What is happening in the heart
Every heartbeat has two electrical phases: a quick downstroke that triggers contraction, and a longer recovery period as the cells reset and prepare for the next beat. On the ECG, that recovery period shows up as the QT interval — measured from the start of the QRS to the end of the T wave.
In long QT syndrome, the QT interval is prolonged because the cells repolarize too slowly. During that extended vulnerable window, a single early beat can land in exactly the wrong place and set off a spiraling, rapidly changing ventricular rhythm called torsades de pointes (“twisting of the points”). Torsades is a form of polymorphic VT, and while it often self-terminates, it can degenerate into ventricular fibrillation and cause sudden cardiac arrest.
Congenital versus acquired
Congenital LQTS
An inherited problem in the genes that code for the ion channels (the tiny molecular gates that move potassium and sodium across heart cells). The three most common subtypes have characteristic personalities:
- LQT1 — potassium channel; triggers are typically exercise, especially swimming.
- LQT2 — different potassium channel; triggers are typically emotional stress, startle, and loud noises (alarm clocks, phones ringing). Postpartum is also a vulnerable period.
- LQT3 — sodium channel; events tend to occur at rest or during sleep.
There are over a dozen other identified subtypes, but these three account for the large majority.
Acquired LQTS
Far more common in everyday practice. The QT interval lengthens because of:
- Medications — certain antiarrhythmics, some antibiotics (azithromycin, fluoroquinolones), antifungals, some antipsychotics and antidepressants, anti-nausea drugs, and many others. The list is long and is maintained at crediblemeds.org.
- Electrolyte abnormalities — low potassium and low magnesium are the usual suspects.
- Bradycardia — very slow heart rates lengthen the QT.
- Other — starvation, hypothyroidism, severe illness.
A patient may have a borderline-long QT at baseline and only run into trouble when a new medication or a bout of vomiting pushes them over the edge.
Why it matters
Most patients with long QT live full, healthy lives. The risk we manage is the small but real chance of a sudden, life-threatening event — particularly in young people who are otherwise healthy. The good news is that with the right diagnosis and treatment, that risk drops dramatically.
How we diagnose it
The cornerstone is a careful 12-lead ECG with the QT interval measured and corrected for heart rate (we call the corrected value QTc). A QTc above roughly 460 ms in women or 450 ms in men is considered prolonged; values above 500 ms carry meaningfully higher risk. Because the QT changes from day to day, we usually look at several ECGs.
Other steps:
- Exercise stress test — many patients with LQT1 fail to shorten their QT normally with exercise, which is a strong clue.
- Holter or patch monitoring — captures the QT across the day and night.
- Family history and genetic testing — when congenital LQTS is suspected. Genetic testing identifies the subtype, guides therapy, and lets us screen relatives.
- Review every medication the patient takes, including over-the-counter and supplements.
How we treat it
Acquired LQTS
Find the cause and remove it. Stop the offending medication if at all possible, replace potassium and magnesium aggressively, and watch the QT shorten back into the normal range. In an acute torsades episode, intravenous magnesium and temporary pacing or isoproterenol are the standard rescue treatments.
Congenital LQTS
A few principles guide therapy:
- Beta-blockers — the foundation. Nadolol and propranolol are the preferred agents and protect against the catecholamine-driven triggers that set off most events. Almost every patient with confirmed congenital LQTS goes on a beta-blocker.
- Trigger avoidance — competitive swimming and high-intensity sports may need to be restricted in LQT1; we counsel LQT2 patients to remove alarm clocks from the bedside and consider gentler ringtones.
- Avoid QT-prolonging medications — every patient is given a list and counseled to check any new prescription.
- ICD — reserved for patients at the highest risk: those who have already had a cardiac arrest or syncope despite beta-blocker therapy, certain high-risk genotypes, and very long QTc values. We do not implant an ICD in every LQTS patient, and the decision is highly individualized.
- Left cardiac sympathetic denervation — a targeted surgical procedure that interrupts the nerve supply to the heart’s left side. Useful in patients who continue to have events despite beta-blockers and in those who want to avoid an ICD.
What to expect at your visit
We’ll review every ECG we have, walk through your symptoms and family history in detail, and look at the full medication list. If congenital LQTS is suspected, we’ll discuss genetic testing — for you and for first-degree relatives. We’ll build a plan that almost always includes a beta-blocker, a list of medications to avoid, and clear instructions on what to do if symptoms occur.