Advanced Patient Assessment Practice Exam

Which of the following is a characteristic of third-degree heart block?

1. There is often a 2:1 ratio between P waves and QRS complexes

2. The atrial rate is the same as the ventricular rate

3. There is no relationship between the P waves and the QRS complexes

4. There is a lengthening PR interval

The correct answer is: There is no relationship between the P waves and the QRS complexes

In third-degree heart block, also known as complete heart block, there is a complete disassociation between the atrial and ventricular electrical activity. This means that the atrial impulses (P waves) are not conducted to the ventricles, resulting in no relationship between the P waves and the QRS complexes on the electrocardiogram (ECG). In this condition, the atria and ventricles beat independently of each other. The atrial rate can be normal, while the ventricular rate is determined by an escape rhythm that is usually slower and originates from the heart's AV junction or ventricles. This lack of coordination is a key characteristic of third-degree heart block and is vital for healthcare providers to recognize during patient assessment. The other characteristics described in the options, such as a 2:1 ratio between P waves and QRS complexes, are more typical of second-degree heart blocks, where some atrial impulses are blocked but others are conducted to the ventricles. The same can be said for the lengthening PR interval, which is indicative of first-degree heart block or type I second-degree heart block. Therefore, the characteristic of disassociation between P waves and QRS complexes accurately defines third-degree heart block.