Local Anesthetics – Study Notes

Local Anesthetics – Comprehensive Study Notes

Goals and Topic Overview

• Lecture by Dr. Marci H. Levine on Local Anesthetics (LA)

• Topics covered: definition, mechanism, factors influencing administration, vasoconstriction (epinephrine), uptake/distribution/elimination, selection of anesthetics, armamentarium, injection techniques, risks and complications (local and systemic).

What are Local Anesthetics?

• Solutions that reversibly depress conduction in peripheral nervous tissues.

• Used locally or systemically; primary goal is to eliminate pain.

• Cocaine was the first LA discovered and is the only naturally occurring LA.

• Evolution of LA enables painless dentistry.

Mechanism of Action (MOA)

• LA produce anesthesia by inhibiting excitation of nerve endings and blocking conduction in peripheral nerves.

• They reversibly bind and inactivate sodium channels.

• Sodium influx is necessary for depolarization and impulse propagation along nerves.

• When a nerve cannot propagate an impulse, there is loss of sensation in the area supplied by that nerve.

Chemical Structure and Classification

• Structure features: an amine end (hydrophilic) and an aromatic ring (lipophilic) connected by an intermediate chain.

• Two classes based on linkage: amino amides and amino esters.

• In dentistry, both amides and esters are used.

Amides vs Esters

• Amides: Lidocaine, Mepivacaine, Prilocaine, Bupivacaine (metabolized in the liver; stable in solution).

• Esters: Benzocaine, Procaine, Cocaine (metabolized in plasma via pseudocholinesterase; less stable in solution; more hypersensitivity potential).

• Practical distinction: Amides vs Esters differ in metabolic pathways and hypersensitivity profiles.

Physiological Activity: Key Pharmacologic Factors

• Function determined by: lipid solubility, diffusibility, affinity for protein binding, percent ionization at physiologic pH, vasodilating properties.

Lipid Solubility

• Directly relates to potency.

• About 90% of nerve cell membrane is lipid; higher lipid solubility → faster penetration and sodium channel blockade.

Diffusibility

• Speed of onset depends on how well the LA diffuses through tissues.

Protein Binding

• Longer duration of action with greater protein binding to the sodium channel.

Percent Ionization at Physiologic pH

• Non-ionized form diffuses across membranes to block sodium channels.

• Greater non-ionized fraction → faster onset.

• Equilibrium pH for given LA near physiologic tissue pH (~7.35–7.45): faster onset when tissue pH is closer to this range.

• Formula/approximate relation: more non-ionized fraction at higher pH → faster onset.

pH Effects and Inflammation

• Decreased tissue pH delays onset by shifting equilibrium toward ionized form.

• LA is slower and less effective in inflamed, acidic environments.

• Sodium bicarbonate can be added to increase pH and enhance onset.

• Different LA solutions may be indicated in acidic situations.

Vasodilators

• Most LA (except cocaine) are vasodilators, causing relaxation of arteriolar smooth muscle.

• Greater vasodilation → faster absorption and shorter duration of action.

• Epinephrine is a vasoconstrictor added to prolong duration and reduce bleeding.

Factors That Influence Administration

• Individual patient characteristics.

• Dose of LA to be administered.

• Presence/absence of epinephrine.

• Speed of administration.

• Vascularity of local tissues.

• Technique of administration.

• Goal: administer the smallest effective dose over the longest period to achieve anesthesia.

Maximum Dosages (with Epinephrine)

• The following values are presented with epinephrine, noting onset and duration:

• Lidocaine: Onset Rapid; Maximum Dose $4.5\ ext{mg/kg} \left(7\ \text{mg/kg}\right)$; Duration $120\ \text{min} \left(240\ \text{min}\right)$

• Mepivacaine: Onset Rapid; Maximum Dose $5\ ext{mg/kg} \left(7\ ext{mg/kg}\right)$; Duration $180\ \text{min} \left(360\ \text{min}\right)$

• Bupivacaine: Onset Slow; Maximum Dose $2.5\ ext{mg/kg} \left(3\ ext{mg/kg}\right)$; Duration $4\ ext{hours} \left(8\ \text{hours}\right)$

• Prilocaine: Onset Medium; Maximum Dose $5\ ext{mg/kg} \left(7.5\ \text{mg/kg}\right)$; Duration $90\ \text{min} \left(360\ \text{min}\right)$

• Note: The values above come from the slide showing “Onset” categories and the paired maximum doses with and without epinephrine, as well as duration with epinephrine in parentheses.

Addition of Epinephrine: Rationale and Benefits

• Allows smaller LA doses to be used safely by causing vasoconstriction.

• Slows absorption from the injection site, increasing local duration of action.

• Improves hemostasis, aiding visibility during dental procedures.

Administration Considerations and Safety

• Use epinephrine in small concentrations (e.g., 1:100,000) but use with caution in patients with cardiac arrhythmias or hypertension.

• Toxicity relates to peak serum concentration; consider patient comorbidities.

• Inject each site sequentially rather than all at once to spread total dose over time.

• If using multiple injections, spread the total dose over time to maintain lower peak serum levels.

