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
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.
• 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 .
• 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.
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.
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.
• 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 .
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.
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
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.