6. Special Techniques
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  • Lidocaine and bupivacaine are the drugs most commonly used in veterinary medicine.
  • Lidocaine patches: effective for pain management of wounds or incisions. Ethyl chloride spray: skin cooling may cause frostbite; used for skin biopsies. Eutectic mixture: lidocaine and prilocaine cream (EMLA cream); applied to shaved skin and covered for 10 minutes; used for minor procedures such as catheterization. Splash block: anesthetic sprays of lidocaine or direct application of bupivacaine in soaked sponges to wounds or open surgical sites. Bupivacaine is instilled through a chest tube during thoracic surgery. Mucous membrane application: conjunctiva, nose, mouth, larynx, or lining of urethra via topical sprays, drops, or ointments.
  • Epinephrine causes local vasoconstriction. Use lidocaine without epinephrine on incisions, ears, tails, or digits; in animals with cardiac disease; and for intravenous techniques. Local anesthetic injections may be painful in an awake patient so sodium bicarbonate can be added to decrease the pain.
  • PPV ensures patient inhales adequate oxygen and exhales adequate carbon dioxide.
  • Animals at risk for developing these problems are those under general anesthesia for >90 minutes, obese animals, those who received neuromuscular blocking agents, those with a preexisting lung disease, those with recent head trauma, those undergoing surgical procedures that involve the chest or diaphragm, horses, and adult ruminants.
  • A mask cannot be used for manual or mechanical ventilation because air being delivered will not meet the patient’s oxygen requirements and some of the air may enter the stomach, which increases the risk of vomiting.
  • The anesthetist must be careful not to overinflate the lungs.
  • When weaning a patient off a ventilator, the anesthetic is turned off and the patient breathes pure oxygen; the number of bmp provided by the ventilator gradually decreases until spontaneous breathing takes over again. Sometimes the patient must be coaxed into spontaneous breathing by pinching the toe pads, gently rubbing the thorax or abdomen (small animals), or twisting an ear (large animals).
  • Neuromuscular blocking agents inhibit normal reflexes. Animals cannot blink so ophthalmic lubricant must be used to prevent corneas from drying. Hypothermia can also be a problem because it slows metabolism and delays anesthesia recovery. Order of appearance of paralysis: first facial and neck paralysis, then tail, limb, and abdominal muscle paralysis, and finally intercostal muscles and diaphragm paralysis.
  • Adverse effects include bradycardia and increased bronchial and salivary secretions.

6. Special Techniques Presentation Transcript

  • 1. Local Anesthesia Assisted and Controlled Ventilation Neuromuscular Blocking Agents Special Techniques Chapter 6
  • 2. Local Anesthesia
    • Use of a chemical agent on sensory neurons to disrupt nerve impulse transmission leading to temporary loss of sensation
    • Indications:
      • Tractable animal
      • General anesthesia is undesirable or high risk
      • Means to deliver general anesthesia are not available
  • 3. Local Anesthesia (Cont’d)
    • Advantages
      • Low cardiovascular toxicity
      • Inexpensive
      • Excellent pain control immediately postoperatively
      • Minimum patient recovery time
    • Uses
      • Ruminant obstetric and abdominal procedures
      • Complement standing sedation in horses
      • In conjunction with general anesthesia for pain control
  • 4. Local Anesthetic Agents
    • Skin infiltration and mucous membrane application:
      • Lidocaine
        • Administer at 0.5% to 2%
        • Dilute with sterile saline if necessary
      • Bupivacaine
        • Administer at 0.25% or 0.5%
        • Slower onset of action; long duration
      • Mepivacaine
      • Procaine
    • Ophthalmic use: tetracaine, proparacaine
  • 5. Characteristics of Local Anesthetics
    • Not general anesthetics
      • Don’t affect the brain and have no sedative effect
    • Few cardiovascular or respiratory effects
      • Better for high-risk patients
    • Exert action close to site of injection
      • Not distributed throughout the body
    • Don’t normally cross the placenta
      • Used for cesarean sections and obstetric manipulations
  • 6. Local Anesthetic Mechanism of Action
    • Drugs that affect primarily sensory neurons
