Local Anesthesia

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Local Anesthesia
Megan McQuade CVM 2005
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Local Anesthesia

• Local Anesthetics
• Why provide local anesthesia?
• Pharmacology
• Mechanism of Action
• Toxicity
• Agents
• Applications
• Topical
• Infiltration
• Regional

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Local Anesthesia

• Why provide local anesthesia?
• Better control of pain (multi-modal analgesia)
• Lower analgesic dose requirements
• Minimize incidence of chronic pain
• Decrease cardiovascular depression associated
  with general anesthesia
• Provide pre-emptive analgesia
• Inexpensive
• Minimal side effects
• Fast onset
• Ability to use with other analgesics

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Preemptive Analgesia

• Long-term changes occur within the peripheral and
  central nervous systems following noxious input.
• These changes have been termed peripheral
  sensitization and central sensitization.
• Peripheral sensitization reduction in the
  threshold of nociceptor afferent peripheral
  terminals
• Central sensitization an activity-dependent
  increase in the excitability of spinal neurons

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Preemptive Analgesia

• They contribute to the post-injury pain
  hypersensitivity found post-operatively.
• This manifests as an increase in the response to
  noxious stimuli and a decrease in the pain
  threshold, both at the site of injury and in the
  surrounding uninjured tissues.
• Recent evidence suggests that preoperative
  regional administration of local anesthetics can
  preempt postoperative pain by preventing the
  establishment of central sensitization.

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Local Anesthetics

• Desirable Characteristics
• Good penetration into body tissues
• High potency so that low concentrations can be
  used
• Rapid onset
• Long duration of action
• Low systemic toxicity
• Not irritating to nerves or other tissues
• Reversible action

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Pharmacology

• Local anesthetic molecules consist of three parts
• Lipophilic aromatic group
• allows penetration into nerve
• Intermediate alkyl chain
• Hydrophilic tertiary amine
• provides water-solubility

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Mechanism of Action

• Local anesthetics are membrane stabilizing agents
• They enter and occupy the membrane channels
  through which ions normally flow
• The most immediate and apparent effect is the
  prevention of the inflow of Na which blocks
  subsequent ionic flow
• Nerve cell depolarization is prevented, slowing
  or stopping the conduction of nerve impulses

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Site of Action

• The drug must diffuse through several physical
  barriers before it reaches individual neurons
  within a peripheral nerve
• Small fibers are affected more rapidly than large
  fibers.
• The larger the nerve, the longer it will take to
  diffuse into the center of the nerve fiber and
  complete the blockade
• Non-myelinated fibers are blocked more rapidly
  than myelinated.
• In myelinated nerves the areas of penetration are
  limited to the nodes of Ranvier

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Types of Nerve Fibers

• Myelinated A-fibers
• a motor, proprioception 12-20mm
• b motor, touch 5-12mm
• g muscle spindles 3-6mm
• d pain, temperature 2-5mm
• Myelinated B-fibers lt3mm
  (preganglionic sympathetic)
• Non-myelinated C-fibers
• Pain 0.4-1.2mm
• Temperature 0.3-1.3mm

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Priority of Blockade

• Nerve fibers
• 1. B-fibers and C fibers
• 2. Ad -fibers
• 3. Aa fibers

• Sensation
• 1. Pain
• 2. Cold
• 3. Warmth
• 4. Touch
• 5. Joint
• 6. Deep pressure

(sensation returns in the reverse order of
blockade)
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Absorption

• Poor absorption through intact skin
• Variable absorption from
• Injured skin
• Mucous membranes
• Serosal surfaces
• Respiratory epithelium
• Intramuscular deposition
• Subcutaneous deposition
• Intravenous administration

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Toxicity

• Usually due to inadvertent rapid intravenous
  injection or excessive extravascular dose.
• Biphasic response of excitation followed by
  depression
• Vasodilation
• Cardiovascular depression
• Clinical signs
• Hypotension
• Cardiovascular collapse
• Seizures
• Methemoglobinemia
• Allergic reaction

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Lidocaine and Bupivicaine
 
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Applications

• Topical anesthesia
• Local infiltration
• Peripheral nerve blocks
• Head
• Forelimb
• Thorax
• Hindlimb
• Intra-articular

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General Considerations

• Use sterile solutions and equipment
• Do not inject into inflamed areas
• Use as small a gauge needle as practical
• Aspirate for blood before injecting
• Use lowest effective concentration
• Use smallest possible amount

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Topical Anesthesia

• Lidocaine is effective when placed topically on
  mucous membranes (not on intact skin).
• Mouth
• Tracheobronchial tree
• Esophagus
• Genitourinary tract
• Proparacaine, a very short acting local
  anesthetic is used to desensitize the cornea.

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Splash Block

• After induction of anesthesia one drop of 2
  Lidocaine is dripped directly over the arytenoids
  in cats
• After one minute intubation occurs without
  laryngospasm.

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Local Infiltration

• The solution is injected extravascularly using a
  22 to 25 gauge needle.
• It is slowly injected, after aspirating for
  blood, advancing the needle along the proposed
  incision line.
• This technique is used for removal of small
  superficial tumors, debridement and suturing of
  wounds, laparotomies, and cesarean sections.

