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Local anesthetics produce desensitization and analgesia of the skin surface (topical anesthesia), tissues( infiltration and field block) and regional structures( conduction anesthesia, intravenous regional anesthesia)
Local anesthetic techniques are an alternative or addition to intravenous and inhalation anesthesia.
A number of anesthetic drugs are available , they vary in potency, toxicity and cost
The most commonly used Local anesthetic drugs are lidocaine, mepivacaine and bupivacaine Hcl
Vasoconstrictors (epinephrine) are occasionally incorporated with or added to lidocaine to increase the intensity of effect and prolong anesthetic activity
Adding the hyaluronidase increases tissue penetration in the region of infiltration and hasten the analgesic activity.
Local analgesia: is a loss of sensation in a limited body area through a selective transient paralytic action on the sensory terminal nerve and nerve endings .
Preemptive local analgesia in animals undergoing general anesthesia :
will reduce the amount of general anesthetic
minimizing the cardiopulmonary depression
leading to quicker recovery.
It provides a useful pain relief, even beyond the full recovery from general anesthesia.
Mechanism of action of LA
It acts by interference with transmission of impulses by preventing the increase in membrane permeability of the nerve cell to sodium ions.
LAs block nerve conduction by inhibiting influx of sodium ions through ion-selective sodium channels in nerve membrane leading to impairment of the generation of action potential.
The sodium channel itself is a specific receptor for local anesthetic molecules.
Mechanism of membrane and impulse conduction
All clinically used LAA are membrane stabilizing agent
They enter and occupy the membrane channels through which ions normally moves.
The most immediate and apparent effect is to prevent the outflow of sodium ions and therefore block all subsequent ionc flow.
They prevent depolarization and therefore stop or retard conduction of impulses
+ + - - + + -- - - + + + + - - Na + + + + + - - - - Resting (Closed**) Open (brief) inactivated Very slow repolarization in presence of LA LA receptor LA have highest affinity for the inactivated form Refractory period **Closed state may exist in various forms as it moves from resting to open. LA have a high affinity for the different closed forms and may prevent them from opening.
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 equipment
3-The technique can be performed by the surgeon himself with no need for anesthetist
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
Use sterile solutions, equipment's, and techniques
Avoid injection into inflamed area (if possible)
Use as small a gauge needle as practical
Aspirate for blood before injecting
Use lowest effective conc. Of LA drug to produce the desired effect.
Wait for onset of analgesia before proceeding
Desirable characters of local AA
Good penetrating quality of tissue
High potency (low conc can be used)
Long duration of action
Low systemic toxicity
No irritation to nerve or other tissue
Reversible action ( reversible block of nerve conduction)
Ease to be sterilized
NONE totally meets these optimally yet!!
Local anesthetics can be divided chemically into two groups:
Esters: procaine, cocaine, and tetracaine
Amides: lidocaine, mepivacaine, and bupivacaine
Esters: Inactivated quickly by non-specific esterases in the plasma and tissue
Amides : More stable, longer plasma half-lifes
Factors affecting activity and potency
Increased drug lipid solubility tends to slow the rate of onset of action, increase the duration of action, and increase potency.
Increased plasma protein binding tends to be associated with increased duration of action.
Pharmacokinetics of local anesthetics
Systemic absorption of a local anesthetic is dependent on a number of factors:
• The greater the dose injected, the greater the systemic absorption and peak blood concentration
2. Injection site
• Administration into a highly vascular area (e.g. mucosal, pleural or peritoneal surfaces) results in rapid absorption and higher blood concentrations compared with injection into less perfused areas (e.g. subcutaneous tissue, perineural fat).
3. Drug–tissue binding
• The more potent local anesthetics with greater lipid solubility and protein binding are associated with lower systemic absorption .
• Decrease rate of systemic absorption .
LA can be widely distributed to all parts of the body including CNS
Distribution is a means of terminating local drug action ........ not metabolism!!
Due to their rapid metabolism and short plasma half-lives, distribution of ester-type agents is limited.
Conversely, amide local anesthetics are widely distributed after intravenous administration.
C. Metabolism and excretion
• Biotransformation of local anesthetics depends on the chemical structure of the drug.
• These agents are rapidly hydrolyzed in the blood (and to a lesser extent in the liver) by non-specific pseudocholinesterases .
