Electrocardiograms (ECGs or EKGs) provide information about heart
activity. They are most
often used by doctors to identify potential arrhythmias. The QT interval
on an ECG is the
time interval representing ventricular depolarization and subsequent repolarization
duration of activation and recovery of the heart). The RR interval on an
ECG is the interval
used to calculate the heart rate.
The QT interval varies with heart rate
(i.e. RR interval); normal QT decreases with increasing
heart rate. Therefore, a QTc interval (corrected QT interval) is often
calculated to adjust for the
heart rate. The most common correction is called the Bazett correction.
This commonly used
correction has been criticized as being inaccurate, and numerous
have been proposed.
A more complicated exponential equation to compute
QTc was found by using nonlinear regression analysis to fit data from
young healthy volunteers in . This equation is given by,
QTc = 425 - 676 · e -0.0037·RR ,
where RR is the RR interval from an ECG (all units are in milliseconds).
Use this equation to solve the three problems
in this section.
Compute the QTc interval
for an individual with an RR interval of 900 ms.
Round you answer to the nearest whole millisecond.
Problem 1- Calculate the corrected QT interval
Problem 2- Calculate
the RR interval
Problem 3- When can you
use this correction?
 Lecocq, B., Lecocq, V., and P. Jaillon
(1989). Physiologic relation between cardiac cycle
and QT duration in healthy volunteers. Am J Cardiol 64:481-486.
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