Wolff-Parkinson-White (WPW)
cautionWolff-Parkinson-White (WPW)
Wolff-Parkinson-White is a sinus rhythm carrying a pre-excitation pattern: an accessory pathway lets the impulse reach the ventricle without passing through the AV node, producing a short PR interval, a slurred delta wave and a widened QRS. It is found in roughly 1 to 3 people per 1000. The pattern is the ECG finding; the syndrome is that pattern in someone who also has the tachyarrhythmias it enables.
The PR interval is short, under 120 ms.
The AV node normally holds the impulse up for about a tenth of a second before releasing it. The pathway skips that wait entirely, so the ventricle is reached sooner than it should be.
The QRS begins with a slow slurred ramp rather than a sharp upstroke.
The pathway delivers the impulse into ordinary ventricular muscle, which conducts slowly from cell to cell. On this trace the upstroke takes about four times as long to rise as a normally conducted beat - that slur is the delta wave.
The QRS is wide, but its later part looks ordinary.
It is a fusion complex. The slow pre-excited start is overtaken once the impulse that went the normal way clears the AV node and races down the His-Purkinje system, so the end of the complex is drawn by normal conduction.
- PR intervalShort - under 120 ms
- Delta waveSlurred start to the QRSMay slur upward or downward depending on where the pathway inserts into the ventricle.
- QRSWide - over 120 msA fusion complex, not a bundle branch block. The widening comes from starting early, not from conducting badly.
- RhythmSinusThis is a pattern seen at rest. It is not itself a tachycardia.
- ST-T changesSecondary, opposing the deltaA consequence of abnormal depolarisation, and not evidence of ischaemia.
- A normal ECGDoes not exclude a pathwayPathways that conduct only from ventricle to atrium show nothing at rest.
An extra strand of muscle connects atrium to ventricle, bypassing the AV node. It has no delay built in, so part of the ventricle fires early; and because it feeds ordinary muscle instead of the fast conducting system, that early activation is slow. The QRS you see is those two wavefronts meeting.
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How much of the complex the pathway draws decides how the beat looks. It gets a head start, but the normally conducted impulse travels much faster once it is through the node, so it catches up and finishes the QRS - which is why the delta appears only at the beginning and why the rest of the complex can look entirely normal. Its direction follows from geography: whichever part of the ventricle the pathway reaches first sets which way that opening slur runs.
The pattern is not the same thing as the syndrome. Many people have the ECG finding and never have an arrhythmia. Equally, a normal resting ECG does not exclude an accessory pathway, because some conduct only backwards, from ventricle to atrium, and are invisible until a tachycardia starts. The property that makes pathways dangerous is that they are plain myocardium: the AV node conducts worse the harder it is driven, which is what protects the ventricles during atrial arrhythmias, and an accessory pathway has no equivalent brake.
The AV node protects the ventricles by conducting progressively worse the faster it is driven. An accessory pathway is ordinary muscle and does not do this, so during atrial fibrillation it can pass impulses through at atrial rates. That single property is why a pathway is worth ablating even in someone whose tachycardia is otherwise easy to live with.
Unstable - cardiovert
Synchronised DC cardioversion for haemodynamic compromise, or whenever drug treatment fails to convert or control the rhythm.
Pre-excited AF - what NOT to give
An irregular, wide, very fast tachycardia in a patient with pre-excitation is atrial fibrillation conducting down the pathway. Give no AV-nodal blocking agent, and adenosine least of all: blocking the node removes the only brake in the system and forces everything down the pathway.
Pre-excited AF - what to give
Intravenous procainamide or ibutilide are preferred. Flecainide and propafenone are alternatives but are not risk-free - they act on the atrium and pathway with little effect on the node, and can accelerate the ventricular response. Amiodarone is no longer recommended: it may enhance conduction down the pathway, with reports of ventricular fibrillation.
Definitive - pathway ablation
Ablating the accessory pathway is curative. It removes the AVRT and the pre-excited-AF hazard together, which is why it is offered readily to anyone whose pathway conducts forwards.
The tachycardia this pattern enables. During orthodromic AVRT the pathway conducts backwards only, so the delta wave disappears and the QRS is narrow - the resting ECG and the tachycardia look nothing alike.
Irregular, wide and very fast, with the QRS width varying beat to beat as conduction shifts between pathway and node. Ordinary atrial fibrillation is irregular but narrow and far slower.
The commonest misdiagnosis of pre-excited AF, and the safer error. VT is regular with a uniform QRS; pre-excited AF is irregular with a QRS that changes shape.
The same hazard by another route. A pathway with no rate-dependent brake can conduct flutter 1:1, taking the ventricles to around 300.