The electrocardiogram (ECG) is a non-invasive tool that evaluates the heart’s electrical activity, providing a visual representation of its rhythm and function. One condition identifiable through an ECG is Supraventricular Tachycardia (SVT), an abnormally fast heart rhythm originating from the upper chambers. This article guides the reader through identifying SVT on an ECG.
Key ECG Components for Identification
Interpreting an ECG tracing relies on understanding its fundamental components, each representing a specific electrical event in the heart. The P wave reflects atrial depolarization, the electrical activation of the heart’s upper chambers (atria) that precedes their contraction. The QRS complex signifies ventricular depolarization, representing the electrical activation of the heart’s lower chambers (ventricles) before they pump blood. The T wave indicates ventricular repolarization, the electrical recovery or relaxation of the ventricles after contraction.
Beyond individual waves, specific intervals on the ECG provide further insights into the heart’s conduction system. The PR interval measures the time from the beginning of atrial depolarization to the start of ventricular depolarization, typically 0.12 to 0.20 seconds. The QRS duration, the time for ventricles to depolarize, normally falls between 0.06 and 0.10 seconds, with less than 0.12 seconds considered narrow. The QT interval spans from the start of the QRS complex to the end of the T wave, reflecting total ventricular depolarization and repolarization time. Analyzing these components allows for heart rate calculation and arrhythmia identification, including SVT.
Identifying the Hallmarks of SVT on an ECG
Identifying SVT on an ECG involves looking for specific features. A rapid heart rate, typically 150 to 250 beats per minute, is a primary indicator. While general tachycardia exceeds 100 bpm, SVT rates are often much higher.
The rhythm in SVT is usually regular, with consistent intervals between QRS complexes. This regularity helps distinguish SVT from other fast, irregular rhythms. The QRS complex is typically narrow, measuring less than 0.12 seconds. A narrow QRS complex indicates the electrical impulse travels through normal ventricular conduction pathways, originating above the ventricles.
P waves, representing atrial activity, are another important aspect. In many SVT cases, P waves may be absent or hidden within the QRS complex, making them difficult to discern. This happens because rapid atrial activation can be closely timed with ventricular activation, obscuring the atrial signal. Sometimes, P waves appear after the QRS complex (retrograde P waves) or exhibit an abnormal shape. Their location and morphology provide clues about the SVT’s origin and mechanism.
Common SVT Subtypes and Their ECG Signatures
SVT is a broad term for several arrhythmias, each with subtle ECG variations.
Atrioventricular Nodal Reentrant Tachycardia (AVNRT)
AVNRT is the most common SVT in adults, characterized by a re-entry circuit within the AV node. On an ECG, AVNRT typically shows a regular, narrow QRS complex tachycardia. P waves are often hidden within the QRS complex or appear immediately after it, sometimes creating a “pseudo-R prime” wave in lead V1 or a “pseudo-S wave” in leads II, III, and aVF.
Atrioventricular Reentrant Tachycardia (AVRT)
AVRT involves an accessory pathway, an extra electrical connection bypassing the normal conduction system. Wolff-Parkinson-White (WPW) syndrome is associated with AVRT; a delta wave (slurring of the initial QRS) may be present during normal sinus rhythm, indicating pre-excitation. During orthodromic AVRT (conduction down normal pathway, back up accessory pathway), the QRS complex is narrow, and retrograde P waves may be visible after the QRS. During antidromic AVRT (conduction down accessory pathway, back up normal pathway), the QRS complex is wide due to abnormal ventricular activation, and P waves may not be visible.
Atrial Tachycardia
Atrial Tachycardia arises from an abnormal electrical focus within the atria. It typically presents with a regular rhythm and a heart rate from 100 to 250 beats per minute. A distinguishing feature is the presence of P waves with an abnormal morphology, different from normal sinus rhythm P waves. These abnormal P waves may precede each QRS complex, and the PR interval might be longer.
Distinguishing SVT from Look-Alike Rhythms
Differentiating SVT from other fast heart rhythms that may appear similar on an ECG is important for accurate diagnosis.
Sinus Tachycardia
Sinus Tachycardia also presents with a heart rate exceeding 100 beats per minute, but it is typically a physiological response to stress, exercise, or fever. Unlike SVT, Sinus Tachycardia has normal, upright P waves that consistently precede every QRS complex, maintaining a consistent PR interval. These P waves originate from the sinus node. The onset and termination of sinus tachycardia are generally gradual, unlike the abrupt changes often seen with SVT.
Ventricular Tachycardia (VT)
Ventricular Tachycardia (VT) with a narrow QRS complex is a less common but important rhythm to distinguish from SVT. While most VTs have wide QRS complexes, some can have a narrow QRS if they originate high in the ventricles or involve specific conduction abnormalities. The primary differentiator is QRS width: SVT maintains a narrow QRS (less than 0.12 seconds), indicating normal ventricular conduction, while typical VT features a wide QRS (0.12 seconds or greater). The presence of atrioventricular (AV) dissociation, where atria and ventricles beat independently, is a hallmark of VT generally absent in SVT.