Dr. James Young Simpson was thoughtful and nervous, as it was a difficult case. His patient was a woman with a deformed pelvis and the challenge of giving birth. James wondered if the baby and mother could hold out. As we know, in 1847, medical advances were precarious. But James had a great idea, he used ether, and the first baby to come into the world under the effects of it was named Anesthesia. As early as 1853, chloroform enjoyed a considerable reputation... and the rest is history.
Local anesthesia in dentistry allows dentists to perform millions of procedures daily in the absence of pain, which is why local anesthetics are of great importance in dental practice. In this article we will briefly talk about how these drugs work and how they provide a temporary block of nerve conduction, to help you as a guide on which one to choose in each case. You're ready? Don't go numb!
How do local anesthetics work?
We will define local anesthesia as a loss of sensitivity in a previously determined area, which is produced by a depression in the excitation in the nerve terminals or by inhibiting the process. conduction in peripheral nerves.
The vast majority of local anesthetic drugs do not irritate the tissues, in normal doses, and their effect is reversible, however, one must be very careful with systemic toxicity because they are absorbed into the system. cardiovascular.
The majority of local anesthetics used today meet two very important criteria such as the speed with which the anesthesia begins to act and an appropriate duration to perform the procedure. clinical procedurethat is required, choosing the appropriate anesthetic and using the nerve block according to the necessary time.
According to Dr. Malamed's Manual of Local Anesthesia, local anesthetics possibly interfere with nerve membrane excitation through one or more of these ways:
- Altering the basic resting potential of the nerve membrane.
- Altering the threshold potential (activation value).
- Slowing down the speed of depolarization.
- Prolonging the speed of repolarization.
It has been determined that local anesthetics mainly affect the depolarization phase of the action potential. The effects of these anesthetics include a reduction in the rate of depolarization, especially in the slow depolarization phase. This results in the cell failing to achieve a level of depolarization sufficient to activate the membrane of a nerve fiber and, as a result, a propagated action potential is not generated. No change in the rate of repolarization is observed.
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Sequence of the mechanism of action of local anesthetics
The process begins with the migration of calcium ions from the sodium channel receptor site, which facilitates the next stage. At this stage, the local anesthetic molecule binds to its specific receptor, which triggers the blockade of the sodium channel and, as a consequence, decreases the capacity for the passage of sodium ions. This leads to a reduction in the speed of electrical depolarization, preventing the cell from reaching the threshold necessary to generate an action potential. As a final result, action potentials do not propagate, which is known as conduction block.
As you well know, local anesthetics can involve great complexity and our blog post is intended to be an elementary reminder, but if you want to deepen or refresh your knowledge, we recommend taking a look at topics related to neurophysiology, principles of generation and transmission of nerve impulses, classification of peripheral nerves according to the size and physiological properties of the fiber, electrophysiology of nerve conduction, impulse propagation, among other topics.
Remember that in Dentaltix you can find everything you need to apply local anesthesia, such as long and short needles , carpule syringes and more.
Most used local anesthetics and their properties
Most injectable local anesthetics are tertiary amines, except for a few such as prilocaine and hexylcaine are secondary amines.
Next, we will review the chemical configuration, physicochemical properties and pharmacological properties of the most used local anesthetics.