      • Drug must be placed in proximity to the neuron
      • Blocks sodium channels and prevents generation of electrical impulses (stops depolarization)
      • Reversal occurs as drug is absorbed into the local circulation
      • Metabolized in liver
  • 7. Local Anesthetic Mechanism of Action (Cont’d)
    • Drugs that affect primarily motor neurons
      • Cause temporary local paresis or paralysis
      • Seen in conjunction with sensory neuron loss of sensation
        • e.g., Epidural block
  • 8. Local Anesthetic Mechanism of Action (Cont’d)
    • Drugs that affect the autonomic nervous system
      • Not always desirable
      • Affect sympathetic neurons between the brain and blood vessels and internal organs
      • Sympathetic blockade = temporary loss of function
      • Sympathetic blockade may affect the heart if local anesthetic diffuses into the thoracic spinal cord
      • Peripheral effect: vasodilation leading to local flushing and increased skin temperature
      • Vasodilation may lead to hypotension
  • 9. Routes of Administration of Local Anesthetics: Topical
    • Applied directly to intact skin
      • Drug molecules can penetrate the epidermis to reach the dermis
      • Less pain relief and shorter duration of effect than if administered by infiltration
    • Clinical applications
      • Lidocaine patches
      • Ethyl chloride spray
      • Eutectic mixture
      • Splash block
      • Bupivacaine instilled through a chest tube
      • Mucous membrane application
  • 10. Routes of Administration of Local Anesthetics: Infiltration
    • Drug injected into tissues in proximity to the target nerve
    • Lidocaine with or without epinephrine is most common
    • Intradermal, subcutaneous, or intramuscular administration
    • Provides analgesia for surgery involving superficial tissues
  • 11. Routes of Administration of Local Anesthetics: Infiltration (Cont’d)
    • Procedure
      • Surgical prep
      • Small-gauge needle (20- or 25-gauge)
      • Amount of drug used varies with species and procedure location
      • Onset of action (lidocaine) = 3-5 minutes
      • Test effectiveness prior to surgery: gently prick skin with a 22-gauge needle
  • 12. Infiltration of Local Anesthetics
    • Effectiveness of local anesthetics
      • Deep tissues are not affected if injection is superficial
      • Scar tissue, fibrous tissue, fat, edema, and hemorrhage impede diffusion of drug
      • Inflammation or infection decrease effectiveness
    • Duration of effect depends on drug used and rate of absorption by local blood vessels
      • Epinephrine may be added to local anesthetic to affect rate of absorption
    • Use of lidocaine without epinephrine
  • 13. Infiltration Techniques: Nerve Blocks
    • Injecting local anesthetic in proximity to a nerve
    • Desensitizes a particular anatomic site
    • Decreases amount of general anesthesia needed
    • Provides short-term analgesia postoperatively
  • 14. Infiltration Techniques: Nerve Blocks (Cont’d)
    • Most commonly used in large animals
      • Lameness examinations in horses
      • Cornual blocks for dehorning cattle
      • Paravertebral blocks for abdominal or obstetric procedures in cattle
      • Dental blocks in dogs and cats
      • Intercostal nerve blocks for chest surgery
      • Limb amputations
      • Cat declaws
  • 15. Infiltration Techniques: Line Blocks
    • A continuous line of local anesthetic placed between the target area and the spinal cord
    • Ring block: line of local anesthetic completely encircles an anatomic part
    • Used in food animal and equine surgery
      • Teat surgery or wound repair
    • L-block: a line block used for laparotomy surgery in ruminants
  • 16. Infiltration Line Blocks (Cont’d)
  • 17. Other Infiltration Techniques
    • Intraarticular administration
      • Local anesthetics injected directly into a joint
    • Regional anesthesia
      • Local anesthetic injected into a nerve plexus or in proximity to the spinal cord
      • Affects a larger area such as an entire limb or caudal portion of the body
  • 18. Other Infiltration Techniques (Cont’d)
    • Paravertebral anesthesia (ruminants only)
      • For standing laparotomies as an alternative to an L-block
      • Local anesthetic blocks the dorsal and ventral branches of spinal nerves T13-L2
      • Provides wide, uniform area of anesthesia
      • May produce hind limb weakness or scoliosis
  • 19. Infiltration Techniques: Epidural Anesthesia
