Heart Education
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Long or short QTc: should you be worried?

ReadMyECG Team
QTc interval: Interpetation, Definition, Causes, Symptoms, and more

Key Takeaways

  • QTc interval measures the time it takes for the bottom part of your heart to contract and then electrically reset itself.
  • Both prolonged and short QTc can lead to abnormal heart rhythms and are associated with symptoms like dizziness and sudden fainting. 

The human heart is an amazing organ. Through cycles of contractions and relaxations, it continuously pumps about 2,000 gallons of blood daily and supplies blood to the human body. No bigger than a fist, it works non-stop. The human heart will have beat more than 2.5 million times in an average human lifetime! 

QTc is an interval that measures the time it takes for the bottom part of your heart to contract and then electrically reset itself. 

The Electrical System of the Heart 

The human heart is an organ made up of a muscle layer. This muscle layer is called the cardiac muscle. It is responsible for making your heartbeat—cardiac muscle contracts in response to electrical impulses that travel through the heart’s electrical pathways. 

An electrocardiogram (ECG or EKG) measures these electrical impulses and can help determine many things about your heart’s condition. 

How Do We Measure the QTc Interval? 

When measured through an ECG, your heart’s electrical activity is identified in “waves.” To identify them more readily, physicians denote these heart waves as either P, QRS, or T waves.

  • P wave: The P wave is the first wave in the ECG tracing. It signifies the contraction of the upper chambers of the heart.
  • QRS complex: This is the easiest identifiable part of an ECG. The QRS complex looks like a spike and represents the contraction of the heart’s lower chambers. 
  • T wave: The T wave comes after the QRS complex, which represents the relaxation of the heart’s lower chambers.

QT Interval in blue 

The QT interval is the length of time from the start of the Q segment until the end of the T wave. This signifies the time it takes for the heart to contract and relax. This will help determine your heart’s time to “reset” itself and get ready for the next heartbeat. We expect this to happen more quickly when the heart beats faster, so we need to correct for the heart rate using a simple mathematical equation. This is where the "c" stands for in QTc. 

What’s a Normal Range for a QTc Interval? 

The normal QTc interval for males is between 350 milliseconds to 450 milliseconds. For females, it is between 350 milliseconds to 470 milliseconds.

What Does Prolonged QTc Interval Indicate? Is It Dangerous?

Prolonged QTC with smartwatch Apple Watch ECG
Apple Watch ECG showing a ventricular bigeminy pattern with a prolonged QTc interval of 531 milliseconds.

The QTc interval is considered prolonged at and above 450 milliseconds for men and at and above 470 milliseconds for women. A prolonged QTc interval means that the heart muscle takes longer than usual to contract and relax. This also means that the heart takes longer to recharge in between beats. A prolonged QTc interval can lead to abnormal heart rhythms and predispose a person to sudden cardiac arrest. Several factors can cause a prolonged QTc interval: 

  • Genetic factors
  • Medications
  • Hormonal imbalance 
  • Electrolyte imbalances 

Although most people who have a prolonged QTc interval do not present with any symptoms, some symptoms associated with a prolonged QTc interval can be any of the following: 

  • Heart palpitations 
  • Dizziness 
  • Sudden Fainting 
  • Seizures 

If your smartwatch can check your ECG reading, it is best to have it interpreted by a professional. Although this condition can be dangerous as it predisposes you to cardiac arrest, it can also be reversed through certain lifestyle modifications and medications. 

What Does a Short QTc Interval Indicate? Is It Dangerous?

Short QTC with smartwatch Apple Watch ECG
Apple Watch ECG showing sustained supraventricular tachycardia (SVT) has a short QTc interval of 317 milliseconds. 

The QTc interval is considered shortened at and below 350 milliseconds. A short QTc interval means that the time it takes for the heart to recharge in between beats is much faster than expected. A short QTc interval is usually congenital, meaning that it is a genetic condition that has been present since birth. Some evidence of it being an acquired disease has come up recently and can be attributed to the following factors: 

  • Electrolyte imbalances 
  • Certain drugs such as digoxin 

A person with a short QTc interval may experience symptoms similar to those of people suffering from a long QTc interval, such as: 

  • Heart pounding 
  • Sudden fainting 
  • Dizziness
  • Abnormal heart rhythms

People diagnosed with a short QTc interval can be treated with a range of heart medications or surgical treatments that involve placing an Implantable Cardioverter Defibrillator to reduce the chances of sudden cardiac death. 

How can a Smartwatch ECG Help?

Maintaining good heart health is key to living your best life. Nowadays, ensuring your heart is healthy can be done at the touch of a button on your smartwatch. Moreover, you can now keep track of your heart health at home with various wearable technology.

Your smartwatch ECG can help you keep your health on track, especially if you do it with the help of experts. ReadMyECG has medical professionals ready to interpret your smartwatch ECG readings within minutes.

Your heart matters; listen to it. 

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