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==The QT interval==
==The QT interval==
[[Image:QRSinterval.jpg|thumb|300px| The QT interval start at the onset of the Q wave and ends where the tangent line for the steepest part of the T wave intersects with the baseline of the ECG. Click on the image for a larger image]]
[[Image:QRSinterval.jpg|thumb|400px| The QT interval start at the onset of the Q wave and ends where the tangent line for the steepest part of the T wave intersects with the baseline of the ECG. Click on the image for a larger image]]
'''The normal QTc interval'''
'''The normal QTc interval'''
The QT interval indicates how fast the ventricles are repolarized and how fast they are ready for a new heart cycle.
The QT interval indicates how fast the ventricles are repolarized and how fast they are ready for a new heart cycle.
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Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is poor as different definitions of the end of the T wave exist.<cite>Viskin</cite> Most QT experts define the end of the T wave as the intersection of the steepest tangent line from the end of the T-wave with the base line of the ECG.<cite>Lepeschkin</cite> This leads to the following stepwise approach:
Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is poor as different definitions of the end of the T wave exist.<cite>Viskin</cite> Most QT experts define the end of the T wave as the intersection of the steepest tangent line from the end of the T-wave with the base line of the ECG.<cite>Lepeschkin</cite> This leads to the following stepwise approach:


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{| class="wikitable" width="750px"
!Stepwise approach to correct measurement of the QT interval
!Stepwise approach to correct measurement of the QT interval
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|-
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In  a (serious) prolonged QT time, it takes longer for the myocardial cells to be ready for a new cardiac cycle. There is a possibility that some cells are not yet repolarized, but that a new cardiac cycle is already initiated. These cells are at risk for uncontrolled depolarization and induce [[Torsade de Pointes]] and subsequent [[Ventricular Fibrillation]].
In  a (serious) prolonged QT time, it takes longer for the myocardial cells to be ready for a new cardiac cycle. There is a possibility that some cells are not yet repolarized, but that a new cardiac cycle is already initiated. These cells are at risk for uncontrolled depolarization and induce [[Torsade de Pointes]] and subsequent [[Ventricular Fibrillation]].


{| class="wikitable"
{| class="wikitable" width="750px"
! Causes of QT prolongation
! Causes of QT prolongation
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