What is QTc (QT interval corrected) prolongation?

Understanding QTc Prolongation: A Simple Explanation

QTc prolongation means your heart takes longer than normal to reset between beats, which can be measured on an electrocardiogram (ECG or EKG) and may increase your risk of dangerous heart rhythm problems. 1

What Is the QT Interval?

• The QT interval represents the time it takes for your heart's lower chambers (ventricles) to electrically activate and then recover before the next heartbeat. 1
• Think of it like the "recharge time" your heart needs between beats—when this takes too long, it's called QT prolongation. 1
• Because heart rate affects this measurement, doctors calculate a "corrected" value called QTc to account for how fast or slow your heart is beating. 1

What Are Normal Values?

• For men, a normal QTc is less than 450 milliseconds (ms); for women, it's less than 460 ms. 2, 3
• When QTc exceeds 500 ms or increases by more than 60 ms from your baseline, the risk of life-threatening heart rhythms increases significantly. 1, 4
• Women naturally have slightly longer QT intervals than men, particularly during younger and middle-aged years. 1

Why Does QTc Prolongation Matter?

• The main danger is a potentially fatal heart rhythm called Torsades de Pointes (TdP), which can cause sudden cardiac death. 2, 3
• For every 10 ms increase in QTc, there is approximately a 5% increase in the risk of dangerous heart rhythm problems. 2
• This abnormal rhythm can cause symptoms like palpitations, lightheadedness, dizziness, fainting, or even sudden death. 5

What Causes QTc Prolongation?

Medications (Most Common Cause)

• Many common medications can prolong the QT interval, including certain antibiotics, antipsychotics, antidepressants, anti-nausea drugs, and cancer treatments. 1
• Cancer drugs like nilotinib and dasatinib cause QTc prolongation in approximately 40% of patients. 6
• Anti-nausea medications like ondansetron, anti-anxiety drugs, and some antidepressants can also prolong QTc. 1

Electrolyte Imbalances

• Low potassium (hypokalemia) and low magnesium (hypomagnesemia) are major contributors to QT prolongation. 1, 4
• Low calcium (hypocalcemia) can also prolong the QT interval. 1, 4

Medical Conditions

• Heart disease, including heart failure and low heart pumping function, increases risk. 1, 5
• Slow heart rate (bradycardia) or heart rhythm pauses make QT prolongation more dangerous. 1, 5
• Kidney or liver failure can increase medication levels and prolong QTc. 1, 5

Patient Characteristics

• Female sex is a significant risk factor, with women being more prone to dangerous rhythms than men. 1, 5
• Older age (over 65 years) increases vulnerability. 2, 5
• Some people have genetic mutations that predispose them to QT prolongation (congenital long QT syndrome). 3, 7

How Is It Monitored?

• Doctors measure QTc using an ECG, looking at the pattern from the start of one heartbeat to when the heart muscle fully recovers. 1
• The measurement should be visually confirmed by a doctor, not just relied upon from computer calculations, because automated readings can be inaccurate. 1
• For patients on high-risk medications, an ECG should be obtained before starting treatment, repeated 7 days after starting, and monitored periodically throughout treatment. 1, 4

What Happens If QTc Is Prolonged?

Mild Prolongation (481-500 ms)

• More frequent ECG monitoring is needed, electrolytes should be aggressively corrected, and doctors may reduce doses of offending medications. 4, 8

Severe Prolongation (>500 ms or increase >60 ms from baseline)

• The causative medication should be temporarily discontinued immediately, electrolytes corrected urgently, and continuous heart monitoring implemented. 1, 4, 8

If Torsades de Pointes Occurs

• Emergency treatment includes 2 grams of intravenous magnesium regardless of blood magnesium level, and potentially electric shock (defibrillation) if the patient becomes unstable. 1, 4

Key Takeaway for Patients

If you're taking medications that can prolong QTc, tell your doctor immediately if you experience palpitations, lightheadedness, dizziness, or fainting—these could be warning signs of a dangerous heart rhythm. 5 Your healthcare team will monitor your heart rhythm with ECGs and check your electrolyte levels regularly to keep you safe. 1