Local anesthesia: overview

Jump to... Definition Classes of local blo... Factors affecting du... Sources

Definition

Top

• Reversible prevention of transmission of nervous impulses from a specific site or area of the body without any direct effect on the central nervous system.
• Many surgical procedures routinely carried out under general anesthesia can be performed using local anesthesia (with or without sedation or light general anesthesia).

• Avoids respiratory and cardiovascular depression.
• Useful for animals with severe systemic disease which would complicate general anesthesia although any sedative used in conjunction with local anesthesia may have side-effects.
  Systemic absorption, inadvertent IV injection or over-dosage may result in cardiovascular or neurological side-effects.
• Care must be exercised with spinal and epidural anesthesia as there is a risk of systemic hypotension.

• No sedation and loss of reflexes.
• Animal can be discharged immediately after minor procedures.
• Agent can provide analgesia after surgical procedure completed (see below for factors affecting duration).

• Reduced peri-operative and post-operative nursing/anesthetic support.
• Relatively simple equipment required.
  Because it is much quicker, a 3 minute general anesthetic in a healthy animal may be less costly than a relatively complicated and lengthy local anesthetic block (see below for factors affecting latency).

• Nervous, aggressive or fidgety animals make poor candidates for local anesthesia without sedation or light general anesthesia.
• A problem if surgical procedure requires a perfectly still and stable site.

• Can be more difficult to handle than under general anesthesia.
  Only attempt procedures which are routine and have very little risk of complication, eg removing a subcutaneous mass on the flank - a similar mass on the chest wall should not be attempted because of potential risk of penetrating the pleural cavity and consequent need for intermittent positive pressure ventilation (IPPV).
• Particular care must be exercised if clotting disorders or hypovolemia is present.
• Local anesthetics should not be injected through areas of local sepsis.

• Needs of individual animal.
• Surgical or other procedure to be undertaken.
  Light general anesthesia can assist local anesthesia by providing restraint and ease of positioning.

Classes of local blocks

Top

• 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.
• 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.

• The agent causes local desensitization when applied to skin or mucous membrane .

Useful agents are:

• Lidocaine ointment and gel .
• Proxymetacaine solution .
• EMLA cream (lidocaine/prilocaine) - takes about an hour to desensitize skin and should have an occlusive dressing placed over it.
  EMLA can make intravenous or arterial cannulation much easier to perform in small or nervous animals because it causes local desensitization and also a degree of vasodilation.
  

Intradermal

• Agent is injected as a bleb into the skin to be anesthetized .
  Best results by injecting into the skin - producing a prominent, raised bleb - rather than beneath it.
• Latency is very short (about 40 seconds).
• Useful technique for:
  • Inserting large cannulae.
  • Epidural injections.
  • Placing chest drains in conscious animals.
  • Removing small cutaneous masses.
    Skin distortion can make suturing 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.
  

Synovial

• Used to relieve synovial pain. a) To locate painful lameness. b) For post-operative analgesia after intra-articular surgery .
• Needle introduced into the synovial cavity and agent injected.
  Place needle during surgery as soon as joint capsule closed.
• Bupivacaine  good because gives prolonged analgesia.
  

Regional

• The agent is placed at a site distant from the surgical field or area of the body to be anesthetized .
  Must identify the limits of the anesthetized area before operating so that non-anesthetized tissues are avoided.
• Does not distort operation site (unlike injection of local anesthetic bleb).
• Provides a degree of muscle relaxation by blocking motor as well as sensory fibers (in addition to analgesia).
• Lidocaine provides rapid onset.
• Bupivacaine provides prolonged duration.
  A lidocaine and bupivacaine mixture gives rapid onset and prolonged duration.
  

Perineural

• Injection of agent close to the nerve supplying the area to be desensitized.
  Safe regional technique requires knowledge of the area supplied by the nerve and of the detailed anatomy of the site of block.
  

Brachial plexus block

• Desensitizes the forelimb to the level of the elbow .
• Includes some sensory fibers from the upper part of the limb.
• A long spinal needle is placed medial to the point of the shoulder and directed caudally between the shoulder joint and the chest wall.
  Do not enter the pleural cavity.
• Inject up to 10 ml of agent at a point level with the center of the shoulder joint.
  To improve needle placement, attach a nerve stimulator to the needle as it is placed. Initial response to the stimulation is a withrawal response, but as the needle enters the sheath of the brachial plexus a definite placement response is seen.
  Stop injecting agent when resistance is felt, ie the sheath is filled.
  

Digital block

• Desensitizes individual digits .
• Place agent 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.
  

