Applied Chemistry and Physics of Anesthetic Gasses

Applied Chemistry and Physics in Anesthesia

Introduction to Anesthetic Gases

  • Anesthetic gases are critical components in the practice of anesthesia, facilitating surgeries and medical procedures by inducing a temporary state of unconsciousness and insensitivity to pain.

  • Key aspects involved in the use and efficacy of anesthetic gases include their absorption in the bloodstream, distribution to target tissues, and elimination from the body.

Overview of the Breathing Circuit

  • Understanding the breathing circuit is crucial for effective gas delivery to patients undergoing anesthesia. Key elements include:

    • Arterial Blood: Refers to blood carried from the heart to the tissues, rich in oxygen.

    • Venous Blood: Blood returning to the heart, often depleted of oxygen and high in carbon dioxide.

Outline/Objectives

  • The following topics will be described and analyzed:

    1. Induction Process: Mechanisms to deliver anesthetic to the alveoli.

    • Key parameters influencing this process include ventilation time constant, uptake, solubility in blood, partial pressure gradient, and cardiac output.

    • Fat solubility and time constant also play significant roles.

    • Vapor pressure equilibrium is described by the relationship between inspired concentration (FI) and alveolar concentration (FA). A crucial equation is: FA = FI * 2.

Concepts in Depth

Anesthetic Induction

  • The goal of induction is to attain a balance or equilibrium between the inspired concentrations of anesthetics and the alveolar concentrations.

    • The relationship is expressed as: F{inspired}/F{alveoli} = 1 when equilibrium is achieved.

End-Tidal Sevoflurane

  • The phenomenon where measured end-tidal sevoflurane indicates 0.5% despite the dial set to 3% raises questions regarding airway adequacy, depth of anesthesia, and the machine’s functionality.

  • Questions for critical evaluation include: "How's the airway? Is the patient asleep? What is the MAC (Minimum Alveolar Concentration)?", highlighting the need for practical skills in laboratory settings.

Anesthetic Delivery Mechanisms

Partial Pressure Gradient

  • Gases move from areas of higher to lower pressure, striving for equilibrium across various interfaces:

    • Between the vaporizer and inflow to the circuit.

    • Between inflow and the circuit itself.

    • Between the circuit and alveoli.

    • Between the alveoli and the blood.

    • Between blood and brain/tissues.

Relationship between FA and FI

  • The interplay between alveolar (FA) and inspired (FI) concentrations is vital because:

    • FI is controllable whereas FA reflects anesthetic partial pressure in the blood.

  • Factors controlling F_I include:

    • Delivery of higher flow rates (influencing time constant).

    • Utilization of high percent concentrations.

    • Expired gas values reflecting alveolar concentrations.

Alveolar Ventilation Effect

  • The induction goal emphasizes achieving FA/FI = 1, opposed by factors such as:

    • Uptake (solubility factors from blood).

  • The rate of change in F_A is rapid, depending on:

    • Tidal volume (TV), respiratory rate (RR), and minute ventilation (VE).

    • With an average minute ventilation of 6 liters (accounting for anatomical dead space), the implications on anesthesia administration are significant.

Anesthetic Solubility Considerations

  • The solubility of gases in blood is relative, emphasizing the following:

    • The Blood-Gas Solubility Ratio impacts how quickly a gas reaches the bloodstream during induction.

    • Insoluble agents foster rapid equilibria, while soluble gases achieve equilibrium slower.

    • Factors such as tissue solubility coefficient, blood flow to tissues, and arterial-tissue partial pressure gradients affect uptake.

Blood/Gas Solubility Ratios

  • Notable anesthetic solubility coefficients:

    • Desflurane: 0.42

    • Nitrous Oxide: 0.47

    • Sevoflurane: 0.6

    • Isoflurane: 1.4

    • Enflurane: 1.9

    • Halothane: 2.4

    • Methoxyflurane: 15.0 (not in US practice by 2025).

Fat/Blood Partition Coefficients

  • Fat solubility directly affects potency, as illustrated by these coefficients:

    • Nitrous Oxide: 2.3 (oil:gas 1.4)

    • Desflurane: 27 (oil:gas 18-20)

    • Isoflurane: 56 (oil:gas 90-99)

    • Halothane: 67 (oil:gas 224)

  • The higher the fat solubility, the greater the agent’s potency, measured by the Minimum Alveolar Concentration (MAC).

Time Constant of Anesthesia

Definition and Calculation

  • The Time Constant (TC) is defined as the time taken to achieve a certain percentage change in a system, calculated by:

    • ext{TC} = rac{ ext{Capacity}}{ ext{Flow Rate}}

  • For example:

    • 1 Time Constant = 63% change

    • 2 Time Constants = 86% change

    • 3-4 Time Constants = 98% change

    • 5 Time Constants = 99.9% change

Implications of Time Constant

  • This concept is crucial in understanding how quickly anesthetic concentrations reach effective levels in the bloodstream and brain.

  • Manipulating flow rates impacts the efficacy of gas delivery.

Vapor Pressure and Anesthetic Concentration

  • According to Dalton's Law, the partial pressure of a gas in a mixture is proportional to its percentage concentration in that mixture.

  • Vapor pressure (VP) is temperature-dependent and must be maximized for effective anesthetic delivery, well exceeding anesthetic concentration needs.

Review Questions and Formulae

  • To consolidate learning, consider:

    • Describe the partial pressure gradients and their significance in anesthesia.

    • Identify factors that oppose rapid increases in F_A.

    • Explore differences in uptake curves for anesthetics and their clinical implications.

    • Identify the significance of vapor pressure.

    • State the formula for time constant and detail changes over time constants.

    • Analyze factors that can shorten the time constant and their clinical relevance.

    • Explore the duration of oxygen administration for optimal pre-oxygenation before induction.