Second-Degree AV Block, Type II (Mobitz II) - Layman's Explanation
Second-degree AV block Type II is a serious electrical problem in your heart where the signal from the upper chambers (atria) suddenly fails to reach the lower chambers (ventricles) without warning, causing your heart to skip beats. This is different from other heart blocks because there's no gradual warning before the skip happens 1.
What's Actually Happening
Think of your heart's electrical system like a relay race. Normally, the electrical signal starts in the upper chambers and travels down a pathway (called the His-Purkinje system) to make the lower chambers pump. In Type II block:
- The signal travels normally most of the time with a constant timing (the PR interval stays the same on the ECG)
- Then suddenly, without any warning signs, a signal completely fails to get through
- The next signal that does get through has the same timing as before - no change 2
This is the key difference: there's no progressive slowing before the skip, it just happens suddenly 1.
Why This Matters
This condition is dangerous because it commonly progresses to complete heart block where no signals get through at all 2. The problem is usually deep in the electrical wiring system (below the AV node), which makes it:
- More likely to get worse suddenly
- Associated with symptoms like dizziness, fainting, or fatigue
- Less responsive to medications 1
What You'll Notice on the ECG
The electrical tracing typically shows:
- A wide QRS complex (the heartbeat looks stretched out on the tracing) in most cases
- Constant PR intervals before and after the skipped beat
- Sudden dropped beats without any pattern of slowing 3, 4
Treatment Approach
Even if you have no symptoms, Type II block almost always requires a permanent pacemaker 2. This is a Class IIa recommendation, meaning experts strongly agree this is the right approach 2. The pacemaker acts as a backup system to ensure your heart doesn't stop if the block becomes complete.
The prognosis is compromised without treatment, and waiting for symptoms is risky because progression can be sudden and unpredictable 2, 4.
Important Caveats
- Not all "skipped beats" are Type II block - there are look-alikes that can fool even experienced doctors 5, 6
- A 2:1 block (every other beat blocked) cannot be classified as Type I or Type II without more information 7, 6
- If you see the timing change after the skipped beat, it's NOT Type II 8
- Hidden extra heartbeats from the electrical system can mimic this pattern (called "pseudo-AV block") 6, 8
The bottom line: Type II block is the heart's electrical system failing at a deep level, and it needs a pacemaker to prevent potentially life-threatening complete heart block 1, 3, 1.