Pharmacology
What this domain tests
Examinations in Cambodia frequently test maximum local anaesthetic doses, antibiotic choice and prophylaxis, analgesic selection, and drug interactions relevant to dentistry.
Core rules
- Amide local anaesthetics (lidocaine, mepivacaine, articaine, bupivacaine) are metabolised in the liver; esters are metabolised by plasma esterases and are more allergenic.
- Lidocaine 2% with 1:100,000 epinephrine: maximum 4.4 mg/kg, absolute maximum 300 mg. Articaine 4%: about 7 mg/kg, absolute 500 mg. Always calculate both the weight-based and absolute limits and use the lower value.
- Vasoconstrictor limits: about 0.2 mg epinephrine for a healthy adult (roughly 11 cartridges of 1:100,000 in 1.8 mL), less in cardiac disease.
- Antibiotics are adjuncts to drainage and removal of the cause. Amoxicillin is first line; amoxicillin–clavulanate for resistant infection; metronidazole for anaerobes; clindamycin in penicillin allergy. Antibiotic prophylaxis is reserved for specified cardiac conditions before invasive dental procedures.
- Analgesia: paracetamol and ibuprofen are first line, and may be alternated. Avoid aspirin in children because of Reye's syndrome. Avoid ibuprofen in asthma, renal disease and peptic ulcer.
- Interactions: metronidazole and erythromycin potentiate warfarin; metronidazole with alcohol causes a disulfiram-like reaction; macrolides raise digoxin levels.
Worked example
How many 1.8 mL cartridges of 2% lidocaine with 1:100,000 epinephrine may a 60 kg patient receive? Reasoning: 2% = 20 mg/mL. Weight limit 4.4 × 60 = 264 mg; absolute limit 300 mg, so 264 mg governs. 264 ÷ 20 = 13.2 mL = about 7.3 cartridges. Epinephrine at 1:100,000 is 0.01 mg/mL, so 13.2 mL delivers 0.132 mg, within the 0.2 mg limit.
Traps
Mixing up milligrams and millilitres; quoting the absolute limit and ignoring body weight; prescribing aspirin to a febrile child; forgetting that metronidazole interacts with warfarin and alcohol.