Mepivacaína
Chemical configuration
- Intermediate chain: Amides
Physicochemical properties
- Molecular weight: 246
- Dissociation constant (pKa) (36º): 7.9
- Base percentage (RN) at pH 7.4: 33
- Approximate onset of action (min): 2-4 (Fast)
Pharmacological properties
- Approximate lipid solubility: 1.0
- Usual effective concentration (%): 2-3
- Protein binding: 75
- Duration: Moderate
Lidocaine
Chemical configuration
- Intermediate chain: Amides
Physicochemical properties
- Molecular weight: 234
- Dissociation constant (pKa) (36º): 7.7
- Base percentage (RN) at pH 7.4: 29
- Approximate onset of action (min): 2-4 (Fast)
Pharmacological properties
- Approximate lipid solubility: 4.0
- Usual effective concentration (%): 2
- Protein binding: 65
- Duration: Moderate
Prilocaine
Chemical configuration
- Intermediate chain: Amides
Physicochemical properties
- Molecular weight: 220
- Dissociation constant (pKa) (36º): 7.7
- Base percentage (RN) at pH 7.4: 25
- Approximate onset of action (min): 2-4 (Fast)
Pharmacological properties
- Approximate lipid solubility: 1.5
- Usual effective concentration (%): 4
- Protein binding: 55
- Duration: Moderate
Articaine
Chemical configuration
- Intermediate chain: Amides
Physicochemical properties
- Molecular weight: 320
- Dissociation constant (pKa) (36º): 7.8
- Base percentage (RN) at pH 7.4: 29
- Approximate onset of action (min): 2-4 (Fast)
Pharmacological properties
- Approximate lipid solubility: 17
- Usual effective concentration (%): 4
- Protein binding: 95
- Duration: Moderate
Etidocaine
Chemical configuration
- Intermediate chain: Amides
Physicochemical properties
- Molecular weight: 276
- Dissociation constant (pKa) (36º): 7.9
- Base percentage (RN) at pH 7.4: 25
- Approximate onset of action (min): 2-4 (Fast)
Pharmacological properties
- Approximate lipid solubility: 140
- Usual effective concentration (%): 0.5-1.5
- Protein binding: 94
- Duration: Long
Bupivacaine
Chemical configuration
- Intermediate chain: Amides
Physicochemical properties
- Molecular weight: 288
- Dissociation constant (pKa) (36º): 8.1
- Base percentage (RN) at pH 7.4: 17
- Approximate onset of action (min): 5 – 8 (Moderate)
Pharmacological properties
- Approximate fat solubility: ND
- Usual effective concentration (%): 0.5-0.75
- Protein binding: 95
- Duration: Long
Tetracaine
Chemical configuration
- Intermediate chain: Esters
Physicochemical properties
- Molecular weight: 264
- Dissociation constant (pKa) (36º): 8.4
- Base percentage (RN) at pH 7.4:7
- Approximate onset of action (min): 10-15 (Slow)
Pharmacological properties
- Approximate lipid solubility: 80
- Usual effective concentration (%): 0.15
- Protein binding: 85
- Duration: Long
Procaine
Chemical configuration
- Intermediate chain: Esters
Physicochemical properties
- Molecular weight: 236
- Dissociation constant (pKa) (36º): 9.1
- Base percentage (RN) at pH 7.4:2
- Approximate onset of action (min): 14-18 (Slow)
Pharmacological properties
- Approximate lipid solubility: 1.0
- Usual effective concentration (%): 2-4
- Protein binding: 5
- Duration: Short
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Factors that may affect local anesthesia
Now that we know more specific information regarding the properties of local anesthetics, let's analyze some concepts and the influence they have on the action of local anesthetics.
The Pka, which is the dissociation constant, can affect the onset of anesthesia, that is, a lower pKa value is related to a faster onset of action, since it implies the presence more RNA molecules available to diffuse through the nerve sheath, which in turn reduces latency.
On the other hand, we have lipid solubility that decisively influences anesthetic potency, that is, the increase in lipid solubility is related to an increase in potency.
Protein binding is also an important factor, especially in the duration of the anesthetic effect, since an increase in the affinity for protein binding leads to a greater binding force of the cations anesthetics (RNH+) to the proteins located in the receptors, which, consequently, prolongs the duration of their action.
Conclusion
It is impossible not to begin this article with a little history of medicine, but if we think about how many bad times and pain anesthesia has saved us, we would appreciate it even more.
In this article we have made a small reminder about some essential concepts and we have classified local anesthetics according to their chemical and pharmacological properties, providing valuable information on their chemical configuration, pKa, lipid solubility and duration of action. action. We have also highlighted how these factors influence the onset of anesthesia, the potency and duration of the effect.
We hope that this complete guide on local anesthetics used in dentistry will help you when making the decision about which anesthetic to use in each case. Remember to follow us on our networks and recommend our articles if you find them interesting. Until next time!
