General and Local Anesthetics

Introduction

Anesthetics are crucial in pharmacology, especially for anesthesia, surgery, emergency medicine, and pain management.
This lecture covers mechanisms, potency, clinical applications, and adverse effects of both general and local anesthetics.
Before anesthesia, surgeries were performed without effective pain relief.

Definition of Anesthetics

An anesthetic produces partial or complete loss of sensation, with or without loss of consciousness.

Types of Anesthetics

General Anesthetics: Induce loss of consciousness and amnesia.
Local Anesthetics: Induce loss of sensation in a specific area without loss of consciousness.

General Anesthetics

Classification of General Anesthetics

Inhalational Anesthetics: Gases like nitrous oxide and volatile agents like isoflurane and sevoflurane.
Intravenous Anesthetics: Propofol, thiopental, and ketamine.

Anesthetic Potency

All general anesthetics have a steep concentration-response curve.
Minimal Alveolar Concentration (MAC): Measures anesthetic potency.
- The lower the MAC, the more potent the anesthetic.
- Most inhaled anesthetics have a low MAC.
- Nitrous oxide has a high MAC, limiting its use as a standalone anesthetic.

Mechanisms of Action

Anesthetics have different mechanisms of action currently being debated.

Lipid Theory

Traditional theory suggests anesthetic potency is linked to lipid solubility. There's a great correlation between the two.

Current Belief

Anesthetic molecules bind to hydrophobic pockets within specific membrane proteins.
GABA A Receptors: Ligand-gated chloride channels with pentameric subunits that inhibit neuronal activity by potentiating receptor activity.
Potassium Channels: Reduce excitability by moving potassium into the cell, decreasing the ability for an action potential to occur.
NMDA Receptors: Receptors of the neurotransmitter glutamate (major excitatory CNS receptor); anesthetics reduce the activity of these receptors.
Voltage-Gated Sodium Channels: Important in local anesthetics but also used in general anesthetics; inhibiting presynaptic channels may inhibit the release of neurotransmitters at excitatory synapses.

Theories of Anesthesia

Anesthetic potency is closely correlated with lipid solubility, not chemical structure.
Recent work favors interaction with membrane-bound ion channels.
Most anesthetics enhance the activity of GABA A receptors and other cysteine-loop ligand-gated ion channels.
Activation of two-pore-domain potassium channels.
Inhibition of excitatory NMDA receptors.

Cellular Effects

Enhancing tonic inhibition (potentiation of GABA).
Reducing excitation by opening potassium channels.
Inhibiting activation of excitatory NMDA receptors.
Depressing neurotransmitter release.
Result: Unconsciousness, loss of autonomic/motor reflexes, muscle relaxation, and analgesia.
Supra-anesthetic doses can cause death by inducing loss of cardiovascular reflexes and respiratory paralysis.

Targeted Brain Regions

Cortex, thalamus, hippocampus, midbrain, and spinal cord.

Inhalational Anesthetics

Halogenated hydrocarbons like isoflurane, sevoflurane, and desflurane.
Nitrous oxide is mainly used in obstetric practice with limited efficacy as a standalone anesthetic.
Xenon offers rapid induction and recovery but is costly.
Advantage: Ability to produce controlled anesthesia.
Effects: Unconsciousness, amnesia, analgesia, decreased blood pressure, respiration depression, and increased intracranial pressure.
Pharmacokinetics: Administered as gases, rapid changes in blood concentration allowing quick adjustments during surgery.
Side Effects: Respiratory depression, increased intracranial pressure, excitement, and euphoria.

Intravenous Anesthetics

Used for rapid induction of anesthesia (act within about 20 seconds).
Commonly used: Propofol, thiopental, and etomidate.
Usually used for induction, followed by inhalational anesthesia.
Preferred over face masks by patients.
Slower metabolism than inhalational drugs, not suitable for maintaining anesthesia.
Propofol: Fast recovery time, poor analgesic, can cause cardiovascular and pulmonary depression.
Ketamine: NMDA antagonist inducing dissociative anesthesia, provides excellent pain relief, may cause hallucinations.
Thiopental: Barbiturate inducing GABA mediated inhibition, has a slower termination of effect.

