TYPES OF ANAESTHESIA:

 Indication of local and regional analgesia:

1.They can be used for standing animals, avoiding the dangers associated with forcible casting and prolonged recumbency especially in large animals.

2.The surgeon can induce it and operate without the assistance of an anaesthetist.

3.The techniques are easy to learn.

4.The toxicity is low.

Local analgesicsThree categories exist based on the relative duration of analgesic action.

1)Procaine has a short analgesic duration of 30 to 60 minutes.

2)Lidocaine represents agent of intermediate analgesic duration 90-180 minutes.

3)Tetracaine, bupivacaine are agents with a long duration of analgesia of 180-300

minutes.

The desirable properties of local analgesics are:1)Good penetrating qualities in body tissues.

2)High potency so that low amount can be used.

3)Rapid onset and long duration of action.

4)Low systematic toxicity.

5)No irritant effect to the nerve or other body tissue.

6)Availability in sterile solution or ease of sterilization.

Potentiation:

1.Vasoconstrictors:• It prolongs the effects of local analgesics by slowing the rate of absorption from the site of action into the systematic circulation. • Systematic toxicity also is reduced. • The classic vasoconstrictor is epinephrine.• The maximum safe concentration of epinephrine is 1: 50,000.

2.Hyaluronidase:• Hyaluronic acid is the substance that prevents foreign materials from diffusing into tissues. • Hyaluronidase is an enzyme that inactivates hyaluronic acid. • Local analgesic drugs diffuse into tissues more rapidly when combined with hyaluronidase and spread to a greater area. • Disadvantages of hyaluronidase include decreased duration of action and increased toxicity, because absorption is more rapid. • It is very expensive so we can not use it in veterinary field.

3.pH adjustments:Local analgesic action may be potentiated carbonated solutions, such as sodium bicarbonate, thus enhancing penetration of local analgesic.

Toxicity of local and analgesic:1.Local toxic effects:Large doses cause local damage to nerves and skeletal muscles, delay healing.

2.Systemic toxic effects:The most frequent and dramatic reactions observed clinically are acute reactions involving the central nervous system and/or cardiovascular system. Central nervous system: - Sedation, later on a various degree of convulsions. - If the convulsions continue for more than 1-2 minutes, thiopentone should be given by I/V injection. Cardiovascular system: - There is a cardiac rhythm disturbances. Other effects: - Respiratory depression, renal and hepatic blood flow may also decrease.

The toxic reactions are avoided by: Limiting of the drug used as possible. Using dilute solution. Adding vasoconstrictor to retarding absorption into general circulation. Repeated aspiration should be carried out before and during the injection to avoid accidental intravascular injection.

The failure or partial failure of local blocks may occur for several reasons, most common of which are: Incorrect anatomic deposition.Inadequate anaesthetic volume.Dilution or hem-dilution of anaesthetic agent. Presence of fibrous connective tissue inhibiting diffusion of anaesthetic agent. Multiple sites of pain.Incorrect location of lameness.

Methods of producing local analgesia:

1.Surface analgesia:

a. Topical applicationsIce causes freezing of the skin and produce local analgesic effect. Ethyl chloride spray, ether spray cause cooling through vaporisation. Lignocaine jell or ointment are applied with friction to the skin or mucous membrane (urethra, larynx, trachea, teat...etc). Aqueous solution of 2% lignocaine or 4% procaine may be applied topically for pain relief in abraded or eczematous areas. The application is made by soaking a piece of absorbent wool or gauze in the solution and placing it on the affected area for about 5 minutes.

b.Instillation:Analgesic instillation may be used to produce analgesia of ocular surfaces (conjunctiva and cornea). For eye drops. lignocaine 4% solution can be applied. One to two drops of the solution are used; the onset of corneal analgesia occurs in about 15 seconds and persists for about 15 minutes.

c.Intra-synovial analgesia: Surface analgesia is also employed for the relief of pain in cases of joint and tendons affections. If the synovial cavity is distended with fluid, it is first drained to insure that the injected solution is not excessively diluted. Analgesia develops within 5-10 minutes after injection and persists for about 1 hour.

2.Infiltration analgesia:

2.Infiltration analgesia:

A-Linear infiltration: It is applied by using needle of about 10 cm. in length inserted s/c at the middle of the proposed line of incision and directed upward.

The injection is carried out as the needle is slowly withdrawn so that a stream of solution is deposited s/c. About 1 ml. of the solution is required for every 1 cm of incision then redirection of the needle to downward and injection is completed.

*To infiltrate several layers of tissue, further advancing of the needle from the same point to the deeper layers is performed. For a laparotomy, for example the subcutaneous tissue and various muscle layers of the flank are infiltrated down to the peritoneum.

B-Field infiltration:Obtained by one of the following:

a-Inverted L.b-Ring block.c-Circular block.

a-Inverted L field block can be produced by 2 linear infiltrations must be carried out. The first line is made parallel to the last rib and the second line vertical to the first one parallel to the vertebral line.

b-Ring block is commonly performed for inducing analgesia of the distal parts of extremities and also at the base of the teat in circular manner injection.

c-Circular field block Used for desensitizing the skin and s/c tissue around a swilling such as hernias, tumours, bursae...etc. The field block methods are preferred than linear infiltration as, deposition of the analgesic solution is away from the incision line prevent oedema, haematoma and possible interference with healing process.

