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What is an electrocardiogram?
Nov 24, 2018

The myocardial cell membrane is a semi-permeable membrane. When resting, a certain number of positively charged cations are arranged outside the membrane. The same number of negatively charged anions are arranged in the membrane, and the extra-membrane potential is higher than the membrane, which is called the polarization state. At rest, the cardiomyocytes in each part of the heart are in a polarized state, and there is no potential difference. The potential curve traced by the current recorder is straight, which is the equipotential line of the surface electrocardiogram. When the cardiomyocytes are stimulated by a certain intensity, the permeability of the cell membrane changes and a large number of cations infiltrate into the membrane in a short time, so that the potential inside the membrane changes from negative to negative. This process is called depolarization. For the whole heart, the potential change of cardiomyocytes from the endocardial to epicardial sequence depolarization, the potential curve traced by the current recorder is called depolarization wave, that is, the P wave and ventricle of the atrium on the surface electrocardiogram QRS wave. After the cell is completely removed, the cell membrane discharges a large number of cations, causing the potential in the membrane to change from positive to negative and return to the original polarization state. This process is performed by the epicardium to the endocardium, which is called repolarization. Similarly, the potential change during the repolarization of cardiomyocytes is described by a current recorder as a polar wave. Since the repolarization process is relatively slow, the repolarization wave is lower than the depolarization wave. The electrocardiogram of the atrium is low in the atrial wave and is buried in the ventricle. The polar wave of the ventricle appears as a T wave on the surface electrocardiogram. After the whole cardiomyocytes were repolarized, the polarization state was restored again. There was no potential difference between the myocardial cells in each part, and the surface electrocardiogram was recorded to the equipotential line.

The heart is a three-dimensional structure. In order to reflect the electrical activity of different parts of the heart, electrodes are placed in different parts of the body to record and reflect the electrical activity of the heart. In routine electrocardiography, only 4 limb lead electrodes and V1 to V66 thoracic lead electrodes are usually placed, and a conventional 12-lead electrocardiogram is recorded. A different lead is formed between the two electrodes or between the electrode and the central potential end and is connected to the positive and negative poles of the electrocardiograph galvanometer through the lead wire to record the electrical activity of the heart. A bipolar lead is formed between the two electrodes, one lead being a positive pole and one lead being a negative pole. Bipolar limb leads include I lead, II lead and III lead; a monopolar lead is formed between the electrode and the central potential end, where the detecting electrode is the positive pole and the central potential end is the negative pole. The central electrical end is The potential difference recorded at the negative electrode is too small, so the negative electrode is the mean of the sum of the potentials of the leads of the other two limbs except for the probe electrode.

The electrocardiogram records the curve of voltage over time. The electrocardiogram is recorded on the coordinate paper, and the coordinate paper is composed of small cells of 1 mm width and 1 mm in height. The abscissa represents time and the ordinate represents voltage. Usually recorded at 25mm/s paper speed, 1 small grid = 1mm = 0.04 seconds. The ordinate voltage is 1 small grid = 1 mm = 0.1 mv. The measurement methods of the electrocardiogram axis mainly include the visual method, the mapping method, and the table look-up method. The heart produces many different galvanic vector vectors in the process of depolarization and repolarization. The galvanic couple vectors in different directions are combined into a vector to form the integrated ECG vector of the whole 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. Help to determine whether the heart’s electrical activity is normal.