Cardiovascular System
First-degree AV block
First-degree atrioventricular (AV) block is a delay rather than a true block: the AV node holds each electrical impulse back for longer than usual, yet every impulse still gets through.
Remember
First-degree block is a long but constant PR interval (the time an impulse takes to travel from the atria to the ventricles), over 200 milliseconds (ms), with no dropped beats: every impulse arrives, only late, and on its own it is usually benign.
The atrioventricular (AV) node is the electrical gateway between the atria and the ventricles. Normally it holds each impulse back for a fraction of a second, so that the ventricles fill before they contract. In first-degree AV block the node holds each impulse back for longer than usual, yet every impulse still gets through.
On the ECG this is a prolonged PR interval. The PR interval is the time taken for the impulse to travel from the atria across the AV node to the ventricles, so a long one means the AV node is delaying the impulse more than usual. A normal interval is up to 200 ms. First-degree block is a consistently longer one, over 200 ms (more than five small squares). Every atrial impulse is conducted, meaning it reaches the ventricles, so no beats are dropped and the rhythm stays regular. This is called 1:1 conduction. The block does not by itself slow the pulse.
A common cause is increased vagal tone (the slowing effect of the vagus nerve on the AV node), as seen with athletic training. Ischaemic heart disease and drugs that slow the AV node, such as beta-blockers, can also cause it. In an athlete with no symptoms and a regular pulse it is generally a benign finding that needs no specific treatment. It can, however, develop into second- or third-degree block, so it is noted rather than ignored. The underlying cause is worth considering, especially when the block is new or the patient has symptoms.
In second-degree block some atrial impulses are not conducted, so they do not reach the ventricles. This happens either after progressive PR lengthening (Mobitz type I) or suddenly with a constant PR interval (Mobitz type II). In third-degree block none are conducted. First-degree block is the mildest form.
Key facts
- PR interval
- consistently longer than 200 ms, which is more than five small squares.
- Conduction
- 1:1, so every atrial impulse reaches the ventricles and no beats are dropped.
- Rhythm
- regular, because the delay is consistent and nothing is dropped.
- Meaning
- a delay in the AV node, not a true block.
- Causes
- increased vagal tone (as with athletic training), ischaemic heart disease and drugs that slow the AV node, such as beta-blockers.
- Severity
- usually benign on its own, but it can develop into second- or third-degree block.
- Longer delay
- the block is described as marked when the PR interval exceeds 300 ms.
Exam traps
- A PR interval that lengthens beat by beat until a beat is dropped is Mobitz type I (Wenckebach) second-degree block, not first-degree block, because first-degree block never drops a beat.
- Intermittent non-conducted impulses with a constant PR interval point to Mobitz type II second-degree block, because in first-degree block every impulse is conducted.
- An accessory pathway conducts impulses directly to the ventricles, bypassing the AV node, so it shortens the PR interval rather than prolonging it.
- Delayed discharge from the sinoatrial node, where each impulse is formed, shifts the whole cardiac cycle later, but the PR interval remains normal.
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