CVS Practice Test

What do both leads I and II present in a normal QRS axis?

1. Both have flat complexes

2. Both have negative complexes

3. Both have upright complexes

4. Both are unavailable for measurement

The correct answer is: Both have upright complexes

In a normal QRS axis, both leads I and II present upright complexes. This is because the electrical activity of the heart during ventricular depolarization is directed primarily towards the left side of the heart and downwards, which is aligned with the axes of leads I and II. In a healthy heart, the direction of the QRS complex should be predominantly positive in these leads, indicating that the heart is depolarizing normally. If we consider what a flat complex would indicate, it suggests a lack of significant electrical activity or conduction blockage, which would not be classified under a normal QRS axis. Negative complexes do occur in certain conditions but indicate either an abnormal axis or other issues, such as myocardial infarction. Lastly, the notion that both leads are unavailable for measurement would imply a technical issue or complete loss of data, which is not relevant when discussing a normal QRS axis. Understanding these principles helps in interpreting ECG readings effectively, which is critical for diagnosing various cardiac conditions.