Thursday, December 23, 2010

ECG Interpretation Review - #9 (Hemiblock, Axis, Ischemia vs Normal T Wave Inversion)

QUESTION: Interpret the 12-lead ECG below, obtained from an older patient with atypical chest pain.
  • Is there a hemiblock?
  • Is there evidence of inferior ischemia?
Figure 1 (ECG reproduced from ECG-2014-ePub )
Note - Enlarge by clicking on Figures -

INTERPRETATION:  The rhythm is fairly regular at 60-65/minute. The PR, QRS, and QT intervals are normal.
  • Axis - There clearly is LAD (Left Axis Deviation) - as determined by the positive QRS complex in lead I - and the predominantly negative QRS in lead aVF. This raises the question as to whether the axis is negative enough to qualify as "pathologic" LAD (which we define as an axis more negative than -30 degrees). To determine this - simply look at Lead II. We know in general that axis is perpendicular to (ie, 90 degrees away from) a lead where the QRS is isoelectric (ie, equal parts positive and negative).  IF lead II (which views the heart's electrical activity from +60 degrees) is more negative than positive - then the axis must be more than 90 degrees away from lead II, or more negative than -30 degrees. This degree of axis deviation qualifies as pathologic LAD (Figure 2).
Figure 2 - Pathologic LAD as defined by Lead II appearance.
(Reproduced from ECG-2014-ePub )
Hemiblocks: A hemiblock is a defect in conduction along one of the two hemifascicles of the left bundle branch. Thus, there are only two types of hemiblock: LAHB (Left Anterior HemiBlock) and LPHB (Left Posterior HemiBlock). Consideration of the following greatly simplifies remembering how to identify the hemiblocks:
  • LAHB - is by far the most common type (~99%). Definitions as to what constitutes LAHB vary, even among experts.  Practically speaking - there is LAHB - IF there is pathologic axis deviation. Thus, Lead II holds the KEY for determining IF there is LAHB (which is present IF the net deflection in lead II is more negative than positive).
  • LPHB - is rare! The left posterior hemifascicle is much thicker than the anterior hemifascicle - and it has a dual blood supply (from the left and right coronary arteries).  LPHB is especially rare as an isolated defect when there is no associated RBBB.
In Figure 1 - there clearly is LAHB based on the presence of a predominantly negative QRS in Lead II (Figures 2,3).
Figure 3 - Blow-up of Figure 1 - showing pathologic LAD with a predominantly negative QRS in lead II.
(Reproduced from ECG-2014-ePub ).
Returning to Our Systematic Approach: 
  • Chamber Enlargement - None.
  • Q-R-S-T Changes:  - Tiny q waves are seen in leads I and aVL of Figure 1.  Transition is normal (occurring between V3-to-V5).  There is no ST segment elevation - but there is ST segment coving and shallow symmetric T wave inversion in lead III, with ST-T wave flattening in lead aVF.
CLINICAL IMPRESSION: Normal sinus rhythm. LAHB. Small, septal q waves in leads I, aVL.  Shallow, symmetric T wave inversion in lead III and ST flattening in aVF - but there are probably no acute changes.  Suggest clinical correlation.
  • PEARL: Certain leads in an adult ECG may normally manifest either Q waves and/or T wave inversion.  These leads are III - aVF - aVL - aVR - and V1.  As suggested by the small green circles in Figure 4 - these leads appear to form a "reverse Z".
Figure 4 - "Reverse Z" to remind which leads may normally show either Q waves and/or T wave inversion (Leads III,aVF,aVL,aVR,V1).
(Reproduced from ECG-2014-ePub).
Final Point: We cannot be certain from Figure 1 that symmetric T wave inversion in lead III does not represent ischemia.  However, the history (chest pain is atypical) and lack of abnormalities in other leads (apart from nonspecific ST flattening in aVF) make ischemia much less likely in this case.
  • Bottom Line: - Isolated T wave inversion or Q waves in leads III, aVF, or aVL may be a normal finding.

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