From the Guidelines
The normal QTc interval in an electrocardiogram (EKG) is considered to be less than 450 ms in men and less than 460 ms in women, as suggested by the most recent evidence 1. The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization, and it varies with heart rate. The QTc is the QT interval corrected for heart rate, typically using formulas such as Bazett's formula (QTc = QT/√RR), which allows for standardized interpretation regardless of heart rate. Some key points to consider when measuring the QT interval include:
- The QT interval is measured from the start of the Q wave to the end of the T wave.
- The QT interval varies inversely with heart rate, increasing as the rate slows and decreasing as the rate increases.
- The corrected QT interval (QTc) estimates the value of QT at a standard rate of 60 beats per minute. Monitoring the QTc is important because prolongation can predispose patients to dangerous ventricular arrhythmias, particularly Torsades de Pointes. Certain medications (such as some antiarrhythmics, antipsychotics, antibiotics), electrolyte abnormalities (especially hypokalemia, hypomagnesemia), and congenital conditions can prolong the QTc interval, requiring careful monitoring and potential intervention to prevent life-threatening complications, as noted in previous studies 1. However, the most recent and highest quality study 1 provides the most relevant guidance on the normal QTc interval.
From the Research
Normal Value of QCT or QTC in EKG
The normal value of QCT or QTC in an EKG is a topic of interest in various medical studies.
- According to 2, the prevalence of abnormal QTc intervals varied by the criterion applied, ranging from 25.2% (using the most lenient criterion of abnormality, QTc > or = 450 msec) to 3.5% (when the most restrictive criterion was applied, QTc > or = 500 msec).
- The study 3 discusses the effects of electrolyte disorders on cardiac ionic currents kinetics, but does not provide a specific normal value for QTC.
- The study 4 defines QTc prolongation as ≥460 ms in women and ≥ 450 ms in men, which can be used as a reference for normal values.
Factors Affecting QTC Interval
Several factors can affect the QTC interval, including:
- Electrolyte disorders, such as hypokalemia, hypomagnesemia, and hypocalcemia, which can prolong the QTc interval 2, 3.
- Serum and dialysate concentrations of calcium, potassium, and magnesium, which have been associated with QTc prolongation in incident hemodialysis patients 4.
- Age, gender, ECG abnormalities, and other covariates, which can also affect the QTc interval 2.