Supraventricular Tachycardia

Supraventricular Tachycardia (SVT) is a type of abnormally fast heart rhythm (tachycardia) that originates in the upper chambers of the heart (atria) or in the atrioventricular (AV) node, which is the electrical pathway connecting the atria and ventricles. During an SVT episode, the heart beats very rapidly, typically between 100 to 250 beats per minute, in a regular rhythm. While often not life-threatening, SVT can cause alarming symptoms like palpitations, dizziness, and shortness of breath, and can significantly impact quality of life. SVT episodes can start and end abruptly and are generally caused by re-entry circuits or abnormal automaticity in the heart’s electrical system.

Types of Supraventricular Tachycardia:

SVT encompasses several specific types of rapid heart rhythms, classified by their exact origin and mechanism.

• Atrioventricular Nodal Reentrant Tachycardia (AVNRT):
  • The most common type of SVT.
  • Involves an extra electrical pathway within or near the AV node itself, creating a “short circuit” that causes the electrical impulse to rapidly re-enter and re-excite the atria and ventricles.
• Atrioventricular Reciprocating Tachycardia (AVRT) / Wolff-Parkinson-White (WPW) Syndrome:
  • Involves an extra, abnormal electrical pathway (accessory pathway) connecting the atria and ventricles, bypassing the AV node.
  • The electrical impulse travels down one pathway (e.g., AV node) and up the accessory pathway (or vice versa), creating a re-entry loop.
  • WPW syndrome is a specific condition where this accessory pathway causes symptoms.
• Atrial Tachycardia (AT):
  • Originates from an abnormal electrical focus (a single irritable spot) in the atria, outside of the sinus node (the heart’s natural pacemaker).
  • This focus fires rapidly and sends impulses to the ventricles.
• Paroxysmal Supraventricular Tachycardia (PSVT):
  • A general term used to describe SVT episodes that start and stop suddenly (paroxysmal).
  • Often used interchangeably with AVNRT or AVRT clinically, as they are common causes of paroxysmal episodes.

Causes and Triggers:

SVT is fundamentally an electrical problem within the heart. While some individuals have structural abnormalities (like extra pathways), triggers can initiate episodes. In many cases, the cause is unknown.

• Abnormal Electrical Pathways or Circuits:
  • As described in the types above, the presence of extra pathways (e.g., in WPW syndrome) or re-entry circuits within the AV node (AVNRT) creates the substrate for SVT.
  • Abnormal automaticity (cells firing spontaneously) can also cause atrial tachycardia.
• Triggers (Can initiate an SVT episode in susceptible individuals):
  • Stress and Anxiety: Emotional stress can activate the sympathetic nervous system.
  • Fatigue and Lack of Sleep.
  • Caffeine: Excessive intake of coffee, tea, energy drinks.
  • Alcohol: Especially binge drinking.
  • Nicotine: From smoking or vaping.
  • Certain Medications:
    • Cold and allergy medications (e.g., pseudoephedrine, phenylephrine).
    • Asthma medications (e.g., bronchodilators).
    • Thyroid medications (if dosage is too high).
    • Stimulants (e.g., for ADHD).
  • Illicit Drugs: Cocaine, amphetamines.
  • Dehydration or Electrolyte Imbalance.
  • Intense Physical Exertion.
  • Hormonal Changes: Pregnancy, menstrual cycle.
  • Underlying Heart Conditions: While often occurring in healthy hearts, SVT can sometimes be associated with heart disease, previous heart attack, or heart failure.
• No Identifiable Cause: Many SVT episodes occur spontaneously without an obvious trigger.

Symptoms:

SVT symptoms can range from mild and bothersome to severe and alarming. They often start and stop suddenly. Severity can depend on the heart rate and the presence of underlying heart conditions.

