6.1 Local Anesthesia Pharmacology, Neurophysiology & Vasoconstrictors

Local Anesthesia Pharmacology, Neurophysiology & Vasoconstrictors

Neurophysiology of Nerve Conduction

To understand how local anesthetics prevent pain transmission during dental hygiene procedures, one must first master the fundamental neurophysiology of nerve impulse generation and conduction along peripheral nerves. Peripheral nerve fibers transmit sensory signals, such as thermal stimuli, tactile sensation, and nociception (pain), from peripheral tissues to the central nervous system (CNS).

Resting Membrane Potential and Ion Distribution

In an unstimulated, resting nerve fiber, a stable electrical potential exists across the nerve cell membrane (axolemma). This resting membrane potential is approximately -70 mV. This negative electrical charge inside the nerve axon relative to the extracellular fluid is actively established and maintained by the ATP-dependent sodium-potassium pump ($Na^+/K^+$ ATPase), which pumps three $Na^+$ ions out of the neuron for every two $K^+$ ions brought inside.

• Extracellular Fluid: High concentration of Sodium ($Na^+$) ions (~140 mEq/L) and Chloride ($Cl^-$) ions.
• Intracellular Fluid (Axoplasm): High concentration of Potassium ($K^+$) ions (~110 mEq/L) and negatively charged intracellular proteins.

Impulse Generation and Action Potential Phases

When a physical, chemical, or thermal stimulus excites a sensory nerve, a sequence of rapid membrane permeability changes occurs:

1. Slow Depolarization Phase: The stimulus causes initial permeability changes in the axolemma, allowing a small influx of $Na^+$ ions through voltage-gated sodium channels. The internal electrical potential moves from -70 mV toward a less negative value.
2. Threshold Potential (Firing Threshold): When the membrane potential depolarizes to approximately -55 mV to -50 mV, the threshold potential is reached. At this critical point, voltage-gated sodium channels undergo a rapid conformational change, opening their activation gates fully.
3. Rapid Depolarization Phase: A massive influx of $Na^+$ ions surges into the axoplasm down both electrical and concentration gradients. The internal charge rapidly reverses from negative to positive, peaking at approximately +40 mV. This reversal represents the action potential.
4. Repolarization Phase: At peak depolarization, sodium channel inactivation gates close, halting $Na^+$ entry. Simultaneously, voltage-gated potassium channels open, allowing $K^+$ ions to rapidly diffuse out of the cell. This efflux of positive charge restores the membrane potential back to -70 mV.
5. Hyperpolarization and Reset: The $Na^+/K^+$ pump actively restores the original ionic concentrations of $Na^+$ extracellularly and $K^+$ intracellularly during the absolute and relative refractory periods.

Nerve Fiber Classification and Saltatory Conduction

Peripheral nerve fibers are categorized by diameter, presence of myelin, and conduction velocity. Pain and temperature sensation in the oral cavity are mediated primarily by two types of fibers:

• A-delta ($ ext{A}\delta$) Fibers: Myelinated, medium-diameter fibers (2-5 $\mu m$) with rapid conduction velocities (6-30 m/s). They mediate sharp, prickling, well-localized first pain.
• C Fibers: Unmyelinated, small-diameter fibers (0.4-1.2 $\mu m$) with slow conduction velocities (0.5-2 m/s). They mediate dull, aching, burning, poorly localized second pain.

In myelinated A-delta fibers, the myelin sheath (formed by Schwann cells) acts as an electrical insulator. Depolarization cannot occur through myelin; instead, action potentials "leap" from one uninsulated gap to the next—a process called saltatory conduction at the Nodes of Ranvier. To achieve complete clinical anesthesia, a local anesthetic agent must bathe at least 8 to 10 millimeters of nerve length (covering 2 to 3 adjacent Nodes of Ranvier) to block impulse propagation completely.

Mechanism of Action of Local Anesthetics

Specific Receptor Theory

Local anesthetics act directly on the nerve membrane to prevent impulse conduction. The widely accepted Specific Receptor Theory dictates that local anesthetic molecules pass through the lipophilic cell membrane into the axoplasm and bind to specific protein receptors located within the internal pore of voltage-gated $Na^+$ channels. Once bound, the anesthetic molecule physically blocks the channel, preventing $Na^+$ influx. Consequently, slow depolarization cannot reach the firing threshold, no action potential is generated, and conduction is blocked.

