PCOL 211

• For centuries, humankind has relied on natural medicines and physical methods to control surgical pain
• Natural medicines used for anesthesia

  • cannabis
  • henbane
  • mandrake
  • opium poppy
• Opium Poppy (Papaver somniferum)


  Meaning: Poppy of sleep – longest standing substance used as anesthesia

  
• Joseph Priestley's discovery of nitrous oxide (N2O)


  1772
• Humphry Davy experimented with the psychotropic properties of N2O


  1790
• Surgery was performed under ether anesthesia


  1842
• First public demonstration of surgical general anesthesia. Considered to be the start of a new era of anesthesia

  1846
• Local anesthesia


  Loss of sensation in a limited region of the body. A drug that causes reversible local anesthesia and a loss of nociception
• Albert Niemann isolated cocaine (from coca plant)


  1859
• Carl Koller discovered that Cocaine have local anesthetic properties and soon became widely used in many types of surgery


  1884
• Mechanism of action of local anesthetics

  Block initiation & propagation of action potential (AP) by preventing voltage-gated Na+ channels. This cause a disruption of afferent neural traffic (sensory)- neurons that carry information from periphery to the brain
• Methods of administration of local anesthesia

  • Surface anesthesia
  • Epidural anesthesia
  • Spinal anesthesia/ Intrathecal
  • Sympathetic block
• Common routes in Dental surgery

  • Nerve block
  • Field block
  • Infiltration anesthesia
• Chemistry of local anesthetics

  Weak bases usually made available clinically as salts (commonly marketed as hydrochloride salts (pH 4.0– 6.0)) to increase solubility and stability. Consist of: 1) lipophilic group - aromatic ring 2) intermediate chain - ester or amide , membrane penetration 3) ionizable group – (amine group) responsible for Na channel blockade
• Ideal characteristics of local anesthetics

  Rapid/fast onset, Long Duration of Action, Reversible & selective blockade of sensory nerves without motor blockade, Minimal local tissue irritation & no systemic toxicities (cardiac & CNS)
• Pharmacokinetics of local anesthetics

  Absorption is higher in highly vascular areas. All local anesthetics cross the BBB & the placenta. Vasoconstrictors like epinephrine are used to slow absorption and prolong action. Amide LAs are metabolized in the liver, ester LAs are hydrolyzed rapidly in plasma

  
• Ester local anesthetics


  • Cocaine
  • Tetracaine
  • Butacaine
  • Benzocaine
  • Hexylcaine
  • Procaine
  • Chlorprocaine
  • Propoxycaine
• Amide local anesthetics


  • Lidocaine
  • Bupivacaine
  • Prilocaine
  • Dibucaine
  • Mepivacaine
  • Etidocaine
  • Ropivacaine
  • Articaine
• EMLA cream


  Combination of lidocaine (2.5%) and prilocaine (2.5%) that permits anesthetic penetration of the keratinized layer of skin, producing localized numbness
• Adverse effects/toxicity of local anesthetics

  • Direct neurotoxicity
  • Central Nervous System effects (early symptoms, convulsions)
  • Cardiovascular effects (depression of cardiac contraction, hypotension)
  • Hematologic effects (methemoglobinemia)
• Transient Radicular Irritation


  Toxicity resulting from pooling of high concentrations of the local anesthetic in the cauda equina (nerve located at the end of the spine), causing pain in the buttocks, posterior thigh, or legs
• General anesthesia


  Acts on the brain, medulla and spinal cord, causing reversible loss of consciousness and insensibility to painful stimuli. Characterized by 5 primary effects: unconsciousness, amnesia, skeletal muscle relaxation, inhibition of autonomic reflexes
• General anesthesia can block both explicit and implicit memory
• Minimum Alveolar Concentration (MAC)

  The partial pressure of an inhalational anesthetic in the alveoli of the lungs at which 50% of a population of non-relaxed patients remained immobile at the time of a skin incision
• Ablation of memory


  • By blocking nerve impulses in hippocampus, amygdala, prefrontal cortex, and regions of the sensory and motor cortices
• General anesthesia


  Can block both explicit and implicit memory at (0.2-0.4 MAC)
• Explicit memory


  Information that you have to consciously work to remember
• Implicit memory


  Information that you remember unconsciously and effortlessly
• Skeletal muscle relaxation


  • Anesthetic immobility is mediated primarily by neural inhibition within the spinal cord
  • 1.0 MAC as the partial pressure of an inhalational anesthetic in the alveoli of the lungs at which 50% of a population of non-relaxed patients remained immobile at the time of a skin incision
• Inhibition of autonomic reflexes


  • Reduction of certain autonomic reflexes (gag reflex, tachycardia, vasoconstriction)
• Gag reflex


  • Contraction of the throat that happens when something touches the roof of the mouth
  • Helps to prevent choking and keeps us from swallowing potentially harmful substances
  • General anesthesia can reduce this reflex to facilitate intubation & ventilation
• Analgesia


  • Insensibility to painful stimuli
  • Loss of sensation of pain that results from an interruption in the nervous system pathway between sense organ and brain
• Properties of ideal anesthetics


  • Rapid and pleasant anesthetic induction and recovery
  • Induce anesthesia smoothly and rapidly while allowing for prompt recovery after its administration is discontinued
  • Rapid changes in anesthetic depth
  • Adequate relaxation of skeletal muscles
  • Wide margin of safety
  • Absence of toxic effects or other adverse properties in normal doses
• None of the currently available anesthesia when used alone can achieve all these desired effects
• The modern practice of anesthesiology relies on the use of combinations of intravenous and inhaled drugs (balanced anesthesia techniques) to take advantage of the favorable properties of each agent while minimizing their adverse effects
• Stages of anesthesia


  • Stage 1 (Cortical Stage)
  • Stage 2 (Delirium/Combative Stage)
  • Stage 3 (Surgical anesthesia)
  • Stage 4 (Medullary depression)
• Stage 1 (Cortical Stage)


  • Starts from beginning of anesthetic inhalation and lasts up to the loss of consciousness
  • The patient initially experiences analgesia without amnesia. Later in stage I, both analgesia and amnesia are produced
• Stage 2 (Delirium/Combative Stage)


  • During this stage, the patient often appears to be delirious and excited but definitely is amnesic
  • Respiration is irregular both in volume and rate
  • Retching and vomiting may occur
  • Heart rate and blood pressure increases due to sympathetic stimulation
• Stage 3 (Surgical anesthesia)

  • This stage begins with the recurrence of regular respiration and extends to complete cessation of spontaneous respiration
  • Plane 1: Roving eye balls. This plane ends when eyes become fixed
  • Plane 2: Loss of corneal and laryngeal reflexes
  • Plane 3: Pupil starts dilating and light reflex is lost
  • Plane 4: Intercostal paralysis, shallow abdominal respiration, dilated pupil
  • The most reliable indication that stage III has been achieved is loss of the eyelash reflex and establishment of a regular respiratory pattern
• Stage 4 (Medullary depression)

  • Severe depression of the vasomotor center in the medulla as well as the respiratory center
  • Without full circulatory and respiratory support, death rapidly ensues
  • Cessation of breathing to failure of circulation and death. Pupil is widely dilated, muscles are totally flabby pulse is thready or imperceptible and BP is very low