A heart block, also known as an atrioventricular (AV) block, is a disruption or delay in the electrical signal traveling from the heart’s upper chambers (atria) to its lower chambers (ventricles). This interruption affects the heart’s coordinated pumping action, often leading to a slower or irregular heartbeat. Recognizing the symptoms and understanding the underlying electrical issue is the first step toward diagnosis, which is confirmed through medical testing.
Understanding Cardiac Electrical Signaling
The heart’s rhythm is controlled by an internal electrical system that ensures the four chambers contract in a coordinated sequence. The process begins in the Sinoatrial (SA) node, the heart’s natural pacemaker, which generates electrical impulses causing the atria to contract. This impulse then travels to the Atrioventricular (AV) node, which acts as a gateway for the signal to pass from the atria to the ventricles.
The AV node slows the signal down for a fraction of a second, allowing the ventricles time to fill with blood before they contract and pump blood out. After this brief delay, the signal continues through the Bundle of His and Purkinje fibers, causing the ventricular contraction. A heart block occurs when damage or interference within the AV node or structures following it causes the electrical message to be delayed or stopped entirely.
Recognizing Common Warning Signs
The symptoms experienced depend directly on the severity of the electrical block and the resulting heart rate. Since a heart block causes the heart to pump blood less efficiently, a frequently reported symptom is fatigue and weakness. This happens because the body’s organs and muscles do not receive an adequate supply of oxygen-rich blood.
Many people feel dizzy or lightheaded, often when standing up quickly, due to insufficient blood flow reaching the brain. A more severe manifestation is syncope, or fainting, which occurs if the heart rate drops suddenly or significantly. Other common signs include shortness of breath, especially during physical exertion, and palpitations, such as a skipped, fluttering, or unusually slow heartbeat.
The Three Degrees of Heart Block
Heart blocks are categorized into three degrees based on the severity of the delay or interruption in the electrical signal. First-degree heart block is the least severe, characterized by a consistent, prolonged delay as the electrical signal passes through the AV node. In this degree, every atrial impulse successfully reaches the ventricles, but the conduction time exceeds the normal 0.20 seconds. This condition rarely causes noticeable symptoms, typically does not require treatment, and is often discovered incidentally during an electrocardiogram (ECG).
Second-degree heart block involves an intermittent failure of the electrical signal to reach the ventricles, causing a dropped beat. This degree is divided into two types: Mobitz Type I and Mobitz Type II.
Mobitz Type I (Wenckebach’s Block)
Mobitz Type I, also known as Wenckebach’s block, involves a progressive pattern. The electrical delay lengthens with each successive beat until one beat fails to conduct entirely, skipping a ventricular contraction. This cycle then resets and repeats. While it can cause dizziness or fainting, it is generally considered the less serious type of second-degree block.
Mobitz Type II
Mobitz Type II is a more serious condition where the electrical delay remains constant. The ventricles suddenly and unpredictably fail to receive the impulse, resulting in an unexpected dropped beat. Unlike Type I, there is no progressive lengthening of the delay before the dropped beat occurs. This type carries a higher risk of progressing to a complete block and is more likely to cause symptoms such as lightheadedness, chest pain, and fainting.
Third-degree heart block, also called complete heart block, represents a total failure of electrical communication between the atria and the ventricles. In this severe state, no electrical signals from the atria are conducted to the lower chambers. The ventricles must rely on their own backup pacemaker cells, resulting in a very slow and independent rhythm, often below 40 beats per minute. This rate is insufficient to maintain proper blood circulation, making complete heart block an emergency situation requiring medical intervention.
Tools Doctors Use for Confirmation
The definitive identification of a heart block relies on the analysis of the heart’s electrical activity. The primary diagnostic tool is the Electrocardiogram, or ECG (also known as EKG), which records the heart’s electrical signals over a short period. The ECG allows a doctor to measure the P-R interval, which represents the time the electrical impulse takes to travel from the atria to the ventricles.
By analyzing the P-R interval and the relationship between the atrial P waves and the ventricular QRS complexes, the exact degree of block is determined. For instance, a first-degree block is confirmed by a consistent P-R interval longer than the normal upper limit of 0.20 seconds. In contrast, a third-degree block is identified by the complete lack of association between the P waves and the QRS complexes, meaning the atria and ventricles beat at entirely independent rates.
For blocks that occur intermittently or are triggered by specific activities, doctors employ secondary monitoring devices. A Holter monitor is a portable ECG device worn for 24 to 48 hours to continuously record every heartbeat during a patient’s normal daily routine. When symptoms are less frequent, an event recorder may be used for several weeks, which the patient activates only when they feel symptoms such as dizziness or palpitations. These longer-term recordings capture transient electrical disturbances necessary to confirm the diagnosis and determine treatment.