The heart’s coordinated pumping action relies on a precise internal electrical system. This article clarifies the heart’s normal electrical activity and explains a specific variation known as first-degree atrioventricular (AV) block.
The Heart’s Natural Pacemaker and Sinus Rhythm
The heart’s electrical activity originates in a specialized cluster of cells located in the upper right chamber, called the sinoatrial (SA) node. This SA node acts as the heart’s natural pacemaker, initiating each heartbeat. These impulses then spread across the upper chambers, the atria, causing them to contract and push blood into the lower chambers.
Following atrial contraction, the electrical signal travels to the atrioventricular (AV) node, situated between the atria and ventricles. From the AV node, the impulse moves through a network of specialized fibers, including the bundle of His and Purkinje fibers, which rapidly distribute the signal throughout the ventricles. When the SA node sets the rhythm and the electrical signals follow this normal pathway, it is known as sinus rhythm. This regular pattern typically results in a heart rate between 60 and 100 beats per minute in adults.
Understanding First Degree AV Block
The atrioventricular (AV) node plays a role in coordinating the heart’s contractions by introducing a brief, natural delay in the electrical signal. This momentary pause allows the atria to fully empty their blood into the ventricles before the ventricles begin their contraction. This sequential pumping action is important for the heart’s overall pumping effectiveness.
In first-degree AV block, the electrical signal experiences a longer-than-usual delay as it passes through the AV node or other parts of the heart’s conduction system. Despite this prolonged delay, every electrical impulse originating from the atria successfully reaches the ventricles. This means no beats are skipped or dropped; the heart continues its regular beat-to-beat pattern, just with an extended pause between atrial and ventricular activity.
This condition is identified on an electrocardiogram (ECG) by a specific measurement called the PR interval. The PR interval represents the time it takes for an electrical impulse to travel from the atria through the AV node to the ventricles. In first-degree AV block, this PR interval is consistently longer than the normal range, typically exceeding 0.20 seconds (200 milliseconds). When the PR interval is notably prolonged, such as beyond 0.30 seconds (300 milliseconds), it is sometimes referred to as a “marked” first-degree AV block.
Identifying First Degree AV Block and Its Causes
First-degree AV block is most frequently discovered incidentally during a routine electrocardiogram (ECG). Often, individuals are unaware they have this condition until an ECG is performed for another reason.
Various factors can contribute to the development of first-degree AV block. It can sometimes be a normal variation, particularly in individuals with increased vagal tone, such as highly conditioned athletes. Aging can also play a role, as the heart’s electrical conduction system can undergo changes and develop some scarring over time.
Certain medications are known to slow electrical conduction through the AV node, including beta-blockers, calcium channel blockers, digoxin, and amiodarone. Underlying heart conditions like coronary artery disease, a heart attack, or inflammation of the heart muscle (myocarditis) can also contribute. Other potential causes include imbalances in electrolytes, infections such as Lyme disease or rheumatic fever, and some systemic conditions like rheumatoid arthritis or sarcoidosis.
What First Degree AV Block Means for You
For most individuals, first-degree AV block causes no noticeable symptoms and is discovered incidentally during a medical examination. Traditionally, this condition has been considered generally benign, meaning it rarely leads to more serious heart problems or requires specific medical intervention. It is uncommon for isolated first-degree AV block to progress to higher-degree heart blocks.
However, some studies indicate that a prolonged PR interval, especially if it is longer than 0.20 or 0.30 seconds, might be associated with slightly increased risks over time. These potential associations include a greater likelihood of developing atrial fibrillation, needing a pacemaker, or experiencing a small increase in overall mortality. Despite these associations, most individuals with first-degree AV block continue to live normal lives without significant complications.
Monitoring or further evaluation may be considered if symptoms like fatigue, dizziness, lightheadedness, palpitations, or chest pain develop. These symptoms can sometimes arise if the delay is very pronounced or if other heart conditions are present. In many cases, no specific treatment is necessary for first-degree AV block itself. Regular medical check-ups are recommended to monitor the heart’s rhythm and overall health, particularly if medications that affect heart conduction are being taken.