WOLFF-PARKINSON-WHITE (WPW) syndrome is a common type of pre-excitation syndrome, an abnormal condition in which electrical impulses enter the ventricles from the atria by using an accessory pathway that bypasses the atrioventricular (AV) node. This results in a short PR interval and a wide QRS complex with a delta wave (an initial slurring of the QRS complex, which represents early, abnormal ventricular depolarization). Because the delta wave prolongs the QRS complex, you may confuse it with a bundle-branch block (BBB).
WPW syndrome
* A delta wave occurs at the beginning of the QRS complex, usually causing a distinctive slurring or hump in its initial slope. A delta wave isn't present in BBB.
* On the 12-lead ECG, the delta wave will be most pronounced in the leads "looking at" the part of the heart where the accessory pathway is located.
* The delta wave shortens the PR interval in WPW syndrome.
BBB
* Carefully examine the QRS complex, noting which part of the complex is widened. A BBB involves a defective conduction of the electrical impulses through the right or left bundle branch from the bundle of His to the Purkinje network, causing a right or left BBB.
* This conduction disturbance results in an overall increase in QRS duration, or widening of the last part of the QRS complex, while the initial part of the QRS complex usually appears normal.
* Carefully examine the 12-lead ECG. With BBB, the prolonged duration of the QRS complexes will generally be consistent in all leads.
* Measure the PR interval. BBB has no effect on it, so it's generally normal. Keep in mind, though, that if the patient has a pre-existing AV conduction defect, such as first-degree AV block, the PR interval will be prolonged.
Source: Interpreting Difficult ECGs: A Rapid Reference, Lippincott Williams & Wilkins, 2006.