Heart rate regulation and electrocardiograms
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Explainer Video
Summary
Heart rate regulation and electrocardiograms
In a nutshell
The heartbeat is regulated by the heart itself, through the sino-atrial node and the atrioventricular nodes. The passage of electrical impulses through the heart can be monitored by electrocardiographs and these can be studied to diagnose heart problems.
Heart rate regulation
The heart is myogenic. This means it contracts and relaxes independently of any nerve signalling. The process of heartbeat regulation is explained below.
|
| A. | The sino-atrial node (SAN) is located in the wall of the right atrium. The SAN generates an electrical impulse that results in a wave of depolarisation spreading over the atria. Note: The SAN is the pacemaker of the heart as it sets the rhythm of the heartbeat. |
| B. | The depolarisation of the atria causes them to contract at the same time (atrial systole). |
| C. | Blood is forced into the relaxed ventricles. |
| D. | Collagen in the heart will prevent the waves of electric charge from being passed to the ventricles. |
| E. | The waves of electrical charge are transferred from the SAN to the atrioventricular node (AVN). |
| F. | After a short delay, the AVN passes the electric charge to the bundle of His. |
| G. | The slight delay ensures the ventricles contract after the atria have emptied. |
| H. | The electrical impulse passes down the heart apex through the Purkyne fibres in the bundle of His. |
| I. | The electrical impulse travels along the Purkyne fibres which branch out into the ventricular muscle. This causes the contraction of the ventricles (ventricular systole). The contraction occurs from the apex and travels up in the direction of the atria. |
| J. | Blood is pushed up and out of the heart through the aorta and pulmonary artery. |
Electrocardiograph
Definition
An electrocardiograph is a machine that measures the electrical activity of the heart. The trace produced by an electrocardiograph is called an electrocardiogram (ECG). Electrodes are placed on the chest to record changes in electrical charge.
When the heart muscle contracts it depolarises. This means that it loses electrical charge. However, when the heart muscle relaxes, it repolarises and regains charge. This can be shown on the ECG and it is explained below.
| | Part of the trace | Description |
| 1. | P wave | Atrial depolarisation and contraction (atrial systole). |
| 2. | QRS complex | A wave of depolarisation resulting in ventricular contraction (ventricular systole). |
| 3. | T wave | Repolarisation (relaxation) of the ventricles during diastole. |
Note: A higher peak indicates more electrical charge is passing through the heart.
A healthy ECG trace is shown below.
Diagnosing heart problems
ECGs can be compared to diagnose heart problems. Some of these are described below.
Heart problem | Description | |
| Tachycardia | Tachycardia is a heart rate of more than (beats per minute). During exercise, this is expected. However, a high resting rate could indicate anxiety or fever. Tachycardia is also associated with coronary heart disease, heart failure and anaemia. |  |
| Bradycardia | Bradycardia is a heart rate lower than . This may indicate a high athletic fitness. However, it can also be associated with heart disease. |  |
| Ectopic heartbeat | An ectopic heartbeat is an extra heartbeat (1.). This may be caused by premature contraction of the atria or the ventricles. |  |
| Fibrillation | The normal electrical activity and rhythm of the heart is disrupted and the heart stops contracting properly. This may cause chest pain or in more serious cases, the loss of a pulse. |  |
FAQs - Frequently Asked Questions
What is an ectopic heartbeat?
An ectopic heartbeat is an extra heartbeat. This may be caused by premature contraction of the atria or the ventricles.
What is an electrocardiograph?
An electrocardiograph is a machine that measures the electrical activity of the heart.
What is tachycardia?
Tachycardia is a heart rate of more than 100 bpm (beats per minute). During exercise, this is expected. However, a high resting rate could indicate anxiety or fever. Tachycardia is also associated with coronary heart disease, heart failure and anaemia.