1-Local analgesia

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I-Local Analgesia…

Many surgical procedures can be satisfactorily performed under the effect of local analgesia. The use of sedation with this technique depends up on the species, temperament, and health of the animal, and magnitude of the operation. Sedation should be avoided in surgical procedures when the animal should not lie down, but sedation could be used when reduction of fear and liability of sudden movement is required for achievement of efficient surgery. Moreover, the dose of sedative drug must be reduced on using certain types of local analgesics like lignocaine as it has systemic sedative effect following its absorption.  

Advantages: -

1-It is suitable for performing surgery on standing animals, accordingly injuries associating casting and prolonged recumbency can be avoided.

2-The technique is simple and requires no expensive or complicated equipments.

3-The technique can be performed by the surgeon himself with no need for anesthetist.

Disadvantages: -

1-Injection shouldn’t be performed in infected area to avoid spreading of infection.

2-Direct injection of the drug at seat of incision causes delay of healing as a result of histotoxic effect of the drug.

3-The amount of used local analgesic drug is relatively higher than other methods like perineural analgesia, accordingly the cost increases.

LOCAL ANALGESICS: -

Desirable characteristics of local analgesic agents: -

1-It should has good penetrating qualities through body tissues                       

2-It should has rapid onset

3-It should be potent so that low concentrations can be used    

4-It should has long duration of action

5-It should has low systemic toxicity                   

6-It shouldn't be irritant to nerve and other body tissues

7-It should has reversible action      

8-It should be available in sterile solution or it can be easily sterilized

Potentiation by vasoconstriction: -

Addition of vasoconstrictor (epinephrine) to local analgesic, at concentration of 1:200,000 allows prolonged analgesic effect by vasoconstriction and delaying absorption of the drug. The maximum safe concentration of epinephrine is 1:50,000; greater concentrations may cause local tissue ischemia and necrosis, accordingly these agents shouldn’t be used in extremities, tail, or teat, etc…to avoid the possibility of ischemia and subsequent necrosis and gangrene.

The exception to this rule is the epidural analgesia where concentration up to 1:10,000 may be safely used. Generally the used of analgesic agents that contain vasoconstrictor is contra-indicated in injured tissue as this tissue might be already ischemic and the further injection with epinephrine may deteriorate the condition of the ischemic tissue and causes gangrene.

Potentiation by hyaluronidase: -

Hyaluronidase is a mucolytic enzyme that hydrolyses hyaluronic acid that is known as the ground substance preventing diffusion of drug in the tissues. Incorporation of that chemical substance in the analgesic solution facilitates diffusion and penetration of the analgesic drug into the tissue and accordingly the drug will acquire faster on set. 

Advantages: -

1-It promotes diffusion and absorption of the local analgesic with which it is mixed

2-It is of particular value in nerve block, especially if the analgesic didn’t deposited accurately around the nerve

Disadvantages: -

1-Toxicity although the ratio of toxic to therapeutic dose is 200:1

2-Reduction of analgesia duration

3-Increased toxicity by analgesic drug itself as a result of rapid absorption

Generally, the last two disadvantages can be counteracted by addition of epinephrine to the solution

AVAILABLE LOCAL ANALGESICS: -

A-Minor local Analgesics: -

1-Ethyl Chloride: -

It is a topical local analgesic, marketed under pressure in containers with a fine capillary nozzle and a control valve that allows the liquid to be sprayed. It has a very superficial and transient analgesic action, and when it is sprayed on the skin, it evaporates leading to freezing of the skin (-20ºC) with induction of surface analgesia for 30-60 seconds. Its use is limited to simple incisions or punctures such as incision of abscess or hematoma.

2-Ethyl alcohol: -

Injection of absolute alcohol around a nerve produces neuritis, degeneration, and sclerosis, however, 30% alcohol temporarily destroys sensory nerves that regenerate again after a variable period, and nerve function will return by then. Duration of block depends on;

1-The size of the nerve                    

2-Degree of destruction

Small-unsheathed nerves may be permanently destroyed, whereas, large heavily sheathed nerves are only temporarily affected.

B-Major local Analgesics: -

Cocaine was the first available local analgesic, but its toxic effect and addictive properties in human restricted its use and availability. Nodaway, many new generations of local analgesics are available, and they vary according to their potency, toxicity and cost. The present three categories are classified according to duration of analgesic action.

