Contributors: John Dodam, Sheilah Ann Robertson, Enzo Vettorato, Claire Waters

 Species: Feline   |   Classification: Miscellaneous

Definition, advantages and disadvantages

Definition

  • Local anesthesia or loco-regional anesthesia is a technique to induce a reversible absence of sensation (anesthesia) in a part of the body (local).
  • Depending on the loco-regional anesthetic technique used, local anesthetic drugs decrease transduction or transmission of a noxious stimulus to the spinal cord and to the brain.

Advantages of local anesthetic techniques

  • Cheap and relatively easy to perform.
  • Produce intraoperative antinociception and, depending on the drugs used and duration of the surgery, postoperative analgesia Analgesia: overview.
  • Might decrease postoperative opioid consumption.
  • Allow performing surgical procedures under sedation Sedation or sedative protocol or lighter plane of anesthesia, therefore decreasing the potential cardio-respiratory effects associated with general anesthesia.
    Minimal systemic side-effects have been reported, if not overdosed or accidently injected intravenously. However hypotension is common after spinal or epidural anesthesia.

Disadvantages of local anesthetic techniques

  • Failure of the block: if the animal is not anesthetized it is advisable assessing the quality and the extension of the block using pin-prick technique or hemostatic forceps before starting the surgery.
  • Toxic doses can produce neurological and cardiovascular signs especially if injected intravenously.
  • Depending on the type of surgery and animal's demeanour, the administration of sedative or general anesthetic in conjunction with local anesthetic technique might be necessary.
  • If the animal is sedated, intraoperative movement can occur even if the local anesthetic technique is 100% successful.
  • Systemic hypotension might occur especially after spinal or epidural anesthesia; blood pressure monitoring is therefore highly advisable.
  • Particular care must be exercised if clotting disorders, hypovolemia or sepsis are present, or if neoplasia are very close to the injection area.

Selection criteria

  • Size and temperament of the animal.
  • Clinical conditions of the animal.
  • Type of surgical procedure.
  • Technical skill of the clinician with the loco-regional technique.
  • Type of surgery.
  • Equipment available.
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Classes of local blocks

Local

  • The agent is placed at the site where desensitization is required.
  • Anesthesia easy to produce and achieved by direct action of agent on nerve endings or nerve sheaths.
  • Disadvantages: no muscle relaxation and can interfere with the surgery because agent is injected into site of the procedure.
    Must keep surgical procedure within the limits of the block.

Surface

Topical

  • Useful agents include lidocaine Lidocaine gel, proxymetacaine Proparacaine drops, EMLA EMLA Cream cream. Anesthesia easy to produce and achieved by direct action of agent on nerve endings.
  • Lidocaine gel can be used for introduction of urinary catheter, nasal or tracheal tube.
  • Proxymetacaine drops (Proparacaine, not available in USA) are useful to provide analgesia of the cornea. The on-set is about 15 seconds, the off-set 10-15 minutes. Does not cause mydriasis. The prolonged use can cause corneal irritation.
  • EMLA cream: is an eutectic mixture of 2.5% lidocaine and 2.5% prilocaine. Can be useful to desensitize the skin and could be used to facilitate the catheterization of peripheral blood vessels (vein or arteries), especially in young animals. A dressing should be applied over the cream to allow skin absorption: 30-60 min of contact necessary. Anesthesia of the skin lasts about 60 min.
  • Disadvantages: no desensitization of deep tissue and no muscle relaxation.

Intradermal 

  • Local anesthetic is injected into the dermis producing a bleb Local anesthesia: intradermal .
  • Latency can be short.
  • Useful for removing small cutaneous skin masses, inserting chest drain in sedated animals.
  • Skin distortion can make suturing more difficult.

Infiltration

  • An extension of intradermal anesthesia.
  • Agent is injected more extensively to anesthetize a larger area of skin (and deeper tissues if required).
    Do not inject perivascularly.Allow a few minutes for desensitization to develop.A large area can be blocked from a single point by using the 'fan' technique through an intradermal weal.

Intratesticular

  • The preoperative intratesticular administration of lidocaine decreases intraoperative nociception and might allow to perform castration under sedation Local anesthesia: intratesticular.