    • A regional anesthetic procedure used in small and large animals
    • Blocks sensation and motor control of the rear, abdomen, pelvis, tail, pelvic limbs, and perineum
    • Uses:
      • Tail amputation Anal sac removal
      • Perianal surgery Urethrostomies
      • Obstetric manipulations Cesarean sections
      • Some rear limb operations
  • 20. Infiltration Techniques: Epidural Anesthesia (Cont’d)
    • Drug choice determined by procedure
      • Local anesthesia
        • 2% lidocaine or 0.5% bupivacaine
      • Postoperative pain control
        • Opioid (morphine)
      • Opioid and local anesthetic mixture
        • Anesthesia and analgesia
      • Opioid and alpha 2 -agonist mixture
        • Epidural anesthesia in horses and cattle
  • 21. Epidural Anesthesia (Cont’d)
    • Location
      • Drug deposited in epidural space between spinal cord and vertebrae
      • Spinal nerves pass through this space and are affected by drugs placed here
      • Dogs: between the last lumbar vertebra (L7) and the sacrum
  • 22. Epidural Anesthesia (Cont’d)
    • Effects:
      • Local anesthetics block sensory and motor neurons
      • Sensory block controls pain
      • Motor block may affect tail and limb mobility
      • Opioids have minimal effect on motor neurons
  • 23. Infiltration Techniques: Bier Block
    • Intravenous regional anesthesia
    • Provides short-term local anesthesia to a limb
    • Lidocaine only
    • Tourniquet applied to proximal superficial vein
    • Lidocaine injected into distal superficial vein
    • Effect lasts less than 1 hour
  • 24. Systemic Constant Rate Infusion
    • Lidocaine administered to healthy anesthetized animals
    • Reduces dose of general anesthesia or analgesic required
    • Used in dogs, cats, and horses
  • 25. Adverse Effects of Local Anesthetics
      • Loss of motor neuron function
      • Loss of nerve function with direct injection
      • Tissue irritation
      • Paresthesia during recovery
      • Allergic reactions from rash or hives to anaphylactic shock
      • Systemic toxicity
      • Trauma to spinal cord or cauda equina
      • Serious toxicity and death with infiltration into the cranial portion of the spinal cord
      • Sympathetic nerve block with infiltration into the cervical or thoracic spinal cord
  • 26. Assisted and Controlled Ventilation
    • Positive-pressure ventilation (PPV)
    • Assisted ventilation
      • Anesthetist delivers an increased volume of air or oxygen/anesthetic gases to the patient
      • Patient initiates inspiration
    • Controlled ventilation
      • Anesthetist delivers all air required by the patient
      • No spontaneous respiratory effort by the patient
      • Anesthetist controls respiratory rate and volume and pressure of gas inhaled
  • 27. Normal Ventilation
    • Physical movement of air into and out of the lungs and upper respiratory passageways
    • Active phase
      • Inhalation
      • Initiated by increased PaCO 2 detected by respiratory center in the brain
      • Thoracic muscles (diaphragm and intercostals) movement causes thoracic cavity to expand
      • Air is pulled into the breathing passages and goes to the alveoli
      • When lungs are “full” the respiratory center stops inhalation
  • 28. Normal Ventilation (Cont’d)
    • Passive phase
      • Exhalation
      • No active muscle movement
      • PaCO 2 begins to rise until respiratory center starts inhalation again
      • Normally twice as long as inspiration
      • Normal tidal volume = 10-15 mL/kg
  • 29. Ventilation in Anesthetized Animals
    • Differs from ventilation in awake animals
    • Reduced amount of air entering and leaving the lungs
    • Tranquilizers and general anesthetics
      • Decrease responsiveness of breathing center to carbon dioxide levels so inhalation doesn’t occur as often
      • Relax intercostals muscles and diaphragm so the chest doesn’t fully expand (V T is reduced)
  • 30. Potential Problems
    • Hypercarbia
      • Related to the breakdown of carbon dioxide to bicarbonate ions and hydrogen ions, which can lead to respiratory acidosis
    • Hypoxemia
      • Related to less oxygen entering the lungs to be absorbed into the blood
    • Atelectasis
      • Related to decreased V T so alveoli don’t fully expand on inhalation, which can lead to partial collapse of the alveoli in some sections of the lung
  • 31. Types of Controlled Ventilation (PPV)
    • Patient is intubated and connected to anesthetic machine
