The QRS complex morphology is very important for many diagnoses, including ventricular hypertrophy, intraventricular conduction disturbances, such as bundle branch block and myocardial infarction.
The QRS complex morphology reflects ventricular depolarization in the frontal and horizontal planes. The initial 0.02 second septal depolarization is left-to-right, inferior-to-superior and posterior-to-anterior.
In the limb leads, this can be reflected by a narrow Q wave in Leads I and aVL or Leads II, III and aVF depending on whether the axis points laterally or inferiorly. In precordial leads V5 and V6, a small initial Q wave may be seen.
The wave then proceeds to the rest of the septum, right ventricle and left ventricle. Because the left ventricle is much thicker than the right ventricle, the resultant force during the last 0.06-0.08 seconds, is a large right-to-left, anterior-to-posterior and superior-to-inferior wave.
In the limb leads, this can be reflected by a prominent R wave after the narrow Q wave in Leads I and aVL, or Leads II, III and aVF, depending on whether the axis points laterally or inferiorly. In the precordial leads V5 and V6, this is reflected by a large R wave following a small Q wave, and in leads V1 and V2, a small R wave followed by a prominent S wave.
Narrow Q waves
Narrow Q waves are usually normal and called septal Q’s. Q waves however can be a sign of myocardial infarction, so it is important to know the difference between normal and abnormal Q’s. Narrow .02 second Q waves are usually normal in both the frontal and horizontal planes. In the frontal plane with a vertical QRS axis, that is, the axis points inferiorly, narrow Q waves may be seen in inferior leads, II, III and aVF, such as in this first example. Similarly, with a horizontal or leftward axis, that is, the QRS points laterally, narrow Q waves may be seen in leads I and aVL, such as in the second example. In the precordial leads, narrow normal septal Q waves are commonly seen in V5 and V6.
Wide Q waves
Now let us discuss wide Q waves. Wide Q waves (.04 seconds or more) can be normal or pathologic. Leads III, aVR, aVL and V1 may have a wide Q wave that, if isolated,
Usually a wide Q wave is pathologic if seen in 2 contiguous leads, and suggests myocardial necrosis. However leads III, aVR, aVL and V1 may have a wide Q wave that, if isolated, can be considered normal. For example, note the isolated wide Q waves in lead aVR and also V1 in the ECG on the left. Usually, a wide Q wave is pathologic if seen in two contiguous leads and suggest myocardial necrosis. For example in the ECG on the right, note the wide Q waves in three contiguous leads V1, V2 and V3.
QRS morphology evaluation
When evaluating the normal QRS morphology in the 12 leads, determine the width and amplitude. In the precordial leads, note whether the R wave height progressively increases at least to V4. Note also the transition zone, that is, when the QRS is most isoelectric: it should be after V2 and before V5, and is usually between V3 and V4. Transition is considered early if V2 is positive, and late if V5 is negative.