Otis Singleton
The human heart is a fascinating organ, with its own built-in electrical system that controls the heartbeat. This electrical system is known as the cardiac conduction system, and it is made up of a network of nodes, cells and signals that work together to keep the heart beating regularly and efficiently. The heart's electrical system is essential to its function, controlling the heart rate and the contraction of cardiac muscle. Each time an electrical impulse completes a circuit, the heart beats, and the number of impulses determines the heart rate. The heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers, ensuring blood is pumped efficiently around the body.
| Characteristics | Values |
|---|---|
| Origin of electrical impulse | Sinus node (also called the sinoatrial node or SA node) |
| Sinus node location | Upper right chamber (atrium) of the heart |
| Sinus node function | Generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions |
| Electrical impulse pathway | From the sinus node to the atrioventricular node (AV node) |
| AV node function | Slows down impulses for a very short period before continuing down the conduction pathway |
| Conduction pathway | Bundle of His into the ventricles, dividing into right and left bundle branches |
| Electrical impulse effect | Causes the heart's ventricles to contract and pump out blood |
| Heart rate control | Electrical impulses determine the heart rate, with each impulse generating one heartbeat |
| Heart contraction sequence | Atria contract first for a short period before ventricles contract, ensuring blood empties into ventricles |
| Heart block | Conduction disorder where electrical signals are weakened or blocked from moving from atria to ventricles |
| Bradycardia | Slow heartbeat caused by conditions like heart block or an irregular sinus node |
| Tachycardia | Abnormally fast heartbeat due to an extra electrical pathway or abnormal "focus" acting like a second sinus node |
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What You'll Learn
The sinus node
Sinus node dysfunction (SND), also known as sick sinus syndrome, can lead to pathological bradycardia and asystolic pauses. It is often associated with atrial tachyarrhythmias such as atrial fibrillation. SND can be diagnosed using electrocardiographic features and is the most common reason for pacemaker implantation.
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Electrical impulses and heart rate
The heart's electrical system, also known as the cardiac conduction system, is essential to its function, controlling the heart rate and the contraction of the heart muscle. The heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers.
The electrical impulse originates in the sinus node, a small mass of specialized tissue located in the right upper chamber (atria) of the heart. This is referred to as the heart's natural pacemaker. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. This electrical pulse then travels down through the conduction pathways, causing the heart's ventricles to contract and pump out blood. The right and left atria are stimulated first and contract to push blood from the atria into the ventricles. As the electrical impulse passes through the atria, it generates the "P" wave on an EKG.
The electrical impulse then reaches the AV node, where it is slowed down for a very short period. This allows the atria to contract a fraction of a second before the ventricles, so their blood empties into the ventricles before the ventricles contract. After passing through the AV node, the electrical current continues down the conduction pathway, through a pathway called the bundle of His, and into the ventricles. The bundle of His divides into right and left pathways (bundle branches) to give electrical stimulation to the right and left ventricles. As the electrical signal travels through the ventricles, it generates the "QRS complex" on the EKG.
The electrical system of the heart causes the heart muscle to contract and send blood to the organs of the body (via the left ventricle) and to the lungs (via the right ventricle). Each time the impulse completes a circuit, the heart beats. This electrical conduction system keeps up a steady, even heart rate and helps the heart speed up or slow down depending on the body's needs.
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The cardiac conduction system
The SA node generates electrical impulses regularly, typically between 60 to 100 times per minute under normal conditions. This rate is influenced by the autonomic nervous system. The sympathetic nervous system increases the firing rate of the SA node, leading to an increased heart rate, while the parasympathetic nervous system has the opposite effect, decreasing the heart rate.
As the electrical impulse passes through the atria, it generates a "P" wave on an EKG, which can be observed as a solid red line on the left side of the image. When the impulse reaches the AV node, or atrioventricular node, it is slowed down briefly before continuing down the conduction pathway. This brief pause in electrical activity is referred to as the PR interval on the EKG.
