Anesthesia Stages Overview

Anesthesia: Depth and Stages

• Stage I: Analgesia
  • Pain sensation decreases due to interference with sensory transmission.
  • Patient is initially conscious and conversational, progressing to drowsiness.
  • Amnesia and reduced pain awareness begin as patient moves toward Stage II.
• Stage II: Excitement
  • Delirium and combative behavior may appear.
  • Blood pressure and respiration become elevated and irregular; risk of laryngospasm.
  • Rapid-acting IV agents are typically administered to reduce this stage’s duration.
• Stage III: Surgical Anesthesia
  • Muscle tone and reflexes diminish as CNS depression deepens, leading to muscle relaxation and loss of spontaneous movement.
  • Breathing becomes regular; skeletal muscle relaxation makes this stage optimal for surgery.
  • Careful monitoring is essential to prevent unintentional transition into Stage IV.
• Stage IV: Medullary Paralysis
  • Extreme depression of respiratory and vasomotor centers.
  • Without immediate support for ventilation and circulation, there is a high risk of fatality.

Inhalation Anesthetics

• Modern inhalation anesthetics are nonflammable and nonexplosive.
• Decrease cerebrovascular resistance, increasing brain perfusion, and cause bronchodilation.
• Reduce spontaneous ventilation and suppress hypoxic pulmonary vasoconstriction.
• Solubility in blood and tissues and blood flow influence both induction and recovery from anesthesia.

Potency of Inhalation Anesthetics

• Minimum Alveolar Concentration (MAC): the concentration of an anesthetic gas in the alveoli that prevents movement in response to a surgical stimulus in 50% of patients.
• Lower MAC values indicate higher potency; higher MAC values indicate lower potency.
• Factors influencing MAC:
  • Increased MAC (lower sensitivity): Hyperthermia, CNS-stimulating drugs, chronic alcohol use.
  • Decreased MAC (higher sensitivity): Aging, hypothermia, pregnancy, sepsis, acute intoxication, concurrent IV anesthetics, and α₂-adrenergic agonists like clonidine.

Uptake and Distribution of Inhalation Anesthetics

• Goal is to achieve steady, optimal brain partial pressure (Pbr) of the anesthetic.
• Achieved by ensuring equilibrium between alveolar (Palv), arterial (Pa), and brain (Pbr) partial pressures.

Alveolar Wash-In:

• Initial process in which inspired anesthetic gas replaces normal lung gases, filling the functional residual capacity (FRC) of the lungs.
• Faster with increased ventilatory rate and smaller lung volume.
• Speed does not rely on gas’s physical characteristics, but on lung and breathing dynamics.

Anesthetic Uptake:

• Depends on blood flow to peripheral tissues and the gas’s solubility in blood.
  • Solubility in Blood: Measured by the blood/gas partition coefficient.

Nitrous Oxide:

• Least hepatotoxic of inhaled anesthetics.
• Does not depress respiration, induce muscle relaxation, or significantly affect cardiovascular function.
• Can lead to diffusion hypoxia during recovery.

Intravenous Anesthetics

• Rapid onset: drug travels from the injection site to the brain.
• Used alone for short procedures or in combination with inhaled agents for longer surgeries.
• Administered at lower doses for sedation.

Induction of IV Anesthesia

• Portion of drug binds to plasma proteins; the remainder remains unbound or "free."
• Highest initial concentrations delivered to vessel-rich organs, particularly the brain.
• Nonionized, lipid-soluble molecules cross the blood-brain barrier most rapidly.

Recovery from IV Anesthesia

• Occurs primarily due to redistribution from the CNS.
• Early recovery is rapid due to initial redistribution.

Effect of Reduced Cardiac Output on IV Anesthetics

• Increased cerebral distribution: More cardiac output is directed to the cerebral circulation, resulting in a higher proportion of the IV anesthetic reaching the brain.
• Reduced dose of anesthetic is often necessary.

Propofol:

• Widely used IV sedative/hypnotic.
• Formulation: Emulsion containing soybean oil and egg phospholipid, giving it a milky appearance.
• Induction occurs smoothly within 30 to 40 seconds after administration; rapid equilibration between plasma and brain tissue.
• Rapidly redistributes; initial redistribution half-life is approximately 2 to 4 minutes.
• Pharmacokinetics remain stable even with moderate hepatic or renal impairment.
• CNS effects: Depresses the CNS, can also elicit excitatory effects.
• Cardiovascular effects: Decreases blood pressure without significantly affecting myocardial contractility and reduces intracranial pressure.
• Neurosurgical monitoring: Less impact on CNS-evoked potentials, making it suitable for procedures requiring spinal cord monitoring.
• Analgesia: Does not provide significant analgesia.
• Side effects: Transient pain at the injection site and a low incidence of postoperative nausea and vomiting due to antiemetic properties.

Opioids

• Provide rapid analgesia compared to morphine.
• Administered intravenously, epidurally, or intrathecally.
• Do not offer significant amnesic effects.
• Can lead to hypotension, respiratory depression, and muscle rigidity.
• Common side effects include postanesthetic nausea and vomiting, reversed with naloxone.

Etomidate:

• Hypnotic agent used for anesthesia induction; lacks analgesic properties.
• Poor water solubility necessitates formulation in a propylene glycol solution.
• Induction is rapid and short-acting.
• Minimal impact on cardiovascular function.
• Adverse effects: Decreased plasma cortisol and aldosterone levels, can persist for up to 8 hours.
• Common side effects include injection site reactions and involuntary skeletal muscle movements.

Ketamine:

• Short-acting, nonbarbiturate anesthetic that induces a dissociated state; patient may appear unconscious but is unaware of pain.
• Stimulates central sympathetic outflow, leading to increased heart rate, blood pressure, and cardiac output.
• Potent bronchodilator.
• Redistributes to other tissues, used in pediatric and elderly populations for short procedures.

Dexmedetomidine:

• Sedative used mainly in intensive care and surgical settings.