MD Calculator QTc

QTc Calculation:

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

QT Interval (ms):

Heart Rate (HR):

bpm

Correction Formula:

Corrected QT Interval (QTc):

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1. What Is QTc Calculation?

QTc (Corrected QT Interval) calculation adjusts the measured QT interval for heart rate, allowing for standardized comparison across different heart rates. MDCalc uses Bazett's formula by default but also supports Fridericia, Framingham, and Hodges formulas for QT correction.

2. How Does The Calculator Work?

The calculator uses various formulas for QT correction:

Bazett's Formula: \[ QTc = \frac{QT}{\sqrt{RR}} \]

Fridericia's Formula: \[ QTc = \frac{QT}{\sqrt[3]{RR}} \]

Framingham Formula: \[ QTc = QT + 0.154 \times (1 - RR) \]

Hodges Formula: \[ QTc = QT + 1.75 \times (HR - 60) \]

Where:

\( QT \) — Measured QT interval from start of Q wave to end of T wave (ms)
\( RR \) — RR interval = 60 / HR (s)
\( HR \) — Heart rate (bpm)

Explanation: Different correction formulas account for the non-linear relationship between QT interval and heart rate in various ways.

3. Importance Of QTc Calculation

Details: Accurate QTc calculation is crucial for detecting prolonged QT intervals, which may indicate increased risk of life-threatening arrhythmias such as torsades de pointes.

4. Using The Calculator

Tips: Enter QT interval in milliseconds, heart rate in beats per minute, and select the desired correction formula. All values must be valid (QT > 0, HR > 0).

5. Frequently Asked Questions (FAQ)

Q1: Why are there different QT correction formulas?
A: Different formulas have varying accuracy across different heart rate ranges and patient populations. Bazett tends to overcorrect at high heart rates, while other formulas may provide better accuracy in specific scenarios.

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

Q3: When should QTc be measured?
A: QTc should be measured in patients taking medications that may prolong QT interval, in those with syncope or family history of sudden cardiac death, and as part of routine cardiac assessment.

Q4: Are there limitations to QT correction?
A: All correction formulas have limitations, especially at extreme heart rates. Clinical judgment should always accompany automated calculations.

Q5: Which formula is most accurate?
A: No single formula is perfect for all situations. Bazett is most commonly used, but Fridericia or Framingham may be preferred in specific clinical contexts or research settings.