Wolff–Parkinson–White syndrome: conduction pathways and ECG patterns
THE CONTEXT
In Wolff–Parkinson–White (WPW) syndrome, ventricular activation through the AV node and accessory pathway produces different ECG appearances during sinus rhythm, orthodromic tachycardia and atrial fibrillation.
Sinus rhythm and pre-excitation
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During sinus rhythm, the ventricles are depolarised through both the atrioventricular (AV) node and the accessory pathway.
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The ECG shows a short PR interval and broadened QRS complexes. Slurring of the QRS upstroke is called a delta wave.
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The degree of pre-excitation is the proportion of activation passing down the accessory pathway. This can vary substantially, so the ECG appearance varies and may sometimes look normal.
Orthodromic tachycardia
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Orthodromic tachycardia is the most common form of tachycardia in WPW syndrome.
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The re-entry circuit conducts antegradely through the AV node and retrogradely through the accessory pathway.
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Because the ventricles are depolarised in the normal way, the resulting narrow-complex tachycardia is indistinguishable from other forms of supraventricular tachycardia.
Atrial fibrillation
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During atrial fibrillation, the ventricles are largely depolarised through the accessory pathway, producing an irregular broad-complex tachycardia.