The cardiac conduction system is a network of specialized cells that generates and carries electrical signals through the heart, triggering each heartbeat in a precise sequence. It works like built-in wiring: one group of cells creates an electrical impulse, and that impulse travels along a specific path to make the upper and lower chambers of the heart contract in the right order, about 60 to 100 times per minute at rest.
The Pathway From Top to Bottom
Every heartbeat begins in a small cluster of cells in the upper right chamber of the heart called the sinoatrial (SA) node. This node continuously fires electrical impulses on its own, which is why it’s called the heart’s natural pacemaker. The impulse spreads across both upper chambers (the atria), causing them to contract and push blood down into the lower chambers (the ventricles).
The signal then arrives at a second cluster called the atrioventricular (AV) node, located between the upper and lower chambers. Here, something critical happens: the signal pauses. This delay lasts roughly 120 to 200 milliseconds and exists for a specific reason. The atria need time to finish squeezing blood into the ventricles before the ventricles contract. Without this pause, the upper and lower chambers would fire almost simultaneously, and the heart would pump far less efficiently.
After the AV node, the signal enters a thick cable of fibers called the bundle of His, which splits into two bundle branches, one running down the left side of the heart and one down the right. These branches fan out into a web of tiny fibers called Purkinje fibers that spread across the walls of both ventricles. Purkinje fibers conduct electricity extremely fast, about 2 to 3 meters per second, compared to just 0.3 to 0.4 meters per second in regular heart muscle. That speed ensures both ventricles contract almost simultaneously in a coordinated squeeze from the bottom up, pushing blood out to the lungs and the rest of the body.
How the Heart Generates Its Own Electricity
The SA node doesn’t need a signal from the brain to fire. Its cells are self-excitable, meaning they automatically drift toward an electrical threshold and then discharge, over and over. This automatic rhythm comes from the movement of charged particles (ions) across cell membranes.
Three ions do most of the work: sodium, potassium, and calcium. Between beats, sodium slowly leaks into pacemaker cells through special channels, gradually raising the electrical charge inside the cell. Once the charge reaches a tipping point (around -40 millivolts), calcium channels snap open, calcium rushes in, and the cell fires. That’s the electrical impulse. Immediately after, potassium flows out of the cell, resetting the charge back down so the cycle can start again. This loop repeats continuously, setting the baseline heart rate.
Regular heart muscle cells work slightly differently. They don’t fire on their own. Instead, they wait for the impulse to arrive, then sodium rushes in much faster, producing a sharp spike in electrical activity. Calcium then flows in during a sustained plateau phase, which is what actually triggers the muscle fibers to shorten and contract. Potassium flowing back out ends the cycle and lets the muscle relax.
How Your Nervous System Adjusts the Pace
Although the SA node sets its own rhythm, the nervous system can speed it up or slow it down depending on what your body needs. Two opposing branches of the nervous system act on the heart like a gas pedal and a brake.
Sympathetic nerves (the “gas pedal”) release adrenaline-like chemicals that make the SA node fire faster, speed conduction through the AV node, and increase the force of each contraction. This is what happens during exercise, stress, or a scare.
The vagus nerve (the “brake”) does the opposite. It slows the firing rate of the SA node, slows conduction through the AV node, and reduces the force of contraction. Vagal activity can decrease the pumping strength of the left ventricle by around 20%, even when sympathetic influence is blocked entirely. When both systems are active at the same time, the vagus nerve tends to win, a phenomenon called accentuated antagonism. Parasympathetic signals suppress the effect of sympathetic signals at the cellular level, so the slowing effect is stronger than you’d expect from simple subtraction.
What the ECG Tells You
An electrocardiogram (ECG or EKG) is a direct recording of the conduction system’s activity from the skin’s surface. Each wave on the tracing corresponds to a specific electrical event inside the heart.
- P wave: A small, rounded bump representing the electrical activation of both atria, triggered by the SA node.
- PR interval: The flat line between the P wave and the next spike. This is the AV node delay, normally 120 to 200 milliseconds.
- QRS complex: A sharp, tall set of waves representing ventricular activation as the signal races through the bundle branches and Purkinje fibers. Because conduction is so fast here, the QRS complex is narrow, typically under 120 milliseconds.
- T wave: A broad wave that represents the ventricles resetting their electrical charge (repolarizing) in preparation for the next beat. Atrial repolarization happens too, but it’s hidden behind the much larger QRS complex.
Doctors use these wave patterns to pinpoint exactly where in the conduction system something has gone wrong.
What Happens When the System Fails
Problems in the conduction system generally fall into two categories based on where the disruption occurs.
AV Block
When the AV node or the bundle of His fails to pass signals properly, the result is an atrioventricular block. These range in severity. A first-degree block simply means the AV delay is longer than normal, stretching the PR interval beyond 200 milliseconds. Most people with first-degree block have no symptoms at all. Second-degree block means some signals get through and others don’t, so the heart occasionally skips a beat. Third-degree (complete) block means no signals pass from the atria to the ventricles. The ventricles then rely on backup pacemaker cells that fire much more slowly, often producing a dangerously low heart rate.
Bundle Branch Block
When one of the two bundle branches is damaged, the signal still reaches both ventricles, but one side activates later than the other. This shows up on an ECG as a widened QRS complex. A right bundle branch block or left bundle branch block can exist alone or in combination with other conduction problems. When a bundle branch block appears alongside a block in one of the smaller fascicles of the left branch, it’s called a bifascicular block, which can indicate more widespread conduction disease.
When a Pacemaker Becomes Necessary
Not every conduction problem needs treatment. Many are harmless and discovered incidentally. The decision to implant an artificial pacemaker depends on two things: the type of block and whether it causes symptoms like fainting, dizziness, or fatigue.
For SA node problems, there is no specific heart rate cutoff that automatically triggers a pacemaker recommendation. What matters is whether symptoms like lightheadedness or passing out can be clearly linked to the slow heart rate. If they can, a pacemaker is indicated.
AV block follows stricter rules. Second-degree Mobitz type II block, high-grade block, and complete (third-degree) block all warrant a permanent pacemaker regardless of whether the person feels symptoms, because these types carry a risk of sudden, dangerous pauses. For milder forms like first-degree block or Wenckebach (Mobitz type I), a pacemaker is only considered when symptoms are clearly tied to the conduction delay.
People with certain neuromuscular diseases, such as myotonic dystrophy, are treated more aggressively. Even moderate conduction delays in these conditions can progress unpredictably, so pacemaker implantation is recommended at lower thresholds than in the general population.