Inhalational Anesthetics & General Anesthesia (Lecture Summary)

Vocabulary flashcards covering the key terms, concepts, and mechanisms related to inhalational anesthetics, their pharmacokinetics, stages of anesthesia, and clinical considerations.

General Anesthesia

A drug-induced, reversible state of unconsciousness with amnesia, analgesia, areflexia, and attenuation of autonomic responses.

Goal of Inhaled Anesthetics

To achieve a specific concentration of anesthetic molecules in the CNS sufficient to produce the anesthetic state.

Guedel’s Classification

Four stages (Analgesia, Excitement, Surgical Anesthesia, Overdose) that describe depth of general anesthesia.

Stage 1 – Analgesia

Induction stage; patient is sedated, conversational, with slow regular breathing and intact consciousness until loss of consciousness.

Stage 2 – Excitement/Delirium

Disinhibition phase with uncontrolled movements, hypertension, tachycardia, intact but hypersensitive airway reflexes; avoid airway manipulation.

Stage 3 – Surgical Anesthesia

Desired level for surgery; eye movements cease, respiratory depression occurs, airway manipulation is safe; divided into four planes.

Plane 3 of Stage 3

‘True surgical anesthesia’—complete intercostal and abdominal muscle relaxation and loss of pupillary light reflex.

Stage 4 – Overdose (Medullary Paralysis)

Excessive anesthetic leads to apnea, flaccid muscles, fixed dilated pupils, severe hypotension, potential death.

Induction (Anesthesia)

Process of achieving unconsciousness, usually with IV agents (propofol, thiopental) plus opioid and muscle relaxant.

Balanced Anesthesia

Use of inhaled agents with IV adjuvants to combine hypnosis, analgesia, and muscle relaxation with hemodynamic stability.

Volatile Anesthetic

Liquid anesthetic (e.g., halothane, isoflurane, sevoflurane, desflurane) vaporized for inhalational delivery.

Nitrous Oxide (N₂O)

Inhaled gas with MAC 104 %; low blood solubility, strong concentration/second gas effects, risk of diffusion hypoxia.

Halothane

Volatile agent, blood-gas partition 2.5, MAC 0.74 %; relatively potent but more soluble—slower induction and recovery.

Isoflurane

Volatile agent, blood-gas 1.4, MAC 1.17 %; produces stable hemodynamics, pungent odor limits mask induction.

Desflurane

Volatile agent, very low blood-gas 0.45, MAC 6 %; fastest induction/recovery among volatiles, irritates airway.

Sevoflurane

Volatile agent, blood-gas 0.65, MAC 2.05 %; non-pungent, suitable for inhalational induction.

Blood-Gas Partition Coefficient

Ratio describing anesthetic solubility in blood vs alveolar gas; lower values mean faster induction and emergence.

Minimal Alveolar Concentration (MAC)

Alveolar concentration at 1 atm preventing movement to surgical stimulus in 50 % of patients—a measure of potency.

MAC Awake

0.15–0.5 MAC; concentration at which 50 % of patients respond to verbal command.

MAC 95 %

1.2–1.3 MAC; concentration preventing movement in 95 % of patients.

MAC-BAR

1.7–2 MAC; concentration blocking autonomic responses to noxious stimuli.

Concentration Effect

Higher inspired concentration (Fi) of a gas causes disproportionately faster rise in alveolar concentration (FA).

Augmented Inflow Effect

Component of concentration effect where uptake-related volume loss draws extra fresh gas into lungs, boosting FA.

Second Gas Effect

High-concentration N₂O accelerates uptake of a concurrently administered potent volatile agent (first gas).

Diffusion Hypoxia (Fink Effect)

Post-N₂O wash-out dilutes alveolar O₂ and CO₂, risking hypoxia; prevented with 100 % oxygen for 3–5 min.

Time Constant (τ)

Volume/Flow; time for gas volume entering a container (lungs or circuit) to equal its capacity—63 % equilibration per τ.

Fresh Gas Flow (FGF)

Rate of gas delivered from anesthesia machine; higher FGF speeds induction and recovery by shortening circuit τ.

FA/Fi Ratio

Alveolar to inspired concentration; its rate of rise indicates speed of induction.

Alveolar Ventilation

Minute ventilation minus dead space; increased ventilation speeds rise of FA and accelerates induction.

Functional Residual Capacity (FRC)

Volume of air remaining after normal exhalation; large FRC dilutes inspired agent, slowing induction.

Vessel-Rich Group (VRG)

Brain, heart, liver, kidneys, spinal cord; 10 % body mass but 75 % cardiac output—equilibrates within minutes.

Muscle Group (Anesthesia)

Skeletal muscles; ~40 % body mass, ~10–15 % cardiac output—slower uptake than VRG.

Fat Group

Adipose tissue; 25 % body mass, 6 % cardiac output, high solubility—important in prolonged anesthesia (delayed emergence).

Pulmonary Phase of Uptake

Determined by inspired concentration, alveolar ventilation, gas solubility, cardiac output, and alveolar–venous gradient.

Circulatory Phase of Uptake

Distribution of anesthetic from blood to tissues based on tissue perfusion and tissue-blood solubility.

Factors Increasing FA/Fi Rise

Low solubility, low cardiac output, high minute ventilation, high PA–Pv gradient.

Factors Decreasing FA/Fi Rise

High solubility, high cardiac output, low ventilation, low PA–Pv gradient.

Recovery/Emergence

Wash-out of anesthetic from tissues to lungs; governed by solubility, ventilation, cardiac output.

Factors Delaying Recovery

High solubility agents, prolonged anesthesia (>4 h), low ventilation, high cardiac output.

Areflexia

Absence of movement or reflexes; one of the goals of general anesthesia (motionlessness).

Attenuation of Autonomic Responses

Blunting of sympathetic reflexes to surgical stimuli—another primary goal of general anesthesia.

Cardiac Output & Uptake

Higher cardiac output increases uptake of soluble agents, slowing FA/Fi rise; low output risks overdose.

Solubility & Uptake

More soluble agents require more dissolved molecules to raise partial pressure, slowing induction.

Time Constant Formula

τ = Volume / Flow; e.g., lung τ = FRC / Alveolar ventilation.

Overdose Signs

Apnea, flaccid muscles, fixed dilated pupils, severe hypotension—hallmarks of Stage 4 anesthesia.