An atrioventricular (AV) block is an issue with the electrical transmission system that coordinates the heart’s chambers. First-degree heart block, the mildest form, is a consistent delay in the electrical signal’s travel time. The impulse successfully moves from the upper chambers to the lower chambers every time, but more slowly than in a typical heart rhythm. This condition is usually recognized during routine heart rhythm monitoring.
Understanding the Heart’s Electrical Circuit
The heart operates on a precise electrical schedule that dictates the timing of its contractions. This electrical activity begins in the sinoatrial (SA) node, which acts as the heart’s natural pacemaker, generating the initial impulse. The signal then spreads across the upper chambers, the atria, causing them to contract and push blood into the lower chambers, the ventricles.
For the ventricles to contract efficiently, the signal must pass through a specialized junction known as the atrioventricular (AV) node. This node functions as a gatekeeper, momentarily pausing the impulse to ensure the ventricles have completely filled with blood before they begin to pump. In first-degree heart block, the electrical signal experiences an extended pause at the AV node before continuing its journey. While the impulse is slowed, every single signal ultimately passes through the AV node to activate the ventricles, maintaining the heart’s regular rhythm.
What First-Degree Heart Block Feels Like
For the vast majority of people, first-degree heart block produces no noticeable physical sensations. The delay is typically so minor that it does not disrupt the heart’s overall pumping efficiency or the blood flow to the body. Due to this absence of symptoms, the condition is most often discovered by chance when a person undergoes an electrocardiogram (EKG) for an unrelated issue.
In rare cases, if the electrical delay is significantly prolonged, it may begin to affect the heart’s function. This “marked” delay can sometimes lead to a lack of proper synchronization between the atrial and ventricular contractions. When this occurs, some individuals may report subtle symptoms such as fatigue, slight dizziness, or reduced tolerance for exercise. These symptoms are uncommon and only tend to appear when the delay is extreme, usually exceeding 300 milliseconds.
The Defining EKG Signature
First-degree heart block is definitively identified using an electrocardiogram, which records the heart’s electrical activity. The EKG trace shows a repeating pattern, where the P wave represents the electrical activation of the atria, and the QRS complex represents the electrical activation of the ventricles. The time it takes for the impulse to travel from the atria to the ventricles is measured as the PR interval.
In a healthy adult heart, the PR interval usually measures between 120 and 200 milliseconds. First-degree heart block is diagnosed when this PR interval consistently measures longer than 200 milliseconds across the entire EKG recording. Despite this prolonged delay, the defining feature of this condition is that every P wave is always followed by a QRS complex. This one-to-one relationship confirms that the signal is only slowed, not blocked or dropped entirely.
Common Underlying Causes and Monitoring
One common cause is a high degree of physical fitness, where the heart is conditioned to beat more slowly due to enhanced vagal tone. Age-related changes can also cause the condition, as fibrosis or scarring can naturally occur within the heart’s electrical pathways over time.
Certain prescription medications are known to slow conduction through the AV node, including beta-blockers and calcium channel blockers, which are often used to manage blood pressure or heart rhythm issues. Other causes include prior heart attacks, infections like Lyme disease, or acute inflammatory conditions such as myocarditis. If a medication is the cause, adjusting the dosage or stopping the drug can often resolve the condition.
Since first-degree heart block is usually benign and asymptomatic, the primary management strategy is simple observation. Routine follow-up is generally recommended to monitor the heart rhythm and ensure the condition does not progress. Treatment, such as the implantation of a pacemaker, is almost never required unless the PR interval is severely prolonged and causing debilitating symptoms. Although some studies suggest a slightly increased risk for developing atrial fibrillation or higher-grade blocks, the overall outlook for patients with asymptomatic first-degree heart block remains favorable.