The heart relies on a precise electrical signaling system to coordinate the pumping action of its four chambers. This electrical pathway begins in the upper chambers (atria) and must pass through the Atrioventricular (AV) node to reach the lower chambers (ventricles). An AV block occurs when this electrical signal faces a delay or interruption traveling from the atria to the ventricles. When this disruption is intermittent—meaning some signals pass through but others do not—it is classified as a Second-Degree AV Block. This block is divided into two distinct patterns, Mobitz I and Mobitz II, and differentiating them is necessary for determining the medical approach.
Understanding Second-Degree AV Block
The severity of an AV block is categorized into three degrees: First-Degree (simple delay), Second-Degree (intermittent failure), and Third-Degree (complete failure). Second-Degree AV Block is characterized by the intermittent failure of atrial electrical impulses to conduct to the ventricles. This results in a “dropped beat” where the ventricles do not contract, causing the heart rate to slow down. This slower rate can sometimes lead to symptoms like lightheadedness or fatigue. Both Mobitz I and Mobitz II share this intermittent failure, but the pattern and location of the failure are different.
Location of the Block and Underlying Mechanism
The fundamental difference between the two Mobitz types lies in where the electrical conduction system is failing. Mobitz I, also known as Wenckebach block, typically involves a functional problem located within the AV node itself. The AV node is an area that can become fatigued with rapid or repetitive stimulation, which is the underlying mechanism for the Mobitz I pattern.
In this scenario, the AV node’s cells become progressively slower at recovering after each successful electrical transmission. The electrical signal encounters increasing resistance with each subsequent beat, eventually leading to a complete breakdown of conduction. After the complete block, the cells have time to recover their full function, and the cycle of progressive fatigue then begins anew.
Mobitz II, by contrast, is caused by a structural or fixed problem located lower in the conduction system, most commonly in the Bundle of His or the Purkinje fibers. This lower system does not exhibit the progressive fatigue seen in the AV node. Instead, the electrical failure is an “all-or-nothing” event that occurs suddenly without prior warning or progressive delay.
Because the block in Mobitz II is located in the bundle branches, it often reflects a more serious underlying structural disease in the heart. This location means the heart’s secondary pacemakers, which might take over if the main signal fails, are also likely to be compromised.
Distinguishing Features on an ECG
The unique physiological mechanisms of Mobitz I and Mobitz II translate into distinct, recognizable patterns on an ECG tracing, which is the primary tool for diagnosis. The P-R interval on an ECG measures the time it takes for the electrical impulse to travel from the atria (P wave) through the AV node to the ventricles (QRS complex). This interval is the key to differentiating the two types of blocks.
Mobitz I is characterized by a progressive lengthening of the P-R interval across several consecutive beats until a QRS complex is completely absent, resulting in a dropped beat. After the dropped beat, the process resets. This progressive lengthening is the hallmark of the Mobitz I or Wenckebach phenomenon.
Mobitz II does not show this progressive lengthening of the P-R interval. For the beats that successfully conduct, the P-R interval remains fixed and constant. The block manifests as a sudden, unexpected failure of the P wave to be followed by a QRS complex. The beat is simply dropped without any prior warning of conduction slowing, reflecting the “all-or-nothing” nature of the block in the His-Purkinje system.
Treatment Implications and Long-Term Outlook
The difference in location and mechanism between the two blocks has significant implications for patient management and long-term prognosis. Mobitz I is generally considered a more benign condition, often occurring in otherwise healthy people, such as athletes, or in response to certain medications. Since the block is functional and located high in the AV node, it is less likely to progress suddenly to a complete heart block (Third-Degree AV Block).
Treatment for Mobitz I is often conservative, focusing on monitoring the rhythm and addressing any underlying causes, such as adjusting medication dosages. If the patient is asymptomatic, active intervention is frequently unnecessary, and the prognosis is generally favorable. A permanent pacemaker is typically reserved only for Mobitz I patients who experience persistent and severe symptoms attributable to the slow heart rate.
Mobitz II, however, is considered a more serious and unstable condition that carries a higher risk of sudden progression to complete heart block. Because the block is lower in the conduction system, it often results from structural damage and can lead to a dangerously slow heart rate. Due to this unpredictable and high-risk nature, Mobitz II frequently requires the implantation of a permanent pacemaker, even if the patient is not currently experiencing significant symptoms. The device ensures a stable heart rate by providing a reliable electrical signal, offering protection against the sudden onset of complete electrical failure.