Definition of local anesthesia
0 They are substances that temporarily block nerve conduction, starting from the application site, when used at appropriate concentrations.
0 They block the permeability of the membrane against Na + and perform membrane stabilization. Therefore, increasing the Na+ concentration in the external environment causes a decrease in its effect.
0 Elevated extracellular calcium levels antagonize local anesthetics. An increase in the potassium level creates an increase in the effect of the local anesthetic.
0 With the use of hydrochloride salt in the local anesthetic solution, the acidity of the environment increases and the effect of the local anesthetic decreases. For the same reason, there is no effect if local anesthetic is infiltrated into infected tissues because the extracellular pH is low.
0 Local anesthetics mostly bind to α-acid glycoprotein (especially those of amide structure).
Effects of local anesthetics on nerve fibers
0 Unmyelinated, small diameter fibers (with the exception of A' fibers) and fibers with slow conduction rates (such as A' and C fibers that transmit pain sensation) are blocked first.
0 Their susceptibility to LAs is ordered as follows: C > B > A> y = ᵝ > a
0 Sequence of loss of sensation with local anesthetics; pain, temperature, touch, proprioception, motor
0 The return of lost senses occurs in the opposite order.
Classification
Those with 0 ester structure; They are broken down in plasma by pseudocholinesterases.
0 Amide structures; They are metabolized in the liver by N-dealkylation.
0 Local anesthetics prevent depolarization by stabilizing the nerve membrane.
0 Action potential generation slows and stops as the electrical excitation threshold rises.
0 Local anesthetics do not change the resting and accompaniment potential of nerve cells and fibers.
0 In local/regional anesthesia:
• Various blocks (surface anesthesia, infiltration and field block, regional intravenous anesthesia)
• Peripheral blocks (nerve, ganglion and plexus blocks)
o Cervical plexus block
o Brachial plexus block (with interscalene, supraclavicular, infraclavicular, axillary approach)
o Paravertebral somatic nerve blocks
o Psoas compartment block
o Intercostal nerve block
• It is classified as central / neuraxial blocks (spinal and epidural anesthesia).
Important Points in Local Anesthesia
Each nerve fiber of a certain diameter has a minimum drug concentration (Cm) that inhibits conduction. Cm is larger in thick nerve fibers. The pH of the medium and Cm are inversely proportional. In the presence of exudate and pus, the pH is 5-6 and local anesthetics are not effective.
local anesthetic effect; It is inversely proportional to the calcium content of the medium and directly proportional to the nerve impulse rate.
Since action potential generation and conduction in myelinated nerves is in the nodes of Ranvier, local anesthetics affect this place.
Loss of pain, temperature, touch, proprioceptive sensation and skeletal muscle tone occurs in sequence with local anesthesia. Since motor fibers are located in the environment, they may be blocked before sensory fibers.
Since the peripheral fibers of the nerve innervate the proximal parts, diffusion starts externally in the extremity block and the proximal part is blocked last. When the local anesthetic is withdrawn, the last proximal function returns.
Local anesthetics are not absorbed through intact skin. They are rapidly absorbed by topical application through the mucous membranes. They easily cross the blood-brain and placental barriers and are not absorbed from the stomach.
Local anesthetics other than cocaine, which is a vasoconstrictor, are dual-acting. In other words, they cause vasoconstriction at low intensity and vasodilation at clinical intensity.
When the local anesthetic is absorbed, the first place it encounters is the lung. Here, most of it is excreted and other organs are prevented from suddenly encountering high doses of anesthetic.
They show their effects by binding to Na+ channels in the neuron membrane. When Na+ channels are open and inactivated, they bind and compete with Na+ and Ca+2 for this binding.
Prolonging and Increasing the Local Anesthetic Effect
0 Provided by the addition of vasoconstrictor or carbonation. Absorption slows down, as a result, the duration of action is prolonged, systemic toxic reactions are prevented and bleeding is reduced.
0 Adrenaline of 1/200,000 is most commonly used as a vasoconstrictor.
0 Since ischemia and gangrene may occur, vasoconstrictor is not used in the procedures in the distal hand, ankle, fingers, earlobe, nose and penis.
0 Local anesthetic without vasoconstrictor is applied in the elderly, hypertensive, those with heart disease, peripheral vascular disease, thyrotoxicosis, those taking MAO inhibitors, tricyclic antidepressants and obstetrics.