Volatile Anesthetics: Comprehensive Study Guide

Volatile Anesthetics: Comprehensive Study Guide

Lecture Objectives

  • Describe the pharmacokinetics and systemic effects of volatile anesthetics.

  • Explain the uptake and distribution of volatile anesthetics.

  • Define MAC (minimum alveolar concentration) and identify factors that can modify MAC.

  • Describe the effects of anesthetic agents on respiratory, cardiovascular, renal, hepatic, and cerebral physiology.

Introduction

  • Volatile anesthetics exert their action by interacting with receptors in the body.

  • Agents discussed include:

    • Nitrous oxide

    • Sevoflurane

    • Desflurane

    • Isoflurane

    • Halothane (discussed despite it being outdated due to historical relevance)

  • These agents produce reversible depression in the central nervous system (CNS).

  • A thorough understanding of inhalational anesthetic pharmacokinetics is essential for safe practice.

History of Volatile Anesthetics

  • Halothane:

    • Synthesized in 1951 and introduced in 1956.

    • Had various drawbacks that led to the search for new anesthetics.

  • Methoxyflurane:

    • Introduced in 1960; problematic due to renal toxicity and prolonged recovery times.

    • Highly lipid soluble with extensive hepatic metabolism leading to nephrotoxicity.

  • Enflurane:

    • Methyl ethyl derivative introduced in 1973; less ideal due to CNS stimulation.

  • Isoflurane:

    • Introduced in 1981; resistant to metabolism and still used today.

  • Desflurane:

    • Introduced in 1992; fully fluorinated, has a high vapor pressure requiring a heated vaporizer.

  • Sevoflurane:

    • Introduced in 1994; background information on its properties.

Physiochemical Properties of Volatile Anesthetics

Basic Characteristic Comparisons

  • All modern volatile anesthetics are ethers except halothane.

  • Vapor Pressure: Proportional to temperature; higher vapor pressure means faster onset.

  • Partial Pressure: Defined as the pressure exerted by a single gas in a mix; depth of anesthesia relates to the partial pressure of the agent in the brain, not volume percent.

Identification of Volatile Anesthetics

  • Isoflurane: 5 fluorine atoms, 1 chlorine atom; potency is increased by chlorine.

  • Desflurane: 6 fluorine atoms; fully fluorinated, increased MAC due to decreased potency, also requires a heated vaporizer.

  • Sevoflurane: Heavily fluorinated with no chlorine; roughly three times more potent than desflurane.

  • Halothane: Only volatile with bromine, associated with liver toxicity risks.

Considerations in Use

  • Modern volatiles' low blood solubility allows rapid induction and recovery, though costs must be managed by using low fresh gas flow rates.

    • Low fresh gas flow rates minimize anesthetic loss and reduce overall consumption.

Pharmacokinetics of Volatile Anesthetics

  • Absorption: Influenced by:

    • Ventilation rates

    • Cardiac output

    • Anesthetic blood solubility

  • Alveolar Uptake::

    • Dependent on the uptake from alveoli into pulmonary blood and distribution throughout the body.

    • Distinct absorption between volatile gases and liquids at atmospheric pressure and room temperature.

Blood-Gas Solubility Coefficients

  • Indicator of anesthetic speed:

    • Isoflurane: 1.4

    • Desflurane: 0.42 (more rapid induction).

  • Higher coefficients = slower onset, lower = quicker effects.

Minimum Alveolar Concentration (MAC)

  • Definition of MAC: Minimum alveolar concentration resulting in a 50% movement response to surgical stimulation.

  • Factors affecting MAC include:

    • Age (decreases with increased age)

    • Hair color (redheads require higher dosages)

    • Co-morbidities like hyponatremia and chronic alcohol use.

  • Variations of MAC:

    • MAC Awake: 0.4-0.5 MAC (consciousness)

    • MAC-bar: Concentration needed to block autonomic responses (approximately 1.5 MAC).

Physiological Effects of Volatile Anesthetics

Respiratory System

  • Depressants leading to:

    • Decreased tidal volume

    • Increased respiratory rate, leading to reduced minute ventilation.

  • Bronchodilation occurs except for nitrous oxide.

Cardiovascular System

  • Volatile anesthetics generally cause:

    • Dose-dependent decrease in systemic vascular resistance (SVR)

    • Reductions in blood pressure in varying degrees based on the specific agent used.

  • Desflurane and Isoflurane: Slight increases in HR due to sympathetic activation.

  • Halothane: Known for myocardial sensitization to catecholamines.

CNS Effects

  • Decrease cerebral metabolic rate of oxygen consumption (CMR o2).

  • Increase cerebral blood flow and intracranial pressure if exceeding 1 MAC dosage.

  • Nitrous oxide: Opposite effects on CMR o2, leading to potential contraindications in neurosurgery.

Renal Effects

  • Associated reductions in renal blood flow and glomerular filtration rates, largely dependent on systemic blood pressure.

  • Preoperative hydration may attenuate these effects.

Hepatic Effects

  • Similar risk concerns across agents; halothane notably associated with hepatitis and significant liver dysfunction in susceptible individuals.

Conclusion

  • Usage of modern volatile anesthetics has advanced significantly.

  • Overall, understanding pharmacodynamics, especially regarding the factors affecting induction and recovery, is key for safe and effective anesthetic practices.

  • Special considerations regarding patient demographics, conditions, and usage of adjuncts can play critical roles in anesthesia management.