AV conduction is the process by which electrical signals travel from the upper chambers of your heart (the atria) to the lower chambers (the ventricles), passing through a small cluster of specialized cells called the atrioventricular node. This relay system ensures your heart’s chambers beat in the right sequence, with a brief, precisely timed pause that lets the upper chambers finish pumping blood downward before the lower chambers contract. When AV conduction works normally, the entire journey from the top of the heart to the bottom takes between 120 and 200 milliseconds.
How the Signal Travels
Every heartbeat starts in the sinus node, a natural pacemaker at the top of the right atrium. This node fires an electrical impulse that spreads across both atria, causing them to contract and push blood into the ventricles. The impulse then reaches the AV node, which sits at the junction between the atria and ventricles.
The AV node is the only electrical bridge between the upper and lower heart. Once the signal passes through, it enters a pathway called the bundle of His, which splits into two branches: one for the right ventricle and one for the left. These branches fan out into a network of tiny fibers that deliver the signal to the muscle cells of both ventricles almost simultaneously, triggering a powerful, coordinated squeeze that pumps blood to the lungs and the rest of the body.
Why the Delay Matters
The AV node deliberately slows the electrical impulse for about 100 milliseconds before passing it along. This happens partly because the cells in the AV node are smaller in diameter than other conduction cells, which physically slows the signal. That fraction-of-a-second pause is critical: it gives the atria time to finish contracting and emptying their blood into the ventricles. Without this delay, the ventricles would start squeezing before they were fully loaded, and the heart would pump less blood with each beat.
On an electrocardiogram (EKG), this entire process shows up as the PR interval, the flat line between the P wave (atrial activity) and the QRS complex (ventricular activity). A normal PR interval ranges from 120 to 200 milliseconds. The average in healthy adults sits around 150 milliseconds.
What Controls Conduction Speed
Your nervous system constantly fine-tunes how fast signals move through the AV node. The vagus nerve, part of the parasympathetic (“rest and digest”) system, slows AV conduction and heart rate. This is why your heart naturally beats slower when you’re relaxed or sleeping. The sympathetic nervous system does the opposite: during exercise, stress, or a surge of adrenaline, it speeds conduction through the AV node so your ventricles can beat faster and deliver more blood to working muscles.
Several classes of medications also affect AV conduction. Beta-blockers and certain calcium channel blockers slow conduction through the AV node on purpose, which is useful for treating abnormally fast heart rhythms. A drug called adenosine can temporarily block AV conduction almost entirely, which doctors use to interrupt certain types of rapid heartbeats and reset the rhythm.
When AV Conduction Slows or Fails
Problems with AV conduction are classified into three degrees of severity, all called “heart block” despite having nothing to do with blocked arteries.
First-degree block means the signal still gets through every time, but it takes longer than normal. The PR interval stretches beyond 200 milliseconds. Most people with first-degree block have no symptoms and may never know about it unless an EKG picks it up.
Second-degree block means some signals fail to reach the ventricles entirely. There are two patterns. In the first (called Mobitz type I or Wenckebach), the delay gets progressively longer with each beat until one signal is dropped completely, then the cycle resets. This pattern is often benign. In the second pattern (Mobitz type II), signals drop without any preceding lengthening of the delay, which is more concerning because it tends to progress unpredictably.
Third-degree block, or complete heart block, means no signals get through at all. The atria and ventricles beat completely independently of each other. The atria keep firing at their normal rate, while the ventricles rely on a much slower backup pacemaker. This can drop the heart rate significantly and is a medical emergency in many cases.
What Causes AV Conduction Problems
The most common cause in older adults is gradual fibrosis, a stiffening and scarring of the conduction tissue that comes with aging. Heart attacks can also damage the AV node or the bundle branches, especially when blood flow to the bottom of the heart is interrupted. In people with heart failure, whether from coronary artery disease or other causes, AV conduction problems are common. Research has found that roughly half of sudden cardiac deaths in heart failure patients may be due to dangerously slow rhythms, including high-degree AV block.
Other causes include infections that inflame the heart muscle (myocarditis), autoimmune conditions like lupus, and congenital differences present from birth. Medications that slow AV conduction can also tip someone into heart block if the dose is too high or if there’s underlying conduction disease that hasn’t been diagnosed.
Symptoms of Impaired Conduction
When the heart beats too slowly because of AV conduction problems, the brain and other organs may not get enough oxygen. Common symptoms include dizziness or lightheadedness, unusual fatigue (especially during physical activity), shortness of breath, confusion or memory difficulty, and fainting or near-fainting episodes. Chest pain can also occur. Many people with mild conduction delays, particularly first-degree block, feel nothing at all.
How AV Block Is Treated
Treatment depends entirely on the type of block and whether it causes symptoms. First-degree block and Mobitz type I usually require no treatment, just monitoring. If a reversible cause is identified, like a medication, adjusting or stopping that drug may be all that’s needed.
For more serious forms, a permanent pacemaker is the standard treatment. Current guidelines recommend pacemaker implantation for anyone with Mobitz type II, high-grade, or third-degree AV block that isn’t caused by a reversible or temporary condition, regardless of whether they have symptoms. For other types of AV block, a pacemaker is generally considered only when symptoms clearly correlate with the conduction problem. The pacemaker monitors the heart’s rhythm continuously and delivers a small electrical pulse to trigger a heartbeat whenever the natural signal fails to arrive on time.