Wolff-Parkinson-White (WPW) syndrome is a congenital heart condition involving an extra electrical connection, known as an accessory pathway, linking the atria and the ventricles. This pathway bypasses the normal electrical delay of the atrioventricular (AV) node, causing electrical signals to travel too quickly (preexcitation). WPW syndrome can cause dangerously fast and chaotic heart rhythms (arrhythmias), leading to lightheadedness, fainting, and, rarely, sudden cardiac arrest. Comprehensive testing is necessary to confirm the diagnosis and accurately determine an individual’s specific risk level for these serious complications.
Initial Screening and Non-Invasive Diagnostics
The initial step in diagnosing Wolff-Parkinson-White syndrome involves non-invasive methods, primarily the 12-lead electrocardiogram (ECG or EKG). The EKG records the heart’s electrical activity and can identify the characteristic “WPW pattern” in a resting patient. This pattern is defined by a short PR interval, indicating the electrical signal reaches the ventricles too quickly. It also includes the presence of a “delta wave,” a slurred upstroke on the QRS complex representing the early activation of the ventricles through the accessory pathway.
Since symptoms of an abnormal heartbeat can be intermittent, ambulatory monitors are often employed to capture electrical activity over a longer period. A Holter monitor is worn for 24 to 48 hours to provide continuous recording of the heart’s rhythm during daily activities. For patients with less frequent symptoms, an event recorder may be used for several weeks, activated only when the patient experiences palpitations or other symptoms. These devices document the presence of preexcitation or the actual occurrence of a rapid heart rhythm, which may not be present on a single EKG.
Another non-invasive diagnostic procedure is the exercise stress test, which records the EKG while the patient walks on a treadmill or uses a stationary bike. During exercise, the heart rate increases, and doctors observe whether the electrical preexcitation pattern abruptly disappears. If the delta wave disappears at faster heart rates, this suggests the accessory pathway has a relatively long refractory period, generally associated with a lower risk profile. Conversely, the persistence of the delta wave throughout maximum exertion is a sign of a higher-risk pathway.
Definitive Diagnosis via Electrophysiology Study
The gold standard for definitively characterizing the accessory pathway is the Electrophysiology Study (EPS), an invasive procedure typically performed in a specialized cardiac catheterization laboratory. Thin, flexible wires called catheters are inserted into a vein, usually in the groin, and threaded up to the various chambers of the heart. These catheters have electrodes that allow cardiologists to map the heart’s electrical system and record signals directly from inside the heart.
The primary diagnostic goal of the EPS is to precisely locate the accessory pathway and determine its electrical properties. Physicians perform controlled electrical stimulation, delivering tiny impulses through the catheters to pace the heart and intentionally induce an arrhythmia, such as supraventricular tachycardia. This induced tachycardia allows for the measurement of the accessory pathway’s effective refractory period (ERP). The ERP is the shortest time interval in which the pathway can conduct an electrical impulse, a critical measure for determining the risk of life-threatening rhythms.
To simulate conditions of stress or exercise, a drug called isoproterenol, which mimics adrenaline, may be administered during the study. This drug shortens the ERP of the accessory pathway, thereby revealing its fastest possible conduction rate. By provoking and analyzing these rhythms, the EPS provides detailed data on the accessory pathway’s capacity to conduct rapid impulses. This information cannot be reliably obtained through non-invasive testing alone.
Interpreting Results and Risk Stratification
Following the diagnostic procedures, the collected data is used to confirm the presence of the accessory pathway and, more importantly, to stratify the patient’s risk. Risk stratification in WPW is the process of determining an individual’s likelihood of experiencing a life-threatening event, such as sudden cardiac death. This event is typically presumed to be caused by a rapid progression to ventricular fibrillation. The most significant metric for risk assessment is the accessory pathway’s effective refractory period (ERP).
A short ERP, generally defined as less than 250 milliseconds, indicates a high-risk pathway that conducts electrical impulses very quickly. This rapid conduction is dangerous because if the patient develops atrial fibrillation, the accessory pathway could transmit the chaotic rhythm to the ventricles at an extremely fast rate, potentially triggering ventricular fibrillation. Another high-risk indicator is the shortest preexcited R-R interval during induced atrial fibrillation of 250 milliseconds or less. Conversely, a longer ERP suggests a lower-risk pathway.