SVT vs VT: Understanding the Difference in Heart Rhythm Disorders

The treatment of arrhythmias, also known as abnormal heart rhythms, is a crucial part of ACLS care. Arrhythmias range from benign to life-threatening, depending on the rate and specific rhythm. Supraventricular tachycardias (SVT) and ventricular tachycardias (VT) are two very common arrhythmias that the ACLS practitioner may encounter.

Although the names sound similar (both SVT and VT represent heart rates over 100 beats per minute) SVT and VT require different evaluation procedures and treatments. Whereas SVT is rarely dangerous, VT is inherently life-threatening and can lead to cardiac arrest if not treated appropriately. Differentiating between SVT and VT on an electrocardiogram is key for the ACLS clinician to provide appropriate care to patients.

Let’s take a closer look at the differences between these two rhythms and how to treat them effectively according to modern ACLS guidelines.

Introduction to Heart Rhythm Disorders

Normal cardiac function is coordinated by a complex electrical system in the heart. A normal heartbeat originates at the sinoatrial (SA) node, conducts through to the atrioventricular node (AV), then travels down the Bundle of His where it depolarizes the left and the right ventricles.

Coordinated cardiac activity between the atria and the ventricles is crucial for normal cardiac contractility. Anelectrical rhythm that does not follow the normal pathway, or is faster or slower than normal, is called an arrhythmia.

Arrhythmias can be categorized into two types:

• Supraventricular -  When the impulses originate above the ventricle.
• Ventricular -  When the cardiac impulses are generated in the ventricles below the level of the AV node.

Explaining SVT (Supraventricular Tachycardia)

Supraventricular tachycardia (SVT) is any heart rate greater than 100 that originates above the ventricles. Some examples of SVT are atrioventricular nodal reentry tachycardia (AVNRT), atrial fibrillation (AF), and multifocal atrial tachycardia (MAT).

Millions of people are affected by SVTs annually, making it a relatively common clinical scenario. While SVT is bothersome and uncomfortable, it is rarely life-threatening. An exception is atrial fibrillation, which puts patients at higher risk of embolic stroke. Otherwise, episodes of SVT can cause palpitations, chest pain, and shortness of breath.

Causes of SVT

The causes of SVT are many and are highly dependent on the underlying cardiac rhythm.

For example, AVNRT is caused by an accessory pathway in the heart. This is usually congenital and is asymptomatic until the episode of AVNRT. Any medication that increases sympathomimetic tone (such as albuterol or caffeine) can cause supraventricular tachycardia. Multifocal atrial tachycardia and atrial fibrillation usually occur in older adults and are caused by right atrial enlargement and increased right atrial pressures from conditions such as COPD.

Identifying SVT

In general, supraventricular tachycardias have a narrow QRS complex on the electrocardiogram (<0.12 milliseconds). Conversely, ventricular tachycardia has a wide-complex QRS (> than 0.12 milliseconds).

However, in the setting of a left bundle branch block or other conduction abnormality, a supraventricular rhythm can appear to be wide, similar to VT. Therefore, when a wide QRS is present, it may be difficult to differentiate SVT from VT. The ACLS practitioner should remember that when the diagnosis of SVT versus VT is unclear, the patient should always be treated as if they have VT which is a life-threatening arrhythmia.

SVT Treatment

SVT can be treated either medically, electrically, or surgically by cardiac ablation. In the acute setting, medications such as beta-blockers or calcium channel blockers may be used to control the heart rate by slowing conduction through the atrioventricular node. If a patient is unstable, the ACLS practitioner should perform immediate synchronized cardioversion.

Chronically, patients may take beta-blockers and calcium channel blockers for this purpose. If AVNRT is the presenting arrhythmia, the clinician may use adenosine per ACLS protocols.  If patients have recurrent episodes of SVT, a cardiac ablation may be considered to destroy the abnormal focus in the heart causing the SVT.

