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6. Special Techniques


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6. Special Techniques

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