Supraventricular Tachycardia (SVT) is a category of rapid heart rhythms that begin in the atria or the atrioventricular (AV) node, meaning they originate “above the ventricles.” These rhythms are characterized by an accelerated heart rate, typically exceeding 100 beats per minute (bpm). Identifying SVT on an electrocardiogram (ECG) involves a systematic approach: assessing rate and regularity, examining the width of the QRS complex, and performing a detailed analysis of the P-wave to determine the specific mechanism.
Establishing the Baseline: Rate and Regularity
The first step in diagnosing any tachycardia on an ECG is establishing the heart rate and its regularity. A rhythm is defined as tachycardia when the rate is greater than 100 beats per minute in an adult. Most SVTs, such as Atrioventricular Nodal Reentrant Tachycardia (AVNRT) and Atrioventricular Reentrant Tachycardia (AVRT), typically generate very fast, regular rates, often ranging from 150 to 250 bpm.
Calculating the ventricular rate is commonly done using the “300 rule” for regular rhythms. This method involves counting the number of large squares between two consecutive R-waves and dividing 300 by that number. For instance, if there are two large squares between R-waves, the rate is approximately 150 bpm (300/2).
While most SVTs are regular, it is important to check the R-R interval consistency, as some forms are irregular. Multifocal Atrial Tachycardia (MAT) is a notable exception, presenting as an irregular rhythm, though it is still classified as supraventricular. The rhythm’s consistency helps narrow the possibilities before moving on to the next diagnostic feature.
The Primary Indicator: QRS Complex Width
The width of the QRS complex is the most important feature for distinguishing SVT from Ventricular Tachycardia (VT). The QRS complex represents the electrical activation of the ventricles, and its width indicates the pathway the electrical signal takes to reach them. A narrow QRS complex is defined as a duration less than 0.12 seconds, or three small squares on the ECG paper.
In SVT, the electrical impulse originates above the ventricles and travels down the normal, highly efficient His-Purkinje conduction system, resulting in a characteristically narrow QRS complex. This narrow complex confirms that the ventricles are being activated through the proper pathways. A significant diagnostic challenge occurs when SVT presents with a wide QRS complex, a phenomenon known as “SVT with aberrancy.”
This widening happens when the fast rate causes one of the bundle branches to momentarily fail to conduct, or when an accessory pathway is used for ventricular activation. While a wide QRS complex (≥ 0.12 seconds) is more commonly associated with VT, wide-complex SVT must still be considered before making a final diagnosis.
Pinpointing the Mechanism: P-Wave Analysis
The final and most detailed step in SVT identification is P-wave analysis, which helps determine the specific mechanism causing the rapid rhythm. The P-wave represents atrial depolarization and provides clues about where the rhythm originates within the atria or AV node. The location and appearance of the P-wave relative to the QRS complex are used to differentiate the three main types of regular SVT: AVNRT, AVRT, and Atrial Tachycardia (AT).
One possibility is that the P-wave is completely hidden or absent from the tracing. This is often characteristic of typical AVNRT, where the electrical signal activates the atria and ventricles almost simultaneously. The atrial activity is therefore buried within the much larger ventricular signal (the QRS complex), making it invisible. Occasionally, the hidden P-wave may slightly distort the QRS complex, creating a small deflection known as a “pseudo r-prime” in lead V1 or a “pseudo S-wave” in the inferior leads.
Another common finding is a retrograde P-wave, which appears inverted in the inferior leads (II, III, aVF), indicating the atria are being activated from the bottom up. In Orthodromic AVRT, the P-wave is typically visible just after the QRS complex, resulting in a short RP interval. The time interval from the R-wave to the next P-wave (the RP interval) is a critical measurement; a very short RP interval (less than 70 milliseconds) strongly favors AVNRT, even if the P-wave is visible, while a longer RP interval suggests AVRT.
The final possibility is a visible, distinct P-wave that appears before the QRS complex but has an abnormal shape. When the P-wave morphology is different from the patient’s normal sinus P-wave, it suggests the rhythm is an Atrial Tachycardia originating from an ectopic focus in the atria. In AT, the RP interval is usually longer than the PR interval, meaning the P-wave is clearly separated from the QRS complex. If P-waves are not initially visible, techniques like vagal maneuvers or administering adenosine can temporarily block the AV node, allowing the underlying atrial activity to become apparent and confirming the diagnosis.