• The rate of hydrolysis varies ( chloroprocaine > procaine > tetracaine ), but clearance is generally rapid and half-lives are measured in minutes.
• Para-aminobenzoic acid (PABA) is a breakdown product of ester metabolism.
• These agents are hydrolyzed by the liver cytochrome enzyme system.
• The rate of hepatic metabolism varies ( lidocaine > mepivacaine > bupivacaine ), but clearance is slower than that of the esters , and half-lives are measured in hours.
– Reduced hepatic biotransformation and clearance of amide local anesthetics may prolong duration of action and increase the risk of toxicity.
Local anesthetic adjuvants and combinations
5 μg/ml (1:200,000) may be added to a local anesthetic solution to decrease local perfusion, delay absorption, and prolong anesthetic action.
When administered into the epidural or subarachnoid spaces, epinephrine may enhance analgesia through interaction with α-adrenergic receptors in the spinal cord and brain.
Effects of vasoconstriction
Increase the margin of safety
Prolong anesthetic activity (up to 5 times)
It may cause disturbance in circulation( teats, distal limbs, penis)
Can increase risk of cardiac arrhythmia and ventricular fibrillation
B. Alpha2 agonists
Effects on peripheral nerve blocks
Studies suggest that the addition of an α2 agonist may potentiate peripheral nerve blocks with local anesthetics in human patients.
Effects on epidural anesthesia
Adding xylazine to lidocaine potentiates and prolongs anesthesia when administered into the caudal epidural space in horses.
This synergism is due to activation of α2 adrenergic receptors in the spinal cord dorsal horn.
This combination is used commonly in clinical equine practice for caudal epidural anesthesia.
This enzyme may be added to a local anesthetic solution to break down hyaluronic acid which functions as mesenchymal tissue cement.
increases tissue penetration in the region of infiltration and hasten the onset of analgesic activity
D. Carbon dioxide
• Carbonation of LA solutions causes local intracellular acidosis which,, increases the amount of intracellular ionized drug available to bind Na+ channel receptors and accelerates the onset of action.
• This practice does not appear to significantly improve block quality in equine patients.
F. Combinations of local anesthetics
• The combination of a quick-onset, short-acting, local anesthetic with a slow-onset, long-acting, local anesthetic has theoretical benefit.
Local anesthetic toxicity
When used correctly, local anesthetics have proven to be safe, and adverse effects are not common.
Toxicities can be divided into three categories:
local tissue toxicity
A. Systemic toxicity
• This is most often the result of inadvertent and rapid intravenous infusions producing a dose-dependent continuum of effects on the brain and heart .
Central nervous system
In horses, procaine is an exception to this rule, demonstrating a low anesthetic potency but a high propensity to cause CNS stimulation .
Low doses of local anesthetics tend to produce CNS depression, while higher doses cause CNS excitation.
Significant CNS excitation is evident after IV boluses of approximately 2.5 mg/kg of procaine in horses (50 ml for 400 kg) and is characterized by deep, rapid and forced inhalations, fine muscle tremors and pawing at the ground.
• Skeletal muscle tremors are usually the first signs of toxicity in horses, and progress to seizures, unconsciousness, coma, and respiratory arrest.
– Muscle tremors are evident at serum lidocaine concentrations of approximately 3 μg/ml in horses , or with IV boluses of approximately 4–6 mg/kg (80 ml for 400 kg).
– Seizures are induced with IV boluses of approximately 6–8 mg/kg.
Skeletal muscle cont….
Adult horses safely tolerate up to 250 ml of 2% lidocaine ( 11 mg/kg) for infiltration of the paralumbar fossa for abdominal surgery.
Benzodiazepines, barbiturates, inhalational anesthetics, and potentially propofol are effective in treating local anesthetic induced seizures and oxygen supplementation are indicated in cases of toxicity.
( Cardiovascular system (CVS
• Significantly higher serum concentrations of local anesthetic are required to produce CVS toxicity compared with CNS toxicity.
• Characteristics of CVS toxicity :
All may decrease myocardial contractility.
The less potent agents ( lidocaine , mepivacaine ) cause bradycardia and hypotension.
The more potent agents ( bupivacaine , ropivacaine ) cause malignant ventricular dysrhythmias that are resistant to treatment.
