Inhaled Anesthetics in Clinical Use

Inhaled Anesthetics in Clinical Use

• Variable-bypass vaporizers are used to deliver most inhaled anesthetics, depending on relative gas flow and vapor pressure.
• Desflurane requires a special vaporizer due to its high vapor pressure and boiling point near room temperature, which heats and injects the agent for predictable output.
• Isoflurane, sevoflurane, and desflurane are the most commonly used inhaled anesthetics.

Nitrous Oxide

• Nitrous oxide is a good adjunct but has side effects, especially during long procedures.
• Risk of diffusion into ETT cuff, leading to increased cuff pressures, tracheal trauma, postoperative discomfort, and cuff rupture.
• Limiting diffusion in the cuff by providing equilibrium between delivered gases and gases that fill the cuff lumen, and using saline to expand the cuff.

Saturated Vapor Pressure and Boiling Point

• Saturated vapor pressure (SVP) is the pressure exerted by molecules from a volatile substance that escape the liquid phase and become vapor.
• SVP is directly proportional to the number of molecules of gas within a container at a certain temperature.
• Boiling point is the temperature at which vapor pressure equals ambient pressure, and the liquid rapidly vaporizes.

Vaporizers

• There are two types of vaporizers: variable-bypass vaporizers and heated vaporizers.

Solubility, Diffusion, and Osmosis

• CO2 is more soluble in tissues than in oxygen.
• Impairments in diffusing capacity are more likely to lead to hypoxemia than hypercarbia.
• Henry's Law states that the number of gas molecules dissolved in a liquid is directly proportional to its partial pressure above the liquid.
• Solubility coefficients include Bunsen and Ostwald coefficients.
• Solubility of liquids and solids increases with temperature, while solubility of gases decreases with temperature.

Partition Coefficient and MAC

• Partition coefficient is the ratio of the concentration of anesthetic in one phase or solvent compared to another.
• Blood-gas and fat-blood partition coefficients are important in anesthesia.
• MAC (Minimum Alveolar Concentration) is related to oil solubility, and high potency correlates with slow onset.

Diffusion and Osmosis

• Fick's Law states that the rate of diffusion is proportional to the gradient of the concentration (liquid) or partial pressure (gas).
• Graham's Law states that the rate of diffusion of gases through a porous membrane is inversely proportional to the square root of the molar mass of the gas.
• Bulk flow consists of ultrafiltration and reabsorption, which helps regulate the distribution of ECF between plasma and interstitial spaces.
• Osmosis is the movement of water through a semi-permeable membrane, and water flows to the more concentrated side.

Osmotic Pressure and Tonicity

• Osmole is a unit of measurement for osmotic activity, equal to 1 Avogadro's number of particles dissolved in a solute that cannot diffuse through a semi-permeable membrane.
• Osmotic pressure is the pressure at which osmosis of a solvent into a semi-permeable membrane is prevented.
• Tonicity is the osmotic pressure of a solution relative to that of other body fluids.
• Effects of mOsm on cells include hypotonic, isotonic, and hypertonic solutions.

Anesthesia and Solubility

• The solubility of an inhaled anesthetic in blood compared to air affects its binding to plasma proteins in the blood.
• A higher solubility in blood leads to a higher blood–gas partition coefficient.

Induction Rate and Solubility

• The induction rate of an anesthetic agent is inversely related to its solubility in blood.
• Induction rate refers to the speed at which an agent produces anesthesia.