Shape and mechanism of wave formation on electrocardiogram

This is an automatically translated article.

The article was professionally consulted by Dr. Tran Quoc Tuan - Emergency Medicine Doctor - Emergency Resuscitation Department - Vinmec Phu Quoc International General Hospital. Doctor Tran Quoc Tuan has more than 12 years of experience in emergency resuscitation.
An electrocardiogram is a graph that records the activity of the heart's electrocardiogram, which plays an important role in diagnosing cardiovascular diseases, monitoring treatment effectiveness, patient's response to drug treatment and monitoring side effects. of drugs,...

1. What is an electrocardiogram?

The heart contracts to a rhythm controlled by a conduction system in the heart muscle. These currents are very small (about a thousandth of a volt) but can be detected by electrodes placed on the patient's arms, legs and chest. These currents are routed to a recorder, amplified by the machine, and recorded on an electrocardiogram. Thus, an electrocardiogram (ECG) is a graph that records the electrical activity of the heart. The electrocardiogram consists of 12 separate leads, including:
6 limb leads: I, DII, DIII, aVR, aVF 6 thoracic leads: V1, V2, V3, V4, V5, V6 These leads help create a Multidimensional view of cardiac electrical activity.
Electrocardiogram has many clinical significance, is used to diagnose heart disease, monitor treatment effectiveness, patient's response to drug treatment and monitor drug side effects,...

2. Mechanism of wave formation on electrocardiogram

The right atrium of the heart has sinus nodes, which are cells that are capable of generating electrical impulses on their own. This electrical impulse is transmitted to the surrounding muscles, causing the two atria to contract, creating a P wave on the electrocardiogram. The current then continues along a specific cell chain to the atrioventricular (AV) node located near the interventricular septum, and then along the Purkinje fibrous cell chain that runs along the septum and spreads to the surrounding muscles (forming the QRS complex). ) causes these two ventricles to contract. The electrical impulse then decreases, the ventricles relax, creating a T wave. Thus, a cardiac cycle manifests on the electrocardiogram as: P wave, QRS complex, T wave and U wave (may appear in the heart) certain diseases).

3. Meaning of the waves on the electrocardiogram

3.1. P wave Normal P wave in:
D1, D2, V3, V4, V5, V6, aVF: always positive D3, aVL, V1, V2: mostly positive, maybe mildly negative, 2 phases aVR: never also negative Whether positive, negative or biphasic, the P wave can be slightly hooked or split. The amplitude of the P wave is usually highest in D2. P waves are formed by atrial depolarization. Normal P waves have an amplitude of less than 2mm (0.2mmV), duration from 0.08-0.11 seconds. When the P wave increases in amplitude or lengthens, it is possible that the patient has a large atrial condition. If the amplitude increases, the patient's right atrium may be enlarged, if the depolarization time is prolonged, the patient's left atrium may be enlarged.

3.2. QRS Complex QRS complex represents the depolarization of the ventricles, normally QRS lasts from 0.06-0.1 seconds. Depending on the direction of depolarization and the placement of the electrodes, different complexes will be shown on the recording paper.
Q wave: is the first sound wave of the QRS complex, formed by depolarization of the interventricular septum. In normal people, Q waves are usually small and short. If the Q wave is large and prolonged negative amplitude may indicate myocardial necrosis. The R wave is the first positive wave of the complex, followed by the negative S wave. These are two waves formed by ventricular depolarization, which are similar in nature. 3.3. T wave The normal T wave is wide, the apex, the flanks asymmetric, the ascending and descending slopes steeper. Always positive in D1, aVF, V3, V4, V5, V6, greatest amplitude in V3, V4 and always negative in aVR. In D2 the majority is positive, a small number is biphasic. In D3, aVL is mostly positive, some biphasic or negative. In V1, most are negative, a few are positive or biphasic.
The T wave is the wave following the QRS complex, representing late repolarization of the two ventricles. The T wave plays an important role in the diagnosis of myocardial ischemia.
3.4. U wave Normally on the electrocardiogram there is no U wave, if there is, it is only a small wave after the T wave. In some diseases such as hypertension, coronary artery disease, valvular heart disease, cardiomyopathy, .. When taking an electrocardiogram, there may be an inverted U wave or a sharp elevation.

3.5. PQ interval The PQ interval is the conduction time from the atria to the ventricles. The way the PQ interval is measured is from the onset of the P wave to the onset of the Q wave or R wave in the absence of Q. In the normal person, the PQ interval lasts from 0.12 to 0.2 seconds. If the PQ is short, the patient may have pre-excitation syndrome (Wolf-Parkinson-White), if a prolonged PQ indicates conduction delay.
3.6. ST segment ST segment is a straight line going from the J point to the T wave origin. The ST segment means early ventricular repolarization phase, normally the ST segment is very little deviated up or down the isoelectric line. In some pathological cases, the ST segment can be elevated or depressed, so the ST segment is of great value in the diagnosis of cardiovascular diseases.
3.7. QT segment The QT segment is the electrical systolic time of the ventricles. The normal value of QT depends on heart rate. If the QT interval is abnormally prolonged, the patient may be at risk for ventricular arrhythmias, especially torsades de pointes.
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