A Fridericia Formula Calculator is a clinical tool used in cardiology to calculate the heart rate-corrected QT interval (QTcF) from an electrocardiogram (ECG). The QT interval on an ECG represents the time it takes for the heart's ventricles to contract and then relax. This interval naturally shortens at faster heart rates and lengthens at slower heart rates. The Fridericia formula is one of several mathematical methods used to adjust the measured QT interval for the influence of heart rate, providing a standardized value that can be accurately interpreted. This corrected value is crucial for cardiologists and other healthcare professionals to assess a patient's risk for certain types of life-threatening cardiac arrhythmias.
formula of Fridericia Formula Calculator
The Fridericia formula is a specific mathematical correction that adjusts the QT interval based on the heart rate.
1. Primary Fridericia Formula
This is the core formula for the calculation. It adjusts the measured QT interval by the cube root of the RR interval.
Formula:
QTcF = QT / (RR^(1/3))
- QTcF: The heart rate-corrected QT interval, according to Fridericia. The result is typically in milliseconds (ms).
- QT: The raw, uncorrected QT interval as measured directly from the ECG, usually in milliseconds (ms).
- RR: The RR interval, which is the time between two consecutive R waves. This value must be in seconds (s) for the formula to work correctly.
- ^(1/3): The cube root operator.
2. Required Sub-Calculation: RR Interval
The RR interval can be calculate from the heart rate in beats per minute (BPM). It is essential to convert this interval to seconds.
Formula to calculate RR interval from heart rate:
RR (in seconds) = 60 / Heart Rate (in BPM)
- Heart Rate (in BPM): The patient's heart rate in beats per minute.
Alternatively, if the RR interval is measure directly from the ECG in milliseconds (ms), you must convert it to seconds.
Formula to convert RR from milliseconds to seconds:
RR (in seconds) = RR (in ms) / 1000
Interpreting the Corrected QT Interval (QTc)
This table provides the generally accepted ranges for the QTc interval in adults and what they signify. These values are a critical part of arrhythmia risk assessment.
| QTc Interval (ms) | Interpretation for Adults |
| < 350 ms | Abnormally Short QTc |
| 350 - 440 ms (Males) | Normal QTc |
| 350 - 460 ms (Females) | Normal QTc |
| 441 - 500 ms (Males) | Borderline Prolonged QTc |
| 461 - 500 ms (Females) | Borderline Prolonged QTc |
| > 500 ms | Markedly Prolonged QTc (Associated with increased risk) |
Example of Fridericia Formula Calculator
A clinician is evaluating the ECG of a female patient.
First, the clinician gathers the data from the ECG readout.
- Heart Rate: 75 beats per minute (BPM)
- Measured QT Interval: 420 milliseconds (ms)
Step 1: Calculate the RR interval in seconds from the heart rate.
RR (in seconds) = 60 / Heart Rate (BPM)
RR (in seconds) = 60 / 75 = 0.8 s
Step 2: Calculate the cube root of the RR interval.
RR^(1/3) = (0.8)^(1/3)
RR^(1/3) ≈ 0.928
Step 3: Apply the Fridericia formula to find the QTcF.
QTcF = QT / (RR^(1/3))
QTcF = 420 ms / 0.928 ≈ 452.6 ms
Therefore, the Fridericia-corrected QT interval (QTcF) is approximately 453 ms. According to the reference table, this falls into the normal range for an adult female.
Most Common FAQs
The duration of the heart's ventricular repolarization (the QT interval) is physiologically dependent on the heart rate. At faster rates, the heart has less time to "reset," so the QT interval naturally shortens. At slower rates, it lengthens. By correcting the measured QT interval to what it would be at a standard heart rate (usually 60 BPM), clinicians can compare a patient's QT value to established normal ranges and accurately assess their risk, regardless of their heart rate at the time of the ECG.
The Bazett formula (QTcB = QT / √RR) is another common method for QT correction. While it is simple and has been used for a long time, it is known to be less accurate at very high and very low heart rates. The Fridericia formula, which uses the cube root of the RR interval, is generally considered to be more accurate across a wider range of heart rates and is often preferred in modern clinical practice and pharmacology studies.
A prolonged QTc interval indicates a delay in the electrical repolarization of the heart's ventricles. This delay can create an unstable electrical environment in the heart, which increases the risk of developing a dangerous type of arrhythmia called Torsades de Pointes. This condition can lead to fainting, seizures, or even sudden cardiac death. Many medications can cause QT prolongation as a side effect, which is why this measurement is so closely monitored.