B. Local tissue toxicity
When applied in excessively high concentrations, all local anesthetics have the potential to be toxic to nerve tissue , especially when administered via the spinal route.
Reports of adverse neurologic sequelae after subarachnoid administration
C. Allergic reactions
• Allergic reactions to aminoamide local anesthetics are extremely rare .
• Aminoester local anesthetics are metabolized to para-aminobenzoic acid (PABA) derivatives which may induce Type I hypersensitivity reactions in a small percentage of patients.
Limiting the total quantity of drugs
Use of dilute solution
Administration of CNS depressant
Repeated aspiration before injection
of local AA How to avoid toxicity
the original local anesthetic ( Isolated in 1860 from Coca leaves)
is the only one to cause vasoconstriction.
It is now not used as a local anesthetic because of its potential for abuse (Schedule II).
Local anesthetic agents commonly used in vet. practice
Procaine (novocaine) largely replaced cocaine ( First synthetic local anesthetic) , and has in turn been displaced by lignocaine.
It presents as white powder and it used as freshly prepared watery solution.
It causes local vasodilation and poorly absorbed from the surface of the MM or skin so it is not used for topical analgesia.
Combined with adrenaline hydrochloride, procaine absorption is slow, solutions may be sterilized by boiling and there is minimal tissue irritation.
• Has an intermediate onset (10–15 min) and short duration (45–60 min).
• A 2% solution has been used for local infiltration, peripheral nerve blocks, and intra-articular anesthesia.
3.Lidocaine (xylocaine or Lignocaine)
Lidocaine has largely replaced procaine as it has the advantage of:
• extreme stability
• more rapid diffusion
• longer duration of action
• useful surface analgesic activity on mucous membranes and cornea as it has the ability to penetrate the MM
• The most commonly used local anesthetic in clinical equine practice.
• Rapid onset (5–10 min) and intermediate duration (60–90 min).
• Available as:
– An aqueous solution in various concentrations (0.5–5%) with or without epinephrine .
– A gel in concentrations of 2–5%.
– A 10% lignocaine spray.
– A transdermal preparation.
• Effective doses for peripheral nerve blocks and caudal epidural anesthesia in horses typically do not exceed 0.2–0.3 mg/kg (i.e. approximately 5–8 ml of the 2% solution in an average adult horse).
it is used as topical anesthesia in the form of jell or oint.
Commonly used for induction of local infiltration and epidural analgesia.
Most common conc. 1- 2%
Doesn’t cause vasodilation
Studies in human patients indicate that systemic lidocaine has a beneficial effect on bowel motility, and this makes it an attractive analgesic adjuvant for the equine colic patient
4.Bupivacaine hydrochloride ( Marcaine)
• Is a potent local anesthetic.
• Variable onset (10–30 min, depending on the route of administration).
• Long duration (180–480 min) of action.
• It is available as an aqueous solution in various concentrations (0.25–0.75%).
• The 0.5% (5 mg|ml) formulation is most commonly used .
• Doses for peripheral nerve blocks and caudal epidural anesthesia are in the range 0.05–0.08 mg/kg (i.e. 5–8 ml of the 0.5% solution in an average adult horse).
Not effective topically.
• Due to its extended duration of action, caution is advised when administering it into the caudal epidural space, as overdose could result in very prolonged periods of hindlimb weakness and motor deficits.
• More cardiotoxic than lidocaine and IV administration is contraindicated.
• Available as a single optical isomer ( levo - bupivacaine ).
– Levo - bupivacaine appears to be associated with 30–40% less systemic toxicity .
5.Cinchocaine (Nupercaine™, Dibucaine®)
is more toxic than procaine, but concentrations for epidural block and surface analgesia are lower (0.5%).
Other properties include:
longer analgesia than with procaine
drug readily decomposed by action of alkalis and therefore syringes and needles should, if not sterile, be boiled in bicarbonate-free water.
• Is similar to lidocaine in its activity profile.
• It has a rapid onset (5–10 minutes).
• Has a somewhat longer duration than lidocaine (120–180 minutes).
• A 2% solution is the most commonly used preparation for infiltration anesthesia, intra-articular anesthesia, peripheral nerve blocks and epidural anesthesia.
• It is administered at doses similar to those for lidocaine .
• It is less effective than lidocaine for topical anesthesia.
The local anesthetic of choice for topical corneal anesthesia in horses.
