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  • What lead from the surface ECG should I measure the QT interval?
    Lead II or V5/V6 are usually the best options of measurement. The reason for this is that the end of the T wave is often more clearly defined in these leads and U waves if present are less prominent.
  • Which formula should i use for QTc calculation?
    Historically many physicians and health care workers use the Bazzett formula. However, Bazzett correction is prone to overcorrection (at slower heart rates) and undercorrection (at faster heart rates. Therefore it is recommended that a linear regression equation (Framingham or Hodges) been used.
  • What is the Tagent T wave Method?
    It is methood used to determine where the T wave ends, defining the end of the QT interval. To do this a tangent drawn on steepest end limb of the T wave. Where this tagent intersects with the baseline defines the end of the T wave.
  • What about U waves?
    The U wave can often add to confusion and uncertainity in defining the end of the T wave. If the U wave is distinct and smaller than the T wave it should NOT included in the measurement.
  • How do i correct for patients in atrial fibriillation
    Unfortantely correction of the QT interval in AF is problametic, as the QT interval varies from beat to beat. Consensus reagrdiding the optimal method for correction does not exist however two methods have been proposed: 1. Take an average over 10 beats 2. Measure the QT interval that follows the shortest and longest R-R intervals. Then use R-R interval preceding it to correct for heart rate. Then take the average of these two QT intervals (longest and shortest).
  • How do i correct if there is a wide QRS (bundle branch block or pacing) ?
    The QT interval will be exaggerated in patients with a wide QRS. Calcuclation of the JT interval has been propsoed as an option (JT = QT - QRS duration) however has largely fallen out of favour due to cocner with resdiual confudjg (ie. highly HR dependent) While there are many forumale vaiavlbe (see below), the easiest approach is to use the following formula; wide QRS adjusted QTc = QTc – [QRS – 100].
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