- Significant arrhythmia (since this may interfere with the interpretation of the electrocardiogram) including Wolff Parkinson-White syndrome, atrial fibrillation, atrial flutter, sick sinus syndrome, significant Atrio-Ventricular heart block, intraventricular conduction defect (QRS > 0.12 seconds) ventricular pre-excitation, bundle branch block, the presence of a pace-maker, the presence of an implanted automatic defibrillator, uncorrected hypokalaemia (potassium < 3.5 mmol/litre)