This ECG was taken from an unknown patient. It shows sinus tachycardia with left bundle branch block. The ECG criteria for left bundle branch block are:
* Wide QRS (.12 seconds or greater)
* Negative QRS deflection in V1
* Positive QRS in Leads I and V6
* Supraventricular rhythm
In addition to these criteria, left bundle branch block will cause repolarization abnormalities. This is because depolarization is altered through the left ventricle, which causes repolarization to also be altered. Instead of the electrical impulse traveling down the left bundle branch to depolarize the left ventricle, it depolarizes the right ventricle first, then spreads cell-to-cell across the larger left ventricle. The ST and T wave changes caused by left bundle branch block are normally “discordant”. That is, the ST segment will be elevated in leads with negative QRS complexes, and depressed in leads with positive QRS complexes. This elevation and depression of the ST segment may “imitate” the changes caused by acute myocardial infarction. They may also work to conceal M.I. changes, as we may not recognize STEMI as we attribute the ST changes to the left bundle branch block itself.
For more on determining the presence of acute M.I. when the patient has left bundle branch block, check out these links: ECG Guru, LBBB with AMI; Life in the Fast Lane, Sgarbossa Criteria; EMS 12-Lead, Sgarbossa Criteria; Dr. Smith's Modified Sgarbossa Criteria.
This ECG has another interesting teaching point. In some leads, part of the QRS is flat or nearly flat with the baseline. Look at Lead I for an example. It is very hard to tell that this represents a wide QRS. Fortunately, the precordial leads, especially V1 through V4, show the width better. In reality, all the QRS complexes represent the time it takes to depolarize the ventricles, and that time is the same, regardless of which lead you are looking in. Some leads are just easier than others. It often helps to look at the QRSD (QRS duration) at the top of the page to see how the computer measured the QRS duration. It is usually accurate. In the illustrated case, the QRSD is measured in milliseconds, 124 ms (or .124 seconds).
You might also notice that the frontal plane axis is shifted to the left. That is, Lead II (viewpoint from left leg) has a negative QRS while Leads I and aVL (from left arm) are positive. This is common in left bundle branch block, because the left ventricle is being depolarized from the right ventricle, which is located inferior and medial. Because axis is affected by many factors, left axis deviation is not a prerequisite for the diagnosis of left bundle branch block.