28th November 2023

ECG Interpretation

No chapter on physical health in a MRCPsych Paper A textbook would be complete without mention of ECGs. For the purposes of the exam, most questions will concentrate on QT prolongation.

The QT is measured from the beginning of the Q wave until the end of the T wave, and formulas ‘correct’ this measurement in the context of heart rate, giving a QTc value. The most common formula you will come across in practice is the Bazett’s formula. Please note that the Bazett’s formula often faces criticism for over-correcting the QTc at extremes of heart rate (Funck-Brentano and Jaillon, 1993).

Image Source: Wikimedia Commons (2018). QT interval, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:QT_interval.jpg (Accessed on: 18 October 2023).

According to The Maudsley Practice Guidelines for Physical Health Conditions in Psychiatry, normal QTc for women is <470ms and <440ms for men (Hindley and Gall, 2021). As you know, a prolonged QTc is important as it can predispose to potentially fatal arrhythmias, including Torsades de Pointes (Al-Khatib et al., 2003).

QTc tends to be longer in females and those of low body weight. Other risk factors for prolonged QT include (Benoit et al., 2021):

• Bradycardia.
• Electrolyte abnormalities – Such as hypokalaemia, hypocalcemia and hypomagnesemia.
• Illicit substances.
• Medications – See below.

Psychotropic medications

The below is adapted from The Maudsley Practice Guidelines for Physical Health Conditions in Psychiatry (Hindley and Gall, 2021):

Please note that other non-psychotropic medications, such as antibiotics and cardiac medications, can have an effect on QTc.

Management

According to The Maudsley Practice Guidelines for Physical Health Conditions in Psychiatry (Hindley and Gall, 2021), the management depends on the extent of the prolongation:

• Prolonged but <500ms – Dose reduction or switch of medication (to a less prolongating medication) and consider referral to cardiology. Repeat ECG.
• >500ms – Stop the medication and consider switching to a less prolongating medication. Repeat ECG and refer to cardiology.

Please note that if any T wave abnormalities are seen, then these should be discussed with cardiology immediately, and a medication review should occur.

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