Pharmacological Effects of Anesthetic Agents

Anesthesia involves unconsciousness, loss of response to painful stimuli, and loss of reflexes.
Super anesthetic doses cause death by loss of cardiovascular reflexes and respiratory paralysis.
Affect synaptic transmission and neuronal excitability.
Main targets: Cortex, thalamus, hippocampus.
Most agents (except ketamine, nitrous oxide, and xenon) produce similar neurophysiological effects.
Cause cardiovascular depression.
Factors affecting the rate of equilibrium of inhalational anesthetics in the body.

Balanced Anesthesia

Combines multiple drugs to achieve unconsciousness, analgesia, amnesia, and muscle relaxation with minimal side effects.
No single drug can achieve this.
Induction: Propofol or ultra-short-acting barbiturates (Thiopental).
Muscle Relaxation: Neuromuscular blockers.
Analgesia: Short-acting opioids (fentanyl).
Amnesia and Anxiolytic Effects: Short-acting benzodiazepines.
Autonomic Stability: Anticholinergic and anti-adrenergic drugs.
This approach allows for full anesthesia with lower doses of each drug reducing toxicity and side effects.

Local Anesthetics

Introduction to Local Anesthetics

Provide regional loss of sensation without affecting consciousness.
Cocaine was one of the first used in surgery in 1884.
Isolated in 1869 and used clinically in 1884 by Karl Kola in ophthalmic surgery.
Addictive nature led to development of procaine in 1905.

Types of Local Anesthetics

Ester-linked (procaine and cocaine).
Amide-linked (lidocaine or bupivacaine).
Amide-linked are more commonly used today due to longer duration and lower risk of addiction and allergy.

Mechanism of Action

Block voltage-gated sodium channels, preventing action potential propagation.
Do not differentiate between nerve types; can also block motor and autonomic functions.
pH-dependent, work better in alkaline environments.
Acidic conditions such as infected tissue reduce their effectiveness.
$Na^+$ channels are important in propagating action potentials by moving extracellular sodium to the inside of the membrane
Local anesthetics bind to different confirmations or states of the sodium channel to stabilize the inactive confirmation.
The activity of local anesthetics are highly pH dependent.
Local anesthetics are alkaline and unionized at alkaline extracellular pH.
Ionized molecules penetrate nerve membranes better.
The compound needs to penetrate the nerve sheath and the axon membrane to reach the inner end of the sodium channel where the local anesthetic binding site resides.
The ionised form is most active inside the cell.

Administration

Topical (spray or cream).
Infiltration (injected into tissues for minor procedures).
Spinal anesthesia (injected into the CSF for lower body surgeries).
Epidural anesthesia (used for pain relief in labor).
Often combined with vasoconstrictors like adrenaline to prolong effect and reduce systemic toxicity.
Reversibly block the conduction of nerve impulses at the axonal membrane and are non-selective.

Clinical Uses

Injected into soft tissue in dentistry (gums) or to block a nerve or nerve plexus.
Co-administration of a vasoconstrictor such as adrenaline to prolong the local effect.
Lipid-soluble drugs (lidocaine) are absorbed from mucous membranes and used as surface anesthetics.
Bupivacaine has a slow onset but a long duration and is often used for epidural blockade using spinal anesthesia as well.

Local Anesthetics in Clinical Use

Reversible inhibition.
Rapid onset (less than five minutes).
Short duration (fifteen minutes to sixty minutes).

Common Routes of Administration

Surface anesthesia sprayed via the nose, mouth, or bronchial tree.
Infiltration anesthesia injected into the tissues.
Spinal anesthesia.
Epidural anesthesia.

Side Effects

Central Nervous System Toxicity: Excitation (restlessness, tremors, dizziness, seizures) followed by depression (drowsiness, respiratory depression, coma).
- Managed with benzodiazepines which enhance GABAergic inhibition reducing excitatory symptoms and preventing seizure progression.
Cardiovascular Effects: Bradycardia, hypotension, heart block.
- Excessive systemic absorption leads to cardiovascular collapse which is managed with administration of intravenous fluids, vasopressors and in extreme cases lipid emulsion therapy to counteract the toxicity.
Allergic Reactions: Rare, more common with ester-linked agents.
Labor Complications: May prolong labor and affect the newborn.

Conclusion

General anesthetics work by modulating ion channels to induce unconsciousness, muscle relaxation, and analgesia.
Inhalation and intravenous anesthetics have different pharmacokinetic and clinical applications.
Balanced anesthesia optimizes patient outcomes by using a combination of drugs.
Local anesthetics block sodium channels to prevent nerve signal transmission, providing targeted pain relief.
Both types of anesthetics have potential side effects that must be managed carefully in clinical settings.