Case 7: A previously healthy 67 year old carpenter with dyspnea and chest pressure of 24 hours duration.
Thought 1: Is there ECG evidence to suggest myocardial ischemia?
Thought 2. Is there ECG evidence to suggest myocardial infarction?
There is no evidence which suggests myocardial ischemia or infarction.
This tracing is a good example of complete left bundle branch block (LBBB). Note the QRS interval which is 154 msec is markedly prolonged (upper limit 100 msec).
When LBBB is present, the QRS and T wave abnormalities used to identify myocardial schemia injury and infarction are often unreliable. It's like hanging a curtain in front of the ECG which obscures changes which are ordinarily used for detecting these abnormalities (ischemia, injury, infarction).
In order to understand why this is so, let's begin with a review of the normal sequence of activation of the heart and then compare it with complete LBBB. We will use chest lead V6 to demonstrate these differences.
A simplified way of looking at the sequence of activation of the heart is to divide it into three phases:
Phase 1 - septurn,
Phase 2 - right ventricular free wall,
Phase 3 - left ventricular free wall.
Note the diagram.
With left bundle branch block everything takes more time because a large part of the His-Purkinje network is out of commission, but the three phases of activation still apply as noted in the diagram.
Notice that the T waves in leads I, aVL, V5 and V6 are markedly inverted and that there is subtle ST depression. This is also typical of LBBB. The T wave inversion is referred to as a "secondary" T wave abnormality; that is, since ventricular activation (polarization) is impaired due to bundle branch block, the recovery process (repolarization) is also abnormal. We expect the T wave polarity in bundle branch block (both left and right) to be opposite that of the major deflection of QRS.
Note in V1, the QRS is negative and the T wave is positive; in V6 the QRS is positive and the T wave is negative. This is "normal" or at least expected in complete LBBB. If the T wave was upright in V6 in this patient we would be concerned that some additional abnormality was reversing the expected secondary T wave change.
Looking at V1, V2, V3, and V4, notice that the R waves are very small and that they do not gradually increase in size as expected. In fact, V1 - V4 looks for all the world like the pattern of anterior infarction. This, however, is also typical of complete LBBB and is sometimes referred to as "pseudo-infarct pattern".
Look also at the ST segments in V1, V2, and V3. They are elevated and are similar to the changes produced by acute anterior injury. However, this is also typical of complete LBBB and should not be interpreted as necessarily due to acute injury. It is usually simply part of the "pseudo infarct pattern".
In order to emphasize the fact that the changes of LBBB are usually purely electrical and not necessarily due to ischemia injury or infarction we will show another example of complete LBBB (ECG below). In this case, the patient's LBBB disappeared with transient slowing of the heart rate and a single beat is recorded which is entirely normal. The changes which suggested "ischemia", "injury" or "infarction" have abruptly disappeared, but they return with the restoration of the LBBB. Very impressive!
So remember the Hippocratic dictum of "Do no harm." and be very conservative about your interpretation of the changes seen with complete LBBB. In future cases we will demonstrate how acute myocardial infarction can be suspected and at times even confirmed electrocardiographically in patients with complete LBBB. However, that is a seperate topic entirely.
Happy learning !!!
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