WPW syndrome is a congenital heart condition caused by an extra, abnormal connection between the heart’s upper and lower chambers. This accessory pathway allows electrical signals to bypass the heart’s natural control center, leading to premature activation of the ventricles. This bypass creates the potential for rapid, potentially dangerous heart rhythms. The electrocardiogram (ECG) is the primary tool used to detect this condition, revealing specific, recognizable patterns that indicate the extra electrical connection.
The Electrical Basis of Wolff-Parkinson-White Syndrome
The heart’s normal electrical system relies on the atrioventricular (AV) node to act as a gatekeeper for impulses traveling from the atria to the ventricles. The AV node introduces a necessary delay, ensuring the atria contract first to fill the ventricles before they are signaled to pump. Electrical activity then moves swiftly through the His-Purkinje system for coordinated ventricular contraction.
In WPW, an accessory pathway forms an additional electrical bridge directly connecting the atria and ventricles, bypassing the AV node’s delay entirely. This pathway allows the electrical signal to take a shortcut. Because this signal arrives at the ventricles earlier than the normal, delayed route, it causes pre-excitation. This premature activation is the underlying cause of the abnormal shapes seen on the ECG.
The Diagnostic Triad: Identifying WPW on an ECG
Ventricular pre-excitation caused by WPW creates three distinct, hallmark changes that form a diagnostic triad visible on the resting ECG. This combination of features identifies the condition in a patient not experiencing a rapid heart rhythm. These findings reflect a fusion of electrical activity: one part travels quickly through the accessory pathway, and the other follows the slower, normal AV node route.
Short PR Interval
The PR interval measures the time required for the electrical impulse to travel from the atria to the ventricles. In a healthy heart, this interval normally ranges between 120 and 200 milliseconds. In WPW, the accessory pathway bypasses the AV node’s normal delay, significantly shortening this travel time. A short PR interval, defined as less than 120 milliseconds, is a defining feature of the WPW pattern.
The Delta Wave
The delta wave is the most distinctive visual sign of pre-excitation, appearing as a slurring or slow upstroke at the beginning of the QRS complex. This slurring represents the initial, slow spread of electrical activation through the ventricular muscle via the accessory pathway. Because the accessory pathway tissue is not as specialized as the His-Purkinje system, this initial electrical spread is gradual, creating the characteristic delayed curve.
Widened QRS Complex
The QRS complex represents the total time for the ventricles to depolarize and normally measures less than 120 milliseconds. In WPW, the QRS complex appears widened, often exceeding 120 milliseconds, due to the fusion of two electrical wavefronts. The first wavefront is the slow, early activation via the accessory pathway (the delta wave). The second is the normal, fast activation that reaches the ventricles through the AV node. This combination results in a broader overall ventricular activation time.
Distinguishing Manifest and Concealed WPW
The accessory pathway’s ability to conduct electricity determines how the WPW pattern appears on the ECG, leading to two classifications.
Manifest WPW
Manifest WPW occurs when the accessory pathway conducts electrical signals forward (antegrade) from the atria to the ventricles. When this happens, the resting ECG displays the full diagnostic triad: the short PR interval, the delta wave, and the widened QRS complex. The presence of these three visible signs indicates the pathway is actively participating in ventricular activation during normal sinus rhythm.
Concealed WPW
Concealed WPW occurs when the accessory pathway can only conduct signals backward (retrograde) from the ventricles to the atria. Because the pathway cannot conduct signals antegrade, the electrical impulse must use the normal AV node pathway for every heartbeat. Consequently, the resting ECG of a patient with concealed WPW appears entirely normal, lacking the short PR interval and the delta wave. This type of pathway is only revealed during episodes of rapid heart rhythm or specialized invasive electrophysiology studies.
Clinical Relevance of Accurate WPW Identification
Accurately identifying the WPW pattern on an ECG is important because the condition carries a risk of serious cardiac arrhythmias. The accessory pathway provides a potential circuit for a rapid, self-perpetuating heart rhythm, such as Paroxysmal Supraventricular Tachycardia (PSVT). The most feared risk occurs if the patient develops Atrial Fibrillation (AFib). Chaotic electrical signals from the atria can be conducted rapidly through the accessory pathway to the ventricles.
Unlike the AV node, which slows excessive electrical impulses, the accessory pathway may conduct signals at dangerously high rates. This rapid conduction can overwhelm the ventricles, potentially causing the heart rhythm to deteriorate into Ventricular Fibrillation, a life-threatening cause of sudden cardiac arrest. Detecting the WPW pattern allows clinicians to assess the risk and consider effective management options, such as catheter ablation, to eliminate the accessory pathway.