Intra-articular 

  • Used for lameness evaluation (less common in dogs and cats) Local anesthesia: synovial Anesthesia: intra-articular.
  • If used pre-operatively can produce intraoperative antinocoception.
  • Depending on the local anesthetic used, such technique can also produce post-operative analgesia.
  • Other agents (ie preservative free morphine) can be administered intra-articularly to provide post-operative analgesia.
  • Needle has to be introduced into the synovial cavity and agent injected in strict aseptic technique Surgery: asepsis.
  • Bupivacaine Bupivacaine, ropivacaine have long lasting effect compared to lidocaine.
    Depending on the local anesthetic used and its concentration chondrotoxic effects have been reported in vitro.

Intravenous regional anesthesia 

  • Intravenous regional anesthesia (IVRA) Local anesthesia: intravenous regional anesthesia (IVRA)  or Bier block anesthesia can be used for surgery of the distal part of the thoracic and pelvic limbs.
  • The technique usually involves exsanguination, which forces blood out of the extremity, followed by the application of a tourniquet to stop blood flow. Local anesthetic is then injected intravenously distally to the tourniquet.
  • IVRA produces reliable intraoperative antinociception if the tourniquet is sufficiently tight.
  • Simple to perform.
  • Cheap equipment required.
  • Once the tourniquet is removed the analgesic effect will wear off, therefore systemic postoperative analgesia should be administered.
  • Local anesthetic commonly used: lidocaine (do not exceed 4 mg/kg).
    Do not use local anesthetic combined with epinephrine.

Perineural or peripheral nerve block

Brachial plexus block

  • Desensitizes the forelimb. 
  • Depending on the approach used (cervical paravertebral or axillary), brachial plexus block Anesthesia: brachial plexus block will desensitize the entire forelimb or the portion distal to the elbow respectively.
  • Peripheral nerve locator can be used to try to improve accuracy.
  • For the axillary approach a blinded technique can be used.

Radial ulnar musculocutaneous median block (RUMM block)

  • Easy to perform without the use of a nerve stimulator.
  • Radial nerve is blocked at the junction between the middle and distal portion of the humerus (lateral aspect) between the lateral head of the triceps and the brachialis muscles.
  • Ulnar, musculocutaneous and median nerves can be blocked on the medial aspect of the humerus, at about one half of the distance between the most prominent craniodorsal point of the great tubercle and the medial epicondyle.
  • The use of bupivicaine or ropivacaine can produce long lasting analgesia.
  • This block is useful for surgical procedure distal to the elbow.

Digital block 

  • Desensitizes individual digits Local anesthesia: digital .
  • Inject local anesthetic into the webbing on either side of the target toe.
    Be sure to include the nerves which run in both the dorsal and plantar/palmer aspects of the webbing.

Sciatic and femoral nerve block

  • The femoral (FN) and the sciatic (SN) nerves are responsible for the majority of the pelvic limb innervation in cats.
  • Several approaches have been described to localize FN and SN using a peripheral nerve stimulator or ultrasound guided technique.
  • The FN can be stimulated at the level of the lumbar plexus within the psoas compartment or cranially to the femoral artery within the femoral triangle delimited by the pectineus, sartorius and iliopsoas muscles.
  • The SN can be localized between the greater trochanter of the femur and the ischiatic tuberosity.

Auriculopalpebral block 

  • Such block does not produce analgesia, but it prevents blinking Local anesthesia: auriculopalpebral .
  • The nerve runs caudal to the mandibular joint at the base of the ear.
  • Palpate the nerve on the dorsal aspect of the zygomatic arch at the point where the arch turns in a medial direction.
  • The execution of this block together with topical anesthesia of the eyes might allow performing small surgical procedures without general anesthesia or sedation.

Infraorbital nerve block

  • Continuation of the maxillary nerve, the infraorbital nerve runs within the intraorbital canal and emerges from the infraorbital foramen.
  • The block of this nerve desensitizes the upper lip, the nostril, the roof of the nasal cavity and the portion of the skin rostroventral to the foramen.
  • If the needle is advanced within the infraorbital foramen, the block can be extended to upper teeth.

Maxillary nerve block 

  • The block of this nerve produces anesthesia of the upper lip, the nostril, teeth, paranasal sinuses and nasal cavity Local anesthesia: intraoral .
  • It can be easily blocked using an intraoral approach advancing a needle perpendicularly just caudal to the last molar tooth.

