What Is an Electrocardiogram?

Electrocardiogram (ECG or EKG) is a technique that uses an electrocardiograph to record the changes in electrical activity generated by the heart during each cardiac cycle from the body surface.

Basic Information

Chinese name: ECG

Foreign name: electrocardiogram, ECG

ECG history

In 1842, the French scientist Mattencci first discovered the electrical activity of the heart; in 1872, Muirhead recorded electrical signals that fluctuated in the heart. In 1885, Dutch physiologist W. Einthoven recorded the ECG waveform from the body surface for the first time. At that time, a capillary electrometer was used. In 1910, a string current meter was improved. This opened the history of body surface electrocardiogram recording. In 1924 Einthoven won the Nobel Prize in Medical Biology. After more than 100 years of development, today's electrocardiogram machine has become increasingly perfect. Not only clear records, strong anti-interference ability, but also portable, and with automatic analysis and diagnosis functions.

The principle of electrocardiogram generation

Cardiac cell membranes are semi-permeable membranes. When resting, a certain number of positively charged cations are arranged outside the membrane, and the same number of negatively charged anions are arranged inside the membrane. The potential outside the membrane is higher than that inside the membrane, which is called the polarization state. In the resting state, since the cardiomyocytes in all parts of the heart are polarized and there is no potential difference, the potential curve recorded by the current recorder is straight, which is the isoelectric line of the body surface electrocardiogram. When myocardial cells are stimulated with a certain intensity, the permeability of the cell membrane changes, and a large number of cations flood into the membrane for a short time, causing the membrane potential to change from negative to positive. This process is called depolarization. For the overall heart, the potential changes during the sequential depolarization of myocardial cells from the endocardium to the epicardium. The potential curve recorded by the current recorder is called the depolarization wave, which is the P wave and the ventricle of the atrium on the body surface electrocardiogram. QRS wave. After the depolarization of the cells is completed, a large number of cations are discharged from the cell membrane, so that the potential inside the membrane changes from positive to negative and returns to the original polarization state. This process proceeds from the epicardium to the endocardium, which is called repolarization. Similarly, the potential changes during the repolarization process of myocardial cells are recorded by the current recorder as repolarization waves. Because the repolarization process is relatively slow, the repolarization wave is lower than the depolarization wave. The repolarization wave in the atrium is low and buried in the depolarization wave of the ventricle, and the body surface electrocardiogram is difficult to recognize. The repolarization wave of the ventricle appears as a T wave on the body surface electrocardiogram. After the entire myocardial cells were repolarized, the polarization state was restored again, there was no potential difference between the myocardial cells in each part, and the body surface electrocardiogram recorded an isoelectric line.

ECG ECG leads

The heart is a three-dimensional structure. In order to reflect the electrical activity on different sides of the heart, electrodes are placed in different parts of the human body to record and reflect the electrical activity of the heart. The location of the heart electrode is shown in the table below. When performing conventional ECG examination, usually only 4 limb lead electrodes and V ₁ to V ₆ 6 chest lead electrodes are placed, and a conventional 12-lead ECG is recorded.

Body surface electrode name and placement

Electrode name	Electrode position
LA	Left upper limb
RA	Right upper limb
LL	Left lower limb
RL	Right lower limb
V ₁	4th intercostal space
V ₂	4th intercostal space left margin of sternum
V ₃	Between leads V ₂ and V ₄
V ₄	5th intercostal space left midclavicular line
V ₅	5th intercostal space left anterior axillary line
V ₆	5th intercostal space on the left axillary midline
V ₇	5th intercostal space on the left posterior axillary line
V ₈	5th intercostal space below the left scapula
V ₉	5th intercostal space on the left paraspinal line
V _3r	Between lead V ₁ and V _4r
V _4r	5th intercostal space right midclavicular line
V _5r	5th intercostal space on right anterior axillary line

