LOCAL ANALGESIA

I-LOCAL ANALGESIA

Many surgical procedures can be satisfactorily performed under the effect of local analgesia alone, and 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, otherwise sedation should be adopted 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 but 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 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

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 skin or gum infiltration  in pets	2 %	5 minutes	1 hour
Epidural injection	1-2.5 %	10 minutes
Skin or 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 and 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, for the cornea, or for mucous membranes (4%), particularly those of the throat and larynx, prior to endotracheal intubation

Disadvantages: -

Toxicity by over dose that is expressed by drowsiness, twitching and respiratory depression, and finally convulsions and hypotension ensue. 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 % with no vasoconstrictor)

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

B-Mepivacaine HCl (Mepacaine®): -

This compound closely resembles lignocaine HCl, and widely used for human dentistry

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-Its toxicity 10 times that of procaine

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

Disadvantages: -

It can't be autoclaved.

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), so it is indicated for use in situation where prolonged analgesia is required like relief of pain in equine during acute laminitis

Concentration: -

Aim of use	Concentration
Infiltration	0.25%
Nerve block	0.5%
Epidural analgesia	0.75%

TYPES OF LOCAL ANALGESIA: -

I-SURFACE ANALGESIA: -

A-Topical analgesia: -

1-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 very painful.

2-Surface analgesia can be performed by using lignocaine ointment that is applied by skin friction for relief of pruritis

3-Surface analgesia may be performed by using lignocaine 2% aqueous solution topically for relief of superficial abraded or eczematous area

4-Surface analgesia of mucous membrane of the glans penis and vulva can be produced by topical use of lignocaine 2% aqueous solution

5-Surface analgesia of urethral mucous membrane can be adopted by lignocaine 2% gel that works as lubricant and analgesic

6-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

7-Surface analgesia of the cornea can be performed by topical instillation of 4% lignocaine or 0.2% Tetracaine®

8-Surface analgesia of the joints can be performed by intra-synovial injection of 2 % lignocaine

B-Intrasynovial: -

Surface analgesia is employed for relief of pain arises from pathological conditions of the joint and tendon sheath. The technique involves direct injection of local analgesic into the joint or tendon sheath with mechanical manipulation of the joint or sheath for spreading of the drug, and when the joint or tendon sheath is distended with synovia, it is recommended to aspirate some the fluid to prevent dilution of analgesic. Analgesia usually arises 5-10 minutes after injection and lasts up to 1 hour, as a result of direct effect of the drug on the surface of the joint or sheath.

Uses: -

1-Therapeutic, like relief of pain during arthritis

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

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.

A-Intra-dermal: -

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. 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 induction of rumenotomy or cesarean section. A simultaneous technique is the creation of insensitive weals beside each other in the form of line.
Advantages: -
1-Simple and easy technique
2-It consumes smaller amount of anesthetic and shorter time than inverted-L technique
Disadvantages: -1-Dealyed healing
2-Changes in the anatomical features
3-Consumption of large amount of drug than paravetribral techniqueC-Field block analgesia: -

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 while is passes caudo-ventrally.

Advantages: -

1-Seat of injection is far from incision line (not interfere with healing)

2-Simple technique and requires no technical skills or complicated equipments

3-It does not cause change of the anatomical features at seat of incision

Disadvantages: -

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

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: -

It is a simple technique usually used in dogs and 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.

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