The QTc Calculator
The following is for information purposes only. Consult a physician if you are having cardiac symptoms or suspect that you have an EKG abnormality.
To calculate the QTc, enter the heart rate in beats per minute, and the QT interval in either milliseconds or small boxes (40 ms). Enter multiple QT values, separated by spaces, to use the mean. The resulting QTc values are sorted smallest to largest.
Normal QTc: 350440.
QTc > 500 increases the risk of torsades de pointes or worse.
The Bazett formula is the most commonly used despite evidence that others such as Fridericia may be more reliable, especially at high or low rates.
QT < half the RR is an older rule of thumb, less reliable at low rates.
Normal PR: 120200.
First degree AV block: PR > 200. Generally benign.
Second degree AV block type I (Mobitz I, Wenckebach): progressive PR prolongation followed by a dropped QRS complex. PR always comes back shorter after the dropped beat. PP interval is regular. Generally benign.
Second degree AV block type II (Mobitz II, Hay): intermittently dropped QRS complexes not preceded by PR prolongation and not followed by PR shortening. Often occurs in a fixed ratio. High-risk.
Third degree AV block: AV dissociation; independent atrial and junctional or ventricular escape rates. High-risk.
Short PR (< 120) may indicate pre-excitation or a junctional rhythm.
Dropped P waves may indicate a sinoatrial block.
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.
The QT interval is measured from the beginning of the QRS complex to the end of the T wave.
The QTc is the QT corrected for heart rate.
The tangent and threshold methods of measuring the QT interval. Use lead II.
One small box = 40 ms.
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