Available as a 0.5% aqueous solution .
When dropped on the cornea it has a rapid onset of action (within 1 minute) and lasts for about 15-30 minutes.
It is non-irritant
does not affect the size of the pupil.
Intravenously administered lidocaine has anesthetic and analgesic properties.
Lidocaine can be administered IV during anesthesia or to the awake horse, or in combination with drugs such as ketamine for analgesia.
The mechanisms underlying systemic lidocaine ’s analgesic and anesthetic actions are not fully understood but it may has a direct action on spinal transmission
Physiological effects of intravenous lidocaine
• Lidocaine has a number of properties which indicate that it may be a beneficial drug to administer in association with general anesthesia or in the awake horse. These include:
minimum alveolar concentration ( MAC )reduction of volatile anesthetics.
MAC is the concentration of the vapour in the lungs that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus.
Physiological effects of intravenous lidocaine cont…..
Reduction of risk of postoperative ileus.
A protection against endotoxemia
A protection against ischemic and reperfusion injury.
Clinical use of intravenous lidocaine
A-As an adjunct to general anesthetics
Used with volatile agents to improve analgesia.
Decrease in MAC is dose-dependent.
Infusion rates of 50 μg/kg/min reduce MAC by ~ 25%.
Infusion rates of 100 μg/kg/min reduce MAC by 40–50%.
Can be infused concurrently with ketamine or alpha2 agonists.
MAC ↓ (60–80%) with ketamine (50 μg/kg/min) and lidocaine (75 μg/kg/min).
Clinical use of intravenous lidocaine cont ….
B. To provide analgesia in the awake horse
Can be safely infused at 50 μg/kg/min for up to 72 h.
A loading dose (1.5 mg/kg, IV) can be infused over 5 minutes.
A dose of 50 μg/kg/min can be safely infused with ketamine (0.2 mg/kg/h).
• The likelihood of systemic toxicity with intravenous lidocaine is related to its plasma concentration.
A. CNS effects
Low concentrations of lidocaine are sedating.
Increasing concentrations result in muscle twitching and ataxia.
The horse may become recumbent temporarily.
Administration of sedatives causes a right shift in dose–response curve.
B. Cardiovascular effects
Occur at higher plasma concentrations than do CNS effects.
Result from a delay in impulse transmission by Na channel blockade. This leads to:
Decreased myocardial contractility.
• Treatment of cardiac toxicity can be unrewarding.
• It is fairly unresponsive to inotropes (e.g. dobutamine ) and parasympatholytic drugs (e.g. atropine ).
Common methods of producing local anesthesia
Surface (topical) anesthesia.
Intravenous regional local anesthesia.
1. Topical or surface analgesia
freezing of the superficial layer of the skin by ethyl chloride spray, carbon dioxide snow or ice.
application of lidocaine oint. on painful eczematous area
instillation of the eye by LA soln as ophthaine or xylocaine 2%
application of lubrication jell contain LAA on the glans penis, vulva and during urethral catheterization.
instillation in the nasal chamber of the horses
Examination of the eye
Removal of foreign bodies
Removal of hypertrophied glands of the third eyelid
Topical anesthetics for instillation:
Lidocaine 2-5% ,Proparacaine 0.5%
Tetracaine 0.5%, Butacaine 0.5%
It gives 15 min anesthesia after single instillation and 2hrs after 3-5 series of instillation with one min intervals.
Diagnosis of lameness caused by joint disorder
To relief pain caused by arthritis
Preparation of the site of arthrocentesis for aseptic injection
Insertion of the injection needle intraarticularly
Aspiration of the excessive synovial fluid to prevent excessive dilution of the LA
Inj of LA dose 5-10 ml according to the size of the joint
The onset of effect begin after 10 min and persist for 60 min.
By this method the nerve endings are affected at the actual site of operation.
Most minor surgery can be done this way, excluding surgery on teats in cattle or small animal digits.
Indicated with or without sedation for surgical interference in ruminant for rumenotomy, C- section, removal of neoplasms, repair of hernia…
Surgical operations require standing position such as medial patellar desmotomy.
Require no greater skill or the knowledge of the anatomy of the site.
muscle relaxation and no interference to healing.
Large volume of LAA in the tissue to be incised and sutured.
Epinephrine in the anesthetic solution may be interfering with the blood supply and retard healing.