 

Analgesia duration

Drug

Duration

1

Short

Procaine

30-60 minutes

2

Intermediate

Lidocaine and mepivacaine

90-180 minutes

3

Long

Tetracaine and bupivacaine

180-300 minutes


1-Short duration analgesic: -

Procaine HCl: -

Procaine HCl is a white, crystalline, water-soluble powder

Advantages: -

1-Its subcutaneous injection has an efficiency approximating that of cocaine, but it has lower toxicity especially when adrenaline HCl is added (10 times less toxic)

2-It is non-irritant

3-Relatively stable solution

4-It can be sterilized repeatedly by boiling without loss of potency

5-It is rapidly and completely detoxicated by the liver when absorbed slowly from injection site (ensured by adding adrenaline), so that a second infiltration can be carried out in the course of an hour without a danger of toxicity by cumulative arising.

Disadvantages: -

1-Toxic when accidentally injected intravenous

2-It has low power of penetration

3-It can not be used for topical application or intra-synovial analgesia as it has very low power of penetration of mucous membrane

4-Decomposed by alkali

Concentration, on set, and duration: -

Use

Concentration

On set

Duration

General infiltration 

2 %

5 minutes

 

1 hour

Epidural injection

1-2.5 %

10 minutes

Perineural use in horses and cattle

4-5 %

10 minutes

2-Intermediate duration analgesic: -

A-Lignocaine or Lidocaine HCl (Xylocaine® or Debocaine®): -

Advantages: -

1-It is extremely stable solution and can be boiled with acid or alkali         

2-It can be sterilized several times even by autoclaving

3-Its onset is twice faster than procaine

4-It has longer duration of action than procaine (90 min alone and 120 min with epinephrine)

5-It has a sedative effect when injected in a large amount and so the dose of tranquilizer must be reduced

6-It has higher penetration power than procaine and so it is preferred in perineural injection and it is unnecessary to add hyaluronidase to it neither for infiltration nor for nerve blocking purposes

7-It can be used for surface analgesia by intra-synovial injection, or topical for the cornea, or mucous membranes (4%), particularly those of the throat and larynx, prior to endotracheal intubation

Disadvantages: -

1-It may cause some local irritation and swelling, which is particularly a problem in the horse.

2-Toxicity happened by over dose that is expressed by drowsiness, twitching and respiratory depression, and finally convulsions and hypotension.

Accordingly the toxic dose is known to be

Animal

Dose in gm

Dose in ml (2%)

Horse and cattle

6

300 ml 2 %

Dog

0.6

30 ml 2 %

Concentration: -

1-General infiltration (0.5:1 % without vasoconstrictor)

2-Epidural and nerve block (2% with or without vasoconstrictor)

B-Mepivacaine HCl or Carbocaine (Mepacaine®): -

This compound closely resembles lignocaine HCl, and widely used for human dentistry. It is widely used in the horse as it causes very little swelling and edema in the area of injection, possibly as it lacks vasodilatory action. 

Advantages: -

1-It is slightly less toxic, even slow intravenous injection over 20 minutes in dog by a dose of 29 mg/kg, produces convulsion that is followed by sedation

2-It has no vasodilatory effect, making the addition of a vasoconstrictor unnecessary. However, a commercial product with levonordefrin is available in market (Mepacaine-L®).

Concentration: -

For infiltration and nerve block (1-2%) is satisfactory, but generally it is available as ampoules of 1.8 ml of 2 % Mepivacaine HCl with or without levonordefrin.

3-Long duration analgesic: -

A-Tetracaine HCl (Pontocaine®): -

Advantages: -

1-The onset of analgesia is 5-10 minutes                           

2-It is 12 times potent than procaine

3-Lesser interference with corneal healing than other agents, so it is the drug of choice for corneal analgesia

Disadvantages: -

1- It can’t be autoclaved

2- Its toxicity 10 times that of procaine

Concentration: -

For the eye (0.2% for 120 min)

For infiltration and nerve block (0.1%)

B-Bupivacaine HCl (Marcaine®): -

Advantages: -

1-Stable solution on boiling with acid and alkali and shows no change on repeated autoclaving.

2-Represented in different concentrations with or without adrenaline

3-More potent 8 times than procaine and 4 times than Lidocaine HCl so it is used as 0.5 % solution         

4-It has greater margin of safety than lignocaine

5-Onset is similar to lignocaine but its effect lasting for 4-6 hours (twice longer period of analgesia). Accordingly it is indicated for use in situation where prolonged analgesia is required like relief of pain in equine during acute laminitis.