• Palpitations: A sudden sensation of a rapid, pounding, fluttering, or racing heartbeat. This is the most common symptom.
• Dizziness or Lightheadedness.
• Shortness of Breath (Dyspnea).
• Chest Discomfort or Pain.
• Anxiety.
• Fatigue.
• Fainting (Syncope) or Near-Fainting: Less common, but can occur if the heart rate is extremely fast and blood pressure drops significantly.
• Sweating.
• Nausea.
• Polyuria: Increased urination during or shortly after an episode (due to increased pressure in the atria).

Diagnosis:

Diagnosing SVT often requires capturing an episode on an electrocardiogram (ECG) to identify the characteristic rapid rhythm originating above the ventricles.

• Medical History and Physical Exam: The doctor will ask about symptoms, their frequency, duration, and any known triggers. Listening to the heart during an episode can reveal a very fast, regular rhythm.
• Electrocardiogram (ECG/EKG):
  • The most important diagnostic tool. It records the electrical activity of the heart.
  • An ECG taken during an SVT episode shows a rapid heart rate (typically 100-250 bpm) with narrow QRS complexes, indicating the electrical impulse originates above the ventricles.
• Holter Monitor: A portable ECG device worn for 24-48 hours (or longer) to record heart activity during daily activities. Useful for capturing infrequent SVT episodes.
• Event Monitor: A portable device worn for weeks or months that can be activated by the patient when symptoms occur, or set to automatically record abnormal rhythms.
• Implantable Loop Recorder (ILR): A small device implanted under the skin for long-term monitoring (up to 3 years) for very infrequent episodes.
• Electrophysiology (EP) Study:
  • An invasive procedure where thin, flexible catheters are threaded through veins into the heart.
  • Used to precisely map the heart’s electrical pathways, locate the abnormal circuit causing SVT, and induce the SVT to confirm diagnosis. Essential for planning ablation procedures.
• Blood Tests: To rule out underlying conditions that can mimic or contribute to SVT (e.g., thyroid disease, electrolyte imbalances, anemia).

Treatment:

Treatment for SVT aims to stop acute episodes, prevent future episodes, and manage underlying causes. The approach depends on the type of SVT, frequency/severity of symptoms, and patient preference.

• Acute Treatment (to stop an ongoing SVT episode):
  • Vagal Maneuvers: Simple techniques that stimulate the vagus nerve to slow heart rate. Examples:
    • Bearing down (Valsalva maneuver) as if having a bowel movement.
    • Gagging.
    • Plunging your face into ice water (diving reflex).
    • Coughing forcefully.
  • Adenosine: An intravenous medication given in a hospital setting. It temporarily blocks the AV node, often stopping the SVT. It has a very short half-life and can cause a brief, uncomfortable sensation.
  • Beta-Blockers or Calcium Channel Blockers (IV): Can also be given intravenously to slow the heart rate.
  • Cardioversion: (Rarely, for unstable patients) A controlled electrical shock to the chest to reset the heart’s rhythm.
• Preventive Treatment (to reduce frequency of episodes):
  • Lifestyle Modifications: Identify and avoid individual triggers like excessive caffeine, alcohol, nicotine, or stress. Ensure adequate sleep.
  • Medications:
    • Beta-Blockers: (e.g., metoprolol, propranolol) Slow heart rate and reduce the likelihood of SVT episodes.
    • Calcium Channel Blockers: (e.g., verapamil, diltiazem) Slow heart rate and block certain pathways.
    • Antiarrhythmic Drugs: (e.g., flecainide, propafenone, amiodarone) Used for more refractory cases, but have more potential side effects.
• Catheter Ablation:
  • A highly effective, often curative procedure for many types of SVT.
  • Performed during an EP study, catheters are used to precisely locate and then destroy (ablate) the small area of heart tissue responsible for the abnormal electrical circuit, using heat (radiofrequency ablation) or cold (cryoablation).
  • Generally recommended for recurrent, symptomatic SVT that impacts quality of life, or for specific types like WPW syndrome.

Management of SVT requires consultation with a cardiologist or electrophysiologist (a heart rhythm specialist) to determine the best treatment strategy for individual circumstances.