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Difficult QT: Difference between revisions
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→QTc is the heart rate corrected QT interval
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===General considerations=== | ===General considerations=== | ||
* The QT interval should be measured in lead II, because the influence of a possible U-wave are smallest in this lead. If lead II is cannot be used for measurements, than leads V4 or V5 can be used alternatively. | * The QT interval should be measured in lead II, because the influence of a possible U-wave are smallest in this lead. If lead II is cannot be used for measurements, than leads V4 or V5 can be used alternatively. | ||
* Draw a line through the baseline. This is the horizontal line between the P wave and QRS complex. If PTa depression is present, the T - P segment is used alternatively. If there is baseline drifting, it can be difficult to determine the proper baseline and measurements will be less accurate. | * Draw a line through the baseline. This is the horizontal line between the P wave and QRS complex. If [[PTa segment]] depression is present, the T - P segment is used alternatively. If there is baseline drifting, it can be difficult to determine the proper baseline and measurements will be less accurate. | ||
* Draw a tangent against the steepest part of the end of the T wave. If the T wave has two humps (as in a camel back), the tallest hump should be picked. If the T wave has a positive and negative hump (or the other way around), the hump with the largest amplitude should be picked (see third example, the first hump has the largest amplitude and should be picked to draw the tangent). | * Draw a tangent against the steepest part of the end of the T wave. If the T wave has two humps (as in a camel back), the tallest hump should be picked. If the T wave has a positive and negative hump (or the other way around), the hump with the largest amplitude should be picked (see third example, the first hump has the largest amplitude and should be picked to draw the tangent). | ||
* The QT interval starts at the beginning of the QRS complex: the beginning of the first deflection (either positive of negative) of the QRS complex. | * The QT interval starts at the beginning of the QRS complex: the beginning of the first deflection (either positive of negative) of the QRS complex. | ||
* The QT interval ends where the tangent and the baseline cross. | * The QT interval ends where the tangent and the baseline cross. | ||
* The QT interval is the time in milliseconds between the start and end of the QT interval. | * The QT interval is the time in milliseconds between the start and end of the QT interval. | ||
* If the QRS interval exceeds 120 | * If the QRS interval exceeds 120 milliseconds, than some specialists advice to deduct the amount of milliseconds that exceed 120 ms from the QT interval (i.e. if QRS is 140 ms and QT is 480, then 20ms (i.e. 140 - 120ms) should be deducted from the QT interval, yielding 460ms). | ||
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=== | ===QTc is the heart rate corrected QT interval=== | ||
<flash>file=QTc.swf|width=300|height=200|quality=best|align=right|salign=R||bgcolor=#FFF5F5</flash> | |||
[[Image: | * The QT interval should be corrected for the heart rate, as the QT interval shortens with increasing heartrate. | ||
* Bazetts formula: [[Image:Formule QTc.png]], can be used to calculate the QTc: the heart rate corrected QT. When entering the formula the RR interval should be entered in seconds. The preceding RR interval should be used. You can also use this [[QTc calculator]] | |||
* Preferably one uses the average of several QTc measurements | |||
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== | ===Examples=== | ||
[[Image:lastigeQT1.png|thumb| The T wave is broad, but the tangent crosses the baseline before the T wave joins the baseline. The QT interval would be overestimated when this last definition of the end of the T wave would be used.]] | |||
[[Image:lastigeQT2.png|thumb| The ECG does not meet the baseline after the end of the T wave. Still, the crossing of the tangent and baseline should be used for measurements.]] | |||
[[Image:lastigeQT3.png|thumb| A bifasic T wave. The tangent to the 'hump' with the largest amplitude is chosen. This can change from beat to beat, making it more important to average several measurements.]] | |||
==References== | |||
<biblio> | <biblio> | ||
#Lepeschkin pmid=14954534 | #Lepeschkin pmid=14954534 | ||
</biblio> | </biblio> | ||