Disadvantages of local infiltration analgesia:1-Incomplete analgesia and muscle relaxation of the deeper layers of the

abdominal wall.2-A possible toxicity from large amount of anaesthetic agents.

II-Regional analgesics•It is produced by blocking sensory nerve conduction to specific area. •It has been used as small amounts to be deposited adjacent to the nerves travelling to the desired region.•It is preferred over local filtration analgesia because the decreased volume lowers the toxicity. •In addition, the introduction of needles or anaesthetic can be away from the surgical site. •Successful regional analgesia requires good knowledge of applied anatomy to inject the agent in the correct location.

Perineural analgesia of the trunk nerves

Paravertebral nerve block (Paralumbar analgesia) (In Bovine)• It is a type of perineural injection of local analgesic solution about the spinal nerves. • It is a technique for obtaining analgesia of the paralumbar fossa in ruminant. • This technique is used for procedures that include most abdominal surgeries, such as rumenotomy, correction of gastrointestinal displacement, intestinal obstruction and volvulus, caesarean section, ovariectomy, liver or kidney biopsy, and others.

Anatomical and physiological considerations:•The flank region bounded anteriorly by the last rib, posteriorly by the angle of the ileum and superiorly by the lumbar transverse processes. • It is innervated by the last thoracic (T13) and first and second lumbar (L1 and L2) spinal nerves. • The spinal nerves, after emerging from the intervertebral foramina, immediately divide into a smaller dorsal and a large ventral branch.

A) The dorsal branch supplies the skin and muscles of the lions but some of its cutaneous branches pass to a considerable distance down to the flank.

B) The ventral branch passes obliquely downwards and backwards between the muscles and comprises the main nerve supply to the skin, muscles and peritoneum of the flank. The ventral branch is also connected with the sympathetic system by a ramus communicans. Blocking of the sympathetic fibers will desensitize the corresponding abdominal pressure.

There are two techniques:1-Proximal paravertebral analgesia:• It is called Farquharson, Hall or Cambridge technique. As the dorsal

and ventral nerve roots of the last thoracic (T13) and 1st and 2nd lumbar (L1 and L2) spinal nerves emerges from the intervertebral foramina in this technique, they are injected.

• In addition, if we desire analgesia of the caudal most part of the paralumbar fossa or mammary gland, the third and fourth (L3 and L4) lumbar nerves are blocked. These blocks result in weakness of the pelvic limb and may cause the animal to lie down.

• The caudal aspect of the transverse processes of the last thoracic and first and second lumbar vertebrae is the site for needle placement.

• If the T13 and L1 transverse processes cannot be palpated, the distance between the more prominent transverse processes of L2 and L3 is measured and the same distance used cranial to L2 and L1 to find the sites for injecting the nerves L1 and T13 respectively.

• A canula with a needle is inserted in the site of injection, to facilitate the passage of the long needle into the longismus dorsi muscle to reduce muscle spasm when the long needle is inserted.

• A spinal needle, is then inserted through these sites until it strikes the cranial border of each transverse process. The intertransverse ligaments are penetrated, and 15 ml of analgesic agent are injected below the ligament to block the ventral branch of each nerve. The needle is then withdrawn 1.0 to 2.5 cm to above the ligament and further 5 ml injected to block the dorsal branch.

• Signs of successful nerve blockade include analgesia of the skin, lateral deviation of the spine toward the other side due to paralysis of paravertebral muscles, and increased skin temperature due to increased blood flow after paralysis of cutaneous vasomotor nerves. Usually present within 10 minutes after injection, analgesia lasts approximately 90 minutes.

the advantages When compared to infiltration analgesia;1. Small doses of analgesic solution.2. A wide and uniform area of analgesia and muscle relaxation.3. Minimal intra-abdominal pressure, increased intestinal tone and motility due to desensitization of sympathetic fibers of rami communicates.4.Absence of local analgesic from the operative wound margins.

The disadvantages of this technique include:1. Technically difficult to be applied.2. Arching of the spine due to paralysis of the back muscles.3. Risk of penetrating vital structures such as the aorta and thoracic longitudinal vein on the left side and the caudal vena cava on the right side.4. Caudal migration of the analgesic solution from the injection site to femoral nerves with subsequent paresis.

2-Distal paravertebral analgesia:• It is called, Magda, Cakala or Cornell technique. It is employs a lateral

approach to the same nerves. The dorsal and ventral rami of the spinal nerves of T13, L1 and L2 are blocked close to the ends of the first, second and fourth transverse lumbar processes respectively.

• A 7.5 cm, 18-gauge needle is inserted ventral to the tips of the respective transverse processes, where approximately 10 to 20 ml of local analgesic solution is injected in a fan-shaped infiltration pattern. The needle is completely withdrawn and reinserted dorsal to the transverse process, in a slightly caudal direction, where the cutaneous branch of the dorsal rami is injected with approximately 5 ml of the analgesic. The procedure is repeated for the second and fourth lumbar transverse processes.

• The advantages include:a) The use of more routine size needles,b) No risk of penetrating a major blood vessel,c) Minimal weakness in the pelvic limb,d) Minimal ataxia, and e) Technical easily.

The disadvantages are that larger doses of anaesthetic are needed and variations in efficiency exist, particularly if the nerves vary in their anatomical pathway.