What makes the PR interval do the "longer, longer, drop" thing? - Why does it do that?
The dysfunction in the AV node is like a tendency to exhaust its conduction capacity. It won't
repolarize adequately by the time the next impulse arrives, which builds, causing the next conduction
to be slower than the previous one, thus making the PR longer. With each consecutive beat, the AV node
becomes more and more exhausted until there is complete refractory in which the impulse is blocked.
The AV node doesn't repolarize and there is no ventricular contraction at all (no QRS).
Let's say you are the gate keeper to the ventricles (the AV node). But you're sick or defective.
The SA node releases the impulse and Mr. P-wave comes running down at you. It's your job to send
him through the AV node gate to conduct the ventricles. However, you are now sick and exhausted.
So it takes you longer to crank open that heavy AV nodal gate (PR widens). The next p-wave arrives
and you still haven't caught your breath yet, so now it takes even longer to crank open that gate
(PR widens more). This continues until you are so exhausted you "drop". The impulse doesn't get
though at all, and the ventricles do not contract (no QRS). The AV node then recovers and the
cycle starts over.
What's the difference between 2nd degree AV block Mobitz1 and Mobitz2 ?
(Location, location, location)
2nd degree Mobitz 1
- Mobitz 1 (Wenckebach) is located at the AV node, and is often referred to as AV nodal block.
- Mobitz 1 is more of a physical conduction delay.
- On an EKG graph the PR interval widens identifying it as 2nd degree type 1 (Mobitz 1).
- Type 1 block is mostly benign, non-threatening, often reversible and rarely needs a pacemaker.
2nd degree Mobitz 2:
- 2nd degree block Mobitz 2 is located down in the bundle branch, or bundle of HIS.
- The EKG graph shows a P-wave with no QRS, however all PR intervals are the same (no PR widening anywhere).
- Mobitz 2 block is often referred to as infranodal block (below the AV node), or HISIAN block (HIS).
- Mobitz 2 is dangerous because it is a sudden electrical short, having a strong chance of no escape rhythm available.
- Mobitz 2 often requires a pacemaker insertion.
Signs and symptoms
- More often, there are no symptoms which is common in patients with type I, such as
welltrained athletes and persons without structural heart disease
- Light-headedness, dizziness, or syncope (more common in type II)
- Chest pain, if the heart block is related to myocarditis or ischemia
- A regularly irregular heartbeat
- Bradycardia may be present
- Symptomatic patients may have signs of hypo-perfusion, including hypotension
Diagnosis
- EKG is used to identify the presence and type of second-degree AV block.
- The most common form of 2nd degree AV block is type I (Winkebach)
- Gradually progressive PR interval prolongation occurs before the blocked sinus impulse
- The greatest PR increment typically occurs between the first and second beats of a cycle.
- Commonly, junctional escape beats occur along with non-conducted P waves
- Shortening of the PR interval occurs after the blocked sinus impulse, provided that the P wave
is conducted to the ventricle.
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