Local Anesthetics

  • Video Focus: Local Anesthetics

  • Series Purpose: Discuss anesthetics, starting with local ones that numb specific areas and later addressing general anesthetics that induce unconsciousness throughout the body.

Neuronal Function Overview

  • Understanding Neurons: Essential before discussing anesthetics. Neurons are the basic building blocks of our nervous system, responsible for transmitting signals throughout the body. Think of them like wires in a circuit, sending electrical impulses that allow us to move, feel, and respond to our environment.

  • Action Potential: Movement of charge along the length of a neuron. This is how neurons communicate with one another.

  • Membrane Potential: Change during firing illustrated in graphs. When a neuron fires, there is a dramatic shift in the charge difference across its membrane, which is recorded in these graphs to help us visualize the process.

Visual Aid

  • Top graph: Membrane potential changes, showing how the charge changes when the neuron is activated.

  • Bottom graph: Permeability of axon membrane to ions (sodium and potassium), depicting how the membrane allows different ions in and out during the firing process.

Neuronal Firing Mechanics

  • Resting Neuron:
      - Resting membrane potential: -70 millivolts. This is the charge of the neuron when it's not sending a signal, working like a battery waiting to be used.

  • Sodium and Potassium Channels:
      - When stimulated, specific sodium channels open, causing sodium influx. When a neuron receives a signal, it triggers channels to open and let sodium ions rush in, making the inside of the neuron more positively charged.
      - Voltage-gated sodium channels lead to rapid sodium influx, causing a membrane potential spike. This rapid change is what we call an action potential, leading to the transmission of the nerve signal.
      - Peak reached approximately at +30 millivolts where:
        - Sodium channels close. As quickly as they open, they also close to stop the influx of sodium.
        - Potassium channels reopen, allowing potassium to flow out and help reset the charge of the neuron back to its resting state.
        - Repolarization occurs. This process returns the neuron to its original resting state, ready for the next signal.

Function of Local Anesthetics

  • Mechanism: Block influx of sodium ions to prevent neuron firing. Local anesthetics work by stopping sodium ions from entering the neuron, which means the neuron cannot send pain signals to the brain.

  • Importance of Sodium Channels:
      - Composed of one large protein with four similar domains, each with six transmembrane helices (S1-S6). These channels are crucial for generating action potentials.
      - Domains vary, leading to different sodium channels identified by location and function. Some sodium channels may perform different roles in various parts of the nervous system.

  • Sodium Channel Structure:
      - 3D structure has a pore, where sodium moves through. This allows us to understand how anesthetics interact with the channel.
      - Local anesthetics interact mainly with specific helices. By doing so, they effectively block the channel from functioning normally.

Genetic Mutations in Sodium Channels

  • Sodium Channel 7: Susceptible to mutations. Mutations in these channels can lead to health issues or change how pain is perceived.
      - Erythromalgia: Mutation causing hyper-excitability leading to:
        - Pain and inflammation. This condition causes intense burning pain, usually in the hands or feet.
        - Symptoms: reddening (sunburn-like) and warmth due to increased blood flow as the body tries to deal with pain signals.

  • Hypo-excitable mutations: Sodium channels that don't open:
      - Result in insensitivity to pain. This may sound beneficial, but it can prevent someone from noticing harmful situations.
      - Pain is clinically useful as it signals harm (sharp objects, heat). Location of sodium channels provides essential pain signals, letting us know when to pull away from harmful stimuli.

Types and Administration of Local Anesthetics

  • Local Anesthetics Delivery Methods:
      - Topical Administration: (e.g., lidocaine cream) applied on skin. This method is commonly used in outpatient settings for minor procedures.
      - Infiltration: Injection under skin. This allows for localized pain relief in a specific area.
      - Nerve Block: Target specific nerve conduction zones. This type of anesthesia can numb larger areas of the body by blocking signals from specific nerves.
      - Epidural/SPinal Block: Affect signaling from spinal cord. These methods are often used during childbirth to manage pain effectively.

