What Are the ECG Characteristics of Sustained Ventricular Tachycardia?

Sustained ventricular tachycardia (VT) is a serious cardiac arrhythmia that originates in the ventricles and is typically defined as three or more consecutive premature ventricular contractions at a rate exceeding 100 beats per minute, lasting for more than 30 seconds. The electrocardiogram (ECG) plays a crucial role in diagnosing this condition, showing several distinct features that help differentiate VT from other types of tachycardia.

Key ECG Features of Sustained Ventricular Tachycardia

1. Rapid Heart Rate with Wide QRS Complexes

The hallmark of sustained VT on an ECG is a heart rate ranging from 150 to 250 beats per minute. This rapid rhythm is accompanied by abnormally wide QRS complexes, which reflect the abnormal electrical conduction through the ventricles due to the origin of the impulse being outside the normal conduction system.

2. Atrioventricular Dissociation

In sustained VT, the atria and ventricles often beat independently of each other, a phenomenon known as AV dissociation. This means that the P waves (representing atrial depolarization) are not related to the QRS complexes (representing ventricular depolarization). Additionally, the T wave, which reflects ventricular repolarization, usually appears in the opposite direction to the main deflection of the QRS complex, a classic sign of ventricular tachycardia.

3. Normal or Prolonged QT Interval

The QT interval, which reflects the time taken for ventricular depolarization and repolarization, may appear normal or slightly prolonged in patients with sustained VT. However, a prolonged QT interval can also be a contributing factor to the development of certain types of VT, such as torsades de pointes.

4. Presence of Fusion Beats and Ventricular Capture Beats

Another notable feature in sustained VT is the appearance of fusion beats and ventricular capture beats. A fusion beat occurs when a supraventricular impulse (originating from the sinus node or atria) and a ventricular impulse coincide, resulting in a hybrid QRS complex. A ventricular capture beat happens when a normal sinus impulse briefly overrides the ventricular rhythm, producing a normal or narrow QRS complex amidst the wide complexes of VT.