An electrocardiogram (ECG) is a record of the heart's electrical activity, particularly its rhythm. It is one of the important biomedical signal. ECGs can be obtained in three different conditions: resting state, stress ECG during exercise, and ambulatory ECG for continuous recording over days or weeks. They are valuable for detecting arrhythmia, identifying myocardial infractions, assessing cardiomyopathy, and diagnosing coronary artery disease caused by the buildup of fatty substances that restrict blood flow.
To capture the heart's electrical activity, a standard 12-lead ECG machine is utilized, involving 10 electrodes. Six electrodes are placed on the chest, while four are positioned on the limbs (arms and legs). Other variations of ECG machines include single lead, three lead, and five lead ECG configurations. A sample ECG displaying QRS complex and T-waves is depicted in the picture below.
%20signal.jpg "electrocardiogram (ECG) signal")
Each heartbeat generates a recognizable pattern consisting of the P-wave, QRS complex, and T-wave. Normal values for different waves and intervals are as follows:
- P-wave: 80 ms
- QRS complex: 80-100 ms
- T-wave: 160 ms
- PR interval: 120-200 ms
- PR segment: 50-120 ms
- ST segment: 80-120 ms
- RR interval: 0.6-1.2 s
The electrical activity of the heart originates from the sino-atrial (SA) node, which acts as the heart's pacemaker. When the SA node generates an impulse, it rapidly spreads through the right atrium (RA) to the atrioventricular (AV) node. The impulse also travels to the left atrium (LA), causing atrial contraction and generating the P-wave, representing atrial depolarization. The time taken for the impulse to travel from the SA node to the AV node is known as the PR segment.
Upon reaching the AV node, the electrical signal continues through the bundle of His, the left and right bundle branches, and the Purkinje fibers, ultimately reaching the ventricular muscles. The first area of the ventricular muscles to be activated is the inter-ventricular septum, leading to the contraction of the lower chambers and resulting in the QRS complex. The QRS complex signifies the rapid depolarization of the right and left ventricles (RVs/LVs). Due to the RVs/LVs having larger muscle mass compared to the RA and LA, the QRS complex typically has a higher amplitude than the P-wave.
The ST segment reflects the time during which the ventricles remain depolarized. Following this, the ventricles start to re-polarize, causing the T-wave to occur.
ECG signals have various applications in diagnosing and monitoring heart conditions, including:
- Detecting suspected heart attacks (myocardial infractions).
- Identifying the presence of murmurs or seizures.
- Monitoring known cardiac arrhythmia.
- Facilitating medical monitoring during anesthesia.
- Enabling continuous or long-term monitoring using Holter monitors.
See also the following articles:
[1] what is Internet of Medical Things(IoMT)
[2] what is EMG sensor and how does it work?
[3] What is PPG(Photoplethysmography) sensor?
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