Ionization Equilibrium and Tissue pH Effects

Local anesthetic agents used in dentistry are synthetic tertiary amines formulated as weak basic compounds. In commercial cartridges, they are combined with hydrochloric acid ($HCl$) to form water-soluble, stable salts dissolved in sterile water. In solution, local anesthetics exist in a dynamic equilibrium between two chemical forms:

$\text{RNH}^+ \rightleftharpoons \text{RN} + \text{H}^+$

• Un-ionized Free Base ($ ext{RN}$): Lipophilic and non-charged. The free base form is essential for penetrating the lipophilic epineurium and neuronal cell membrane.
• Ionized Cation ($ ext{RNH}^+$): Hydrophilic and positively charged. The cationic form is the biologically active species that binds to the sodium channel receptor inside the axoplasm.

The ratio of $ ext{RN}$ to $ ext{RNH}^+$ is governed by the drug's dissociation constant ($ ext{pKa}$) and the pH of the surrounding tissue, expressed by the Henderson-Hasselbalch equation:

$\text{pH} = \text{pKa} + \log\left(\frac{[\text{RN}]}{[\text{RNH}^+]}\right)$

• Normal Tissue pH (7.4): At physiological tissue pH, local anesthetics with a lower pKa (e.g., mepivacaine $\text{pKa} = 7.6$, lidocaine $\text{pKa} = 7.7$) have a higher proportion of un-ionized free base ($ ext{RN}$), enabling rapid membrane penetration and a faster onset of anesthesia.
• Infected/Inflamed Tissue pH (5.5 - 6.0): Tissue inflammation releases acidic metabolic byproducts. In an acidic environment, the abundance of hydrogen ions ($H^+$) shifts the equilibrium heavily toward the ionized cationic form ($ ext{RNH}^+$). Because $ ext{RNH}^+$ cannot cross the lipophilic nerve sheath, very few molecules enter the axon, resulting in delayed onset or complete failure of local anesthesia.

Pharmacokinetics and Chemical Classifications

Local anesthetics are structurally divided into two major chemical groups based on the intermediate chain connecting their lipophilic aromatic ring and hydrophilic tertiary amine group: Esters and Amides.

Ester Pharmacology

Ester anesthetics (e.g., procaine, tetracaine) are rapidly hydrolyzed in blood plasma by the enzyme pseudocholinesterase (plasma cholinesterase). A major metabolic byproduct of ester hydrolysis is Para-Aminobenzoic Acid (PABA). PABA is a known allergen capable of triggering Type I anaphylactic reactions and Type IV delayed cell-mediated hypersensitivity. Patients with atypical pseudocholinesterase (a hereditary genetic deficiency) cannot metabolize ester anesthetics efficiently, leading to prolonged blood levels and heightened toxicity risks. Today, esters are used in dentistry primarily as topical anesthetics (e.g., 20% benzocaine gel).

Amide Pharmacology and Specific Drug Profiles

Amide local anesthetics are metabolized primarily in the liver by hepatic microsomal P450 enzymes (CYP1A2 and CYP3A4) and excreted via the kidneys.

1. Lidocaine (Xylocaine - 2% with 1:100,000 or 1:50,000 Epinephrine):
   • The "gold standard" dental anesthetic.
   • $\text{pKa} = 7.7$; rapid onset (3-5 minutes); intermediate duration (60 minutes pulpal, 180-300 minutes soft tissue).
   • Liver metabolism; half-life ~90 minutes.
2. Mepivacaine (Carbocaine - 3% Plain or 2% with 1:200,000 Levonordefrin):
   • $\text{pKa} = 7.6$; fastest onset (2-3 minutes).
   • Produces the least amount of vasodilation of all amides. Mepivacaine 3% plain is the drug of choice for patients in whom vasoconstrictors are contraindicated or for short procedures.
3. Prilocaine (Citanest - 4% Plain or 4% with 1:200,000 Epinephrine):
   • $\text{pKa} = 7.9$; onset 3-5 minutes.
   • Metabolized in both the liver and lungs. A major metabolite, ortho-toluidine, can oxidize hemoglobin iron from the ferrous ($Fe^{2+}$) state to the ferric ($Fe^{3+}$) state, inducing methemoglobinemia at doses exceeding 600 mg.
4. Articaine (Septocaine - 4% with 1:100,000 or 1:200,000 Epinephrine):
   • $\text{pKa} = 7.8$; onset 1-3 minutes.
   • Unique hybrid structure containing a thiophene ring (increasing lipid solubility) and an ester side chain.
   • Hydrolyzed 90-95% in blood plasma by plasma carboxylesterases and only 5-10% in the liver. Has the shortest elimination half-life (~45 minutes) and superior lipid diffusion through dense cortical bone.
5. Bupivacaine (Marcaine - 0.5% with 1:200,000 Epinephrine):
   • $\text{pKa} = 8.1$; slower onset (6-10 minutes).
   • Highly lipophilic with 95% protein binding capacity, conferring long duration of pulpal anesthesia (up to 90-180 minutes) and soft tissue anesthesia (up to 8 hours). Indicated for lengthy procedures or post-operative pain management.

