The heart operates through an electrical system that dictates its rhythm and pumping action. A “heart block” broadly refers to an interruption or delay in these electrical signals as they travel through the heart. This disruption means signals don’t move efficiently, potentially affecting heart chamber contraction.
The Heart’s Electrical Pathway
The heart’s journey of electrical signals begins in the sinoatrial (SA) node, often called the natural pacemaker. Located in the upper right chamber of the heart, the SA node generates electrical impulses that spread across the atria, causing them to contract and pump blood into the ventricles. These impulses then travel to the atrioventricular (AV) node, a relay station situated near the center of the heart. The AV node briefly delays the electrical signal, allowing the atria to fully empty their blood into the ventricles before they contract. This coordinated timing ensures efficient blood flow through the heart and to the rest of the body.
What First Degree Heart Block Reveals
First-degree heart block is typically not “seen” or felt by an individual, as it usually causes no symptoms. Instead, it is identified through an electrocardiogram (ECG or EKG), a common test that records the heart’s electrical activity. An ECG records the heart’s electrical impulses, allowing assessment of its rhythm and timing. In the context of first-degree heart block, the ECG specifically reveals an abnormally prolonged PR interval.
The PR interval on an ECG measures the time it takes for an electrical signal to travel from the atria, through the AV node, and into the ventricles. Normally, this interval ranges from 0.12 to 0.20 seconds (120 to 200 milliseconds). With first-degree heart block, this interval consistently measures greater than 0.20 seconds, indicating a delay in the electrical signal’s passage through the AV node. Despite this delay, every electrical impulse from the atria still reaches the ventricles, meaning no beats are skipped. This prolonged PR interval is the defining characteristic of first-degree heart block, representing a slowed but uninterrupted conduction.
Why First Degree Heart Block Happens
First-degree heart block can arise from various factors, and in many instances, it is considered a normal variation. For example, individuals who engage in regular intense athletic training often exhibit first-degree heart block due to increased vagal tone, which naturally slows heart conduction. Certain medications can also cause this condition, particularly those that affect the AV node, such as beta-blockers, calcium channel blockers, and cardiac glycosides. These drugs can slow the electrical impulses as a side effect.
Other underlying heart conditions may contribute to first-degree heart block, including damage from a heart attack, inflammation of the heart muscle (myocarditis), or certain types of heart disease. Electrolyte imbalances, such as abnormal levels of potassium or magnesium, can also disrupt the heart’s electrical system and lead to this delay. Additionally, as people age, fibrosis or scarring of the heart’s conduction system can occur, which may result in a first-degree heart block.
Understanding Its Importance
For most individuals, first-degree heart block is considered a benign condition that typically causes no symptoms and does not require specific treatment. It is frequently discovered incidentally during a routine ECG performed for other reasons. The condition rarely progresses to more severe forms of heart block, such as second- or third-degree heart block.
While specific treatment is generally unnecessary, ongoing monitoring may be recommended, especially if other heart conditions are present. In rare cases where the PR interval is significantly prolonged (e.g., greater than 0.30 seconds), some individuals might experience mild symptoms like lightheadedness or fatigue due to less efficient heart pumping. If new or concerning symptoms arise, or if there is a history of other heart issues, consulting a medical professional is important to determine if any intervention or further evaluation is needed.