From the Guidelines
A normal QTc interval is less than 0.46 second in women and 0.45 second in men, as indicated by the most recent and highest quality study 1.
Definition of Normal QT Interval
The normal QT interval on an electrocardiogram (ECG) is typically measured from the beginning of the QRS complex to the end of the T wave, representing the time from the start of ventricular depolarization to the end of ventricular repolarization.
- The QT interval varies with heart rate, so it's often corrected (QTc) using formulas like Bazett's formula.
- A normal QTc interval is crucial for assessing the risk of dangerous arrhythmias like torsades de pointes.
Importance of QT Interval Monitoring
Regular monitoring of the QT interval is important, especially when prescribing medications known to prolong it, such as certain antiarrhythmics, antipsychotics, antibiotics, and antidepressants.
- Patients with a history of QT prolongation, relevant cardiac disease, treated with QT-prolonging drugs, bradycardia, thyroid dysfunction, or electrolyte abnormalities should be monitored by repeated 12-lead ECG, as recommended by the European Society of Cardiology 1.
- Conditions known to provoke torsades de pointes, especially hypokalaemia and extreme bradycardia, should be avoided in patients with drug-induced QT prolongation.
Clinical Implications
A prolonged QT interval can indicate an increased risk of dangerous arrhythmias like torsades de pointes, while a shortened QT interval may also be pathological.
- The QT interval naturally changes with age and can be affected by medications, electrolyte abnormalities (particularly potassium, calcium, and magnesium), and various cardiac and non-cardiac conditions.
- Consider treatment discontinuation or alternative regimens if the QTc is ≥ 0.50 ms, QTc prolongation is ≥ 0.60 ms, or dysrhythmias are encountered, as suggested by the European Society of Cardiology 1.
From the Research
Normal QT Interval
The normal QT interval is a crucial aspect of electrocardiogram (ECG) interpretation.
- A study published in the International Journal of Cardiology in 2014 2 established new age- and sex-specific criteria for QT prolongation, which are essential for accurate diagnosis and treatment.
- According to this study, the criteria for prolonged QTc are:
- Age < 40 years: men 430 ms, women 440 ms
- Age 40 to 69: men 440 ms, women 450 ms
- Age ≥ 70 years: men 455 ms, women 460 ms
- These criteria highlight the importance of considering age and sex when evaluating QT prolongation.
Factors Influencing QT Interval
Several factors can influence the QT interval, including:
- Medications: certain anti-arrhythmic drugs, such as quinidine, sotalol, and amiodarone, can prolong the QT interval 3
- Electrolyte imbalances: hypokalemia can increase the risk of QT prolongation 4
- Age: QTc shortens in men during adolescence and becomes 13 ms shorter than in women at age 20-29 years 2
- Sex: sex differences in QTc originate from shortened QT in adolescent males 2
Clinical Significance
QT prolongation can lead to life-threatening arrhythmias, such as torsades de pointes.
- A study published in Heart Rhythm in 2004 4 reported two cases of ciprofloxacin-induced acquired long QT syndrome, highlighting the importance of monitoring QT interval in patients receiving certain medications.
- Another study published in The Korean Journal of Pain in 2018 5 emphasized the risk of QT prolongation associated with pain medications, such as NSAIDs, opioids, and anticonvulsive drugs.