From the AV node, the electrical impulse travels through the bundle of His and Purkinje fibres, spreading the wave impulses along the ventricles and causing them to contract. This contraction of the ventricles results in the generation of the "QRS complex" on the EKG. The electrical system of the heart ensures that the heart muscle contracts in a coordinated manner, sending blood to the organs of the body via the left ventricle and to the lungs via the right ventricle.
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Heart palpitations and arrhythmia
The heart's electrical system controls the heartbeat. Electrical impulses trigger heartbeats, and each impulse generates one heartbeat. The sinus node, a small mass of specialized tissue located in the right upper chamber (atria) of the heart, generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. This causes the heart's ventricles to contract and pump out blood.
However, sometimes the heart's electrical system may not work properly due to heart disease, certain medications, or unknown reasons. Changes in the heart's electrical system can lead to abnormal heart rhythms, known as arrhythmias. Arrhythmias can cause the heart to beat too fast (tachycardia) or too slow (bradycardia), or have a varied rhythm.
Heart palpitations are a sensation of the heart racing, thumping, fluttering, pounding, or skipping beats. They are often short-lived and occasional palpitations that do not affect one's general health and are usually not a cause for concern. However, if palpitations occur more frequently or consistently, they may be associated with a serious abnormal heart rhythm and require medical attention.
Palpitations can be triggered by various factors, including stimulants such as caffeine, nicotine, cocaine, amphetamines, and certain medications. They can also be caused by emotional or physical stress. While occasional palpitations are normal, if they are accompanied by symptoms such as chest pain, discomfort, nausea, cold sweats, dizziness, or shortness of breath, it is important to seek immediate medical assistance.
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Electrocardiograms (ECG/EKG)
Electrocardiograms (ECG or EKG) are a simple, quick, and non-invasive way to evaluate the heart's electrical activity. The test involves placing electrodes (small, sticky plastic patches) on the skin of the chest, arms, and legs. These electrodes are connected to an ECG machine via lead wires, and the electrical activity of the heart is then measured, interpreted, and printed out. The procedure is straightforward and carries minimal risks. It is performed by trained medical personnel such as technicians, nurses, or doctors, and the results are interpreted by clinicians, often with confirmation from a cardiologist.
The origins of the ECG can be traced back to 1902 when Dutch physician Willem Einthoven invented the technique. Einthoven, sometimes referred to as the "father of electrocardiography", was awarded the Nobel Prize in Medicine in 1924 for his groundbreaking work. The development of the ECG has led to its use as a diagnostic and screening tool, making it the gold standard for diagnosing various cardiac diseases.
During an ECG, the patient lies flat on a table or bed, remaining still and quiet to ensure accurate results. The procedure is typically performed in a temperature-controlled environment to prevent shivering, and the patient wears a gown to expose the necessary skin for electrode placement. Before the test, it is important to disclose any allergies to adhesive gel, and chest hair may need to be shaved or clipped to ensure proper adhesion of the electrodes.
The ECG itself is a brief process, and the patient will not feel anything during the recording. Once the electrodes are attached and connected, the machine records the electrical impulses generated by the heart. These impulses are then visualized as waves on the ECG readout, with distinct patterns like the "P" wave and "QRS complex" providing valuable information about the heart's function.
ECGs are an invaluable tool for assessing cardiac function and diagnosing heart-related issues. They are often used as a baseline for future comparisons and play a crucial role in the initial evaluation of patients suspected of having cardiac problems.
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Frequently asked questions
Electrical impulses trigger heartbeats. Each time an impulse completes a circuit, the heart beats.
In a normal sinus rhythm, the sinus node generates an electrical impulse 60 to 100 times per minute.
The number of electrical impulses determines the heart rate. The electrical conduction system regulates blood flow by controlling the heartbeat and contraction of the heart muscle.
The electrical impulse originates in the sinus node, which is located in the right upper chamber (right atrium) of the heart. From there, it spreads across the right and left atria, causing them to contract and push blood into the ventricles. The impulse then travels through the AV node, where it is slowed down, before continuing into the ventricles.
An extra electrical pathway between the atria and ventricles can cause an abnormally fast heartbeat (tachycardia). This occurs when the electrical impulse makes a continuous loop, flowing down the normal path and then returning to the atrium along the extra pathway.
