Questions: 10. What does a narrow QRS complex indicate on an electrocardiogram (ECG)? The impulse starts in the ventricles. The impulse starts in the atria. The rhythm is fast. The rhythm is slow.

The answer is the second one: The impulse starts in the atria.

Explanation for each option:

1. The impulse starts in the ventricles.
   This is incorrect. A narrow QRS complex indicates that the electrical impulse is conducted through the normal His-Purkinje system, which typically means the impulse originates above the ventricles, usually in the atria.

2. The impulse starts in the atria.
   This is correct. A narrow QRS complex on an ECG suggests that the electrical impulse originates in the atria or the atrioventricular (AV) node and is conducted normally through the ventricles.

3. The rhythm is fast.
   This is incorrect. A narrow QRS complex does not necessarily indicate the speed of the rhythm. It only indicates the origin and conduction pathway of the impulse. The rhythm could be fast or slow with a narrow QRS complex.

4. The rhythm is slow.
   This is incorrect. Similar to the previous option, a narrow QRS complex does not provide information about the speed of the rhythm. It only indicates that the impulse is conducted through the normal pathway.

In summary, a narrow QRS complex on an ECG indicates that the impulse starts in the atria.