The heart functions as a muscular pump, circulating blood to deliver oxygen and nutrients. Its continuous pumping relies on a sophisticated internal electrical system that coordinates its rhythm. This network ensures the heart’s chambers contract in sequence for efficient blood flow.
The Heart’s Electrical System and Its Pacemakers
The heart possesses specialized cells capable of generating their own electrical impulses, acting as natural pacemakers. The primary pacemaker is the sinoatrial (SA) node, located in the upper right atrium. This cluster of cells initiates the electrical signal for each heartbeat, setting the heart’s rhythm.
The electrical impulse from the SA node travels across the atria, causing them to contract. These signals converge at the atrioventricular (AV) node, which serves as a relay station between the heart’s upper and lower chambers. While the SA node is the dominant pacemaker, the AV node can also generate electrical impulses, functioning as a secondary or backup pacemaker.
Defining the AV Node’s Intrinsic Rate
The atrioventricular node is a compact structure found in the lower interatrial septum, positioned near the opening of the coronary sinus. Its “intrinsic rate” refers to the natural pace at which it generates electrical impulses if it were to act as the sole pacemaker, without receiving signals from the SA node. This inherent rate for the AV node ranges from 40 to 60 beats per minute (bpm).
This intrinsic rate is considerably slower than the SA node’s inherent pace, which generates impulses at 100 to 110 bpm. The SA node’s cells have a faster rate of spontaneous depolarization, meaning they reach the threshold for firing an electrical impulse more quickly than AV nodal cells. Consequently, the faster SA node normally overrides the slower intrinsic rhythm of the AV node and other potential pacemaker sites.
The AV Node’s Dual Role in Heart Function
Beyond its capacity as a backup pacemaker, the AV node performs two primary functions. Its most prominent role is to introduce a brief delay in the electrical signal as it travels from the atria to the ventricles. This delay, lasting approximately 0.09 to 0.12 seconds, is essential for coordinated heart function.
The delay ensures that the atria have sufficient time to fully contract and empty their blood into the ventricles before the ventricles begin their own contraction. Without this pause, the ventricles might contract prematurely, reducing the amount of blood they can pump out to the body. This coordinated action maximizes the heart’s pumping efficiency and helps prevent irregular heart rhythms. The AV node also acts as a protective gate, preventing excessively rapid electrical signals from the atria from reaching the ventricles, a property known as decremental conduction.
When the AV Node Sets the Heart’s Pace
Under certain conditions, the AV node’s intrinsic rate can become the dominant heart rhythm, leading to what is known as an AV nodal rhythm or junctional rhythm. This occurs when the SA node, the heart’s primary pacemaker, fails to generate impulses or generates them too slowly. It can also happen if the electrical signals from the SA node are blocked and cannot reach the ventricles, a condition referred to as heart block.
When the AV node takes over, it paces the heart at its inherent slower rate of 40 to 60 bpm. While this rhythm provides a life-sustaining heartbeat, this slower rate might not adequately meet the body’s metabolic demands, especially during physical activity. Individuals experiencing an AV nodal rhythm might exhibit symptoms such as dizziness or fatigue due to reduced blood flow.