Anesthesia & Gas Laws Study Notes

Minimum Alveolar Concentration (MAC)

• Definition: % of volatile anesthetic at 1 atm required to prevent movement in 50% of subjects to surgical stimulus.

• Values:

  • Sevoflurane: 2%

  • Desflurane: 6%

  • Isoflurane: 1.2%

  • Halothane: 0.75%

Vapor & Gas Concepts

• Vapor: Gas below critical temp.

• Gas: Gas above critical temp.

• Critical Temperature (CT): Temp above which vapor cannot be liquefied by pressure.

• Volatile liquid: High vapor pressure at room temp.

• Vapor Pressure (VP): Pressure exerted by vapor over liquid in closed container.

Factors affecting VP

• ↑ Temperature → ↑ VP

• ↑ Intermolecular forces → ↓ VP

• Surface area has no effect

VP vs Boiling Point

• Boiling Point (BP): Temp at which VP = atmospheric pressure.

• ↑ VP → ↓ BP

Vapor Pressure @ 20°C

• Sevo: 157 mmHg

• Iso: 240 mmHg

• Halo: 244 mmHg

• Des: 669 mmHg

Boiling Points (°C)

• Sevo: 58.5

• Iso: 48.5

• Halo: 50.2

• Des: 22.8

Solubility & Partition Coefficients

• VP vs Solubility: Inversely proportional.

• Blood/Gas Partition Coefficient (BGPC):

  • Sevo: 0.65

  • Iso: 1.46

  • Halo: 2.5

  • Des: 0.42

• Henry’s Law: Amount of gas dissolved ∝ partial pressure.

• ↑ Gas pressure → ↑ dissolved gas

• ↑ Temperature → ↓ solubility

Solubility coefficients

• O₂: 0.003 mL/dL/mmHg

• CO₂: 0.067 mL/dL/mmHg

Gas Movement & Diffusion

• Diffusion: High → low concentration.

• ↑ Temp → faster diffusion.

• Brownian Motion: Random molecular movement.

• Graham’s Law: Rate of effusion ∝ 1/√MW.

• Fick’s Law:

  • ∝: Pressure gradient, solubility, surface area

  • ∝ 1/: Membrane thickness, molecular weight

Osmosis & Pressures

• Osmosis: Water diffusion across membrane.

• Osmotic Pressure: Force needed to stop osmosis.

• Oncotic Pressure: Plasma proteins retaining water (≈ 28 mmHg).

• Hydrostatic Pressure: Pushes water out of vessels.

Physical Constants

• BP of Water: 100°C (212°F).

• Absolute Zero: 0 K, molecular motion stops.

Critical Temperatures (CT)

• CO₂: 31°C

• N₂O: 36.5°C

• O₂: -119°C

• Air: -140°C

Vaporization & Energy

• Vaporization: Liquid → gas.

• Rate depends on: Temp, VP, PP above liquid.

• Latent Heat of Vaporization: Energy needed for phase change.

• Adiabatic Process: Gas expands/compresses too fast for heat exchange.

• Joule-Thompson Effect: Rapid expansion → cooling.

• Compression (Energy Concentration): Rapid compression → ↑ Temp.

Physics: Force & Motion

• Gravity: 9.81 m/s².

• Mass: Amount of matter.

• Weight: Mass × gravity.

Newton’s Laws

1. Inertia.

2. F = m × a.

3. Action = Reaction.

• Force unit: Newton (kg·m/s²).

• 1 dyne: 1 g × 1 cm/s².

Pressure Units

• Pressure = Force/Area.

• Standard Pressure Equivalents:

  • 1 atm = 760 mmHg = 760 torr = 100 kPa = 1 bar = 1020 cmH₂O = 14.7 psi

• Absolute Pressure: Atmospheric + Gauge.

• Bourdon Gauge: Zeroed at atm pressure.

• Critical Pressure: Pressure to liquefy gas at CT.

Gas Laws

• Universal Gas Law: PV = nRT

  • P = Pressure

  • V = Volume (L)

  • n = moles

  • R = 0.0821 L·atm/K·mol

  • T = Kelvin

Specific Gas Laws

• Boyle’s Law (P–V): P₁V₁ = P₂V₂

• Charles’ Law (V–T): V₁/T₁ = V₂/T₂

• Gay-Lussac’s Law (P–T): P₁/T₁ = P₂/T₂