The electrocardiogram (ECG) is a simple, non-invasive test that records the heart’s electrical activity, which appears as a series of waves and complexes. The delta wave is an abnormal finding on this recording, representing a distinct electrical pattern known as pre-excitation. Its presence signifies that a portion of the heart’s lower chambers is being activated too early by an electrical shortcut.
Identifying the Delta Wave on an ECG
The delta wave appears as a slurred, slow rise at the very beginning of the QRS complex. The QRS complex represents the electrical activation of the ventricles, the heart’s main pumping chambers. Normally, the QRS complex begins with a sharp, immediate upward deflection, but the delta wave smooths out this initial spike.
This slurring is accompanied by a shortened PR interval, which measures the time it takes for the electrical signal to travel from the upper chambers (atria) to the ventricles. The shortcut causes the signal to arrive prematurely, shortening this interval to less than 120 milliseconds. This premature ventricular activation also results in a QRS complex that is wider than normal, often exceeding 100 milliseconds.
The Electrical Cause Pre-excitation
Electrical impulses originate in the sinoatrial (SA) node, the heart’s natural pacemaker, and travel to the atrioventricular (AV) node. The AV node acts as a gatekeeper, slowing the impulse momentarily to ensure the atria have time to finish pumping blood into the ventricles before the ventricles contract.
Pre-excitation occurs when an individual possesses an extra bundle of muscle tissue called an accessory pathway. This pathway is essentially an electrical connection between the atria and ventricles that bypasses the AV node. Since this accessory pathway lacks the delay mechanism of the AV node, the electrical signal reaches a portion of the ventricles too early.
The resulting QRS complex is known as a fusion beat because the ventricles are activated by two different signals simultaneously. The initial, slurred part of the QRS complex (the delta wave) is caused by the slow, muscle-to-muscle spread of the impulse that traveled via the accessory pathway. The remainder of the QRS complex is formed when the normal, delayed impulse finally arrives through the AV node, spreading rapidly through the specialized His-Purkinje system.
Associated Condition Wolff-Parkinson-White Syndrome
Wolff-Parkinson-White (WPW) Syndrome is defined by the delta wave pattern on an ECG combined with a history of rapid heart rhythms. This condition is a congenital abnormality, present from birth, although it may not cause symptoms until adulthood. The estimated prevalence of WPW syndrome is approximately one to three people per 1,000 worldwide.
The accessory pathway creates a risk for tachycardia because it can form a self-perpetuating electrical loop, or re-entrant circuit. The impulse travels down one pathway and immediately returns up the other, causing the heart to beat fast, often between 150 and 250 beats per minute. The greatest danger arises if the patient develops atrial fibrillation, a chaotic rhythm in the atria.
The AV node normally blocks most of these rapid, chaotic impulses from reaching the ventricles, protecting the heart. However, the accessory pathway has no such protective filtering mechanism and can conduct these rapid signals directly to the ventricles. This can overwhelm the ventricles, potentially leading to ventricular fibrillation, a life-threatening rhythm. The risk of sudden cardiac death is low, but it is the primary concern associated with WPW syndrome.
Clinical Implications and Management
Individuals with WPW syndrome often experience symptoms related to rapid heart rhythms, such as palpitations, dizziness, lightheadedness, or fainting (syncope). Some patients, however, may have the delta wave pattern on their ECG but never experience any symptoms; this is referred to as WPW pattern rather than syndrome. For asymptomatic individuals, risk stratification is performed to determine if the accessory pathway poses a high risk for dangerous rapid rhythms.
The long-term management strategy for symptomatic patients involves a curative procedure called catheter ablation. This minimally invasive procedure involves guiding thin wires, or catheters, through blood vessels up to the heart. Once the accessory pathway is precisely located, energy (such as radiofrequency or cold energy, cryoablation) is delivered to destroy the small area of muscle tissue responsible for the shortcut. Catheter ablation has a high success rate, permanently eliminating the pathway. Antiarrhythmic medications can also be used to control the heart rhythm and slow conduction through the accessory pathway.