What is a normal QT (quinidine time) interval?

From the Guidelines

The normal QT interval on an electrocardiogram (ECG) is typically considered to be less than 450 milliseconds for men and less than 460 milliseconds for women when corrected for heart rate (QTc). This is based on the most recent and highest quality study available, which emphasizes the importance of correcting the QT interval for heart rate to accurately assess the risk of dangerous arrhythmias like Torsades de Pointes 1.

Key Considerations

• The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization, essentially measuring how long it takes for the heart's electrical system to activate and reset the ventricles.
• Certain medications, electrolyte abnormalities, and congenital conditions can affect the QT interval, making monitoring crucial when prescribing medications known to prolong QT.
• The QTc interval is calculated using formulas such as Bazett’s or Fridericia’s to normalize the QT interval to a heart rate of 60 bpm, denoted as the corrected QT or QTc.

Clinical Implications

• Monitoring the QT interval is important because prolongation can predispose patients to dangerous arrhythmias.
• Baseline and follow-up ECGs are recommended to monitor for potentially dangerous changes when prescribing medications known to prolong QT.
• QTc intervals greater than 500 ms and a change from baseline of more than 60 ms are considered to be of particular concern because torsades de pointes rarely occurs when QTc is less than 500 ms 1.

Measurement and Correction

• The QT interval should be measured from the beginning of the QRS complex to the end of the T wave.
• The end of the T wave can be difficult to determine, especially when there is a partial superimposition of the T- and U-waves.
• Bazett’s formula (QTcB = √(QT/RR)) and Fridericia’s formula (QTcF = QT/√(RR)) are commonly used to correct the QT interval for heart rate.

From the Research

Normal QT Interval

The normal QT interval is a topic of discussion in several studies.

• A study published in 1999 2 mentions that QTc values average 0.49 seconds and vary somewhat by genotype, with approximately 12% of long QT gene carriers having a normal QTc, ≤0.44 seconds.
• Another study from 2011 3 found that the average QTc was 428 ± 28 milliseconds, with approximately one-third of patients having QTc values of ≥440 milliseconds.
• A 1993 study 4 states that a QT interval corrected for heart rate (QTc) of >0.44 seconds is considered abnormal in stable sinus rhythm.

QT Interval Measurement

The measurement of the QT interval is crucial in determining what is considered normal.

• The 1993 study 4 explains that the QT interval extends from the beginning of the QRS complex to the end of the T wave and that lead II has been chosen to measure the QT interval by convention.
• The same study 4 also mentions that numerous attempts at correction have been made, but Bazett's correction (QT/square root of RR) remains the current standard.

Factors Affecting QT Interval

Several factors can affect the QT interval, including:

• Autonomic tone and state of wakefulness, with the QT interval being approximately 19 msec longer in sleeping patients with a heart rate of 60 beats/min than in awake patients with the same heart rate 4.
• Perioperatively administered drugs, such as isoflurane, methadone, and ketorolac, which can cause significant QT-interval prolongation 5.
• Postoperative body temperature, which has a weak negative correlation with ΔQTc 5.