What is the most likely diagnosis based on the…
Pharmacology questions in the ADC written exam test applied prescribing decisions, not rote recall of drug names. This exam-level vignette tests your ability to work through clinical details and arrive at the correct management.
Mr. Sam Reeves, a 37-year-old man, is having four teeth restored with composite fillings under bilateral inferior alveolar nerve blocks plus infiltrations. The total local anaesthetic administered is 7 cartridges of 4% articaine with adrenaline 1:100,000. Twenty minutes into the second quadrant, the dental nurse notices the patient's skin has developed a slate-grey discolouration. His SpO2 reading on the pulse oximeter shows 82% but he is conscious and reports feeling 'a bit short of breath.' He has no allergies and no medical history. What is the most likely diagnosis based on the slate-grey skin discolouration and reduced SpO2 after multiple articaine injections?
- A. Vasovagal syncope causing peripheral cyanosis
- B. Methaemoglobinaemia — a rare but serious complication of articaine and prilocaine, presenting as slate-grey cyanosis unresponsive to supplemental oxygen
- C. Anaphylaxis to the local anaesthetic causing cyanosis
- D. Acute myocardial infarction causing circulatory failure
- E. Acute asthma attack triggered by the dental procedure
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Methaemoglobinaemia presents as slate-grey cyanosis (distinct from the blue cyanosis of hypoxia) that characteristically does NOT respond to supplemental oxygen. It occurs when haemoglobin iron is oxidised from ferrous (Fe²⁺) to ferric (Fe³⁺) state, preventing oxygen binding. Articaine and prilocaine carry the highest risk among dental local anaesthetics. The SpO2 reading of 82% reflects the methaemoglobin's effect on pulse oximetry.
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