Different leads are formed between the two electrodes or between the electrode and the central potential end, and the leads are connected to the positive and negative electrodes of the electrocardiograph ammeter through the lead wires to record the electrical activity of the heart. A bipolar lead is formed between the two electrodes, one lead is positive, and one lead is negative. Bipolar limb leads include lead I, lead II, and lead III; a unipolar lead is formed between the electrode and the central potential terminal, and the detection electrode is the positive electrode at this time, and the central potential terminal is the negative electrode. The avR, avL, avF, V1, V2, V3, V4, V5, and V6 leads are all unipolar leads. Because avR, avL, avF are far from the heart, the potential difference recorded when the central electrical terminal is the negative electrode is too small, so the negative electrode is the average of the sum of the potentials of the two limb leads except the probe. Because this recording increases the potential of the avR, avL, and avF leads, these leads are also referred to as pressurized unipolar limb leads.

ECG lead names and composition of positive and negative electrodes

Schematic diagram of ECG lead connections

Lead name	positive electrode	negative electrode
I	LA	RA
II	LL	RA
III	LL	LA
avR	RA	1/2 (LA + LL)
avL	LA	1/2 (RA + LL)
avF	LL	1/2 (LA + RA)
V ₁	V ₁	Central potential terminal
V ₂	V ₂	Central potential terminal
V ₃	V ₃	Central potential terminal
V ₄	V ₄	Central potential terminal
V ₅	V ₅	Central potential terminal
V ₆	V ₆	Central potential terminal

The limb lead system reflects the projection of the cardiac potential on the sagittal plane. Including leads I, II, III, avR, avL and avF. The chest lead system reflects the cardiac potential projection level including: V ₁ , V ₂ , V ₃ , V ₄ , V ₅ , and V ₆ leads. These leads are further grouped to reflect electrical activity in different parts of the heart.

Grouping of ECG leads

I high sidewall lead	avR	V ₁ anterior wall lead	V ₄ anterior wall lead	V ₇ positive posterior wall lead	V _3r right ventricular lead
II inferior lead	avL high sidewall lead	V ₂ anterior wall lead	V ₅ left wall lead	V ₈ positive posterior wall lead	V _4r right ventricular lead
III inferior lead	avF lower wall lead	V ₃ anterior wall lead	V ₆ left wall lead	V ₉ positive posterior wall lead	V _5r right ventricular lead

Central potential terminal: Also called Wilson central electrical terminal, it is generated by connecting the RA, LA, LL electrodes through a resistor network, which represents the average voltage of the body. This voltage is close to the maximum (ie, 0).

ECG electrocardiogram recording paper

An electrocardiogram records a curve of voltage over time. The electrocardiogram was recorded on graph paper, which consisted of small grids 1 mm wide and 1 mm high. The abscissa indicates time, and the ordinate indicates voltage. Usually use 25mm / s paper speed record, 1 grid = 1mm = 0.04 seconds. The ordinate voltage is 1 grid = 1mm = 0.1mv.

Composition of each wave band of the electrocardiogram

1.P wave

Electrical activation of the normal heart begins in the sinoatrial node. Since the sinoatrial node is located at the junction of the right atrium and the superior vena cava, the excitability of the sinoatrial node is first transmitted to the right atrium and transmitted to the left atrium through the room bundle, forming a P wave on the ECG. The P wave represents the atrial excitement, the first half represents the right atrium excitement, and the second half represents the left atrium excitement. The P-wave time limit is 0.12 seconds and the height is 0.25mv. When the atrium is enlarged and the conduction in the two chambers is abnormal, the P wave can appear as a high-point or bimodal P wave.

2.PR interval

The PR interval represents the time required for the excitability generated by the sinoatrial node to reach the ventricle via the atrium, the atrioventricular junction, and the atrioventricular bundle, and cause the ventricular muscles to begin to excite. The normal PR interval is 0.12 to 0.20 seconds. When the conduction from the atrium to the ventricle is blocked, it appears as the PR interval is extended or the ventricular wave disappears after the P wave.

