The heart functions as a muscular pump, circulating oxygenated blood throughout the body. Its continuous pumping action is regulated by an intricate electrical conduction system, where specialized cells generate and transmit signals to coordinate chamber contractions.
A consistent heartbeat ensures all organs receive necessary blood supply. Deviations from this normal rhythm can affect the heart’s pumping ability. This article explores second-degree AV block, a specific heart rhythm disturbance.
Understanding Second-Degree AV Block
An atrioventricular (AV) block occurs when electrical signals from the atria (upper chambers) are delayed or interrupted before reaching the ventricles (lower chambers). Normally, impulses originate in the sinoatrial (SA) node, travel through the atria, and pause at the AV node. This pause allows the atria to fill the ventricles before ventricular contraction.
Second-degree AV block means some, but not all, atrial electrical signals fail to reach the ventricles, causing skipped heartbeats. This condition has two main types: Mobitz Type I (Wenckebach) and Mobitz Type II. The difference lies in the pattern of signal blockage.
Mobitz Type I (Wenckebach) is characterized by a progressive lengthening of the PR interval on an electrocardiogram (ECG) until a beat is dropped. The PR interval measures the time for an electrical impulse to travel from the atria to the ventricles. This delay increases with successive beats until an atrial impulse fails to conduct, resulting in a dropped QRS complex.
In contrast, Mobitz Type II AV block shows a consistent PR interval for conducted beats, followed by a sudden, abrupt dropped QRS complex. This occurs when an atrial impulse fails to conduct to the ventricles without prior PR interval lengthening. Mobitz Type II indicates a more significant disruption in the heart’s electrical pathway, often below the AV node. It also carries a higher risk of progressing to a complete heart block.
Causes and Symptoms
Second-degree AV block can result from various factors affecting the heart’s electrical conduction system. Common causes include age-related degeneration of electrical pathways, making signal transmission less efficient. Underlying heart conditions like coronary artery disease or a prior heart attack can also contribute.
Certain medications, such as beta-blockers and calcium channel blockers, can interfere with heart signals and induce AV block. Electrolyte imbalances, particularly abnormal potassium or calcium levels, can disrupt heart cell electrical stability. Inflammatory conditions affecting the heart, like myocarditis, may also impair the conduction system.
Symptoms of second-degree AV block vary. Some individuals, especially with Mobitz Type I, may have no noticeable symptoms, with the condition discovered during routine exams. When symptoms occur, they often relate to the heart’s reduced ability to pump sufficient blood.
Common symptoms include dizziness or lightheadedness from decreased blood flow to the brain. Individuals may also feel unusually tired or fatigued. Shortness of breath can occur, especially during physical activity, as the heart struggles to meet oxygen demands. Chest discomfort or palpitations, like skipped heartbeats, are possible. In severe cases, fainting (syncope) may occur due to significant blood pressure drops from prolonged heart pauses.
Diagnosis and Treatment Approaches
Diagnosing second-degree AV block primarily involves evaluating the heart’s electrical activity. An electrocardiogram (ECG or EKG) is the main diagnostic tool, recording electrical signals to observe characteristic patterns of Mobitz Type I or Mobitz Type II. Distinct ECG findings, like progressive PR interval lengthening in Mobitz Type I or sudden dropped beats in Mobitz Type II, help differentiate the types.
For intermittent blocks not consistently present on a standard ECG, continuous monitoring devices are used. Holter monitors record heart activity over 24 to 48 hours, while event recorders can be worn longer, activated by the patient during symptoms. These devices capture irregular rhythms missed during brief office visits. Additional diagnostic tests, such as blood tests or stress tests, may identify underlying causes like electrolyte abnormalities or coronary artery disease.
Treatment for second-degree AV block is individualized, depending on the block type, symptom severity, and underlying causes. Asymptomatic individuals with Mobitz Type I and no other significant heart conditions may not require active treatment, often adopting watchful waiting and regular monitoring.
Mobitz Type II often requires proactive management, even without severe symptoms, due to its higher risk of progressing to complete heart block. In these cases, a permanent pacemaker is a common and effective treatment. A pacemaker is a small device surgically placed under the skin that sends electrical impulses to maintain a regular heart rhythm when the natural conduction system fails. Managing underlying conditions, such as optimizing coronary artery disease treatment or adjusting contributing medications, is also important.