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Health Education

What is the QT interval and why is it important?

The QT interval is the time it takes for your heart to contract and then reset for the next beat. It's important because if the QT interval is too long or too short, it can lead to serious heart rhythm problems
By | Aug. 18, 2024
A stethoscope with a heart shaped stress ball between the earpieces: What is the QT interval and why is it important?

Key takeaways

  • The QT interval is a measured time frame determined on an electrocardiogram, and it reflects the time when our heart’s pumping chambers are depolarizing and then repolarizing with electrical energy during each heartbeat.

  • There are normal ranges for how long one’s QT interval should last, and the normal range is different between children and adults, as well as between adult males and females.

  • Having a QT interval that takes an abnormally prolonged amount of time poses a risk of dangerous heart arrhythmias that can provoke passing out spells or even sudden cardiac death.

  • Prolonged QT or long QT syndrome can be congenital from birth or be acquired due to medication side effects, electrolyte disturbances, or health conditions.

  • Treatments for long QT syndrome depend on the cause and may involve problematic medication discontinuation, electrolyte replacement, therapeutic drugs, lifestyle measures, and implanted cardiac devices.

In this article:

The QT interval is a time frame in the heart’s electrical cycle which can be vulnerable to life-threatening rhythm disturbances with each beat. That vulnerability is present if the QT interval lasts too long. Medications, health conditions, and genetic disorders can all lead to a prolonged QT interval and, as a result, a heightened risk of sudden cardiac death. Getting informed about long QT syndrome may provide you with life-saving knowledge. 

What is the QT interval and why is it important?

An electrocardiogram (ECG) measures the heart’s electrical activity. Normal heartbeats begin with an electrical impulse from the right atrium, one of the two upper cardiac chambers. When recorded by an ECG, the start and spread of electricity through the atria is labeled as the P-wave. Following that, the electrical impulse passes down and through the bottom pumping chambers, the cardiac ventricles. The QT interval on an ECG lasts from the start of electrical depolarization in the ventricles until the end of the repolarization of those cells. Shortly thereafter, the cycle restarts.

The QT interval deserves all the attention it gets. If an extra, abnormal heartbeat occurs during the QT interval, dangerous heart rhythm disturbances can result. If there is a prolongation of the QT interval, the period of vulnerability is longer, and the risk escalates quickly. The only way to know if you have a long QT is to get an ECG.

How is the QT interval measured? 

QT intervals are routinely measured on every ECG. Typically, an ECG is done if a patient has symptoms that could be coming from the heart, has abnormal heart findings on a physical, or is required to have one for a sport, job, or upcoming surgery. Long QT syndrome (LQTS) is often found incidentally on an ECG done for a reason unrelated to it.

The test itself is simple and painless. It can be done in a medical office or hospital. After electrode stickers are placed on the chest and limbs, wires are then clipped onto the stickers. The ECG machine registers and records your heart’s electrical activity through each lead in order to generate a 12-lead ECG report.

Part of the report’s measurements, the QT interval is the time that elapses from the start of the Q-wave, when ventricular depolarization starts, to the end of the T-wave, when ventricular depolarization ends. The QT interval actually changes depending on heart rate, getting longer if the heart rate is slower and shorter if the heart rate is faster. Based on that little complexity, a QT interval corrected for heart rate or QTc must be calculated using Bazett’s formula.

What are normal QT intervals?

The QT interval does not differ between genders before puberty. After puberty, females have a slightly longer QT interval. Historically, defining normal required measuring a whole lot of ECGs and using statistics to determine who were the significant outliers. According to a statement from the American Heart Association and the American College of Cardiology, an abnormal corrected QT interval is over 470 milliseconds (ms) for males and over 480 ms for females, because 99% of the population has a QT under those values. Values close to those but under the threshold can be termed borderline, and values further from the cutoff are normal, as laid out in the following table.

Normal and abnormal QT values
Age and gender Normal QT Borderline QT  Long QT
Pre-pubertal male or female <450 ms 450-459 ms ≥460 ms
Post-pubertal male <460 ms 460-469 ms ≥470 ms
Post-pubertal female <460 ms 460-479 ms ≥480 ms

Anyone with a borderline or long QT interval deserves consultation with their doctor to determine what action and precautions are needed. In addition, the previously referenced expert statement does mention that QTc prolongation longer than 500 ms is highly abnormal and risky for abnormal heart rhythms. Specifically, the feared arrhythmias are ventricular tachycardia and torsades de pointes, which can degenerate into fatal ventricular fibrillation.

Long QT syndrome (QT prolongation)

If someone’s QT interval is prolonged, they have long QT syndrome or prolonged QT syndrome. Some are born with it and will always have it. They are classified as having congenital LQTS. Typically, there are other family members with it too, and genetic testing can be done to look deeper into the syndrome. 

People can develop it as a result of a health condition, often involving kidney, liver, or heart disease. These individuals have acquired long QT syndrome and will have it forever unless the underlying condition improves or resolves.

Acquired LQTS can be temporary in other cases. For example, it can be brought on by transient electrolyte imbalances, namely hypokalemia (low potassium) and hypomagnesemia (low magnesium). 

In other cases, medicines can lengthen the QT interval, potentially pushing someone from a normal or borderline QT to a long QT. Many medications have this insidious side effect. Common offenders come from drug classes like antiarrhythmics, selective serotonin reuptake inhibitors, other antidepressants, antipsychotics, and macrolide and quinolone antibiotics, and they include the following examples:

Whatever the cause, long QT is a reason for concern. If temporary and related to electrolyte disturbances or medication, action can still be taken to lower the risk of syncope (passing out) and cardiac arrest from heart arrhythmias. Permanent acquired and congenital long QT have more sustained risk, and despite their permanence, treatment is still available to lessen the risk of complications.

Long QT syndrome treatment

The treatment of long QT must be individualized. Your healthcare provider will first determine the cause of the issue in order to then set a treatment plan. If a medication is to blame, an alternative might be substituted. If a low potassium or low magnesium level is lengthening the QT, replacing the deficient electrolyte may help. For those with more permanent LQTS, options can be presented.

Medications

Beta blockers are a common treatment for congenital long QT. They reduce the risk of syncope and sudden death. Inderal LA (long-acting propranolol) or Corgard (nadolol) are often chosen as effective and inexpensive options. A SingleCare discount card can help if price is an obstacle.

Mexiletine is an antiarrhythmic drug that can shorten the QT. It does not work for everyone with congenital long QT, and its use will depend on the particulars of each case.

Lifestyle changes

Athletic participation is the primary lifestyle element in question. Guidelines are in place to help determine if someone with long QT should be disqualified from competitive sports. The risk of participating must be addressed. If athletics are continued, safety measures can be put in place, like having an automated external defibrillator (AED) on hand.

Medical devices

A cardiologist may recommend a pacemaker or automated implantable cardioverter defibrillator (AICD) in some cases of long QT. The specifics of one’s prolonged QT syndrome and whether they have a history of symptoms like syncope are taken into account before either of these medical devices is advised.