An electrocardiogram (ECG) is a graphic recording of the heart’s electrical activity, providing a visual representation of its rhythm and function. Supraventricular Tachycardia (SVT) refers to an abnormally fast heart rhythm originating from the upper chambers of the heart. Understanding how SVT appears on an ECG tracing is important for its identification and appropriate management.
Understanding the Basics of an Electrocardiogram
An electrocardiogram translates the heart’s electrical signals into a visual tracing composed of distinct waves and segments. Each wave corresponds to a specific electrical event within the heart. The P wave represents the electrical activation of the atria, the heart’s upper chambers, as they contract.
Following the P wave, the QRS complex indicates the electrical activation of the ventricles, the heart’s lower chambers, as they contract to pump blood. The QRS complex is typically a tall, sharp deflection on the ECG. The T wave then signifies the electrical recovery of the ventricles, as they relax and refill with blood.
The time between these waves and their shapes provides information about the heart’s electrical conduction system. The heart’s rhythm refers to the regularity of these electrical impulses, while the rate indicates how many times the heart beats per minute. A typical resting heart rate for an adult ranges from 60 to 100 beats per minute.
What is Supraventricular Tachycardia?
Supraventricular Tachycardia (SVT) is an abnormally rapid heartbeat. The term “supraventricular” indicates that the electrical impulse originates from above the ventricles, specifically in the atria or the atrioventricular (AV) node.
During an SVT episode, the heart rate typically ranges from 150 to 250 beats per minute. This rapid rate can prevent the heart chambers from filling completely between contractions, potentially reducing the amount of blood pumped to the body. SVT can begin and end suddenly, and while it may not always cause symptoms, some individuals might experience palpitations, chest pain, or lightheadedness.
SVT is a broad term encompassing several types of fast rhythms, all originating from the upper regions of the heart. These can include atrioventricular nodal reentrant tachycardia (AVNRT), which is a common type, and atrioventricular reciprocating tachycardia (AVRT). The underlying cause often involves an electrical short circuit or an abnormal pathway within the heart’s conduction system.
Key ECG Characteristics of Supraventricular Tachycardia
Identifying Supraventricular Tachycardia on an ECG involves recognizing distinct features related to heart rate, QRS complex, and P waves. The heart rate in SVT is consistently fast, typically ranging from 150 to 250 beats per minute. This rapid rate is a primary indicator, distinguishing it from a normal resting heart rate.
A defining characteristic of most SVTs on an ECG is a narrow QRS complex, generally measuring less than 0.12 seconds (120 milliseconds) in duration. A narrow QRS complex indicates that the electrical impulse travels through the ventricles via the heart’s normal conduction pathways. This narrow complex helps differentiate SVT from other fast heart rhythms originating in the ventricles, which typically show a wider QRS complex.
The appearance or absence of P waves is another important, yet variable, feature in SVT. In some SVT types, P waves may be absent or hidden within the QRS complex because the atria and ventricles activate almost simultaneously. When visible, P waves might appear immediately after the QRS complex, or within the ST segment. These P waves often display a retrograde pattern, meaning they are inverted in certain ECG leads (like leads II, III, and aVF), indicating that atrial activation moves backward from the AV node towards the atria.
For example, in atrioventricular nodal reentrant tachycardia (AVNRT), P waves are frequently buried within the QRS complex, leading to their apparent absence, or they may create a small distortion at the end of the QRS complex, sometimes referred to as a “pseudo-R prime” wave in lead V1 or a “pseudo-S” wave in inferior leads. In other SVT forms, such as atrioventricular reciprocating tachycardia (AVRT), retrograde P waves are typically seen following the QRS complex. The regularity of the rhythm is also a key observation, as SVT usually presents as a regular, fast rhythm.
Why Accurate ECG Interpretation Matters
Accurate interpretation of an ECG showing Supraventricular Tachycardia is important for guiding appropriate medical assessment. The ECG provides a tangible record of the heart’s electrical activity during an episode, often serving as the primary tool for diagnosis. Correctly identifying SVT and distinguishing it from other types of rapid heart rhythms allows medical professionals to understand the origin and nature of the abnormal electrical activity.
The precise ECG findings help in characterizing the specific type of SVT, which can influence decisions regarding potential interventions. While some SVT episodes may resolve on their own, others might require medical attention. Additional diagnostic steps, such as a 12-lead ECG during an active episode or comparison with a previous normal ECG, are taken to confirm the diagnosis and determine the underlying mechanism.