Long QT syndrome (LQTS) is a heart rhythm condition where the heart’s electrical system takes longer than normal to recharge between beats. This delay, measured on an electrocardiogram (EKG) as the “QT interval,” can trigger dangerously fast, chaotic heartbeats that lead to fainting, seizures, or in rare cases, sudden cardiac arrest. The condition affects roughly 1 in 2,000 people, though many never know they have it.
What Happens in the Heart
Every heartbeat involves two electrical phases: a charging phase that triggers the heart muscle to contract, and a recharging phase that resets the system for the next beat. The QT interval on an EKG captures both of these phases. In LQTS, the recharging phase takes too long, creating a window where the heart is electrically unstable and vulnerable to abnormal rhythms.
The recharging process depends on tiny channels in heart cells that move charged particles (ions) in and out. Potassium channels are the main drivers, pushing potassium out of the cell to reset it. When these channels don’t work properly, or when sodium or calcium channels let too much current flow inward, the recharging phase stretches out. That prolongation is what gives the condition its name.
Inherited vs. Acquired Forms
LQTS comes in two broad categories. The inherited form is caused by gene mutations that alter how ion channels function. Over a dozen genetic subtypes have been identified, but three account for the vast majority of cases.
The acquired form develops in people with normal genetics when something external disrupts the heart’s electrical balance. A surprisingly long list of medications can prolong the QT interval, including certain antibiotics (erythromycin, clarithromycin, moxifloxacin), antifungals (fluconazole, voriconazole), antipsychotics (haloperidol, risperidone, chlorpromazine), antidepressants (citalopram, escitalopram), anti-nausea drugs (ondansetron), and some heart rhythm medications themselves. Low potassium or magnesium levels, which can happen during vomiting, diarrhea, or certain diets, also lengthen the QT interval. Older age, female sex, and existing heart disease increase vulnerability to the acquired form.
The Three Main Genetic Types
Each genetic subtype of LQTS affects a different ion channel and responds to different triggers. Knowing which type you have directly shapes how the condition is managed.
LQT1
LQT1 involves mutations in the gene that builds a key potassium channel activated during physical exertion. Because this channel normally helps the heart keep pace when your heart rate rises, people with LQT1 are most vulnerable during exercise. About 68% of dangerous cardiac events in LQT1 patients happen during physical activity. Swimming is an especially high-risk trigger: 99% of LQTS-related swimming incidents occur in people with this subtype.
LQT2
LQT2 affects a different potassium channel that handles the rapid phase of heart cell recharging. People with this type are particularly sensitive to sudden startling noises, like a ringing telephone or alarm clock. Most dangerous events occur during rest rather than exercise, making LQT2 a distinctly different risk profile from LQT1.
LQT3
LQT3 involves a sodium channel that stays open longer than it should, allowing extra current to flow into heart cells during the recharging phase. Events in LQT3 tend to happen during sleep or rest. This type is less common but carries a higher risk per event.
Symptoms and When They Appear
Fainting is the hallmark symptom. It can strike with little to no warning, often during moments of physical exertion, emotional excitement, anger, or fear, depending on the genetic type. Some people also experience seizures caused by the heart’s temporary inability to pump blood to the brain. Rapid or pounding heartbeats are another common sign.
Many people with LQTS never have symptoms at all and only discover the condition through a routine EKG or after a family member is diagnosed. When symptoms do appear, the timeline varies. Babies born with LQTS can show signs within the first weeks or months of life, while other cases surface later in childhood. Most people with the inherited form experience their first symptoms by age 40. Symptoms can also occur during sleep, which makes the condition easy to miss if nighttime episodes go unwitnessed.
How It’s Diagnosed
Diagnosis starts with an EKG, which measures the QT interval in milliseconds. Because heart rate affects the QT interval, doctors use a corrected value called the QTc. The diagnostic thresholds differ by sex, since women naturally have slightly longer QT intervals. A QTc above 470 milliseconds in men or above 480 milliseconds in women is considered prolonged and raises suspicion for LQTS.
A single EKG isn’t always conclusive. Some people with LQTS have a normal QT interval on any given day, so repeat testing, exercise stress tests, or monitoring over 24 to 48 hours may be needed. Genetic testing can confirm the diagnosis and identify the specific subtype, which is valuable for guiding treatment and screening family members. If one person in a family is diagnosed, close relatives are typically offered testing because the condition follows a pattern of inheritance where each child of an affected parent has a 50% chance of carrying the mutation.
Treatment and Daily Management
Current guidelines recommend beta-blockers for all patients with congenital LQTS, whether or not they’ve ever had symptoms. Beta-blockers work by blunting the heart’s response to adrenaline, which reduces the chance of triggering a dangerous rhythm. They’re especially effective in LQT1 and LQT2. LQT3 responds less reliably to beta-blockers alone, and additional strategies may be needed.
For people at higher risk, particularly those who’ve had cardiac arrest or who continue fainting despite medication, an implantable defibrillator (ICD) can detect dangerous rhythms and deliver a corrective shock within seconds. This is a small device placed under the skin near the collarbone, similar to a pacemaker.
Daily management goes beyond medication. All people with LQTS should avoid drugs that prolong the QT interval. The website CredibleMeds.org maintains a searchable database of these medications, and it’s worth checking before starting any new prescription or over-the-counter drug. Replacing electrolytes during illnesses that cause vomiting or diarrhea is important, since low potassium and low magnesium can worsen the condition. Bringing down a fever promptly also helps, as elevated body temperature can affect heart rhythm in susceptible people.
Exercise recommendations depend on the genetic type. People with LQT1 are generally advised to avoid competitive swimming and strenuous exercise, while those with LQT2 may need to manage their exposure to sudden loud noises (some use vibrating alarms instead of audible ones). These lifestyle adjustments, paired with consistent medication, allow most people with LQTS to live full, active lives with a very low risk of serious cardiac events.