Pain Management (Anesthetics)

• Classification of Anesthetic Agents

  • Pre anesthetic agents
  • General Anesthesia
  • Intravenous Anesthesia
  • Inhaled Anesthesia
  • Local Anesthesia
• Pre Anesthetic Medications

  • Anticholinergics
  • Antiemetics
  • Anti-histamines
  • Barbiturates
  • Benzodiazepines
  • Muscle relaxants
  • Opioids
• General Anesthetics - Intravenous

  • Barbiturates
  • Benzodiazepines
  • Etomidate
  • Ketamine
  • Opioids
  • Propofol
• General Anesthetics - Inhaled

  • Desflurane
  • Enflurane
  • Halothane
  • Isoflurane
  • Nitrous oxide
  • Sevoflurane
• Local Anesthetics

  • Bupivacaine
  • Lidocaine
  • Procaine
  • Tetracaine
• General Anesthesia


  Anesthesia or anaesthesia is a state of temporary induced loss of sensation or awareness
• Functions of adjuncts to anesthesia

  • Relieve Anxiety (Benzodiazepines)
  • Sedation (Barbiturates)
  • Prevent allergic reactions (Anti-histamines)
  • Provide Analgesia (Opioids)
  • Anticholinergic drugs
  • Muscle Relaxants
• Stages of General Anesthesia

  • Induction
  • Maintenance
  • Recovery
• Induction


  The period of time from the onset of administration of the anesthetic to the development of effective surgical anesthesia in the patient
• Induction


  • Depends on how fast effective concentration of anesthetic reach the brain
  • Minimum alveolar concentration (MAC)
  • Blood-gas partition coefficient
  • Oil/gas partition coefficient
• Maintenance


  Provides sustained anesthesia, usually maintained by the administration of volatile anesthestics (minute-to-minute control)
• Recovery


  The time from discontinuation of anesthetic until consciousness and protective reflexes return
• Recovery


  Depends on how fast the anesthetic diffuses from the brain
• Stages / Depth of Anesthesia

  • I. Analgesia
  • II. Excitement
  • III. Surgical Anesthesia
  • IV. Medullary paralysis/ depression
• Stage I: Analgesia


  Loss of pain sensation results from interference with sensory transmission in the spinothalamic tract, mild CNS depression, suitable for surgical procedures not requiring muscle relaxation
• Stage II: Delirium/Excitement


  Displays delirium and possibly combative behaviour, irregularity in BP and respiration, risk of laryngospasm
• Stage III: Surgical Anesthesia


  Gradual loss of muscle tone and reflexes as the CNS is further depressed, ideal stage for surgery, careful monitoring is needed to prevent undesired progression to stage IV
• Stage IV: Medullary Depression


  Severe depression of the respiratory and vasomotor centers occurs, ventilation and/or circulation must be supported to prevent death
• Inhalation Anesthetics

  • Methoxyflurane
  • Halothane
  • Enflurane
  • Isoflurane
  • Sevoflurane
  • Desflurane
  • Nitrous oxide
• Inhalation Anesthetics


  Inhaled gases are used primarily for maintenance of anesthesia after administration of an IV agent
• Inhalation Anesthetics


  • Activate K+ channels
  • Block Na+ channels
  • Disrupt membrane lipids
  • Minimum alveolar concentration (MAC) determines potency
  • In general, all general anesthetics increase the cellular threshold for firing, thus decreasing neuronal activity
  • Higher MAC = lower potency, Nitrous oxide - least potent, Methoxyflurane - most potent
• Ether (CH3CH2-O-CH2CH3) is spontaneously explosive, irritant to respiratory tract, high incidence of nausea and vomiting during induction and post-surgical emergence
• Nitrous Oxide


  Laughing gas, rapid onset, good analgesia, used for short procedures and in combination with other anesthetics, supplied in blue cylinders, MAC: 100%, side effects: megaloblastic anemia, hypoxia
• Halothane


  Volatile liquid, relatively potent, narrow margin of safety, low blood-gas partition coefficient, less analgesia and muscle relaxation, choice for pediatric anesthesia, side effects: hepatotoxic, reduced cardiac output, increased sensitization of myocardium to catecholamines, malignant hyperthermia
• Methoxyflurane


  Used for obstetrics practice, safe for pregnant, most potent, MAC: 0.16%, side effect: nephrotoxicity
• Enflurane


