Long QT syndrome (LQTS) is a heart rhythm condition where the heart’s electrical system takes too long to recharge between beats. This delay, measured on an electrocardiogram (ECG), creates a window of vulnerability during which the heart can slip into a dangerous, chaotic rhythm. The condition affects roughly 1 in every 2,000 to 2,500 live births, though many people carry it without knowing.
What Happens Inside the Heart
Every heartbeat is driven by a wave of electrical activity. Charged particles (ions) flow in and out of heart muscle cells through tiny gates called ion channels. Sodium ions rush in to start the beat, then potassium ions flow out to reset the cell for the next one. In LQTS, either the potassium channels don’t let enough potassium out or the sodium channels stay open too long. Both problems delay the electrical reset, stretching out what’s called the “QT interval” on an ECG.
That prolonged reset phase is more than a measurement quirk. While the heart cell is stuck in its in-between state, stray electrical signals can fire prematurely and trigger a rapid, twisting rhythm called Torsades de Pointes. During this arrhythmia, the heart quivers instead of pumping. Most episodes stop on their own within seconds, causing a brief faint. But if the rhythm accelerates further, it can degenerate into ventricular fibrillation, which is cardiac arrest.
Congenital vs. Acquired Forms
LQTS comes in two broad categories. The congenital form is inherited, caused by mutations in genes that build ion channels. More than 15 genetic subtypes have been identified, but three account for the vast majority of cases. The acquired form develops later in life, usually triggered by medications or shifts in the body’s electrolyte balance.
Hundreds of medications can lengthen the QT interval. The most common culprits include certain antidepressants (particularly older tricyclics and citalopram), antipsychotics, anti-nausea drugs, some antibiotics, and even common acid-reflux medications like proton pump inhibitors. Low levels of potassium, magnesium, or calcium in the blood amplify the risk considerably, which is why people taking diuretics (water pills) need periodic blood work. In most cases of acquired LQTS, multiple risk factors stack together: an older patient taking two or more QT-prolonging drugs while running low on potassium, for example.
The Three Main Genetic Types
Type 1 (LQT1)
LQT1 is the most common subtype. It involves mutations in the KCNQ1 gene, which builds a potassium channel responsible for one of the key currents that resets the heart. Over 600 different mutations in this single gene have been identified. The defining feature of LQT1 is that physical exercise is the primary trigger for dangerous rhythms. Swimming carries a particular and poorly understood risk: in one major study, exercise triggered 62% of cardiac events in LQT1 patients, and swimming was disproportionately represented. European cardiology guidelines specifically recommend that people with LQT1 avoid strenuous swimming, partly because fainting in water carries an obvious drowning risk.
Type 2 (LQT2)
LQT2 results from mutations in the KCNH2 gene (also called hERG), which builds a different potassium channel. The triggers here are emotional rather than physical. Sudden startling noises, alarm clocks, and emotional stress are the classic provocations. People with LQT2 are generally advised to avoid situations where they might be jolted awake by a loud sound, and to keep potassium levels from dipping low, since even mild deficiency can worsen the electrical delay.
Type 3 (LQT3)
LQT3 accounts for 5 to 10% of congenital cases and works through a different mechanism entirely. Instead of too little potassium leaving the cell, the problem is too much sodium staying inside. Mutations in the SCN5A gene prevent sodium channels from closing properly, allowing a persistent trickle of current that prolongs the heartbeat. The dangerous events in LQT3 tend to happen at rest or during sleep, which makes this subtype particularly unsettling for families. Over 300 SCN5A variants linked to LQT3 have been cataloged.
How It’s Diagnosed
The starting point is a standard 12-lead ECG, where a doctor measures the corrected QT interval (QTc). Normal values differ slightly by sex. Current guidelines consider a QTc at or above 450 milliseconds in adult males and 460 milliseconds in adult females to be prolonged. For children under 15, the upper limit is generally set at 460 milliseconds. Values between 440 and 470 milliseconds are often labeled “borderline,” and a QTc above 480 milliseconds scores heavily on diagnostic scales.
A single ECG isn’t always enough. Some people with confirmed LQTS mutations have a normal QT interval on any given day, which is why a point-based scoring system combines ECG findings with personal symptoms and family history. Genetic testing can confirm a specific mutation and, importantly, identify which subtype a person has, since this shapes treatment and lifestyle decisions. When a pathogenic mutation is found in one family member, cascade screening of first-degree relatives (parents, siblings, children) with both ECG and genetic testing is strongly recommended.
Treatment Options
Beta-blockers are the cornerstone of treatment. These medications blunt the effect of adrenaline on the heart, reducing the likelihood that a surge of stress hormones will kick off a dangerous rhythm. Nadolol is generally the preferred choice, though propranolol is also widely prescribed. Beta-blockers are effective enough that most people with LQTS can live normal lives on medication alone.
For patients who still experience fainting spells despite taking beta-blockers, or for anyone who has survived a cardiac arrest, an implantable cardioverter-defibrillator (ICD) is typically recommended. This small device, placed under the skin near the collarbone, continuously monitors heart rhythm and delivers a shock to restore normal beating if a life-threatening arrhythmia occurs. In select cases, a surgical procedure called left cardiac sympathetic denervation can reduce the nerve signals that trigger arrhythmias. This is usually reserved for patients who can’t tolerate beta-blockers or continue having events despite other treatments.
Living With LQTS
Daily management revolves around knowing your specific subtype and its triggers. If you have LQT1, the conversation centers on exercise. Competitive and intense sports are generally considered high-risk, particularly swimming. That doesn’t necessarily mean a sedentary life. Many cardiologists work with patients to find moderate activity levels that are safe, especially when beta-blockers are on board. If you have LQT2, the practical adjustments look different: replacing a jarring alarm clock with a vibrating one, keeping electrolytes in check, and being mindful of sudden loud noises. For LQT3, awareness that events happen during rest means family members may need training in CPR and the use of automated external defibrillators.
Across all subtypes, medication awareness is critical. You’ll want to keep an updated list of drugs known to prolong the QT interval. The nonprofit CredibleMeds (formerly AzCERT) maintains a searchable database of over 280 such medications. Before starting any new prescription, over-the-counter drug, or supplement, checking it against this list is a practical habit. Staying well hydrated and avoiding prolonged vomiting or diarrhea without replacing electrolytes matters too, since potassium and magnesium losses can tip an otherwise stable heart into trouble.
Family planning is another consideration. LQTS is typically inherited in an autosomal dominant pattern, meaning each child of an affected parent has a 50% chance of carrying the mutation. Genetic counseling can help families understand this risk and decide whether to pursue prenatal or early newborn testing.