Source: https://blog.clinicalmonster.com/2018/06/06/ventricular-tachycardia/

Explaining VT (Ventricular Tachycardia)

Ventricular tachycardia (VT) is defined as any tachycardic rhythm (heart rate greater than 100 bpm) that originates below the level of the atrioventricular (AV) node. When VT occurs, the ventricles pump out of sync with the atria. As a result, there is abnormal decreased cardiac output which can lead to cardiogenic shock. Although the ACLS guidelines divide ventricular tachycardia into stable tachycardia and unstable tachycardia, the clinician should understand that VT is always life-threatening and can degenerate into ventricular fibrillation at any time.

Ventricular tachycardias include monomorphic VT (from one focus in the ventricle) and polymorphic VT (originating from more than one focus in the ventricles). On an electrocardiogram, VT appears as a wide-complex QRS (>0.12 milliseconds).

Causes of VT

The causes of VT can be structural, electrical, or medication-related. Any condition that causes a change in the heart structure, such as cardiomyopathy or scar tissue resulting from a myocardial infarction, can also lead to VT.

In addition, there are congenital conditions such as Brugada syndrome or congenital long QT syndrome, that place patients at greater risk of developing VT. Medications that prolong the QT interval can lead to polymorphic VT (torsades de pointes).

Treatment of VT

The treatment of ventricular tachycardia may consist of medications, synchronized cardioversion, or defibrillation — the correct approach depends on the specific rhythm. Here are some common scenarios:

• Ventricular tachycardia without a pulse - The ACLS provider should immediately perform unsynchronized cardioversion.
• Unstable VT with a pulse - The clinician should perform synchronized cardioversion per ACLS guidelines.
• Stable VT - The ACLS provider may choose to perform synchronized cardioversion or opt to administer medications such as amiodarone or procainamide. All antiarrhythmic medications have medication interactions and side effects such as hypotension. On the other hand, synchronized cardioversion requires sedation of a patient to prevent discomfort when delivering the shock which could lead to airway compromise. Therefore, the decision to use synchronized cardioversion versus an antiarrhythmic medication should be based on a patient’s medical history, current medications, and other patient factors.
• Polymorphic VT due to prolonged QT – In this case, magnesium sulfate is the medication of choice.

Long-term management of ventricular tachycardia may include placement of an automated internal cardiac defibrillator (AICD).

Key Differences Between SVT and VT

The key difference between SVT and VT is where the electrical impulses originate in the heart. Here are the most important points to remember:

SVT	VT
The electrical impulse originates at the level of the atrioventricular node or above.	The electrical impulse originates below the level of the atrioventricular note, in the ventricle.
Rarely a life-threatening arrhythmia.	Always life-threatening, as it can quickly degenerate into ventricular fibrillation.
Usually treated with medications, as patients are generally not unstable.	Usually treated with synchronized cardioversion rather than medications, because of the risk of degenerating into a lethal arrhythmia.

Similarities Between SVT and VT

SVT and VT are both tachycardias with ventricular rates greater than 100 bpm. Both also have the potential to be stable or unstable rhythms. Both SVT and VT can be treated with medications or synchronized cardioversion if the patient is unstable.

NOTE: If SVT coexists in a patient with a bundle branch block, it can appear similar to VT as it will show as a wide-complex tachycardia on electrocardiogram.

Final Thoughts

Where supraventricular tachycardia is rarely life-threatening or unstable, ventricular tachycardia is inherently unstable and can quickly degenerate into ventricular fibrillation. Although both VT and SVT can be treated with medications, unstable SVT and VT should always be treated with synchronized cardioversion.

As an ACLS clinician, it is very likely you’ll encounter both of these relatively common arrhythmias at some point in your career. To be prepared, it's important to understand the differences, causes, and treatment options necessary to help patients recover. Misidentifying these rhythms can lead to improper treatment and may cause harm to patients.

To learn more about the drugs, clinical scenarios, and ECG rhythms associated with ACLS, AMRI’s study materials will help you succeed. Having helped over one million professionals become certified or recertified in ACLS, BLS, or PALS, AMRI is well-qualified to prepare you for any life support situation. Register for your exam today and gain access to study guides, practice tests, and more.