How to Calculate QTc on ECG

Bazett's Formula:

\[ QTc = \frac{QT}{\sqrt{RR}} \]

Corrected QT Interval (QTc):

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1. What is QTc Calculation?

The QTc (Corrected QT Interval) calculation adjusts the measured QT interval for heart rate using Bazett's formula. This provides a standardized measure of ventricular repolarization that can be compared across different heart rates.

2. How Does the Calculator Work?

The calculator uses Bazett's formula:

\[ QTc = \frac{QT}{\sqrt{RR}} \]

Where:

\( QT \) — Measured QT interval from start of Q wave to end of T wave (ms)
\( RR \) — RR interval = 60 / HR (seconds)
\( HR \) — Heart rate (beats per minute)

Explanation: The formula corrects the QT interval for heart rate variations, providing a standardized measure of ventricular repolarization time.

3. Importance of QTc Calculation

Details: Accurate QTc calculation is crucial for assessing risk of arrhythmias, particularly torsades de pointes, and for monitoring patients on medications that may prolong QT interval.

4. Using the Calculator

Tips: Enter QT interval in milliseconds and heart rate in beats per minute. Both values must be positive numbers. Measure QT from the start of Q wave to the end of T wave, using the previous RR interval.

5. Frequently Asked Questions (FAQ)

Q1: Why use Bazett's formula for QTc calculation?
A: Bazett's formula is the most widely used and clinically accepted method for correcting QT interval for heart rate, though it may overcorrect at very high heart rates.

Q2: What are normal QTc values?
A: Normal QTc is generally <440 ms for men and <460 ms for women. Values above these may indicate prolonged QT interval.

Q3: How should QT interval be measured on ECG?
A: Measure from the beginning of the QRS complex to the end of the T wave, preferably in lead II or V5 where T waves are most clearly defined.

Q4: Are there limitations to Bazett's formula?
A: The formula tends to overcorrect at high heart rates and undercorrect at low heart rates. Alternative formulas like Fridericia's may be used in specific cases.

Q5: When is QTc monitoring important?
A: QTc monitoring is crucial for patients on medications known to prolong QT interval, those with electrolyte imbalances, or with congenital long QT syndrome.