Interpret a paediatric ECG — OSCE

Candidate brief

You have this station to interpret the 12-lead ECG of a 14-year-old girl with two syncopal episodes, apply the systematic age-referenced read, recognise the prolonged QTc, give the differential and the disposition, and outline the technical principles of paediatric ECG acquisition. Treat this as a calm clinic consultation, with an explicit systematic read and a defensible plan. [1] [4]

Key teaching and management objectives

Begin with the systematic read of the ECG in the order rate, rhythm, axis, intervals, chambers, repolarisation, clinical context — and at every step compare the measurement with the value expected for a 14-year-old. The rate is 75 (normal for an adolescent); the rhythm is sinus; the axis is normal; the PR and QRS intervals are within the adolescent range. The QTc is 490 milliseconds by Bazett, which is prolonged in a 14-year-old female (upper normal 460 milliseconds), and combined with the syncopal triggers is diagnostic of long QT syndrome until proven otherwise. [1] [2]

Explain why the QT interval is corrected for heart rate with Bazett's formula (QT divided by the square root of the RR interval in seconds), acknowledge that Bazett overcorrects at fast heart rates and undercorrects at slow rates, and note that at a heart rate of 75 (RR 0.8 seconds) the correction is close to accurate. State that for drug-safety monitoring over time the Fridericia correction is preferred, and that an unexpectedly long QTc at a fast heart rate should be rechecked at a slower rate. [5] [4]

Give the differential of a prolonged QTc: congenital long QT syndrome (the most likely here given the triggers and the absence of an alternative explanation), a QT-prolonging drug (ask about macrolides, fluoroquinolones, ondansetron, antipsychotics, methadone), electrolyte disturbance (hypokalaemia, hypomagnesaemia, hypocalcaemia), and the Bazett overcorrection at fast heart rate. Elicit the key history features: a family history of sudden death before the age of 40, congenital deafness (the Jervell and Lange-Nielsen phenotype), and any QT-prolonging medication. [4]

Close with the disposition: urgent paediatric cardiology referral the same day, a Holter and an exercise test to characterise the QTc behaviour, genetic testing for the common long QT genes (KCNQ1, KCNH2, SCN5A), the institution of beta-blockade and activity modification, and clear written and verbal advice to the family that the girl should avoid swimming alone and competitive sport until the cardiology review. The long-term prognosis of treated long QT syndrome is excellent. [4] [5]

Marking domains

• Systematic age-referenced read (4 marks). Names the seven-step sequence; quotes the adolescent normal ranges; correctly identifies the QTc of 490 milliseconds as prolonged.
• QTc correction and its limits (3 marks). Explains Bazett's formula, its rate-dependent artefact, and the Fridericia alternative; states that at a heart rate of 75 the 490 milliseconds is reliable.
• Differential and focused history (3 marks). Names long QT syndrome, drug effect, electrolyte disturbance, and Bazett overcorrection; asks about family history of sudden death, deafness, and QT-prolonging drugs.
• Disposition and safety advice (3 marks). Urgent paediatric cardiology referral; Holter, exercise test, genetics; beta-blockade and activity modification; advice against swimming alone and competitive sport.
• Technical principles of paediatric ECG acquisition (2 marks). Names the 25 mm per second and 10 mm per millivolt conventions, the calibration square, and the correct electrode placement; emphasises a calm child. [2] [4]