Vasoconstrictors and Dosage Calculations

Physiology of Vasoconstrictors

Local anesthetics are inherently vasodilators. Absorbed into the systemic circulation, un-constricted local anesthetics lead to rapid vascular uptake, shortened duration of anesthesia, inadequate depth of pain control, and increased risk of systemic toxicity. Vasoconstrictors (sympathomimetic amines) are added to local anesthetic cartridges to counteract vasodilation.

• Mechanism: Vasoconstrictors stimulate Alpha-1 ($\alpha_1$) adrenergic receptors on vascular smooth muscle, causing microvascular constriction at the injection site. This reduces local blood flow, slows systemic absorption, decreases peak blood concentration, prolongs anesthesia duration, and enhances hemostasis.
• Systemic Effects: Vasoconstrictors also stimulate Beta-1 ($\beta_1$) receptors on the heart (increasing heart rate, stroke volume, and cardiac output) and Beta-2 ($\beta_2$) receptors in bronchioles (causing bronchodilation) and peripheral vasculature (causing vasodilation at low doses).
• Preservative: Vasoconstrictor-containing cartridges include sodium bisulfite or sodium metabisulfite as an antioxidant to prevent epinephrine oxidation. Bisulfites lower cartridge pH to 3.3-5.5 (causing a mild stinging sensation upon injection) and represent a true allergen in sulfite-sensitive patients (e.g., steroid-dependent asthmatics).

Epinephrine and Levonordefrin Limits

1. Epinephrine (Adrenalin):
   • Healthy (ASA I) Maximum Dose: 0.2 mg per appointment (~11 cartridges of 1:100,000 epi).
   • Cardiac (ASA III/IV / Severe Cardiovascular Disease) Maximum Dose: 0.04 mg per appointment (approximately 2 cartridges of 1:100,000 epi or 4 cartridges of 1:200,000 epi).
2. Levonordefrin (Neo-Cobefrin - 1:20,000):
   • Synthetic vasoconstrictor, 1/5th as potent as epinephrine.
   • Healthy Maximum Dose: 1.0 mg per appointment (~11 cartridges).
   • Cardiac Maximum Dose: 0.2 mg per appointment (~2 cartridges).

Maximum Recommended Dosage (MRD) Calculations

Standard dental cartridges contain 1.8 mL of solution (or 1.7 mL in select manufacturer specifications). To calculate dosage safely, multiply the concentration percentage by 10 to yield milligrams per milliliter (mg/mL):

• 2% Lidocaine: $2% \times 10 = 20\text{ mg/mL}$. A 1.8 mL cartridge contains $20\text{ mg/mL} \times 1.8\text{ mL} = \mathbf{36\text{ mg}}$ of lidocaine.
• 3% Mepivacaine: $3% \times 10 = 30\text{ mg/mL} \times 1.8\text{ mL} = \mathbf{54\text{ mg}}$ of mepivacaine.
• 4% Articaine / Prilocaine: $4% \times 10 = 40\text{ mg/mL} \times 1.8\text{ mL} = \mathbf{72\text{ mg}}$ of drug.
• 0.5% Bupivacaine: $0.5% \times 10 = 5\text{ mg/mL} \times 1.8\text{ mL} = \mathbf{9\text{ mg}}$ of bupivacaine.

$\text{Patient Weight (lbs)} \times \text{MRD (mg/lb)} = \text{Total Allowed Dose (mg)}$ $\text{Cartridges Allowed} = \frac{\text{Total Allowed Dose (mg)}}{\text{mg of drug per cartridge}}$

Example Calculation: For a 110 lb healthy patient receiving 2% Lidocaine with 1:100,000 epinephrine ($ ext{MRD} = 3.2 ext{ mg/lb}$, absolute max 500 mg): $\text{Max Dose} = 110\text{ lbs} \times 3.2\text{ mg/lb} = 352\text{ mg}$ $\text{Max Cartridges} = \frac{352\text{ mg}}{36\text{ mg/cartridge}} = 9.77 \rightarrow \mathbf{9.5\text{ cartridges}}$

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Exam Note: Competing Lidocaine MRD Conventions

Board candidates will see two widely taught MRDs for 2% lidocaine with epinephrine:

• FDA / current Malamed absolute maximum: 7.0 mg/kg (3.2 mg/lb), absolute max 500 mg — use this chapter's calculations unless a stem states otherwise.
• Conservative pediatric / older dental-hygiene textbook figure: 4.4 mg/kg (2.0 mg/lb) — often used for children to build a safety margin.

When an NBDHE-style stem specifies a mg/kg or mg/lb rate, use the rate given in the stem. Do not mix conventions inside one calculation.