Supraventricular tachycardia (SVT) is a type of abnormal fast heart rhythm that originates above the ventricles. It is characterized by a rapid heart rate that can cause palpitations, dizziness, shortness of breath, and even fainting in some cases. One of the most effective ways to diagnose SVT is through an electrocardiogram (ECG), which records the electrical activity of the heart. But what exactly should one look for in a supraventricular tachycardia ECG? This article explores the key ECG findings in supraventricular tachycardia, helping patients and healthcare professionals understand this condition better.
Understanding Supraventricular Tachycardia
Before diving into ECG interpretations, it's essential to understand what SVT is and how it manifests. SVT refers to a group of arrhythmias that originate in the atria or the atrioventricular (AV) node. These arrhythmias often occur due to abnormal electrical circuits or triggers that cause the heart to beat excessively fast.
Common types of SVT include:
· Atrioventricular Nodal Reentrant Tachycardia (AVNRT) – The most common type, caused by a reentrant circuit within the AV node.
· Atrioventricular Reciprocating Tachycardia (AVRT) – Involves an extra electrical pathway connecting the atria and ventricles, often seen in Wolff-Parkinson-White (WPW) syndrome.
· Atrial Tachycardia – Originates from abnormal electrical signals in the atria.
The hallmark of these conditions is an abnormally fast heartbeat (often between 150-250 beats per minute) that begins and ends suddenly.
Key ECG Findings in Supraventricular Tachycardia
1. Narrow QRS Complex (≤120 ms)
One of the primary ECG findings in supraventricular tachycardia is a narrow QRS complex. This occurs because SVT originates above the ventricles, meaning the electrical impulse travels through the normal conduction pathway of the heart. The QRS complex in supraventricular tachycardia ECG is typically less than 120 milliseconds, indicating a properly functioning ventricular conduction system.
However, in some cases, particularly if there is underlying bundle branch block or an accessory pathway, the QRS complex can appear wide (>120 ms), mimicking ventricular tachycardia (VT). This is called SVT with aberrancy and can make differentiation from VT challenging.
2. Regular Rhythm
A key feature of most SVTs is a regular heart rhythm. The R-R intervals (the distance between consecutive R waves on an ECG) remain constant. This helps distinguish SVT from atrial fibrillation (AF), where the rhythm is typically irregular.
3. Absent or Abnormal P Waves
· In AVNRT, P waves are often hidden within the QRS complex or appear as retrograde P waves (seen immediately after the QRS in leads II, III, and aVF).
· In AVRT, retrograde P waves may be seen at a fixed distance after each QRS.
· In atrial tachycardia, the P waves are present but may have an abnormal morphology, as the impulse originates from an ectopic atrial focus rather than the sinus node.
4. Short RP Interval (in AVNRT and AVRT)
· The RP interval (distance between the R wave and the subsequent P wave) can be short, particularly in AVNRT, where the P wave is very close to or within the QRS complex.
· In AVRT, the RP interval varies depending on the type of accessory pathway involved.
5. Sudden Onset and Termination
SVT episodes usually begin and end abruptly, distinguishing them from sinus tachycardia, which has a gradual onset and resolution. This is a crucial ECG feature that helps in diagnosing SVT.
6. High Heart Rate (150-250 BPM)
A rapid heart rate is the most apparent ECG finding in SVT, typically ranging between 150-250 beats per minute. This is faster than sinus tachycardia (which usually does not exceed 150 BPM in adults) but slower than ventricular tachycardia.
7. Pseudo-S Waves and Pseudo-R’ Waves
In some cases, pseudo-S waves (negative deflections in leads II, III, and aVF) or pseudo-R’ waves (positive deflections in lead V1) may be observed, particularly in AVNRT. These are retrograde atrial activities and can help confirm the diagnosis of SVT.
Differentiating SVT from Other Arrhythmias
SVT vs. Atrial Fibrillation (AF)
· SVT: Regular rhythm, narrow QRS, absent/retrograde P waves.
· AF: Irregularly irregular rhythm, absence of distinct P waves.
SVT vs. Ventricular Tachycardia (VT)
· SVT: Narrow QRS, fast but regular rate, absent or retrograde P waves.
· VT: Wide QRS, may have AV dissociation, fusion, and capture beats.
Diagnosis and Management
ECG in Acute SVT Episodes
If a patient presents with palpitations, obtaining an ECG during the episode is crucial. If the episode terminates before the ECG is recorded, a Holter monitor or event recorder can be used to capture transient arrhythmias.
Acute Treatment
· Vagal maneuvers (such as the Valsalva maneuver or carotid sinus massage) can help terminate certain SVTs, especially AVNRT.
· Adenosine is a first-line medication that temporarily blocks AV nodal conduction and helps identify the arrhythmia.
· Beta-blockers or calcium channel blockers may be used for rate control.
· Electrical cardioversion is considered if the patient is hemodynamically unstable.
Long-Term Management
· Lifestyle modifications (reducing caffeine, stress, and alcohol) may help prevent episodes.
· Medications like beta-blockers or antiarrhythmics can help in recurrent cases.
· Catheter ablation is a definitive treatment for frequent or symptomatic SVT.
Where to Seek Expert Care?
If you experience symptoms of SVT, timely diagnosis and management are essential. The heart.ae offers state-of-the-art cardiac services, including advanced ECG interpretation and treatment options for arrhythmias like SVT.
Conclusion
An ECG is a powerful tool in diagnosing supraventricular tachycardia. Recognizing the key ECG findings—such as a narrow QRS, regular rhythm, abnormal P waves, and rapid heart rate—can help differentiate SVT from other arrhythmias and guide appropriate treatment. If you suspect SVT, consulting a heart specialist is crucial for proper evaluation and management.
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