6-This drug has a prolonged duration of action therefore it is used whenever long action is required (post-operative analgesia; prolonged surgery etc)

Concentration: -

Aim of use

Concentration

Infiltration

0.25%

Nerve block

0.5%

Epidural analgesia

0.75%


Toxicity of local analgesic:-

Toxic reactions to local analgesic drugs arise when:

1-The drugs are absorbed into the general circulation at a rate greater than that at which they can be broken down by the body. Rapid absorption occurs from any hyperaemic or inflamed tissue and the rate of absorption is increased by the use of solutions which contain spreading agents such as hyaluronidase.

2- Accidental intravenous injection. When the concentration of local analgesic in the general circulation reach the toxic level, two types of symptoms may be seen either singly or together. These are cardiovascular and central nervous types.

A- The cardiovascular type of reaction: is caused by decrease cardiac output due to depression of the myocardium. It characterized by sudden collapse, pale mucous membrane, hypotension and tachycardia.

Treatment: vasopressors administered intravenously will overcome the hypotension.

B- The central nervous system type of reaction: usually, sedation is the first obvious sign but a further increase in the brain concentration produces tonic-clonic seizures.

Treatment: Barbiturates are used to counteract the convulsive action of the local analgesic.

Types of Local Analgesia…

I- SURFACE ANALGESIA

A-Topical analgesia: -

Surface analgesia can be produced by freezing of superficial layers of skin by ethyl chloride, ether, or carbonic acid snow, as a result of their rapid volatilization. Their action is superficial and transient, and their use is limited to simple surgical interferences like incision of an abscess. Excessive use may lead to necrosis, and the thawing after their use is painful in man.

Surface analgesia can be performed by using lignocaine ointment that is applied by skin friction for relief of pruritis. Surface analgesia may be performed by using lignocaine 2% aqueous solution topically for relief of superficial abraded or eczematous area. Surface analgesia of mucous membrane of the glans penis and vulva can be produced by topical use of lignocaine 2% aqueous solution

Surface analgesia of urethral mucous membrane can be adopted by lignocaine 2% gel that works as lubricant and analgesic. Surface analgesia of the nasal mucous membrane can be performed by lignocaine 4% spray for trans-nasal passage of stomach tube in dog or for surgical procedures of the nasal chamber in the horse. Surface analgesia of the cornea can be performed by topical instillation of 4% lignocaine or 0.2% Tetracaine®.

B-Intra-synovial: -

Surface analgesia is also employed for the relief of pain arising from pathological conditions involving joints and tendon sheaths. Surface analgesia of the joints can be performed by intra-synovial injection of 2 % lignocaine. The local analgesic is injected into the synovial cavity and then
dispersed throughout the cavity by manipulation of the limb. If the synovial
cavity is distended with fluid, it is first drained to ensure that the injected
solution is not excessively diluted. Analgesia develops within 5 to 10 minutes
after injection and persists for about 1 hour. The injection renders the synovial
membrane insensitive but it is not known whether the nerve endings in the
underlying structures are affected.

Uses: -

1-Therapeutic, like relief of pain during arthritis

2-Diagnostic, for diagnosis of arthritis or lameness (therapeutic diagnosis)

1-Intra-articular injection: -

  A-Horse..

1-The distal interphalangeal joint (pedal or coffin joint): -

A 20 gauge needle 7 to 10 cm long is inserted about 2 cm above the coronary band and 2 cm away from the midline, and directed onwards in a medial direction to contact the second phalangeal bone. It is run downwards over the surface of this bone until synovial fluid flows from it. About 5 ml of solution are injected and the horse is then walked to distribute the injected fluid throughout the joint. By diffusion the synovial membrane around the navicular bone is also affected. The horse's foot should be held off the ground while the needle is being inserted in order to facilitate entry into the joint.

2-The proximal interphalangeal joint (pastern joint): -

The limb is held up and flexed, a 20 gauge needle is inserted 1 cm proximal to the joint space in the midline, or slightly paramedian, and directed horizontally or slightly distally into the joint space. 5 ml of the local analgesic is enough to anaesthetize the pastern joint.

3-The metacarpo-phalangeal / metatarso-phalangeal joint (fetlock joint): -

This joint is anaesthetized by 10-15 ml local analgesic. A 20 gauge needle is inserted in the Proximopalmar or proximoplantar pouch of the joint capsule in the triangular space formed by the third metacarpal / metatarsal bone, the proximal sesamoid bone and the suspensory ligament immediately proximal to the condyle of the third metacarpal/metatarsal bone. This injection is made while the horse is standing on the limb.

