An electrocardiogram (EKG or ECG) measures the heart’s electrical system, and one specific value from this test is the QT interval. This measurement represents a distinct phase of the heart’s activity, capturing the process that allows the heart to beat. For many people, seeing this term on a medical report can be confusing. Understanding what it signifies is the first step toward appreciating why healthcare professionals monitor its duration.
The Heart’s Electrical Cycle and the QT Interval
The heart’s rhythm is driven by an electrical sequence, which an EKG records as a series of waves. This pattern shows the heart’s “squeeze and relax” cycle. The main components include the P wave, showing the upper chambers (atria) contracting, and the QRS complex, representing the contraction of the lower chambers (ventricles). Following the QRS complex is the T wave, signaling that the ventricles are electrically resetting.
The QT interval measures the total time from the start of the QRS complex to the end of the T wave. This duration corresponds to the entire period of ventricular activation and recovery—how long it takes for the heart’s main pumping chambers to contract and then recharge. Because this interval shortens when the heart beats faster and lengthens when it slows, doctors use a standardized value to ensure accuracy.
This standardized measurement is the corrected QT interval, or QTc. By using a formula that accounts for heart rate, the QTc provides a value as if the heart were beating at a steady 60 beats per minute. This correction allows for a consistent and reliable assessment, regardless of a person’s heart rate. The QTc value is what is used to determine if the interval is normal.
Abnormal QT Interval Durations
The length of the QTc interval is measured in milliseconds (ms) and is an indicator of cardiac electrical stability. For adult men, a QTc is considered normal when it is less than 440 ms, and for adult women, the normal threshold is less than 460 ms. A QTc above 450 ms in males or 470 ms in females is considered abnormal.
When the QTc is persistently long, it is a condition known as Long QT Syndrome (LQTS). This syndrome signifies a delay in the heart’s electrical recharging process, which can leave the heart vulnerable to chaotic heart rhythms. The most well-known of these is Torsades de Pointes, a very fast, irregular heartbeat that can cause sudden fainting, seizures, or cardiac arrest.
While less common, an unusually short QT interval can also be a concern. This condition, called Short QT Syndrome (SQTS), is diagnosed when the QTc is less than 350 ms. Similar to LQTS, a significantly shortened repolarization time increases the risk of developing serious arrhythmias. Both conditions underscore the importance of the QT interval’s duration.
Causes of an Abnormal QT Interval
The reasons for an abnormal QT interval are either congenital (present from birth) or acquired (develops later in life). Congenital Long QT Syndrome and Short QT Syndrome are genetic disorders caused by mutations in genes that code for ion channels in heart cells. These channels control the flow of charged particles, and when they don’t function correctly, the heart’s electrical recharging is disrupted.
More frequently, an abnormal QT interval is acquired. A primary cause is medication, and several classes of common drugs are known to prolong the QT interval. These include certain antiarrhythmics, some antibiotics (like macrolides and fluoroquinolones), antipsychotic medications, and some antidepressants. The potential for these drugs to affect the heart’s electrical cycle necessitates careful monitoring.
Electrolyte imbalances in the blood are another cause. Low levels of potassium, magnesium, or calcium can interfere with the electrical signaling in heart muscle cells, leading to a prolonged QTc. Other medical conditions can also contribute, such as an underactive thyroid gland (hypothyroidism) or structural heart diseases.
Diagnosis and Management
An abnormal QT interval is most often identified on a standard 12-lead EKG. If a doctor suspects the issue is intermittent, they may recommend a Holter monitor. This is a portable EKG worn for 24 hours or longer to continuously record the heart’s electrical activity. If a congenital cause is suspected based on family history, genetic testing may be used to identify specific gene mutations.
Management strategies are tailored to the underlying cause and the degree of risk. A foundational step is often to discontinue any medications known to prolong the QT interval. If the cause is an electrolyte imbalance, correcting low levels of potassium or magnesium is an effective treatment.
For individuals with a higher risk of arrhythmias, medications such as beta-blockers may be prescribed to help stabilize the heart’s rhythm. In cases with a high risk of sudden cardiac arrest, particularly with congenital LQTS, an implantable cardioverter-defibrillator (ICD) may be recommended. This small device is placed under the skin and can detect a life-threatening arrhythmia, delivering an electrical shock to restore a normal heartbeat.