 ### Description

The Q-T interval becomes longer in bradycardia and shorter in tachycardia. This is due to the shortening or lengthening of the myocardial refractory period. The corrected Q-T interval (referred to as the "Q-Tc") is an attempt to give the theoretical Q-T interval if the heart rate was 60 beats per minute. Bazett proposed a formula for this purpose in 1920.

corrected Q-T interval in ms^(0.5) =

= (measured Q-T interval) / SQRT(R-R interval)

corrected Q-T interval in ms^(0.5) =

= (measured Q-T interval) * SQRT((heart rate) / 60)

where:

• R-R interval is the measurement between 2 consecutive R-R waves in seconds.

Interpretation:

• A QTc above a certain cutoff (varies between 420 to 440 milliseconds) indicates a prolonged QTc (Luo et al).

The R-R interval depends on the heart rate. The following equation approximates the R-R interval if the heart rate is known, based on the data from Friedman (1985; see documentation).

R-R interval =

= (0.1349829 * ((HR / 60)^4)) - (1.131181* ((HR / 60)^3)) + (3.5907994 * ((HR / 60)^2)) - (5.373251 * ((HR / 60))) + 3.7807829

where:

• HR is the heart rate in beats per minute.

Limitations of Bazett's correction (Strevel et al):

(1) The formula is nonlinear.

(2) It tends to overcorrect the QT at fast heart rates.

(3) It tends to undercorrect the QT at slow heart rates. This may result in underestimating a patient's risk of torsade de pointes.