4-The carpal joint: -

Only the proximal (radiocarpal) and middle (intercarpal) joints need to be injected since the dista1(carpometacarpal) joint communicates with the middle one between the third and fourth carpal bones. Both joint cavities can easily be palpated when the limb is flexed. A 20 gauge needle is inserted in the dorsal aspect of each joint, medial or lateral to the extensor carpi radialis tendon. 10-15 ml of local analgesic is injected in each joint.

5-The elbow joint: -

This joint is entered either in front of, or behind its lateral ligament. This ligament can easily be palpated as a tense, cord-like structure about 1 cm in diameter. The needle should be introduced in front of this ligament just under the easily palpated lateral condyle of the humerus and advanced obliquely and inwards along the bone until synovial fluid appears at the hub of the needle. The injection of 10 to 20 ml of local analgesic solution can then be made directly into the joint cavity. If the needle is introduced behind the lateral ligament, the solution is injected into the communicating bursa under the lateral flexor of the carpus.

6-The shoulder joint: -

Entry into this joint is made between the anterior and posterior parts of the lateral tuberosity of the humerus. The needle is directed backwards and inwards in a horizontal plane until synovial fluid appears at the hub of the needle, 10 to 20 ml of local analgesic solution can then be made directly into the joint cavity.

The tendon of the infra- spinatus muscle can be palpated running just behind the site where the needle is introduced.

7-The tibio-tarsal joint: -

The best site for puncture of this joint capsule is located on the dorsomedial aspect of the joint between the extensor tendons and the medial ligament of the joint, but care must be taken to insert the needle medial or lateral to the saphenous vein.

8-The stifle joint: -

This joint has three synovial sacs, one at the femoro-patellar articulation and two (one medial and one lateral) for the femoro-tibial articulation. The femoro-patellar joint capsule can be entered on either side of the middle ligament of the patella. The medial joint sac is entered just behind the medial patellar ligament and just above the tibia.

The lateral sac is approached between the lateral femoro-tibial ligament and the tendon of the long digital extensor. The needle should be inserted just above the margin of the lateral condyle of the tibia behind the groove for this tendon.

Injection into the femoro-patellar joint capsule also affects the medial half of the femoro-tibial joint in those animals where these two joint capsules communicate.

9-The hip joint: -

A needle at least 15 cm long and 2 mm in diameter must be used and it is introduced through the notch between the anterior and posterior parts of the great trochanter of the femur. The skin is incised under infiltration analgesia at the site of introduction of the needle and the needle is run along the neck of the femur until cartilage is touched or synovial fluid issues from its hub.

B-Cattle and dogs..

The techniques described for the horse are, with only minor modifications, applicable to cattle and dogs.

2-Injection into synovial sheaths: -

A needle can be introduced into synovial sheaths quite easily when they are distended with synovial fluid, but entry into a normal sheath is not easy. When searching for a synovial sheath the exploring needle should be connected to a syringe containing local analgesic solution and a slight pressure maintained on the syringe plunger. As soon as the needle enters the sheath the resistance to injection disappears and some of the solution enters the synovial cavity, lifting its wall away from the underlying tendon.

II- INFILTRATION ANALGESIA

This technique can be used for minor operations or even for major operation either alone or in adjunction with sedation or basal narcosis.

Infiltration analgesia is contraindicated to use in:

1- Infected area: to avoid spreed of infection.

2- inflamed tissue: to avoid rapid absorption and toxicity, as well as the acidic condition in the inflamed area impair dissociation of the analgesic solution.

3- Malignant tumour: as infiltration procedure may transplant tumour cells to other normal tissue.

A-Intra-dermal or sub-dermal infiltration: -

It is a process through which analgesic drug is injected intra-dermal to facilitate injection in animals. The main point of this technique is the humanity as it reduces pain during subsequent procedure of infiltration analgesia.

B-Linear infiltration: -

This method can be performed by creation of insensitive intra-dermal weal through which the needle is inserted subcutaneously into two opposite directions to create analgesic line, and by this method, a line of analgesia that has double length of the needle can be created with minimal skin bricking.

Usually the drug is injected while the needle is dragged out of the subcutaneous tissue and the amount of required analgesic is 1 ml/cm2. Before injecting any local analgesic solution, aspiration is attempted to ascertain that the needle point has not entered a blood vessel. If blood is aspirated back into the syringe, the needle is partially withdrawn and reinserted in a slightly different direction

It is always better to overdo local infiltration than to apply it inadequately and to use more of a dilute rather than less of a concentrated solution of local analgesic.