Mental nerve block 

  • Continuation of the mandibular nerve, the mental nerve runs within the mandibular canal and emerges from the mental foramen Local anesthesia: mental . The block of this nerve desensitises the lower lip rostral to the foramen. If the needle is advanced within the mental foramen, the block can be extended to the lower teeth.

Mandibular nerve block

  • This nerve can be blocked before it enters the mandibular foramen on the medial aspect of the vertical ramus of the mandible.
  • The block of this nerve desensitizes all lower teeth.

Retrobulbar 

  • Desensitizes the structures of the eye and paralyzes the external ocular muscles, providing a globe which is anesthetized and central Local anesthesia: retrobulbar .
  • Can be used as an alternative to neuromuscular blockade Anesthesia: non-depolarizing neuromuscular blockade for intraocular surgery.
    A degree of proptosis will be caused by the volume of local anesthetic which is injected caudal to the eye.
  • Disadvantage: the volume of agent required presses on the globe from behind and increases intraocular pressure.
  • Two approaches to this block:
    • A curved needle is introduced into the medial canthus of the eye and passed around and behind the orbit.
    • A needle is inserted dorsal to the zygomatic arch and passed medially to a point behind the orbit.
  • With each approach, local anesthetic solution is injected until the eye proptoses slightly.Beware that agent is not injected into the vasculature (the site is very vascular) or the optic nerve sheath.

Intercostal block

  • Useful for post-operative analgesia following thoracotomy Thorax: thoracotomy (intercostal). If performed preoperatively, is also helpful to decrease intraoperative nociception. 
  • Local anesthetic is injected just caudal to the rib.
  • At least a couple of spaces cranially and caudally to the surgical site should be blocked. 
    Do not inject into the vessels which run with the nerves.

Spinal

  • Local anesthetic is injected in the subarachnoid space.
  • A spinal needle is generally positioned between L4-L5 or L6-L7 vertebrae and advanced until CSF is noticed from the hub of the needle.
  • In cats CSF can be found also at L7-S1 intervertebral space.
  • Anesthetic and analgesic effect shorter compared to epidural.
  • Preservative free solutions should be used for this technique.
  • Opioids, eg morphine Morphine can be administered with local anesthetic in order to increase the duration of postoperative analgesia.
  • Extension of the block depends on the volume of local anesthetic administered.
  • Might cause severe hypotension, therefore careful monitoring of arterial blood pressure is advisable.
  • Procedure contraindicated in the presence of sepsis, clotting disorder, hypovolemia and neurologic dysfunction.

Epidural 

  • Local anesthetic is injected into the epidural or extradural space Anesthesia: epidural .
  • Usually performed at L7-S1 interspace with the animal in lateral or sternal recumbency.
  • Desensitizes the nerves which leave the spinal column in the area of spread of the local anesthetic.
  • Commonly used local anesthetics are bupivacaine, ropivacaine or lidocaine. Opioids and alpha-2 agonists can be added to these to prolong the duration of postoperative analgesia.
  • If blood is observed from the hub of the needle, do not inject.
  • If CSF is observed from the hub of the needle it has been positioned in the subarachnoid space. The volume to be injected should be reduced (see spinal anesthesia).
  • Can cause hypotension, therefore monitoring of arterial blood pressure is advisable.
  • Monitoring postoperative urination especially if morphine has been injected.

Factors affecting duration and latency

Drug

  • Each drug has a variable duration depending on its pharmacokinetic properties and the type of block performed Local anesthesia: overview.
    The relative duration of an agent compared with other agents is always the same for different blocks.

Closeness to site of action

  • Maintaining constant type of drug, its concentration and volume, the closer the drug is injected to the nerve the quicker will be the onset and the longer will be the offset.

Concentration

  • The higher the concentration of anesthetic solution, the greater the diffusion gradient, and hence more rapid onset and duration of action.

Volume

  • The greater the volume injected, the greater the chance of blocking the nerve. If neuroaxial block (spinal or epidural) is performed, the higher the volume injected, the more cranial is the spread.

Potentiating drugs

  • Vasoconstrictors and hyaluronidases can be added to local anesthetics:
    • Vasoconstrictors, eg epinephrine, increase block duration by reducing perfusion to the injection site and by slowing removal of the agent from it.
    • Hyaluronidases reduce the latency of the block by aiding perfusion of the agent through the tissues.