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765 bytes added ,  00:56, 5 January 2008
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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.
 
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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The QT interval is defined as follows: <cite>Lepeschkin</cite> The time between the beginning of the Q until the point where the steepest tangent line from the end of the T-wave intersects with the base line of the ECG.  
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'''A often used definition of the QT interval is the time between the beginning of the Q until the point where the steepest tangent line from the end of the T-wave intersects with the base line of the ECG.'''<cite>Lepeschkin</cite> 
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{| class="wikitable" align="left" width="400px"
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!Stepwise approach to correct measurement of the QT interval|-
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|-
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|
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#Use lead II. Use lead V5 alternatively if lead II cannot be read.
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#Draw a line through the baseline (preferably the PR segment)
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#Draw a tangent against the steepest part of the end of the T wave. If the T wave has two positive humps, the tallest hump should be picked. If the T wave is biphasic, the end of the tallest wave should be picked.
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#The QT interval starts at the beginning of the QRS interval and ends where the tangent and baseline cross.
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#If the QRS duration exceeds 120ms the amount surpassing 120ms should be deducted from the QT interval (i.e. QT=QT-(QRS width-120ms) )
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#Calculate QTc according to Bazett:
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[[Image:Formule_QTc.png]]
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|}
    
The difficult part is that the QT interval gets shorter if the heart rate increases. This cab be solved by correcting the QT time for heart rate using the Bazett formula::
 
The difficult part is that the QT interval gets shorter if the heart rate increases. This cab be solved by correcting the QT time for heart rate using the Bazett formula::
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[[Image:Formule_QTc.png]]
 
[[Image:Formule_QTc.png]]
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''at an RR interval 1 second, the  (heart frequency 60/min) QTc=QT''
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''At an RR interval 1 second, the  (heart frequency 60/min) QTc=QT''
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Using the QTc calculator on the right, the QTc is easy extractable.
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The 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>
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On the 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..
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On the 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.
    
The following formula is indicative for normal values of QT time (uncorrected):
 
The following formula is indicative for normal values of QT time (uncorrected):
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