Second-degree atrioventricular (AV) block is a condition affecting the heart’s electrical system. In a healthy heart, electrical signals originate in the atria and travel through the AV node to the ventricles, prompting a contraction. An AV block occurs when this signal transmission is delayed or interrupted. Second-degree AV block is an intermediate level of interference where some, but not all, electrical impulses reach the ventricles. This intermittent failure results in an irregular and often slower heart rhythm.
The Mechanism of Intermittent Block
The heart’s rhythm is controlled by electrical impulses, which are visible on an electrocardiogram (ECG) as distinct waves. The P-wave represents the electrical activation of the atria, while the QRS complex represents the activation and contraction of the ventricles. The atrioventricular node acts as a gatekeeper, managing the flow of electricity between the upper and lower chambers. In second-degree AV block, the electrical signal successfully triggers the atria but occasionally fails to pass through the AV node to initiate a ventricular contraction.
This failure means that a P-wave occurs without a corresponding QRS complex, which is commonly referred to as a “dropped beat.” The intermittent nature of the block defines the second degree, differentiating it from a first-degree block, which is only a consistent delay, or a third-degree block, which is a complete lack of conduction. The issue lies in the temporary inability of the AV node or the tissue immediately below it to recover quickly enough to transmit the next impulse. This results in the ventricles beating slower than the atria, which can compromise the heart’s pumping efficiency.
Mobitz Type I and Mobitz Type II
Second-degree AV block is further categorized into two distinct patterns, Mobitz Type I and Mobitz Type II, which are differentiated by their electrical signature and the location of the conduction disturbance.
Mobitz Type I (Wenckebach)
Mobitz Type I, also known as Wenckebach periodicity, is characterized by a gradually increasing delay in conduction before a beat is dropped. On an ECG, this pattern appears as a progressive lengthening of the PR interval—the time it takes for the signal to travel from the atria to the ventricles—over several consecutive beats. This delay continues until the AV node becomes temporarily exhausted and the next atrial signal is completely blocked, producing a P-wave without an accompanying QRS complex. The cycle then resets, and the conduction pattern begins again with a shorter PR interval.
The block in Type I is typically located within the AV node itself. Because the delay is progressive and often related to temporary fatigue of the nodal cells, this type is considered a more benign form. It can sometimes occur in healthy individuals, such as highly conditioned athletes, due to increased vagal tone, and does not carry a high risk of progressing to a complete block.
Mobitz Type II
Mobitz Type II is a more concerning condition, distinguished by a sudden, unpredictable failure of conduction. Unlike Type I, the PR interval of the conducted beats remains constant before a beat is dropped. The dropped beat occurs without warning, representing an “all-or-nothing” failure of the electrical pathway.
The block in Type II is usually located lower in the heart’s electrical system, specifically in the Bundle of His or the Purkinje fibers below the AV node. This location often suggests structural damage to the conduction system. Because of this underlying pathology, Mobitz Type II carries a higher probability of deteriorating into a complete heart block.
Identifying and Treating the Condition
The diagnosis of second-degree AV block relies primarily on an electrocardiogram, which captures the heart’s electrical activity and reveals the characteristic patterns of dropped beats. While some individuals, especially those with asymptomatic Mobitz Type I, may not experience noticeable effects, others present with symptoms linked to a reduced heart rate. These include fatigue, lightheadedness, and dizziness. More pronounced symptoms can involve fainting spells, or syncope.
The condition may be a consequence of aging, underlying heart disease, or an imbalance of electrolytes. Certain medications, such as beta-blockers and calcium channel blockers used to treat high blood pressure, can also slow AV node conduction and contribute to the development of this block.
Treatment strategies are dictated by the specific type of block and the presence of symptoms. For Mobitz Type I, if a person is not experiencing symptoms, the condition may require observation and regular monitoring. If a reversible cause, such as a medication, is identified, adjusting or discontinuing the drug may resolve the issue. However, if the patient is symptomatic or if the block occurs after a heart attack, intervention is required.
Mobitz Type II necessitates a proactive approach due to the higher risk of progression to a complete heart block. Patients with symptomatic Type I or those diagnosed with Type II are evaluated for the implantation of a permanent pacemaker. This small device delivers electrical impulses to the heart muscle, ensuring a consistent and appropriate heart rate and rhythm. Pacemaker placement manages high-risk or symptomatic second-degree AV blocks.