3.QRS complex

Agitating downward through the Heath bundle and left and right bundle branches simultaneously excite left and right ventricles to form QRS complexes. The QRS complex represents the depolarization of the ventricles, and the duration of activation is less than 0.11 seconds. When conduction block of left and right bundle branches of the heart, ventricular enlargement or hypertrophy occur, the QRS complex appears widened, deformed, and prolonged.

4.J point

The intersection where the QRS wave ends and the ST segment begins. All ventricular myocytes are depolarized.

5.ST segment

All ventricular muscles are depolarized, and repolarization has not yet started. At this time, the ventricular muscles in each part are in a depolarized state, and there is no potential difference between the cells. Therefore, under normal circumstances, the ST segment should be on the equipotential line. When there is ischemia or necrosis in a part of the myocardium, there is still a potential difference in the ventricle after depolarization is completed. At this time, the ST segment on the electrocardiogram is shifted.

6.T wave

The subsequent T wave represents the repolarization of the ventricle. In the lead of the QRS wave main wave, the T wave should be in the same direction as the QRS main wave. Changes in T waves on the ECG are affected by a number of factors. For example, myocardial ischemia can be manifested as a T-wave low-level inversion. T-wave towering can be seen in hyperkalemia, the acute phase of acute myocardial infarction, and so on.

7.U wave

U waves can be seen after T waves on some leads, which are currently thought to be related to ventricular repolarization.

8.QT interval

Represents the time from ventricular depolarization to repolarization. The normal QT interval is 0.44 seconds. Since the QT interval is affected by heart rate, the concept of a corrected QT interval (QTC) was introduced. One calculation method is QTc = QT / RR. Prolongation of the QT interval is often associated with the occurrence of malignant arrhythmias.

ECG band	Significance of corresponding ECG activity
P wave	Atrial depolarization
PR interval	Atrioventricular conduction time
QRS complex	Ventricular depolarization
ST segment	Ventricular depolarization completed
T wave	Ventricular repolarization
U wave	May be related to repolarization
QT interval	Ventricular depolarization to full repolarization time

ECG ECG vector axis

The measurement methods of the ECG axis mainly include visual inspection method, drawing method and look-up table method. The following table is a visual assessment of the orientation of the ECG axis. The heart is a three-dimensional structure made up of countless myocardial cells. During the depolarization and repolarization process, the heart generates many different galvanic vectors. The galvanic vectors of different directions are integrated into a vector to form a comprehensive electrocardiogram vector of the entire heart. The heart vector is a three-dimensional, vector with frontal, sagittal, and horizontal planes. Commonly used clinically is the direction of the partial vector projected on the frontal plane during ventricular depolarization. Helps determine whether the heart's electrical activity is normal.

The forehead electric axis uses a six-axis system. The coordinates are marked with an angle of ± 180 °, with 0 ° on the left, positive in the clockwise direction, and negative in the counterclockwise direction. Each lead is divided into positive and negative halves from the center point, and the included angle between each adjacent lead is 30 °. If the electrical axis of the QRS wave frontal plane falls at 0 + 90 °, the electrical axis is normal; 0 -30 ° is a slight left deviation of the electrical axis; -30 ° -90 ° is a significant left deviation of the electrical axis; + 180 ° is the right deviation of the electric axis; + 180 ° + 270 ° is extremely right deviation of the electric axis.

^{ECG axis offset}	^I			^{ECG axis range}
^normal	⁺	⁺	⁺	⁰ ^{to + 90 °}
^{Slight left deviation}	⁺	⁺		⁰ ^{to -30 °}
^{Obvious left}	⁺			^{-30 °} ^{-90 °}
^{Right axis deviation}		^±	⁺	^{+ 90 °} ^{to + 180 °}
^{Extremely skewed right axis}				^{+ 180 °} ^{to + 270 °}