What is ECG vector?

First, ECG vector and comprehensive ECG vector

The quantity used to express quantity and directionality in physics is called vector, also called vector. In the process of depolarization and repolarization, myocardial cells form galvanic couples, which are both quantitative and directional, and are called galvanic vectors. The electric couple vector can be regarded as the ECG vector of a single myocardial cell, and its number is the electromotive force of the electric couple, which depends on the number of charges accumulated at the two poles of the electric couple. The more numbers, the greater the electromotive force, and vice versa. The direction of ECG vector is the direction of electric couple. The electric couple vector can be represented by an arrow, the length of the shaft represents the size of the vector, the arrow represents the direction of the vector (power supply), and the tail of the arrow represents the electric hole (Figure 14-2- 1. Because the depolarization of myocardium starts from the endocardial surface and points to the epicardial surface, the direction of the vector is that the power source is in front (arrow) and the electric point is in the back (arrow). When repolarizing, the electric point is in front of the power supply because the part that depolarizes first repolarizes first. Myocardial repolarization starts from epicardium and points to endocardium, so repolarization vector is consistent with depolarization.

A piece of myocardium is made up of multiple myocardial cells. During depolarization and repolarization, many electrical coupling vectors will be generated, which are superimposed to form an electrical coupling vector, which is the comprehensive ECG vector. The heart consists of several parts of the myocardium. In the process of depolarization, the electric couple vectors in different directions move at the same time, and each produces electromotive force in different directions. Combining several ECG vectors in different directions into one vector represents the comprehensive ECG vector of the whole heart. We take figure 14-2-2 as an example to illustrate the comprehensive vector of simultaneous depolarization of left and right ventricles. A represents the depolarization vector of the left ventricle, pointing to the left. Because of the thickness of left ventricular wall, the depolarization potential is large and the arrow shaft is long; B stands for right ventricular depolarization vector, which refers to the right anterior part. Because the right ventricular wall is thin and the depolarization potential is small, the arrow shaft is short. Put one; B is one side of the parallelogram and intersects with C. The diagonal CD of the parallelogram ABCD is the composite vector of the two (pointing to the left rear).

2. Instantaneous synthesis of ECG vector and spatial ECG vector loop.

During the period of ECG activity, the depolarization and repolarization of different parts of the myocardium have a certain order, and there are ECG activities of different parts of the myocardium at all times (Figure 14-2-3). For example, when the ventricular partition is 0.0 1s, the ECG vector of 0.02s~0.08s will produce many electric coupling vectors in different directions at a certain moment.

The heart is a three-dimensional organ, and its instantaneous vectors face all directions of space. If the tip of the instantaneous integrated ECG vector forms a point, the circular trajectory connecting the moving points will form a space ECG vector ring, that is, space vector electrocardiogram, over the whole ECG cycle.

Three, plane ECG vector diagram and its basic graphics

The vector loop of space vector is a three-dimensional figure, and it is difficult to draw three-dimensional figures on plane paper. It is usually observed by the projection of the vector ring of space vector ECG on three different mutually perpendicular planes (Figure 14-2-5). The so-called projection means that parallel rays perpendicular to a certain plane irradiate the ECG vector ring, and the image formed by this vector ring on this plane is called projection (Figure 14-2-6). Then the shape projected on each surface is drawn as a plane, and the three planes form a spatial three-dimensional image. This is the vectorcardiogram recorded routinely in clinic. Also known as the first projection of the space vector ring.

Three planes (frontal plane, transverse plane and sagittal plane) commonly used in clinic are composed of three axes (Y axis, X axis and Z axis) (see figure 14-2-5 and notes). Taking the cross section as an example, the basic graphics of ECG vector diagram are briefly described.

P-ring, also known as P-vector ring, represents the depolarization process of atrial muscle, and the direction of its comprehensive vector (P-axis) points to the lower left slightly ahead.

QRS ring represents the depolarization process of ventricular muscle. The ring is oval and runs counterclockwise, with a total time of about 0.08s, of which one third is located in front of the X axis and two thirds is located behind the X axis. The direction of its comprehensive vector (QRS axis) points to the left rear. According to the order of depolarization, it can be divided into: ① ventricular septal depolarization, also known as initial vector or 0.0 1s vector. Ventricular depolarization starts from left to right at the left side of ventricular septum 1/3, and the depolarization vector refers to the right front (about 1 10). ② Tip depolarization. When the ventricular depolarization reaches 0.02 seconds, the impulse extends to the apex. At this time, the left and right apex of the heart are depolarized at the same time, and its comprehensive vector points forward and downward. ③ The left ventricular depolarization started about 0.04s after depolarization, and most of the interventricular septum and right ventricle have been depolarized, only the depolarization of the left ventricular sidewall and the basal part of the right ventricle continues, so it is also called the 0.04s vector or the maximum vector, and its direction points to the left posterior. ④ When the basic depolarization reaches 0.06s, only a small amount of basic depolarization is still going on in the left ventricular posterior basement and interventricular septum, so it is also called terminal vector, and its direction refers to the right posterior (equivalent to about 265 degrees).

T-ring represents the repolarization process of ventricular muscle, and its comprehensive vector direction points to the left front, which is basically consistent with the electric axis direction of QRS ring, reflecting that T-wave is upright in the lead dominated by R-wave in ECG. This seems to contradict the statement that the depolarization of a single myocardial cell is opposite to the complex vector. At present, it is considered that the process of ventricular repolarization is different from that of depolarization, which has nothing to do with conduction system, but is closely related to myocardial metabolic function. Generally speaking, cells repolarize faster in areas with high temperature, low pressure and good blood supply. Compared with endocardium, epicardium meets these three conditions, so epicardium recovers very quickly. Because epicardium is earlier than endocardium repolarization, the power supply of its electric couple vector is on epicardium side, and the electric point is on endocardium side, that is, the ventricular repolarization vector points to epicardium, so the direction of ventricular depolarization is the same as that of repolarization.