Corrected QTC Calculator

What does QTc stand for?

The QT interval measures the time between ventricular repolarization and depolarization. It starts at the beginning of the q wave and ends at the T wave’s conclusion. If there’s no q wave, the R wave serves as the starting point. Many factors influence the QT interval, such as heart rate, autonomic nerve tone, sympathomimetics, electrolyte levels (especially calcium), medications, age, sex, and even sleep. This measurement is often overlooked by paramedics due to the complexity of assessing and interpreting it. However, neglecting it is unfortunate because the QT interval can help in spotting life-threatening conditions early.

The QT interval spans from the onset of the QRS complex, representing ventricular depolarization, to the end of the T wave, resulting from ventricular repolarization. Though normal QT lengths vary in reports, it’s generally accepted that a normal QT interval is less than 400–440 milliseconds (0.4–0.44 seconds). Women usually have a longer QT interval than men, and a slower heart rate also extends the interval. A quick way to check for QT prolongation is to see if the T wave ends after the midpoint of the RR interval. If so, the QT interval is prolonged.

What is the normal range for QTc?

In adults, the QTc should typically range between 0.33 to 0.44 seconds. A prolonged QT interval may indicate a risk for ventricular arrhythmias, fainting, or sudden death. If the QT interval exceeds 0.44 seconds, it signals potential myocardial electrical instability. The QT interval is longer in women than in men and is also influenced by heart rate. According to literature, the normal QTc interval is between 450 milliseconds (ms) for men and 460 ms for women. A QTc exceeding 500 ms is considered significantly prolonged and increases the risk of arrhythmias.

What is the difference between QT and QTc?

The QT interval and heart rate are inversely related. Typically, QT intervals are adjusted for heart rate; for example, at a heart rate of 60 beats per minute, the RR interval equals one second, and the QTc equals the QT. To determine the corrected QT interval (QTc), multiply the QT interval by the square root of the preceding R-R interval. The standard QTc is around 0.42 seconds.

How to use Bazett’s formula?

The Bazett formula is the most commonly used method to calculate the QTc: QTc = QT / √RR. First published by Bazett in 1920, it was later refined in 1947 by Taran and Szilagyi. The Bazett formula undercorrects the QT interval at slower heart rates and overcorrects it at faster ones. Due to these limitations, especially at slower heart rates, alternative formulas have been explored to improve accuracy. A prolonged QT interval increases the risk of torsades de pointes (TdP). The Bazett formula "corrects" the observed QT interval to a heart rate of 60 beats per minute. This formula works best for patients with a heart rate between 60 and 100 beats per minute. For heart rates outside this range (below 60 or above 100 bpm), the Framingham method is more accurate.

How do you calculate QTc using the Fridericia method?

The Fridericia formula (QTc = QT / RR1/3) was introduced in 1920 and is used to calculate the QT interval from the electrocardiogram (ECG) adjusted for heart rate. Fridericia suggested that QT intervals over 0.43 seconds should be considered prolonged. Compared to Bazett’s formula, the Fridericia formula has shown less sensitivity in detecting QT prolongation, especially at higher heart rates.

How to calculate QTc using Hodges and Framingham formulas?

In 1983, Hodges developed a linear QTc formula to improve on the non-linear Bazett and Fridericia formulas: QTc = QT + 0.00175 * (HR - 60). Later, the Framingham linear regression formula, introduced by the Framingham Heart Study in 1992, goes as follows: QTc = QT + 0.154 * (1 - RR). This method was based on data from over 5,000 participants with an average age of 44 years.

The general rule applies to heart rates between 62 and 66 beats per minute. If the QT interval is under 500 ms at a heart rate of 60 bpm (where the RR interval is 1,000 ms), then the corrected QT should match the observed QT interval. However, this rule overestimates the QT interval for heart rates over 66 bpm. Manual measurement is still the best option, especially when the computer’s QT calculation seems too long. A "Half the QT" rule states that if the QT exceeds half the RR interval, it’s prolonged. However, this rule should be applied with caution, especially in heart rates below 60 bpm.