A nurse is reviewing cardiac rhythms and heart blocks for a critical care course. The nurse is aware that which of the following describes first degree heart block?
A.
The PR interval gets longer then drops.
B.
PR interval greater than 0.20 seconds in duration
C.
There are absent P waves
D.
R-R is irregular
The Correct Answer and Explanation is:
The correct answer is B. PR interval greater than 0.20 seconds in duration.
Explanation:
First-degree heart block is a type of atrioventricular (AV) block where there is a delay in the conduction of electrical impulses from the atria to the ventricles. This condition is characterized by a prolonged PR interval on the electrocardiogram (ECG).
- PR Interval Prolongation:
- In first-degree heart block, the PR interval is consistently prolonged. Specifically, the PR interval exceeds 0.20 seconds (200 milliseconds). This indicates a delay in the conduction through the AV node, which is responsible for the delay in electrical impulse transmission from the atria to the ventricles. On an ECG, this appears as a longer than normal time between the onset of the P wave (representing atrial depolarization) and the onset of the QRS complex (representing ventricular depolarization).
- Absence of PR Interval Drop:
- It’s crucial to note that in first-degree heart block, the PR interval is prolonged but remains consistent. There is no pattern of the PR interval getting longer and then dropping, which is characteristic of second-degree heart blocks (either Type I or Type II).
- Absence of P Waves:
- The presence or absence of P waves is not directly related to first-degree heart block. In this type of block, P waves are typically present and are followed by a QRS complex; the issue lies in the length of the PR interval.
- Irregular R-R Interval:
- First-degree heart block does not result in an irregular R-R interval. The R-R intervals remain regular because the block does not affect the rhythm’s regularity; it only prolongs the PR interval.
In summary, first-degree heart block is primarily identified by a PR interval greater than 0.20 seconds, reflecting a delayed conduction through the AV node. This condition often does not present significant clinical symptoms but is important to recognize for accurate diagnosis and management in critical care settings.