A short PR interval is less than 3 mm (120 ms) at standard settings. This is usually caused by a pre-excitation syndrome where the impulse can travel from the atria through to the ventricles abnormally fast, or from a junctional rhythm.
See also: PR interval & segment
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This ECG is from a 12 year old girl with a history of mild intermittent asthma who presented with a week of worsening shortness of breath. Her usual nebuliser treatments had not been working. The shortness of breath was occurring intermittently, with no apparent trigger, and these episodes were increasing in frequency. There was no wheeze or cough.