If the flank laparotomy is being done the LA must not only infiltrated SC but into the fascia and the muscle as well.
Types of infiltration analgesia
2.Inverted "L" or "T" or inverted "V "block
Preparation of the area to be anaesthetized for aseptic interference.
Define the intended line of the incision by one of the following methods :
1. scratching of the skin by the tip of the injectable needle
2. intradermal injection of 1 ml of the LA at both ends of the line to make blebs.
3. using of surgical marker
Area blocked: skin and muscles layers of the flank and parietal peritoneum along the line of incisions.
Needle: 18 G, 7-10 cm
Anesthetic: 10-100 ml of 2% lidocaine
Methods: make multiple SC injections of 0.5 to 1 ml of AA, 1-2 cm apart then infiltrate the muscle layer and the partial peritoneum through the desensitized skin.
The inverted red L is the site of inverted L block and the green line is the site of linear infiltration for rumenotomy
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.
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 paravertebral
4- lack of muscle relaxation
5-incomplete blocking of the deeper layer of the abdominal wall
6-formation of haematoma along the incision line
8-toxicity after inj to peritoneal cavity( more than 250 ml (5g) of 2% lidocaine.
Linear infiltration analgesia for rumenotomy
2.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 .
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
It is a field block technique through which only the dorsal and anterior aspects of the flank region are injected subcutaneously with LA solution to produce complete analgesia of the flank ( for 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.
Area blocked: flank caudal and ventral to site of injection
Site : a line along the caudal border of the last rib and a long a line ventral to the lumber transverse process from the last rib to the 4 th lumber vertebra.
Needle: 18 G, 7 cm
Anesthetic: up to 100 ml of 2% lidocaine in adult cattle
The inverted red L is the site of inverted L block and the green line is the site of linear infiltration for rumenotomy
1-Seat of injection is far from incision line (not interfere with healing)
2-Simple technique and requires no technical skills or complicated equipment's
3-It does not cause change of the anatomical features at seat of incision
1-It consumes larger amount of anesthetic than linear infiltration and paravertebral
2-It consumes longer time than linear infiltration
3. incomplete block of the deeper layer of the abdominal wall(particularly the peritoneum).
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.
INDICATION: The technique is useful for surgical repair of presternal bursitis in buffalo calves, umbilical hernia, amputation of digit ,operations of cows teats.
Vasoconstrictors should not be added on solutions used to produce ring block in teats, for prolonged vasoconstriction may results in ischemic necrosis of the end of the teat.
Ring block (olecranon bursitis in buffalo)
( Ring block (umbilical hernia
Intravenous regional local anesthesia(IVRA)
In this technique, a limb vein is catheterized.
The limb is then exsanguinated (Esmarchs bandage), and a tourniquet placed around the limb.
Local anesthetic (preferably without epinephrine) is then injected into the vein.
After a period of 15 minutes the area distal to the tourniquet is anesthetized until the tourniquet is removed.
Intravenous regional anesthesia is commonly used in cattle for amputation of a digit.
The analgesic technique is particularly effective in the hind limb.
It is also used to a lesser extent for dogs to enable amputation of the digit combined with systemic sedation.
No special knowledge of anatomy is needed.
Only one injection is required
Little risk of introducing bacteria
Rapid onset of analgesia( 5-10 min)
Rapid recovery after removal of tourniquet(5-10 min).
Potential problems of IVRA
Difficulty in finding the vein .
Cardiac arrhythmias or even arrest. This is due to an inadequate tourniquet.
Failure to take effect. Common reasons are inadequate tourniquet, inadequate time, and lack of exsanguinations .
Collapse when tourniquet is removed. This is because of anoxic waste products re-entering circulation. It is preferable if the animal is recumbent at this time.
Damage as a result of the tourniquet being left on too long. This is rare. It can be left on for 1-1.5 hours on the limb of cattle and dogs
IVRA (teat surgery)
Local anesthesia for standing laparotomy in cattle
1. infiltration analgesia
2. proximal paravertebral anesthesia
3. distal paravertebral anesthesia
4. segmental dorsolumber epidural anesthesia
Abdominal surgeries in which these anesthetic techniques can be used:
Rumentomy, cecotomy, correction of abomasal displacement, intestinal obstruction, volvulus, cesarean section, ovariectomy and liver or kidney biopsy.