Supraventricular Tachycardia (SVT) is an abnormally fast heart rhythm originating in the upper chambers of the heart, above the ventricles. This rapid rate is caused by a malfunction in the heart’s electrical signaling system. An SVT episode causes the heart to suddenly speed up, typically beating significantly higher than the normal resting rate of 60 to 100 beats per minute. While episodes can be alarming, SVT is generally not considered life-threatening unless a person has underlying heart damage or another severe heart condition.
Physical Symptoms During an Episode
SVT often begins and ends with an abrupt, startling sensation in the chest. The most common symptom is a sudden, racing heartbeat, known as palpitations, often described as a pounding, fluttering, or thumping feeling. During an episode, the heart rate typically ranges from 150 to 220 beats per minute, which impairs the heart’s ability to properly fill with blood between beats.
Because the heart cannot pump blood efficiently at such a high rate, the body may not receive enough oxygen, leading to secondary symptoms. Individuals often report feeling dizzy, lightheaded, or weak during the episode. A pounding sensation may also be felt in the neck due to the forceful contractions of the upper heart chambers.
The rapid heart rate can also cause shortness of breath or chest discomfort, which may feel like pressure or pain. These intense sensations often trigger anxiety, which can further exacerbate symptoms. In rare or prolonged episodes, an exceptionally fast heart rate can cause a severe drop in blood pressure, leading to fainting or loss of consciousness (syncope).
How Doctors Confirm SVT
Confirming SVT relies on capturing the heart’s electrical activity during an episode, primarily through an electrocardiogram (ECG or EKG). The signature of most SVTs on an ECG is a regular, rapid rhythm, usually between 150 and 250 beats per minute. The defining feature separating SVT from other tachycardias is the appearance of the QRS complex, which represents the electrical activation of the ventricles.
In most SVT cases, the QRS complex is “narrow,” meaning its duration is less than 120 milliseconds. This narrow width indicates the electrical impulse is traveling normally down the lower chambers’ conduction system, confirming the rhythm originates above the ventricles. The behavior of the P wave, which represents the upper chambers’ activation, is also scrutinized.
The rapid rate often causes the P wave to be hidden within or just following the QRS complex, or it may appear abnormally shaped. In the most common form of SVT, the P wave can be buried or create a subtle notch on the QRS complex, sometimes called a “pseudo-R wave” or “pseudo-S wave.” Analyzing the P wave’s exact position relative to the QRS complex allows doctors to distinguish between different types of SVT.
Since SVT episodes are often paroxysmal (starting and stopping suddenly), the rhythm may not be present during a standard, brief in-office ECG. To capture the event, doctors rely on ambulatory monitoring devices. These tools, such as Holter monitors or event recorders, are worn for extended periods (24 hours up to several weeks) to record the heart’s electrical activity when an unexpected episode occurs.
Immediate Steps When SVT Occurs
When an SVT episode starts, a person can attempt to slow the heart rate by performing specific physical actions known as vagal maneuvers. These techniques stimulate the vagus nerve, which runs from the brain to the chest and abdomen, slowing the conduction of electrical impulses through the heart’s atrioventricular (AV) node. The most common self-administered technique is the Valsalva maneuver, which involves forcefully exhaling against a closed airway.
To perform the Valsalva maneuver, a person should sit or lie down, take a deep breath, and then bear down for about 10 to 15 seconds. A modified version, which includes lying on the back and elevating the legs immediately after straining, is often more effective. Other maneuvers include forceful, continuous coughing or splashing ice-cold water onto the face, which triggers a reflex that stimulates the vagus nerve.
While these maneuvers are safe and can quickly restore a normal heart rhythm in certain SVT types, it is important to know when to seek professional help. Emergency medical attention should be called if an episode lasts longer than a few minutes, if the rapid heartbeat is accompanied by severe chest pain, or if fainting occurs. Medical care is also advised if the heart rate does not slow down despite attempting the vagal maneuvers.
What Causes Supraventricular Tachycardia
The underlying cause of most SVT episodes is an electrical short circuit within the heart’s upper chambers or the junction between the upper and lower chambers. This mechanism is often a re-entry circuit, where an electrical impulse gets trapped in a looping pathway instead of traveling in a straight line. This creates a self-perpetuating cycle that rapidly fires electrical signals, driving the heart rate up.
The most common type of SVT involves two pathways of electrical conduction near the AV node, allowing the impulse to circle endlessly. Other forms involve an extra, abnormal electrical pathway connecting the upper and lower chambers that is often congenital. While the underlying wiring problem is present from birth, the rapid rhythm usually needs a trigger to start.
Common external factors can initiate an SVT episode in susceptible individuals. These triggers include excessive caffeine or alcohol consumption, high emotional stress, and intense physical exertion. Certain over-the-counter medications, such as cold or allergy remedies containing stimulating ingredients, can also provoke an episode, as can lack of sleep or dehydration.