Cardiology doctors Mathikere – BELCardiologist in Doddaballapur Road, Bangalore • SVT The resting ECG is usually normal and this certainly does not exclude the diagnosis. When the ECG is normal between attacks, the accessory pathway responsible for the problem is said to be ‘concealed’ . Sometimes the ECG will show a short PR interval This can be a normal variant, but for patients with intermittent rapid and regular palpitations it suggests the presence of an intra-nodal accessory pathway. This is called Lown–Ganong–Levine syndrome.4 Fibres close to, or within, the atrioventricular node bypass the normal slow conduction between the atria and ventricles and allow impulses to overtake the normal conduction. The atrial impulse reaches the ventricles early and the PR interval is short. During an episode of SVT one pathway conducts impulses anterogradely—that is, from the atria to the ventricles—and the other conducts impulses retrogradely. This enables a circuit of electrical activity to cause atrial and then ventricular contraction independently of the sinus node and very fast t all patients with WPW on the surface ECG develop symptoms of SVT. During an episode of tachycardia the QRS morphology returns to normal since anterograde (forward) conduction is usually (> 90% cases) via the normal pathway The opposite is true when AF and (much rarer) atrial flutter occur; such patients are at quite high risk of sudden death. VT Again, the resting ECG is usually normal between attacks but brief runs of VT may be present, or there may be evidence of ischaemia (e.g. an old infarct), which suggests an ischaemic substrate for VT.