure PB is: Fmax = SVP / PB.
  • Example method: if SVP = 32 kPa at 20°C and PB = 101 kPa, then Fmax ≈ 0.32 → 32% (before dilution/controlled splitting in a vaporizer).
• SVP is independent of PB, but Fmax depends on PB because PB sets the denominator.

Anaesthetic agent comparisons (qualitative)

• High SVP at room temperature → high potential vapour concentration; requires design features to control output.
  • Desflurane has an SVP close to atmospheric pressure at room temperature → would boil; hence heated (~39°C) and pressurised vaporizer to deliver stable concentrations.
• Lower SVP agents (e.g., isoflurane/sevoflurane) are suitable for variable-bypass vaporizers with temperature compensation.

Water vapour (humidification) as a common FRCA application

• At 37°C, fully saturated gas contains water vapour at PH2O ≈ 6.3 kPa (47 mmHg).
• Inspired oxygen partial pressure in the trachea: PIO2(tracheal) = (PB − PH2O) × FiO2.
  • At sea level PB 101 kPa and FiO2 0.21: PIO2 ≈ (101 − 6.3) × 0.21 ≈ 19.9 kPa.