An atrioventricular (AV) block represents a delay or interruption in the electrical impulse traveling from the heart’s upper chambers (atria) to its lower chambers (ventricles). This conduction failure can result in a slower heart rate and skipped beats. Second-degree AV block is further categorized into two subtypes, Mobitz I and Mobitz II, which are distinguished by their unique electrical patterns and underlying causes.
Understanding Second-Degree Atrioventricular Heart Blocks
A second-degree AV block occurs when the electrical signal generated in the atria occasionally fails to pass through the AV junction to activate the ventricles. This intermittent failure means that some atrial impulses are successfully conducted, resulting in a beat, while others are blocked, leading to a momentarily dropped ventricular contraction. This phenomenon interrupts the typical sequence of the heartbeat, causing the heart to beat irregularly or too slowly.
The two patterns of second-degree block, Mobitz I and Mobitz II, describe the distinct ways this intermittent failure manifests. Mobitz I is also commonly known by its alternate name, Wenckebach block.
The Distinct Electrical Signatures
The most straightforward way to differentiate between the two types is by observing their patterns on an electrocardiogram (EKG). The PR interval represents the time it takes for the impulse to travel from the atria to the ventricles. The appearance of the PR interval just before a dropped beat is the defining characteristic separating Mobitz I from Mobitz II.
Mobitz I displays a predictable, cyclical pattern known as the Wenckebach phenomenon. In this block, the PR interval progressively lengthens with each consecutive beat until an atrial impulse is completely blocked, and no corresponding ventricular contraction occurs. After this dropped beat, the conduction system recovers, and the PR interval resets to its shortest duration, beginning the cycle anew.
In contrast, Mobitz II is characterized by a constant PR interval for all conducted beats, with the dropped beat occurring suddenly and without warning. The PR interval remains consistent and does not show the progressive lengthening seen in Mobitz I. An atrial impulse simply fails to conduct to the ventricles, resulting in an unexpected, non-conducted P wave and a dropped QRS complex.
The distinction extends to the R-R interval, which measures the time between consecutive ventricular contractions. In Mobitz I, the R-R interval progressively shortens leading up to the dropped beat, despite the lengthening PR interval. Mobitz II, conversely, has a consistent R-R interval between conducted beats, with an abrupt, longer pause when a beat is dropped.
Anatomical Location and Block Predictability
Mobitz I, or Wenckebach block, occurs high up, typically within the AV node itself. The AV node is rich in cells that temporarily fatigue when stimulated repeatedly, but which also recover relatively quickly.
This temporary electrical “exhaustion” of the AV nodal cells causes the progressive lengthening of the PR interval. With each impulse, the node takes slightly longer to recover until a point is reached where an impulse is blocked entirely. This allows the node to fully recover, resetting the cycle and resulting in the characteristic progressive nature of Mobitz I.
Mobitz II is located lower in the conduction system, usually within the bundle of His or the Purkinje network, which are collectively referred to as the infra-nodal system. The block in this area is generally structural or organic, rather than functional.
This all-or-nothing behavior below the AV node is why Mobitz II is considered less predictable and more abrupt in its presentation.
Clinical Importance and Treatment Approaches
Mobitz I is often considered a more benign condition because the block occurs at the level of the AV node, which is capable of temporary, rate-dependent failure. Many patients with Mobitz I are asymptomatic and may only have the block due to high vagal tone, such as during sleep or in highly trained athletes.
Because Mobitz I rarely progresses suddenly to a complete heart block, treatment is often unnecessary unless the patient is symptomatic with dizziness or fainting. If intervention is required, it typically involves addressing any underlying reversible causes, such as stopping certain medications that slow AV nodal conduction. Permanent pacemaker implantation is seldom required for Mobitz I.
Mobitz II is viewed as a more serious condition due to its infra-nodal location, which suggests underlying structural disease in the conduction system. This block carries a higher and more sudden risk of progressing to a third-degree AV block, where no impulses reach the ventricles, leading to cardiac arrest.
For patients diagnosed with Mobitz II, the standard of care is the implantation of a permanent pacemaker. The pacemaker ensures a stable heart rate and prevents the progression to complete heart block. This definitive, prophylactic intervention highlights the difference in clinical urgency between Mobitz I and Mobitz II.