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Field Block

• A diamond-shaped block for removal of small
  tumors or lesions.

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Ring Block

• A field block in which the soft tissues around an
  extremity are infiltrated to induce anesthesia
  distally.
• Onychectomies
• Tail amputations
• Caution with volume of local used in puppies
• Do not use lidocaine combined with epinephrine.
  Vasoconstriction can lead to necrosis

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Radial-Median-Ulnar Nerve Block

• 0.1-0.3ml of local anesthetic at each site
• Medial to the accessory carpal pad on the palmar
  surface
• Medial and palmar branch of the ulnar nerver
• Lateral and proximal to the accessory carpal pad
  on the palmar surface
• Dorsal branch of the ulnar
• Dorso-medial aspect of the proximal carpus
• Radial nerve

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Regional Anesthesia of the Head

• Infraorbital B. Maxillary C. Ophthalmic
• D. Mental E. Mandibuloalveolar

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Anesthesia of the Upper Lip and Nose

• Blocking the infraorbital nerve provides
  anesthesia to the upper lip and nose, roof of
  nasal cavity and skin ventral to the infraorbital
  foramen.
• The needle is inserted either intraorally or
  extraorally 1 cm cranial to the bony lip of the
  infraorbital foramen. It is then advanced to the
  infraorbital foramen.
• The infraorbital foramen can be palpated between
  the dorsal border of the zygomatic process and
  the gum of the canine tooth.

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Anesthesia of the Maxilla

• Blocking the maxillary nerve provides anesthesia
  to the maxilla, upper teeth, nose, and upper lip.
• The needle is inserted through the skin at a 90
  degree angle ventral to the border of the
  zygomatic process and approximately 0.5cm caudal
  to the lateral canthus of the eye.

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Anesthesia of the Lower Lip

• Blocking the mental nerve provides anesthesia to
  the lower lip.
• The needle is inserted over the mental nerve,
  rostral to the middle mental foramen at the level
  of the second premolar tooth.

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Anesthesia of the Eye

• Blocking the lacrimal, zygomatic, and ophthalmic
  nerves provides anesthesia to eye, orbit,
  conjunctiva, eyelids, and forehead skin.
• The needle is inserted ventral to the border of
  the zygomatic process at the lateral canthus of
  the eye. It should be approximately 0.5 cm
  cranial to the anterior border of the vertical
  portion of the ramus of the mandible. The needle
  is advanced medial to the ramus in a mediodorsal
  and somewhat caudal direction until it reaches
  the orbital fissure.

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Anesthesia of the Mandible

• Blocking the inferior alveolar branch of the
  mandibular nerve provides anesthesia to the cheek
  teeth, canine, incisors, skin, and mucosa of the
  chin and lower lip.
• The needle is inserted at the lower angle of the
  jaw approximately 1.5 cm rostral to the angular
  process. It is advanced 1.5 cm dorsally against
  the medial surface of the ramus of the mandible
  to the palpable lip of the mandibular foramen.

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Anesthesia of the Forelimb

• Brachial Plexus Block
• Blocking the radial, median, ulnar, and axillary
  nerves provides anesthesia within or distal to
  the elbow.
• The needle is inserted at the point of the
  shoulder, medial to the shoulder joint and
  directed toward the costochondral junction, and
  parallel to the vertebral column. The anesthetic
  is injected slowly as the needle is withdrawn.
• Anesthesia occurs within 20 minutes and can last
  for up to 2 hours.

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Brachial Plexus Block
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Anesthesia of the Thorax

• Intercostal nerve blocks are performed after
  lateral thoracotomies.
• A minimum of two adjacent intercostal spaces both
  cranial and caudal to the incision must be
  blocked due to overlap of nerve supply.
• The needle is inserted at the caudal border of
  the rib near the intervertebral foramen.

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Anesthesia of the Thorax

• Intrapleural block
• A splash block of the pleura can be performed
  during thoracotomies.
• Local anesthetic can also be injected into a
  chest tube post-op. The animal is then often
  positioned so that the side with the incision is
  dependent.

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Anesthesia of the Hindlimb

• Selective blockade of the saphenous, common
  peroneal, and tibial nerves will provide
  anesthesia distal to the hip.

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References

• Atchinson, W. (2002) Pharmacology of Local
  Anesthesia. Veterinary Pharmacology PHM 556.
  Michigan State University.
• Greene, S. A. (2003) Local Anesthetic
  Techniques. Proceedings of the Western
  Veterinary Conference. www.vin.com
• Ilkiw, J. (2001) Local Anesthesia and Local
  Anesthetic Techniques. World Small Animal
  Veterinary Association World Congress
  Proceedings. www.vin.com
• Lumb, W. V., Jones, E. W. (1984). Veterinary
  Anesthesia. Philadelphia Lea Febiger.
• Muir, W. W., Hubbell, J. A., Skarda, R. T.,
  Bednarski, R. M. (2000). Handbook of Veterinary
  Anesthesia, Third Edition. St. Louis Mosby.
• Stein, R. (2003) Ring Blocks. Veterinary
  Anesthesia Support Group. http//www.vasg.org/ring
  _blocks.htm
• Warren, R. G. (1983). Small Animal Anesthesia.
  St. Louis Mosby.

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