Supraventricular tachycardia (SVT) is caused by abnormal electrical signaling in the upper chambers of the heart, most often due to a short-circuit loop that sends electrical impulses around and around instead of following the normal path. This causes the heart to beat rapidly, typically between 150 and 250 beats per minute, in sudden episodes that can last seconds to hours. The specific cause depends on where the electrical problem originates and what type of SVT you have.
How the Heart’s Electrical System Misfires
Your heart has a built-in electrical system that sends signals from the top chambers (atria) down to the bottom chambers (ventricles) through a relay point called the AV node. Normally, each heartbeat follows this single pathway in an orderly sequence. In SVT, something disrupts that sequence in one of three ways.
The most common mechanism is called re-entry. This happens when an electrical impulse doesn’t stop after triggering a heartbeat. Instead, it loops back around and re-excites heart tissue that has already recovered, creating a self-sustaining circuit. Think of it like a car stuck on a roundabout with no exit: the signal keeps circling and firing the heart with each pass. For this loop to work, the signal has to travel slowly enough that the tissue ahead of it has time to recover and respond again.
The second mechanism is enhanced automaticity, where a group of heart cells starts firing on its own, faster than the heart’s natural pacemaker. The third is triggered activity, where residual electrical ripples from a normal heartbeat accidentally set off an extra beat, which can then set off another. Re-entry circuits account for the vast majority of SVT cases.
The Three Main Types of SVT
SVT is an umbrella term. The three most common subtypes each have a distinct electrical cause, and knowing which one you have matters for treatment.
AVNRT: A Loop Inside the AV Node
Atrioventricular nodal reentrant tachycardia is the most common form of SVT. It happens because the AV node, the electrical relay between the upper and lower chambers, contains two pathways instead of one. One pathway conducts signals quickly but recovers slowly. The other conducts slowly but recovers quickly. Under the right conditions (often triggered by a premature heartbeat), an electrical impulse gets caught in a loop between these two pathways. About 90% of AVNRT cases follow a “slow-fast” pattern, where the signal travels down the slow pathway and loops back up the fast one.
AVRT: An Extra Electrical Bridge
Atrioventricular reciprocating tachycardia involves an accessory pathway, an extra strand of electrically conductive tissue that bridges the atria and ventricles outside the normal conduction system. This extra bridge forms during fetal heart development when the upper and lower chambers don’t fully insulate from each other. The most well-known version is Wolff-Parkinson-White (WPW) syndrome. In WPW, the accessory pathway bypasses the AV node’s natural delay, allowing electrical signals to arrive at the ventricles early. This creates a larger re-entry loop: the signal goes down through one route and back up through the other, over and over.
Some accessory pathways only conduct signals in one direction (from ventricle back to atrium) and don’t show up on a resting EKG. These “concealed” pathways can still cause SVT episodes even though the heart looks normal between episodes.
Atrial Tachycardia
This less common type originates from a specific spot in the atria that fires rapidly on its own, independent of the AV node. It can be caused by enhanced automaticity or by a small re-entry circuit within the atrial tissue itself. Atrial tachycardia is more often seen in people with structural heart disease or after heart surgery.
Who Is More Likely to Develop SVT
SVT can happen at any age, but certain patterns are clear. Women have a 7% higher overall risk of SVT compared to men, based on a nationwide analysis of over 23 million emergency medical calls in the U.S. That elevated risk held across all age groups and racial backgrounds. Women were also 23% more likely to have SVT severe enough to require medication to stop the episode. The average age at presentation was 57, though SVT frequently begins in younger adults as well.
People born with accessory pathways (like in WPW) typically experience their first episode in their teens or twenties. AVNRT tends to appear a bit later but can occur at any age. Having a structurally normal heart does not protect you. Most people with SVT have no underlying heart disease at all.
Medical Conditions That Set the Stage
An overactive thyroid is one of the most important non-cardiac causes of SVT. Excess thyroid hormone makes heart cells more excitable in several ways: it speeds up the heart’s natural pacemaker cells, amplifies the heart’s response to adrenaline, increases sympathetic nervous system activity while dialing down the calming vagal tone, and alters how calcium moves in and out of heart muscle cells. All of these changes lower the threshold for abnormal rhythms to take hold. In some cases, SVT is the first sign that leads to a diagnosis of hyperthyroidism.
Electrolyte imbalances, particularly low potassium and low magnesium, can also make the heart electrically unstable and more prone to SVT. These imbalances commonly result from dehydration, diuretic use, or prolonged vomiting and diarrhea. Anemia, fever, and significant emotional or physical stress can all increase the heart’s workload and trigger episodes in susceptible people.
Everyday Triggers
Many people with SVT notice that specific substances or situations set off their episodes. The relationship between caffeine and heart rhythm is more nuanced than most people expect. Coffee can increase premature heartbeats. One randomized trial found that coffee drinkers had a 54% increase in premature ventricular contractions compared to those who avoided caffeine. Yet large population studies have found that people who drink more than about three cups of coffee per day actually have a lower incidence of atrial fibrillation than lighter drinkers. The effect likely depends on individual susceptibility rather than a universal dose threshold.
Alcohol and nicotine both raise arrhythmia risk, particularly with heavy use. Stimulant drugs, including cocaine and amphetamines, are well-known triggers. Even intense exercise, bending over suddenly, or bearing down (like during a bowel movement) can set off an episode by changing the pressure in your chest or stimulating the vagus nerve.
Medications That Can Trigger SVT
Over 2,400 distinct medications have been linked to SVT in adverse event databases, with more than 22,000 documented cases between 2004 and 2023. The highest-risk categories include respiratory medications, nervous system drugs, and, paradoxically, some cardiovascular drugs used to treat other heart rhythm problems.
Asthma medications are among the most common culprits. Inhaled bronchodilators like albuterol (salbutamol) and formoterol stimulate receptors in the heart that increase heart rate and excitability. High doses of albuterol are a particular risk factor. In studies of formoterol, 13% of patients experienced SVT. Theophylline, an older asthma drug, can trigger rapid heart rhythms even at standard doses in people who are predisposed.
Certain antidepressants also carry risk. Paroxetine, an SSRI, and venlafaxine, which affects both serotonin and norepinephrine, have both been linked to SVT. With venlafaxine, the risk is highest at elevated doses or in overdose situations. Over-the-counter decongestants containing pseudoephedrine or phenylephrine act as stimulants and can provoke episodes in people with underlying conduction abnormalities, even if those abnormalities have never caused symptoms before.
What Keeps the Cycle Going
Understanding the cause of your SVT matters because it determines how well it can be treated. Re-entry circuits, the cause behind most AVNRT and AVRT, are highly treatable because the loop has a fixed location that can be targeted. Accessory pathways and dual AV node pathways don’t go away on their own, so people with these conditions will typically continue to have episodes unless the pathway is disrupted.
Triggers like caffeine, stress, or medications can be modified, and treating an underlying condition like hyperthyroidism can eliminate SVT entirely in some cases. But for people whose SVT is driven by a structural electrical abnormality they were born with, the episodes will recur until that pathway is addressed, usually through catheter ablation, a procedure that uses heat or cold to disable the abnormal circuit.