Tissue Vascularity and Absorption

• Tissue vascularity affects rapid absorption of LA.

• Oral mucosa is highly vascular and can increase absorption risk; technique and aspiration reduce this risk.

• Aspiration is critical to prevent inadvertent intravascular injection.

Absorption, Distribution, and Metabolism (Pharmacokinetics)

• Absorption: influenced by chemical structure, dose, epinephrine presence, speed of administration, local tissue vascularity, injection technique.

• Distribution: occurs in three phases; initially rapidly taken up by highly vascular tissues (lungs, kidneys), then appears in less vascular tissues (muscle, fat); drug then is metabolized.

• Metabolism:

  • Amide LA: Hepatic metabolism with renal excretion.

  • Ester LA: Plasma pseudocholinesterase metabolism.

Adverse Reactions and Toxicity

• Toxicity can be local (overdose) or systemic via absorption, distribution, or metabolism.

• Adverse reactions may also be allergic in nature.

Local and Systemic Adverse Reactions

• CNS: lightheadedness, tinnitus, circumoral numbness, metallic taste, double vision, drowsiness, slurred speech, nystagmus, anxiety, tremors, seizures, CNS depression with hypoxia/acidosis, respiratory arrest, death.

• Cardiac: decreased rate of depolarization, negative inotropic effects causing bradycardia, ventricular fibrillation, asystole, hypotension (from direct vasodilation).

• Tissue and systemic toxicity can occur if dose is excessive or intravascular injection occurs.

Allergy and Hypersensitivity

• Hypersensitivity to preservatives in LA solutions is a common cause; ester group hypersensitivity is more common due to fruit allergies; amide group hypersensitivity is extremely rare.

• Reactions include anxiety, panic, vasovagal responses, or intravascular injections.

• True allergic reactions account for <1% of LA reactions.

• Question about allergies to vasoconstrictors: assess with an allergist if needed.

Hypersensitivity Reactions to PABA (Para-aminobenzoic acid)

• Some hypersensitivity is related to PABA breakdown products from esters.

• PABA is antigenic and can trigger Type I (allergic/anaphylactic) or Type IV (contact dermatitis) reactions.

• Testing and a controlled-dose challenge should be done under supervision; refer to an allergist if prior LA reaction.

Immediate Management of Adverse Reactions

• Immediately stop the procedure.

• Notify faculty and call for help; assign someone to call for emergency assistance (e.g., medical emergency protocols).

• Stay with the patient until help arrives and monitor status.

Armamentarium and Pre-Injection Preparation

• Armamentarium: syringe, carpules, needles, disinfectants, topical anesthetic, aspiration equipment, suction, vasoconstrictor with LA.

• Prepare to inject:

  • Proper patient positioning: head and heart parallel to floor; feet slightly elevated to reduce vasovagal syncope.

  • Apply topical anesthetic (20% benzocaine) for 1–2 minutes to outer mucosa before injection.

  • Communicate with patient: phrase choices to reduce anxiety (e.g., avoid words like “shot” or “pain”).

  • Establish firm hand rest; keep tissues taut.

  • Hold syringe out of patient’s sight; insert needle with bevel toward bone; do not redirect needle after insertion; avoid forcing through resistance.

  • Maintain eye contact with the syringe; inject several drops of anesthetic preliminarily (optional) for surface-to-periosteum anesthesia; often reduces patient anxiety.

  • Advance to target slowly and steadily.

  • Use aspiration to check for intravascular placement, then inject cautiously.

  • For multiple injections, aspirate and inject gradually (about 1–2 minutes per cartridge).

Injection Techniques in Dentistry

• Main techniques: Local Infiltration (Supraperiosteal), Field Block (Diffuse Local Infiltration), Nerve Block.

Supraperiosteal Injection (Local Infiltration)

• Targets: Incisive foramen and adjacent region; used for extraoral soft tissue and alveolar mucosa, and palatal soft tissue and bone near affected teeth.

Field Block (Diffuse Local Infiltration)

• Used to anesthetize a broader field around a tooth or area via infiltration along tissue planes.

Inferior Alveolar Nerve (IAN) Block (Nerve Block for Mandibular Dentistry)

• Distribution: Affects mental foramen, lingual soft tissue and bone, tongue, alveolar mucosa, extraoral soft tissue.

• Nerves anesthetized by IAN: Incisive nerve, mental nerve, mandibular nerve, auriculotemporal nerve, lingual nerve, inferior alveolar nerve, mylohyoid nerve.

Landmarks for IAN Block

• Step 1: Coronoid Notch

• Step 2: Pterygomandibular raphe

• Step 3: Contralateral premolar region

• Step 4: Mandibular occlusal plane landmarks (inferior alveolar nerve, artery, lingual nerve, target)

• Additional reference: mental foramen location and buccinator/oris region relationships.

Other Nerve Blocks in the Mandible

• Mental nerve: anterior soft tissue anesthesia in mandible.

• Lingual nerve: lingual soft tissue of all mandibular teeth.