  • Comparison of Local vs. Systemic Effects:
      - Local anesthetics act at specific neuron locations; systemic absorption can lead to undesired effects, such as affecting other parts of the body.

Structure of Local Anesthetics

  • Prototypical Structure:
      - Hydrophobic tail with a phenyl ring. The hydrophobic part helps the anesthetic to pass through cell membranes.
      - Ionizable head connected via a linker. This structure allows anesthetics to interact with sodium channels effectively.

  • Types of Local Anesthetics:
      - Ester Types: Contain ester bonds (e.g., benzocaine). These are typically shorter-acting anesthetics.
      - Amide Types: Contain amide bonds (e.g., lidocaine). Amide anesthetics are usually more stable and longer-lasting.

  • Special Note: Benzocaine lacks an ionizable head but is used as a local anesthetic. This unique characteristic means it works differently from others.

Chemistry and Mechanism of Action

  • Entry into Sodium Channels:
      - Local anesthetics can be hydrophilic or lipophilic, influencing their action. This affects how they travel within the body and their ability to block sodium channels.
      - Mechanism differences:
        - Hydrophilic: Moves through the membrane into the intracellular fluid, establishes equilibrium, and blocks sodium channels.
        - Lipophilic: Interacts with membrane, can collapse pore structure or wiggle into channel through protein.

  • Binding Characteristics:
      - Local anesthetics bind more effectively to open/inactive sodium channel states. This means they are more effective when the channels are being used.
      - Quickly firing neurons capture local anesthetics more effectively. Neurons transmitting signals rapidly are more likely to be affected by anesthetics.

  • Variability in Binding:
      - Bupivacaine: Binds to slowly opening channels (cardiotoxic). This means it can have more side effects on the heart.
      - Lidocaine: Binds to rapidly opening channels (fewer cardiac effects). This makes lidocaine a safer option in many situations.

Ionization of Local Anesthetics

  • Importance of Ionization:
      - The ionized form (active) acts in blocking channels; unionized form permeates membranes better. This is important for the effectiveness of the anesthetic.

  • Entry Process:
      - Local anesthetic traverses:
        1. Nerve sheath.
        2. Axon membrane to reach axoplasm, then blocks sodium channels. This journey is necessary for the anesthetic to start working effectively.

Systemic Considerations of Local Anesthetics

  • Local action preferred; systemic effects problematic. When anesthetics enter the bloodstream, they can cause various side effects that are undesirable.

  • Use of Vasoconstrictors (Epinephrine):
      - Increases local retention of anesthetic, minimizing systemic absorption. This helps to keep the anesthetic in the targeted area longer.
      - Quick metabolism in the body limits systemic side effects. The body breaks down the anesthetic to minimize potential problems.

  • Potentially severe adverse reactions if systemic absorption occurs:
      - CNS effects range from convulsions, respiratory depression, cardiac issues, and potential death due to overdose. It highlights the importance of careful dosing.

Pharmacodynamics of Local Anesthetics

  • Relationship between plasma concentrations and effects:
      - Side effects vs. desired effects differentiation. Understanding this helps healthcare professionals use anesthetics safely.
      - Numbness, lightheadedness, and more from low concentrations; convulsions and respiratory arrest significantly higher. These highlights show how sensitive the body can be to these drugs.
      - Careful monitoring necessary, as overlaps exist in response thresholds to drugs. This is crucial for patient safety during procedures.

Metabolism

  • Primary Metabolism Location: Liver. This is where most local anesthetics are processed.

  • Mechanisms for amide drugs:
      - Dealkylation: Cutting off parts of ionizable head.
      - Hydrolysis: Breaking down hydrophobic tail.

  • Half-life determinants: Speed of entry into systemic circulation; prolonged by vasoconstrictors. This influences how long the effects of the anesthetic last.

Conclusion

  • Local anesthetics focus: Block pain signals at chosen locations. They are crucial tools in medicine for managing pain during and after procedures.

  • Subsequent topic: General anesthetics, which affect the whole body intentionally. These are more complex and involve different considerations for safety and effectiveness.