What is the correct order of the intrinsic conduction system?

Have you ever wondered what causes your heart to beat? Your heart beats as a result of the generation and conduction of electrical impulses. Cardiac conduction is the rate at which the heart conducts electrical impulses. These impulses cause the heart to contract and then relax. The constant cycle of heart muscle contraction followed by relaxation causes blood to be pumped throughout the body. Cardiac conduction can be influenced by various factors including exercise, temperature, and endocrine system hormones.

The first step of cardiac conduction is impulse generation. The sinoatrial (SA) node (also referred to as the pacemaker of the heart) contracts, generating nerve impulses that travel throughout the heart wall. This causes both atria to contract. The SA node is located in the upper wall of the right atrium. It is composed of nodal tissue that has characteristics of both muscle and nervous tissue.

The atrioventricular (AV) node lies on the right side of the partition that divides the atria, near the bottom of the right atrium. When the impulses from the SA node reach the AV node, they are delayed for about a tenth of a second. This delay allows atria to contract and empty their contents into the ventricles prior to ventricle contraction.

The impulses are then sent down the atrioventricular bundle. This bundle of fibers branches off into two bundles and the impulses are carried down the center of the heart to the left and right ventricles.

At the base of the heart, the atrioventricular bundles start to divide further into Purkinje fibers. When the impulses reach these fibers they trigger the muscle fibers in the ventricles to contract. The right ventricle sends blood to the lungs via the pulmonary artery. The left ventricle pumps blood to the aorta.

Cardiac conduction is the driving force behind the cardiac cycle. This cycle is the sequence of events that occur when the heart beats. During the diastole phase of the cardiac cycle, the atria and ventricles are relaxed and blood flows into the atria and ventricles. In the systole phase, the ventricles contract sending blood to the rest of the body.

Disorders of the heart's conduction system can cause problems with the heart's ability to function effectively. These problems are typically the result of a blockage that diminishes the rate of speed at which impulses are conducted. Should this blockage occur in one of the two atrioventricular bundle branches that lead to the ventricles, one ventricle may contract more slowly than the other. Individuals with bundle branch block typically don't experience any symptoms, but this issue can be detected with an electrocardiogram (ECG). A more serious condition, known as heart block, involves the impairment or blockage of electrical signal transmissions between the heart's atria and ventricles. Heart block electrical disorders range from first to third degree and are accompanied by symptoms ranging from light-headedness and dizziness to palpitations and irregular heartbeats.

The heart is, in the simplest terms, is a pump made up of muscle tissue. Like all pumps, the heart requires a source of energy and oxygen in order to function. The heart’s pumping action comes from an intrinsic electrical conduction system.

Parts of the Electrical System

Your heart's electrical system includes 3 important parts:

1. S-A node (sinoatrial node)-known as the hearts natural pacemaker. The S-A node initiates each heartbeat.
2. A-V node (atrioventricular node) - the bridge between the atria and the ventricles. Electrical signals are passed from the atria down to the ventricles through the A-V node.
3. His-Purkinje system carries the electrical signals throughout the ventricles. The His-Purkinje system consists of the following parts:

• His Bundle (the start of the system).
• Right Bundle Branch.
• Left Bundle Branch (2 Tracts).
• Purkinje fibres (the end of the system).

The Path of an Electrical Signal

A system of electrical pathways in your heart connects one part to another the S-A node to the A-V node for instance.

This movement of the signals causes the heart to contract (or beat) and relax. The number of electrical signals controls the speed of your heartbeat. The more signals passing through the heart, the faster the heartbeat.

Usually, 60-100 signals per minute travel these pathways. This equates to a heartbeat of 60-100 beats a minute.

Step 1. The S-A node creates an electrical signal.

Step 2. The electrical signal follows the natural electrical pathways through both atria. The movement of electricity causes the atria to con-tract, pushing blood into the ventricles.

Step 3. The electrical signal reaches the A-V node. There, the signal pauses to give the ventricles time to fill with blood.

Step 4. The electrical signal spreads through the His-Purkinje system. The movement of electricity causes the ventricles to contract, pushing blood to your lungs and body.

Note that the atria contract a fraction of a second before the ventricles do. After your heart contracts, it relaxes for a moment before the process begins again.

When working correctly, your conduction system automatically responds to your body's changing need for oxygen:

• When you climb stairs, carry heavy groceries, or take a walk, you need more oxygen, therefore your heart beats at a faster heart rate.
• When you are sitting or sleeping, you need less oxygen, therefore your heart beats at a slower rate.

Potential Problems with the Conduction System

Sometimes there are problems with the electrical conduction system:

• The S-A node doesn’t produce the right number of signals.
• Another part of your heart takes over as the natural pacemaker.
• The electrical pathways are interrupted.
• Sometimes the heart beats too slowly or too rapidly. Heartbeats that are too fast or too slow are called arrhythmias.

Slow Arrhythmias

When the heart beats too slowly it is called bradycardia (brady=slow cardia=heart). Slow arrhythmias can be a problem because they cause the oxygen and nutrient rich blood to travel more slowly to your organs. Your body may not receive enough oxygen to function properly, often making you dizzy or breathless.

Fast Arrhythmias

When the heart beats too fast it is called tachycardia (tachy=fast, cardia=heart). During tachycardia, the heart is not able to pump blood to the body as well as it should. Fast rhythms in the upper chambers may not be life-threatening, but may contribute to other problems that are more serious. Fast arrhythmias in the lower chambers, the ventricles, can be very dangerous, even fatal.

What causes these problems?

Interruptions in the pathways can occur for a number of reasons:

• Heart disease causes changes in the heart tissue.
• Ageing of the heart muscle can also change the heart tissue.
• Infection and scarring.
• Physical problems such as diabetes, smoking, high blood pressure, and excessive alcohol or drug use, can affect the heart tissue.
• Inherited heart problems.
• Evidence of heart failure or a heart attack.

People who have had a heart attack or have heart failure often have hearts that don’t pump blood as well as they should. The Ejection Fraction (EF) is a measurement of how well the heart is pumping. A low EF also may mean that these people are at a higher risk for arrhythmia.

Updated by: Thomas S. Metkus, MD, Assistant Professor of Medicine and Surgery, Johns Hopkins University School of Medicine, Baltimore, MD. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.