Overview
This lecture covers the delivery of inhaled anesthetics to patients, focusing on partial pressures, solubility, partition coefficients, and the process of induction and uptake.
Delivery and Equilibration of Inhaled Anesthetics
- The aim is to achieve the same anesthetic partial pressure in the brain, blood, and alveoli at equilibrium.
- Rapid transfer of anesthetic gases ensures equal partial pressures across compartments at equilibrium.
- If alveolar partial pressure (PA) is known, brain partial pressure (PCNS) can be inferred at equilibrium.
- Anesthetics are commonly described by fractional concentration (e.g., 1% isoflurane), referencing atmospheric pressure (760 mmHg at sea level).
- Physiological effect depends on partial pressure in the brain, not just fractional concentration.
- Vaporizers are calibrated to deliver partial pressure, adjusting for altitude except for desflurane vaporizers.
Solubility and Partition Coefficient
- Solubility describes how much gas dissolves in a solution, measured by the partition coefficient (λ or Lambda).
- The blood-gas partition coefficient (BG) is the ratio of anesthetic in blood to gas phase at equilibrium.
- High BG (>1): agent is highly soluble in blood; more agent dissolves before achieving needed partial pressure.
- Low BG (<1): agent is less soluble in blood; partial pressure increases quickly with less agent.
- Other partition coefficients describe agent movement between other compartments (e.g., blood–fat, blood–brain).
Induction and Uptake Process
- Induction is delivering anesthetic from the machine to the brain to induce anesthesia.
- The anesthesia machine, breathing circuit, and lungs work together to equilibrate agent concentrations.
- Fi is the inspired fractional concentration; FA is alveolar fractional concentration.
- At induction start, FA/Fi ratio is zero; equilibrium (and steady anesthesia) is reached when FA/Fi = 1.
- Uptake is the transfer of agent from lungs to blood, influenced by blood-gas partition coefficient.
- Blood distributes the agent to tissues (muscle, fat, brain) based on respective tissue-blood partition coefficients.
- Blood returning from tissues to the lungs can absorb more agent, continuing uptake.
Examples of Partition Coefficients
- Nitrous oxide: blood-gas partition coefficient 0.47 (not very soluble in blood, prefers gas phase).
- Halothane: blood-gas partition coefficient 2.4 (more soluble in blood than air).
Key Terms & Definitions
- Partial Pressure — the pressure exerted by a specific gas in a mixture, driving anesthetic effect.
- Fractional Concentration (F or Fi) — percent of anesthetic in a gas mixture.
- Partition Coefficient (λ or Lambda) — ratio describing how much anesthetic dissolves between two phases (e.g., blood/gas).
- Blood-Gas Partition Coefficient — reflects solubility of an agent in blood relative to gas.
- Induction — process of delivering anesthetic to the brain to induce anesthesia.
- Uptake — absorption of anesthetic from alveoli into blood and distribution to tissues.
Action Items / Next Steps
- Review and understand all discussed concepts, especially partition coefficients and their impact on anesthetic delivery.
- Prepare questions for clarification before the next lecture.