Although sensation is mainly confined to the skin, but in some circumstances it is recommended to infiltrate the muscular layer beneath the skin as sensory nerves pass through it and this will achieve better analgesia, moreover, involvement of motor nerves that passes through the muscles reduces movement of the muscles during incision. A clear example of this is the linear infiltration of the left flank in cattle that involves both subcutaneous tissue and underlying muscles for performing rumenotomy or cesarean section. A simultaneous technique is the creation of insensitive weals beside each other in the form of line.

To infiltrate several layers of tissue, the procedure is to inject, from one puncture site, first the subcutaneous tissue and then, in succession by further advancing the needle, the deeper tissues.

Advantages: -

1-Simple and easy technique

2-It consumes smaller amount of anesthetic and shorter time than field block technique

Disadvantages: -

Deposition of the local analgesic at the incision site lead to:

1-Dealyed healing                                 

2-Oedema and hematoma formation that may lead to change in the anatomical features.

C-Field block analgesia: -

In field block all the nerves entering the surgical field are  desesitized. The advantage of field block in compared to linear infiltration is depostion of the analgesic solution away from the incision site, thus minimizing oedema, hematoma, and possible interference with healing.

The disadvantages of local infiltration analgesia include incomplete analgesia and muscle relaxation of the deeper layers of the abdominal wall and its possible toxicity after injection of significant amount of analgesic solution.

1-Cup shape field block: -

It is an inverted pyramidal shape analgesic area that is created by two punctures, and can be used when the pass of nerve supply is not exactly known. Usually it is applied to an area of bulky musculature.

Advantages: -

1-Absence of anatomical distortion at seat of incision

2-When the drug contains vasoconstrictor, it will produce efficient ischemia

3-Complete muscular relaxation                   

4-No retardation of healing

2-Inverted-L block: -

It is a field block technique through which only the dorsal and anterior aspects of the flank region are injected subcutaneously with local analgesic solution to produce complete analgesia of the flank for induction of rumenotomy or cesarean. The main point of neglecting the posterior aspect is that the nerves pass to the flank from the dorsal and anterior aspects caudo-ventrally.

Disadvantages: -

1-It consumes larger amount of anesthetic than linear infiltration and paravertebral block

2-It consumes longer time than linear infiltration

3-Ring block: -

It is a technique used for induction of analgesia by injection of analgesic drug in a ring manner at one level like in teat or digit. On induction of analgesia of the teat, adrenaline shouldn’t be incorporated in the injected solution as vasoconstriction may cause necrosis of the compromised teat. The technique is useful for surgical repair of presternal bursitis in buffalo calves, umbilical hernia, amputation of digit etc…

III- INTRAVENOUS REGIONAL ANALGESIA (IVRA or BIER’S BLOCK)

It is a simple technique usually used in cattle, small ruminant and dog to produce analgesia of the digit and it can be used combined with systemic sedation. In dog it can be performed by injection of 2-3 ml of 1% lignocaine intravenous in the cephalic vein after application of tourniquet on the forearm. Analgesia allover the limb can be achieved and the effect can be reversed just the tourniquet is removed.

This technique can be performed in cattle through catheterization of the limb vein, then the limb is exsanguinated (usually by Esmarch's bandage) and a tourniquet placed around the limb at a pressure adequate to prevent arterial circulation (> 150 mmHg) at the top of the limb.

Local anesthetic (preferably without epinephrine) is then injected into the vein and after a period of 15 minutes the area distal to the tourniquet is anesthetized until the tourniquet is removed.

Disadvantages: -

1-Absence of vein after the limb is exsanguinated (so it is best to have a catheter in place first)

2-Cardiac arrhythmias or arrest due to an inadequate tourniquet (in man there are more problems when using bupivacaine than Lidocaine)

3-Failure to take effect due to inadequate tourniquet, inadequate time, and lack of exsanguination (it doesn't work so well without exsanguination)

4-Collapse when tourniquet is removed because of anoxic waste products re-entering circulation. It is preferable if the animal is recumbent at this time.

5-Damage as a result of leaving the tourniquet for more than 1-1.5 hours

IV- LOCAL ANALGESIA OF FRACTURE

It is a simple technique performed by injection of 2-5 ml of 1% lignocaine (small animals) or 10-15 ml of 1% lignocaine (large animals) into the hematoma as near as possible to the ends of bone. Analgesia will ensue within 5 minutes after injection.

 

References: Hall, C.W. and Clarke, K.W. (1983), Veterinary anesthesia, 8th edition; Hall, L.W. (1978), Wrights veterinaray anesthesia and analgesia, 7th edition