  Similar to halothane, potent and moderate speed of induction, associated with reversible reduction in glomerular filtration rate, advantages: rapid and smooth adjustment of depth of anesthesia, less arrhythmias, nausea, post-operative shivering, side effects: seizure, blood loss, malignant hyperthermia
• Isoflurane


  Volatile liquid, provides adequate muscle relaxation greater than halothane, stimulates airway reflexes, can increase secretions, coughing and laryngospasm, decreases mean arterial pressure, advantages: depth of anesthesia is easily controlled, no hepatic and renal toxicity, preferred in neurosurgery, arrhythmias are uncommon
• Desflurane


  Fluorine atom replaces chlorine atom on the alpha ethyl carbon of isoflurane, typically administered following IV induction, recovery twice as rapid as isoflurane, preserved cardiac output, advantages: no malignant hyperthermia, no seizure activity, no renal toxicity
• Sevoflurane


  Low blood solubility, high potency allows excellent anesthesia control, same pharmacological properties as desflurane, very commonly used, increased fluoride levels rarely associated with kidney or renal damage, metabolized to substance A, sweet smell, for children
• Intravenous Anesthetic Agents

  • Thiopental
  • Propofol
  • Barbiturates
  • Benzodiazepines
  • Opioids
  • Etomidate
  • Ketamine
  • Dexmetomidine
• Intravenous Anesthetics


  Cause rapid induction, may be used for short procedures or administered as infusions to help maintain anesthesia during longer cases
• Propofol


  IV sedative/hypnotic used for induction and/or maintenance of anesthesia, widely used and has replaced thiopental as the first choice, poorly water soluble, supplied as an emulsion, interacts with GABA-A channels, onset is smooth and occurs 30-40 seconds after administration, rapidly equilibrate between plasma and brain, plasma levels decline rapidly, prolonged period of hepatic metabolism and renal clearance, half-life: 2-4 hours, depresses the CNS accompanied by excitatory phenomena, can decrease blood pressure without depressing the myocardium, reduces intracranial pressure, does not provide analgesia, has antiemetic effects
• Opioids


  Commonly combined with other anesthetics, induce analgesia more rapidly than morphine, given IV, epidurally or intrathecally, not good amnesics, can cause hypotension, respiratory depression, muscle rigidity, postanesthetic nausea and vomiting, antidote: naloxone
• Etomidate


  A hypnotic agent used to induce anesthesia, lacks analgesic activity, water solubility is poor, formulated in a propylene glycol solution, induction is rapid, short acting, little or no effect on the heart and circulation, usually only used for patients with CAD or cardiovascular dysfunction, adverse effects: decreased plasma cholesterol and aldosterone levels, injection site reaction and involuntary skeletal muscle movements are not uncommon, given benzodiazepines and opioids
• Ketamine


  Short-acting, nonbarbiturate anesthetic, induces a dissociated state in which the patient is unconscious but may appear awake and does not feel pain, blocks glutamate receptors, provides sedation, amnesia, and immobility, stimulates central sympathetic outflow causing stimulation of the heart with increased blood pressure and cardiac output, potent bronchodilator, beneficial in patients with hypovolemic or cardiogenic shock and in asthmatics, contraindicated in stroke and hypertensive patients, used for children and elderly adults for short procedures, can cause hallucinations and dream-like state similar to PCP
• Dexmedetomidine


  Sedative used in intensive care settings and surgery, can provide sedation without respiratory depression, alpha2 receptor agonist in certain parts of the brain, has sedative, analgesic, sympatholytic and anxiolytic effects that blunt many cardiovascular responses
• Local Anesthetics


  Drugs that are used to prevent or relieve pain in specific regions of the body without loss of consciousness, block nerve conduction of sensory impulses and in high concentrations, motor impulses from the periphery to the CNS, blockade of voltage-gated sodium channels, weak bases
• Local Anesthetics


  • Intact consciousness, mechanism of action: Na+ channel blockers, decrease depolarization, increase repolarization, CNS stimulation, CNS depression, (-) inotropic effect (bradycardia), blood vessels: vasodilation (except cocaine)
• Local Anesthetics - Benzoic acid derivatives (Esters)


  • Tetracaine
  • Procaine
  • Cocaine
  • Benzocaine
• Local Anesthetics - Aniline derivatives (Amides)


  • Bupivacaine
  • Lidocaine
  • Mepivacaine
  • Ropivacaine
  • Articaine