The Heart Conduction System
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:
- S-A node (sinoatrial node)-known as the hearts natural pacemaker. The S-A node initiates each heartbeat.
- 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.
- 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.
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.
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.