• Long buccal nerve: buccal soft tissue of molars; often combined with IAN for complete buccal soft tissue anesthesia.

• Ant/post superior alveolar nerves: regional maxillary injections (for maxillary teeth and related soft tissues; listed for reference in compatibility tables).

• Nasopalatine nerve: palatal soft tissue and bone of maxillary incisors and canine; ant palatine nerve blocks can affect palatal tissues.

Long Buccal Nerve Block

• Regional note: In the region of the mandibular second premolar, buccal soft tissues are innervated primarily by the mental branch of the IAN and terminal branches of the long buccal nerve.

• Practical use: Supplement IAN block with long buccal nerve block to achieve adequate anesthesia of buccal soft tissue for extraction of a lower second premolar.

• Techniques: Often illustrated with A and B diagrams illustrating steps.

Type of Injection Necessary by Nerve/Tissue (Summary)

• Inferior alveolar nerve: All mandibular teeth; buccal soft tissue of PMs, canines, incisors; lingual soft tissue of all mandibular teeth.

• Lingual nerve: Lingual soft tissue of all mandibular teeth (and surrounding region).

• Long buccal nerve: Buccal soft tissue of molars; adjacent buccal mucosa.

• Ant. superior alveolar nerve: Maxillary incisors and canine; buccal soft tissues of incisors and canines.

• Middle superior alveolar nerve: Maxillary premolars and portion of first molar; buccal soft tissue of PMs.

• Posterior superior alveolar nerve: Maxillary molars (except portion of first molar); buccal soft tissue of molars.

• Anterior palatine nerve: None for teeth; lingual soft tissue of molars and PMs.

• Nasopalatine nerve: Palatal soft tissue and bone of maxillary incisors and canine.

Local Complications of LA Use

• Needle breakage, pain on injection, burning on injection, persistent anesthesia or paresthesia, trismus, infection, edema, sloughing of tissues, soft tissue injury, facial nerve paralysis, post-anesthesia intraoral lesions.

Systemic Complications of LA Use

• Toxicity may arise from direct pharmacologic effects (side effects, overdose), systemic distribution, metabolism, or allergic responses.

• Distinct pathways: absorption, distribution, metabolism, all contributing to systemic toxicity.

Overdose: Signs, Symptoms, and Stages

• Early signs: dizziness, tinnitus, paresthesias of mouth and tongue, drowsiness.

• Progressive stages (as per NYSORA):
  1) Drowsiness
  2) Paresthesias in mouth and tongue
  3) Tinnitus, auditory symptoms
  4) Muscle spasms
  5) Seizures
  6) Coma
  7) Respiratory arrest
  8) Cardiac arrest

• Clinically, monitor peak serum levels to prevent progression.

Iatrogenic Nerve Injury

• Incidence of IAN or lingual nerve injury from LA is low: approximately $1:850{,}000 ext{ to } 1:20{,}000$.

• Reported risk: around 0.15 ext{%} ext{ to } 0.54 ext{%} for temporary injury; permanent injury risk: 0.0001 ext{%} ext{ to } 0.01 ext{%}.

• Lingual nerve injury occurs at least twice as often as IAN injury.

• References to Cummings et al. (Clin. N. Am. 2011), Morris et al. (JOMS 2010), Hillerup et al. (Int J Oral Maxillofac Surg 2006).

Educational Integration: Oral Surgery Experience

• Local anesthesia training is integrated across the curriculum:

  • D2: Introduction to Oral Surgery, Pain & Anxiety Control.

  • Landmarking sessions in clinics.

  • Virtual reality simulations.

• Skills are practiced and refined throughout the DDS program.

• LA is fundamental to painless dentistry.

References for Further Information

• Malamed, Handbook of Local Anesthesia.

• Peterson's Principles of Oral Surgery.

Quick Reference: Practical Takeaways

• Always assess patient factors and medical history prior to LA administration.

• Use the smallest effective dose; consider adding epinephrine to prolong action and improve hemostasis.

• Be mindful of tissue pH and inflammation; consider buffering with sodium bicarbonate when indicated.

• Always aspirate before injecting to avoid intravascular administration; inject slowly (about 1–2 minutes per cartridge).

• Be fluent with IAN block landmarks and long buccal nerve block indications to achieve full mandibular anesthesia.

• Prepare for possible adverse reactions; have emergency protocols ready and know when to involve medical staff.

• Ongoing training and hands-on practice are essential for safe mastery of LA techniques.

$ext{pH considerations: } pH <br>ightarrow ext{non-ionized fraction} <br>ightarrow ext{onset time decreases as non-ionized fraction increases}$
$ext{Epinephrine concentration: } 1:100{,}000 ext{ is common}; ext{ used to extend duration and reduce bleeding}$
ext{Maximum doses (with epinephrine): } egin{cases} ext{Lidocaine}: 4.5 \ 7 \ ext{Mepivacaine}: 5 \ 7 \ ext{Bupivacaine}: 2.5 \ 3 \ ext{Prilocaine}: 5 \ 7.5 \ ext{Duration (with epi)}: 240, 360, 8h, 360 \ ext{min}}
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