33.2 UK Local Anaesthetic Formulations

Formulation Specifics in UK Dental Practice

In the United Kingdom, dental local anaesthetics are packaged in single-use glass or plastic cylindrical cartridges containing either 2.2 mL (standard UK dental cartridge) or 1.8 mL of solution.

Clinical Pharmacology of Individual Agents

1. 2% Lidocaine with 1:80,000 Adrenaline:

   • The benchmark dental anaesthetic in the UK. Provides rapid onset (2–3 minutes), reliable pulpal anaesthesia lasting 45–60 minutes, and soft tissue anaesthesia for 2–3 hours. The adrenaline concentration (1:80,000 $\approx 12.5\text{ mcg/mL}$) is higher than the North American standard (1:100,000), offering superior local haemostasis for surgical extractions and periodontal surgery.

2. 4% Articaine with 1:100,000 or 1:200,000 Adrenaline:

   • Articaine's thiophene ring imparts superior lipid solubility, allowing diffusion through thick cortical bone. Buccal infiltration of 1.8–2.2 mL of 4% articaine in the adult mandible achieves successful pulpal anaesthesia in first molars and premolars without requiring an inferior alveolar nerve block.
   • The Paresthesia Debate: Retrospective reporting historically suggested a slightly elevated incidence of prolonged paresthesia (predominantly involving the lingual nerve) following IAN blocks with 4% solutions (articaine and prilocaine) compared to 2% lidocaine. Although prospective randomized controlled trials have demonstrated comparable safety, many UK clinicians prefer 4% articaine for infiltrations while continuing to use 2% lidocaine for mandibular block injections.

3. 3% Mepivacaine Plain:

   • Mepivacaine possesses minimal intrinsic vasodilatory properties, enabling reliable anaesthesia without an added vasoconstrictor. It provides 20–40 minutes of pulpal anaesthesia and is the agent of choice when vasoconstrictors are contraindicated or when preservative-free solution is required.

4. 3% Prilocaine with Felypressin (0.03 IU/mL):

   • Felypressin (Octapressin): A synthetic polypeptide analogue of vasopressin (antidiuretic hormone). It acts directly on venous vascular smooth muscle $V_1$ receptors to produce venous constriction. Because it does not stimulate cardiac $\alpha$ or $\beta$ adrenergic receptors, it causes no direct arrhythmias or tachycardia, making it useful in patients with severe ischaemic heart disease.
   • Methaemoglobinaemia Risk: Prilocaine is metabolised in the liver and lungs into ortho-toluidine (o-toluidine). Ortho-toluidine oxidizes normal ferrous iron ($Fe^{2+}$) within haemoglobin into the ferric state ($Fe^{3+}$), creating methaemoglobin. Methaemoglobin cannot bind oxygen and causes an allosteric leftward shift of the oxygen dissociation curve, impairing peripheral tissue oxygen delivery. Clinical cyanosis typically emerges when methaemoglobin levels exceed 10–15% (manifesting with slate-grey cyanosis of lips and nail beds unresponsive to 100% oxygen, fatigue, and chocolate-brown arterial blood). Treatment of severe toxic methaemoglobinaemia ($>30%$ or symptomatic) consists of intravenous methylene blue (methylthioninium chloride) 1–2 mg/kg administered over 5 minutes.
   • Pregnancy Warning: Felypressin causes oxytocic uterine smooth muscle contraction, theoretically risking reduced placental perfusion or induction of labour; prilocaine-felypressin should therefore be avoided in pregnant patients.

Choosing an Agent for the Patient in Front of You

The formulation is selected from the patient's medical history and the procedure, and examiners build stems around exactly those choices. For routine restorative and surgical work in a healthy adult, 2 per cent lidocaine with 1:80,000 adrenaline is the default. For a mandibular posterior tooth requiring infiltration only, or where profound anaesthesia has been difficult to achieve, 4 per cent articaine with adrenaline diffuses through cortical bone more reliably. For a patient in whom adrenaline is best avoided or minimised — significant uncontrolled cardiovascular disease, uncontrolled hyperthyroidism, phaeochromocytoma, or a patient taking a non-selective beta-blocker or a tricyclic antidepressant where interaction is a concern — 3 per cent mepivacaine plain or 3 per cent prilocaine with felypressin is appropriate, accepting a shorter duration and less haemostasis.

Cautions and Contraindications Worth Memorising

Prilocaine at high dose can cause methaemoglobinaemia, presenting as cyanosis unresponsive to oxygen with a low measured saturation and chocolate-brown blood; it is avoided in congenital methaemoglobinaemia, in glucose-6-phosphate dehydrogenase deficiency, in infants, and alongside other oxidising drugs. The felypressin in Citanest is a vasopressin analogue with weak oxytocic activity, so it is conventionally avoided in pregnancy. Sulfite preservatives are present in adrenaline-containing cartridges and are the relevant consideration in a patient with sulfite sensitivity, which is why a plain solution is chosen rather than a different local anaesthetic. True allergy to an amide local anaesthetic is rare; most reported "allergy" is vasovagal syncope, an adrenaline response, or a reaction to the preservative, and a proper history distinguishing the reported symptoms is the examinable first step rather than avoiding all local anaesthesia.

Aspiration in two planes before injection is a standard requirement, because inadvertent intravascular injection is the commonest cause of systemic toxicity and of the transient palpitations and anxiety patients describe after a block.