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Conduction: Difference between revisions
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→The QT interval
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==The QT interval== | ==The QT interval== | ||
[[Image:QRSinterval.jpg|thumb| 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 bigger image]] | |||
The QT interval indicates how fast the ventricles are repolarized and how fast they are ready for a new heart cycle. The normal value for QTc(orrected) is: '''440-450ms for men and 450-470ms for women'''. <cite>Moss</cite> | |||
A prolonged QT interval can lead to arrhythmias such as [[Torsades de Pointes]] and subsequent [[Ventricular fibrillation]]. The QT interval is prolonged in congenital [[long QT syndrome]], but QT prolongation can also occur as a consequence of (a.o.) medication, electrolyte imbalances and ischemia. QT prolongation is often treated with beta blockers. | |||
The QT interval comprises the QRS-complex, the ST-segment, and the T-wave. One difficultly of QT interpretation is that the QT interval gets shorter if the heart rate increases. This can be solved by correcting the QT time for heart rate using the Bazett formula:: | |||
[[Image: | [[Image:Formule_QTc.png]] | ||
Thus at a heart rate of 60 bpm, the RR interval is 1 second and the QTc equals QT/1. | |||
On modern ECG machines, the QTc is given. However, the machines are not always capable of recognizing the correct QT time. Therefore, it is important to check this manually. | |||
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. 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: | |||
{| class="wikitable" | {| class="wikitable" | ||
!Stepwise approach to correct measurement of the QT interval | |||
!Stepwise approach to correct measurement of the QT interval | |||
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The | The following QTc calculator can facilitate QTc calculation. | ||
<flash>file=QTc.swf|width=300|height=200|quality=best|align=right|salign=R||bgcolor=#FFF5F5</flash> | <flash>file=QTc.swf|width=300|height=200|quality=best|align=right|salign=R||bgcolor=#FFF5F5</flash> | ||
In a (serious) prolonged QT time, is 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 a [[Arrhythmias#Torsade_de_pointes|torsade de pointes]], a ventricular tachycardia. | |||
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{{clr}} | {{clr}} | ||