Auriculopalpebral block

• To prevent blinking during ophthalmic surgery .
• The nerve runs caudal to the mandibular joint at the base of the ear.
• The nerve is not sensory, but blocking it prevents blinking during delicate ophthalmic surgery.
• Palpate the nerve on the dorsal aspect of the zygomatic arch at the point where the arch turns in a medial direction.
  

Mental

• Desensitizes the rostral mandible on the blocked side .
• Palpate the mental foramen and inject agent into it (the mental nerve leaves the mandible on the lateral surface caudal to the lower canine).
• Can also block the nerve at a more caudal site through the mandibular foramen although this is harder to locate.
  Use the needle tip to search for the mandibular foramen on the surface of the mandible.
  

Retrobulbar

• Desensitizes the structures of the eye and paralyses the external ocular muscles, providing a globe which is anesthetized and central.
• Can be used as an alternative to 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 round 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.
• Agent injected around the intercostal nerves of the ribs on at the surgical site and at least one space either side of the surgical site.
• Agent usually injected just before closing the chest wall.
• Block the nerves as dorsally as possible from within the pleural cavity.
  Do not inject into the vessels which run with the nerves.
  Bend the needle to an angle of about 30° to allow easier injection without placing the whole syringe in the chest cavity.
  

Intravenous

• Intravenous injection of local anesthetic below a tourniquet placed round a limb.
• Usual dose of agent - 0.5 ml 2% lidocaine per 10 kg.
• Desensitizes the whole area distal to the tourniquet.
• Specially useful for surgery of the foot - the tourniquet also reduces bleeding.
• Tourniquet must be tight enough to occlude arteries and veins.
  Check that no pulse distal to the tourniquet, ie that tourniquet sufficiently tight.
• Analgesia persists until tourniquet removed.
  Never leave tourniquet in place for >1 hour.
  Do not use bupivacaine - relatively high toxicity if enters the blood circulation.
  Never use agents combined with adrenalin.
  Esmarch's bandage is useful aid.
• Place an appropriate venous cannula, apply the bandage and then inject agent through the cannula.
  Cannula tip must be distal to the bandage.
  

Spinal

• Regional anesthesia in which agent is injected around the spinal column.
• Opioids, eg morphine  and buprenorphine  and alpha-2 agonists in conjunction with local anesthetics provide long-lasting analgesia by spinal injection .
• Preservative free solutions must be used in this site.
• Procedure contraindicated in the presence of sepsis, clotting disorder, hypovolemia and neurologic dysfunction.
  

Epidural

• Agent is placed into the epidural space .
• Usually performed at L7-S1 interspace.
• Desensitizes the nerves which leave the spinal column in the area of spread of the local anesthetic.
• Most of block thought to occur where the nerves exit the dura.
• Usual dose of agent - 1 ml per 5 kg.
  Do not exceed 10 ml even in very large dogs.
• Use bupivacaine  , lidocaine  or a mixture of the two. Opioids or alpha-2 agonists can be added to these.
  Take care to avoid subdural or intravenous injection.
  

Sub-dural

• Deliberate placement of agent into cerebrospinal fluid (CSF) space.
• Injection usually at the L3-L4 interspace, but can be at any level .
• Usual dose of agent - 10% of that for epidural anesthestic.
• Desensitizes spinal column.
• Block includes all body areas caudal to injection site.

Factors affecting duration and latency

Top

• Each drug ('agent') has a variable duration depending on the type of anesthetic block performed .
  The relative duration of an agent compared to other agents is always the same for different blocks.

• Local anesthetic applied close to the site of action will have a shorter onset time ('latency') and longer duration because agent level in the nerve fibers will develop more quickly and peak concentration will be higher.

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

• A larger volume of agent will usually produce a prolonged action, but will not reduce latency because the diffusion gradient is the same.

• Vasoconstrictors and hyaluronidases can be added to local anesthetics:
  • Vasocontrictors, 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.

Sources

Top

• Vickers M D, Morgan M & Spencer P S J (1991) Drugs in Anesthetic Practice. Butterworth Heinemann (Good on the pharmacology of local anesthetic agents).
• Muir W W, Hubbell J A E & Skarda R (1989) Handbook of Veterinary Anesthesia. Mosby Press (Useful source of diagrams).

• Dr John Dodam DVM PhD , A303 Clydesdale Hall, 379 East Campus Drive, College of Veterinary Medicine, University of Missouri-Columbia, Columbia, MO 65211, USA.

Back to top
© Copyright Vetstream

CANIS FRE00001

---