Supraventricular Tachycardia (SVT) describes an abnormally rapid heart rhythm originating within the heart’s upper chambers or central conduction system, specifically above the ventricles. SVT is an umbrella category for several fast rhythms sharing a similar origin point. Identifying the specific type of SVT requires detailed analysis of the electrical signals recorded by an electrocardiogram (ECG). The ECG tracing provides a visual map of the heart’s electrical activity necessary for distinguishing these rhythms from one another and from more dangerous ventricular rhythms.
Understanding the Basic ECG Components
An ECG tracing is composed of distinct waves and intervals reflecting the sequence of electrical activation within the heart during a single beat. The P wave represents the depolarization of the atria. The PR interval measures the time the electrical signal takes to travel from the atria through the atrioventricular (AV) node down to the ventricles. The QRS complex signifies the rapid electrical activation and depolarization of the ventricles. A narrow QRS complex indicates the signal traveled through the normal ventricular conduction system, while the T wave represents the electrical resetting of the ventricles.
Establishing the Tachycardia Criteria
The initial step in identifying SVT is confirming tachycardia, defined as a heart rate exceeding 100 beats per minute (bpm) at rest in adults. Sustained SVT rates typically fall between 150 and 250 bpm, which often causes electrical waves to overlap and makes tracing difficult to interpret. The second feature of SVTs is a narrow QRS complex, typically measuring less than 0.12 seconds in duration. This narrow width confirms the electrical impulse utilized the fast-conducting His-Purkinje system to activate the ventricles. A narrow QRS complex is the primary indicator that the rhythm originates above the ventricles, establishing it as a narrow-complex tachycardia requiring further analysis.
P-Wave Presence and Location
The most detailed analysis centers on the P wave, as its presence, shape, and timing relative to the QRS complex reveal the specific mechanism driving the SVT. In many common SVTs, the rapid re-entry circuit causes the atria and ventricles to activate nearly simultaneously. This often results in the P wave being completely obscured, or “hidden,” within the QRS complex. If atrial activation happens immediately after the ventricles, the P wave can appear as a subtle deflection, often fusing with the end of the QRS complex or the beginning of the T wave. In Atrioventricular Nodal Reentrant Tachycardia (AVNRT), this P wave can create a small notch, sometimes called a pseudo-R’ wave in lead V1 or a pseudo-S wave in the inferior leads.
These P waves often travel backward, or retrograde, toward the atria, causing them to appear inverted in leads where they would normally be upright (e.g., leads II, III, and aVF). The R-P interval, the time from the peak of the QRS complex to the start of the subsequent P wave, is a differentiating tool. In Atrioventricular Reentrant Tachycardia (AVRT), the P wave often appears later and distinctly separate from the QRS complex, creating a longer R-P interval. This later appearance results from the impulse traveling a longer distance through an accessory pathway before reaching the atria. Analyzing the R-P interval helps differentiate between SVT mechanisms that share a narrow QRS and fast rate.
Recognizing Common Variants and Look-Alikes
Although SVT is defined by a narrow QRS complex, it can occasionally present as a wide-complex tachycardia. This occurs when the supraventricular impulse encounters a pre-existing or rate-related bundle branch block, a phenomenon called aberrancy. The resulting delayed ventricular conduction widens the QRS complex, causing the SVT to mimic Ventricular Tachycardia (VT). Clues suggesting SVT with aberrancy include a QRS pattern during tachycardia identical to a known bundle branch block pattern on a patient’s previous normal ECG. Detailed QRS morphology features, such as a sharp downstroke in lead aVR, can also favor SVT over VT.
Other rhythms, such as Atrial Fibrillation and Atrial Flutter, are also types of SVT but have distinct ECG signatures separating them from typical paroxysmal SVT. Atrial Flutter is characterized by a distinctive “sawtooth” pattern of flutter waves, often visible in the inferior leads, reflecting rapid, organized atrial activity around 300 bpm. Atrial Fibrillation shows a chaotic, wavy baseline with no discernible P waves and an irregularly irregular rhythm, reflecting disorganized atrial activation.