Saturday, December 18, 2010

ECG Interpretation Review - #4 (QT Interval, List #3, Causes of Long QT, Tachycardia, Torsades)

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QUESTION: Interpret the 12-lead ECG below.
  • What is the rhythm?
  • What is the most remarkable finding on this tracing?
  • Clinically - What else may be going on?
Figure 1 (ECG reproduced from ECG PB book - pg 29A)
- Note - Enlarge by clicking on Figures -




         








INTERPRETATION:  The rhythm is rapid and regular.  It appears that the R-R interval is ~2 large boxes, so that the rate is ~150/minute.  The QRS complex is narrow, and upright P waves are present in lead II with a fixed PR interval.  The rhythm is therefore sinus tachycardia.
  • We fully acknowledge that it is not easy to be certain of the rhythm from inspection of lead II alone (the small upright deflection preceding QRS complexes in lead II could be a P wave, T wave, or both). Lead V1 is often the next best lead to look at when assessing a patient's rhythm - and one clearly sees a P wave with negative deflection and consistent PR interval preceding the QRS in lead V1 (thus confirming the rhythm as sinus).
Regarding Intervals:  - Both the PR and QRS intervals are normal.  The QT interval is difficult to measure because of the rapid rate. Typically - the QT interval is not more than half the R-R interval (but this relationship is less reliable with tachycardia).  That said - in at least several leads (such as leads V3,V4,V5) - it looks like the QT is clearly much more than half the R-R interval.  Thus despite the very rapid rate, we strongly suspect QT prolongation.
  •  List #3 (Figure 2helps us recall the likely reasons for QT Prolongation (= "Drugs - Lytes - CNS" ).  Clinical correlation is needed to determine which of these may be relevant for the patient in question.
Figure 2Common Causes of QT Prolongation.
(This is List #3 of our 6 "Essential Lists" - reproduced from ECG PB book - pg 29;
Click HERE for more on Long QT and Torsades - reproduced from our ECG Web Brain)
Returning to Our Systematic Approach: The axis in Figure 1 is normal (+15 degrees). There is no chamber enlargement. ST-T wave changes are dominated by the prolonged QT interval.
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CLINICAL IMPRESSION:  We interpret this ECG as showing sinus tachycardia at ~150/minute.    There are diffuse non-specific ST-T wave abnormalities - and a markedly prolonged QT interval.  Clinically - QT prolongation in the absence of infarction, ischemia or a conduction defect should make one think, "Drugs - Lytes - CNS" (the causes in List #3).
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NOTE - Click HERE for more information on Long QT and Torsades de Pointes in our FREE Demo sample of Chapter 7 from our ECG Web Brain.
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