    • Manual ventilation or bagging
      • Anesthetist bags patient every 2-5 minutes
    • Intermittent mandatory ventilation
      • Patient requires bagging throughout the anesthetic period
    • Mechanical ventilation
      • Lungs are filled with oxygen by pressure of gas from a ventilator
  • 32. Manual Ventilation
    • Lungs are filled with oxygen by pressure of gas entering airways
      • Anesthetist is squeezing the reservoir bag
      • Pop-off valve is fully or partially closed
      • Exhalation is passive and occurs when positive pressure is discontinued and pop-off valve is fully opened, which allows the lungs to empty
      • Bag one to two breaths every 2-5 minutes (sighs)
      • Bag is squeezed for 1-1.5 seconds (inhalation time)
      • Pressure manometer reading: <20 cm H 2 O (small animals); <40 cm H 2 O (large animals)
  • 33. Intermittent Mandatory Manual Ventilation (Cont’d)
    • Periodic bagging doesn’t provide enough ventilation
    • Animals with preexisting heart or lung disease; diaphragmatic hernias
      • Patients show shallow breaths and respiratory rate <6 bpm
      • May be used prior to placing animal on the ventilator
      • Bagging superimposes positive pressure over patient’s spontaneous breathing efforts, which will stop in about 1 minute
  • 34. Intermittent Mandatory Manual Ventilation (Cont’d)
    • Assisted ventilation rate
      • Initially 8-20 bpm depending on size of the patient
      • 6-12 bpm after control of respiration has been established
    • Wean patient off assisted ventilation near the end of the surgical procedure
  • 35. Mechanical Ventilation
    • Similar to intermittent mandatory manual ventilation
    • Breathing is controlled by the ventilator, which replaces the reservoir bag and becomes part of the breathing circuit
      • Ventilator bellows automatically compresses and forces oxygen and anesthetic into the patient’s airways
      • Gases delivered on inspiration: pressure cycle, volume cycle, or time cycle ventilators
  • 36. Mechanical Ventilation (Cont’d)
    • Normal beginning settings
      • RR = 6-12 bpm
      • Duration of inspiration = 1-1.5 seconds
      • Duration of expiration = 2-6 seconds
      • Inspiratory/expiratory ratio = 1:2 to 1:3
    • Ventilator settings vary with the needs of the patient
  • 37. Ventilators
  • 38. Risks of Controlled Ventilation
    • Ruptured alveoli
    • Decreased cardiac output
    • Respiratory alkalosis
    • Ventilator delivers more inhalant anesthetic to patient
    • Anesthetist monitoring
  • 39. Neuromuscular Blocking Agents
    • Muscle-paralyzing agents
      • Animals remain conscious and feel pain
    • Limited use in veterinary medicine
      • Animals on mechanical ventilation
      • Orthopedic surgery and ophthalmic surgery
      • Cesarean sections
      • Facilitate difficult intubation
      • Balanced anesthesia techniques
    • Administered when animal is unconscious and respiration is controlled by intermittent mandatory mechanical ventilation
  • 40. Neuromuscular Blocking Agents: Depolarizing
    • Interrupt normal impulse transmission from motor neuron to muscle synapse
    • Succinylcholine
    • Single surge of activity at neuromuscular junction
    • Followed by period when the muscle end plate is refractory to further stimulation
    • Fast onset; short duration of effect
    • Useful for rapid intubation
  • 41. Neuromuscular Blocking Agents: Nondepolarizing
    • Interrupt normal impulse transmission from motor neuron to muscle synapse
    • Gallamine, pancuronium, atracurium besylate, cisatracurium
    • Block receptors at end plates
    • No initial surge of activity
  • 42. Neuromuscular Blocking Agents
    • Concurrent use with other drugs may affect potency
    • Administered by slow IV injection
    • Effect seen within 2 minutes and lasts 10-30 minutes
    • Repeated doses or continuous infusion can be used with some agents
    • Makes assessment of anesthetic depth difficult
    • Only voluntary skeletal muscles are affected
  • 43. Neuromuscular Blocking Agents: Reversal Drugs
    • Nondepolarizing agents reversed with a anticholinesterase agent
      • Edrophonium, neostigmine, pyridostigmine
      • Maintain at light anesthesia until reversal is complete
      • Make sure spontaneous breathing has started or support respiration
      • Pretreat patient with atropine or glycopyrrolate to prevent adverse effects
    • Depolarizing agents have no effective reversal drugs