What Does the QT Interval Represents?

If you’ve ever had an EKG, your doctor may have mentioned that they looked at your QT interval. But what does that mean, exactly? Let’s take a closer look at the QT interval – from A to Z – so you can understand what it is and why it’s essential for the health of your heart. Knowing about the QT interval can help you get regular checkups and seek medical help if something looks irregular or concerning to this measurement.

Overview of ECG Components

An ECG (electrocardiogram) is a test used to measure the electrical activity of your heart. It records the heart’s rhythm and rate. It measures other things such as the size and position of your heart chambers, any signs of enlargement or thickening, how fast the electrical impulses travel through your heart and the duration of a single heartbeat. The ECG trace is composed of several different waves, which are labeled.

  • P
  • QRS
  • T.

The P- wave represents the electrical activation of the heart’s right and left upper chambers (atria).

The QRS complex is a combined waveform indicating the electric impulse that travels through the bundle of His and then spreads via both ventricles (the lower pumping chambers).

Lastly, the T-wave is an indicator of ventricular repolarization, when all of the cells in the ventricles become depolarized.

QT interval

The QT interval reflects the time it takes for your ventricles (heart chambers) to contract and relax. It starts from the beginning of the Q wave (the first downward movement of the wave) and ends at the end of the T wave (the last upward direction). It’s measured in milliseconds and is usually between 350 to 450 milliseconds depending on various factors.

The QT interval can help determine if you’re at risk for developing an arrhythmia (irregular heart rhythm) or other cardiac problems. In some cases, an abnormally long QT interval may be a sign of a heart disorder, so it’s important to discuss any changes in your QT interval with your doctor.

The duration of the QT interval can also be affected by medications (such as anti-arrhythmic drugs and certain antidepressants) and certain medical conditions (such as hypocalcemia, which is low blood calcium). Certain lifestyle factors can also contribute to an abnormal QT interval, such as drinking excessive alcohol, using illegal drugs, or having a high-stress level.

Finally, if your doctor notices that your QT interval is too long or short, they may recommend further testing to identify the underlying cause. It’s essential to take any changes in the QT interval seriously so you can receive appropriate treatment. Regular checkups with your doctor are vital for monitoring any changes in the QT interval while ensuring you remain healthy and heart-healthy. Knowing about the QT interval can help give you peace of mind regarding your heart health.

How do we Measure the QT Interval?

A healthcare provider uses an electrocardiogram (ECG) to measure the QT interval. During the test, electrodes are placed on your chest and connected to an ECG machine. The machine records the electrical activity of your heart, which is displayed as waves on the ECG tracing paper. The healthcare provider measures the length of time from the beginning of the Q wave to the end of the T wave and then calculates your QT interval.

So, How Do We Correct the QT Interval?

Corrected QT interval (QTc)
  • The corrected QT interval (QTc) estimates the QT interval at a standard heart rate of 60 bpm
  • This allows comparison of QT values over time at different heart rates and improves detection of patients at increased risk of arrhythmias

There are multiple formulas used to estimate QTc. It is not clear which formula is the most useful:

  • Bazett formula: QTC = QT / √ RR
  • Fridericia formula: QTC = QT / RR 1/3
  • Framingham formula: QTC = QT + 0.154 (1 – RR)
  • Hodges formula: QTC = QT + 1.75 (heart rate – 60)

Note: The RR interval is given in seconds (RR interval = 60 / heart rate).

Normal QTc values
  • QTc is prolonged if > 440ms in men or > 460ms in women
  • QTc > 500 is associated with an increased risk of torsades de pointes
  • QTc is abnormally short if < 350ms
  • A helpful rule of thumb is that a regular QT is less than half the preceding RR interval. QT interval abnormalities

What Causes a Prolonged QT Interval?

Lots of things can cause a prolonged QTc…here are just a few:

  • Drugs (i.e., certain antibiotics, antiarrhythmic medications, antidepressants)
  • Electrolyte imbalance (low potassium or magnesium levels)
  • Genetic disorders (such as Long QT Syndrome)
  • Heart failure
  • Hypothyroidism
  • Myocardial infarction

When evaluating patients, it is essential to consider all possible causes of prolonged QTc.


Long QT syndrome (LQTS)

Long QT syndrome (LQTS) is the most common cause of prolonged QT interval and can lead to arrhythmias, fainting episodes, and sudden cardiac death. Other causes of a long QT interval include electrolyte imbalances (low calcium or magnesium levels), certain medications, and certain medical conditions.

The QT is prolonged…what’s the worst that could happen?

The one-lines answer is. Your patient could die.

Prolonged QT intervals can lead to arrhythmias and other heart rhythm disturbances, such as torsades de pointes. These arrhythmias can cause palpitations, lightheadedness, fainting episodes, or even sudden cardiac arrest in some cases. If you have any concerning symptoms or abnormalities in your QT interval, it’s essential to seek medical advice as soon as possible.

If you have any questions about your QT interval or what it means for the health of your heart, don’t hesitate to speak with your healthcare provider. Knowing about the QT interval can help ensure that you receive appropriate treatment and regular checkups. It’s also important to be aware of any lifestyle changes or medications that could compromise your QT interval and take steps to address them before they become a problem.

Causes of Prolonged QT Interval


  • Congenital Long QT Syndrome (LQTS)
  • Jervel and Lange-Nielsen syndrome


  • Antiarrhythmics – amiodarone, sotalol, procainamide, quinidine
  • Antidepressants – amitryptiline, dothiepin, citalopram, escitalopram
  • Antipsychotics – risperidone, haloperidol, clozapine, droperidol, chlorpromazine
  • Antiemetics – ondansetron, domperidone
  • Macrolides – azithromycin, clarithromycin, erythromycin
  • Quinolones – ciprofloxacin, moxifloxacin
  • Antifungals – fluconazole, ketoconazole
  • Antimalarials – chloroquine, mefloquine
  • Anesthetic gases – halothane, sevoflurane
  • Methadone


  • Hypokalaemia
  • Hypomagnesaemia
  • Hypocalcaemia
  • Severe hypothermia
  • Severe bradycardia – sick sinus syndrome, complete heart block
  • Cardiovascular disease – tako-tsubo cardiomyopathy, MI, CCF
  • Cerebrovascular disease – intracranial/subarachnoid hemorrhage, stroke
  • Hypothyroidism

Short QT syndrome (SQTS)

Short QT Syndrome (SQTS) is a rare genetic disorder that can cause arrhythmias and sudden cardiac death. A gene mutation usually causes it and can be inherited from family members with the mutated gene. Other causes of a short QT interval include electrolyte imbalances, certain medications, and congenital heart defects.

What are you going to DO about it?

When a patient’s QTc is prolonged but not excessively long, informing the medical doctor and monitoring it closely is essential. Possible causes should be identified, and the interval should be measured regularly.

If it exceeds 500 milliseconds, any medications that may be contributing should be stopped, and any other potential issues addressed. Electrolytes out of balance must be replaced, intracranial pressure reduced, and an amiodarone drip for atrial fibrillation with RVR discontinued if necessary.

All measures taken should be carefully documented, along with the patient’s response. This way, one can confidently measure and treat a prolonged QTc in the clinical setting. It is, therefore, essential not to ignore this ECG measurement.


  • https://en.wikipedia.org/wiki/QT_interval
  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4291996/

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