Department of Clinical Sciences, Faculty of Veterinary Medicine, University of Montreal, Saint-Hyacinthe, QC, Canada
What They Are
Local anaesthetics (LAs) are drugs that reversibly bind to Na+ channels and block impulse conduction in nerve impulses. LAs contain an aromatic ring (lipophilic) at one end and an ionizable group at the other and an intermediate chain in between, which can be either an ester (tetracaine, procaine, benzocaine) or an amide (lidocaine, bupivacaine, ropivacaine, mepivacaine, prilocaine and their respective mono-isomers). Potency is directly related to lipid solubility while onset is inversely associated with pKa and lipophilicity; increased protein binding and potency and the vasoactivity of the LAs correlate with increased duration of action. The intermediate chain determines their metabolism (amides, liver; esters, liver and pseudocholinesterases in plasma). Local anaesthetics are the main drugs used for loco-regional anaesthesia and analgesia.
How They Work
LAs stabilize the cell membrane, rendering it non-susceptible to electrical stimuli by altering Na+ -channel conformation. LAs block small unmyelinated C-fibres and myelinated Aδ fibres before other sensory and motor fibres (unmyelinated Ay, Aβ and Aα). In a neuraxial blockade (epidural, intrathecal) from least to most sensitive to LAs are: autonomic, pain, proprioception, and motor fibers. Recovery of sensation is expected to be in the reverse order. In a peripheral nerve block (brachial plexus), in contrast, fibre sensitivity from least to most is: motor, proximal sensitive, distal sensitive.
When administered systemically, lidocaine blocks the ectopic afferent neural activity at the NMDA receptor within the dorsal horn.
Topical anaesthesia. To aid intubation, lidocaine spray. To desensitize only the skin and upper subcutaneous layers, eutectic mixture of local anaesthetics (EMLA) cream (lidocaine and prilocaine) (generally used to aid vascular catheterization). To desensitize mucous membranes, lidocaine gel. Studies in humans and animal models have indicated that lidocaine patches provide analgesia of the skin and underlying tissues and can reach deep tissues to provide perioperative analgesia; for example, for joint surgery and large surgical wounds.
Infiltration anaesthesia: consists of injection of LAs into tissues that surround the area of interest or into joints. Blocking transmission of stimuli in defined, specific, peripheral nerves represents the larger part of loco-regional applications. These techniques can be accomplished by using anatomical knowledge or, in case of some distal limb blocks, even by palpation of the nerve itself. LAs can also be delivered through diffusion (wound soaker) catheters placed within large wounds, especially amputation sites. This technique is best applied as part of a multimodal analgesic protocol.
Neuraxial blockade can be achieved by either epidural or intrathecal application of LAs. Other drugs (opioids, alpha-2 agonists, and others) can also be applied via these routes either alone or together with LAs provided they are sterile and preservative free. It should be noted that single use of morphine with preservative has been used epidurally without problems - repeat dosing should be avoided.
Systemic: lidocaine can be administered intravenously either as a bolus or as a constant-rate infusion to provide pro-kinetic, anti-arrhythmic, inhalant-anaesthetic sparing and anti-inflammatory effects.
Ester (prilocaine, benzocaine) LAs may cause allergic reactions in some animals and methemoglobinemia in cats. Loco-regional, particularly neuraxial techniques, should not be performed if there is skin infection at the puncture site. Other contraindications for neuraxial blockade include coagulation disorders, spinal cord trauma, hypovolemia and septicemia. Toxicities usually result from high plasma concentrations affecting the central nervous system first (except bupivacaine) before cardiovascular depression and death). Central nervous toxicity may manifest as head pressing, star gazing and with increasing doses as stupor and coma. Due to its cardiotoxicity, bupivacaine should not be used intravenously. Other signs of toxicity may include allergic reactions and range from urticaria to anaphylaxis. If clinical signs of toxicity occur, the administration/infusion should be stopped immediately, and in severe cases of cardiac signs, an intravenous lipid emulsion (4 mL/kg bolus, followed by 10 min of 0.5 mL/kg/min of Intralipid® can be administered to augment chances of survival in cardiac arrest after bupivacaine overdose).
When combining different local anaesthetics, do not exceed the maximum dose of either drug.
Adrenaline (epinephrine) may be added as a local vasoconstrictor to decrease tissue absorption (1:200,000 = 5 µg/mL; 1:400,000 = 2.5 µg/mL) and increase duration of effect; with erroneous intravascular injection this may cause short-lived tachycardia. This formulation must not be injected into the extremities due to the risk of tissue necrosis.
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