Electrocardiogram: principles of electrocardiogram analysis
Any change in this impulse conduction can cause an abnormality in the electrical potential of the heart and consequently a change in the shape of the waves on the electrocardiogram.
Use vectors to represent potentials
To understand how abnormalities of the heart affect the lines on the electrocardiogram, it is first necessary to be familiar with concepts such as vector, vector analysis to apply to cardiac potentials.
At one point in the cardiac cycle, the electrical current of the heart moves in a certain direction. A vector is an arrow indicating the direction of the potential generated by the cardiac current, with the direction of the arrow indicating the direction of the positive potential. Similarly, by convention, the length of the arrow is plotted relative to the potential it represents.
Composite vector of the heart at one point in the cardiac cycle
Figure. Average vector of ventricular depolarization.
The dark areas and negative marks in Figure 12-1 represent the depolarization of the ventricles and the endocardial tips of the interventricular septum. At the time of cardiac stimulation, current flows between the depolarized region inside the heart and the non-depolarized region outside the heart, shown by the long elliptical arrow. Some currents also travel inside the chambers of the heart in the direction from the depolarized region towards still polarization. In general, there is a significantly greater amount of descending currents from the base of the ventricles to the apex of the heart than there is going up. Thus, the sum of vectors emitted at a time, known as the instantaneous mean vector, is represented by a black arrow that travels between the two ventricles in the base-to-cardiac direction. Furthermore, because a significant amount of currents is synthesized, the potential is large, and the vector is long.
The direction of the vector represented by the measure
Figure. The vector is plotted to represent the potential of several different hearts, and the axis of the potential (expressed in degrees).
When the vector is horizontal and pointing to the left of the body, it points to 0o, as shown on illustration 12-2. From the 0o point, clockwise, when the vector goes straight from top to bottom, it points to +90o; when the vector goes from the left side to the right side of the body, it points to +180o; and when the vector is pointing straight up, it points to -90o or +270o.
In the normal human heart, the mean direction of the vector at the onset of the ventricular depolarization wave, known as the QRS mean vector, is only about 59°, and is represented by vector A. That is, in most waves. depolarized, the top of the heart is positive relative to the bottom
The axis of each bipolar and unipolar lead
Figure. Axes of 3 bipolar leads and 3 unipolar leads
Three bipolar leads and three unipolar limb leads. Each lead is a pair of electrodes connected to the body at opposite sides of the heart, and the direction from the cathode to the anode is called the axis of the lead. Lead I was recorded from two electrodes at the respective forearms. Since the electrode is horizontal, with the anode to the left, the axis of lead I is 0o.
When recording lead II, electrodes were placed on the right forearm and left leg. The right forearm connects to the torso in the upper right-hand corner, and the left leg joins the lower left-hand corner. Hence the direction of the lead is about +60o.
Similar analysis lead III has an axis of about +120o; lead aVR about +210; lead aVF about +90o; and lead aVL is about -30o. The orientations of the axes of all these leads are shown in the figure, which is known as the Hexagonal Axis System. The polarity of the electrodes is indicated by plus and minus signs. The reader must learn these axes and their polarization, especially the limb dipole leads I, II, and III, in order to understand the rest of this chapter.
Analysis of potential vectors on different leads
Figure. Determine that vector B lies along the axis of I when vector A represents the instantaneous potentials of the ventricles.
The figure shows a partial polarization of the heart, with vector A representing the instantaneous average direction of the current in the ventricles. In this case, the direction of the vector is about +55o, and the magnitude of the potential is represented by the length of vector A, which is 2mV. In the diagram below the heart, vector A is again shown, and a line is drawn representing the axis of lead I with direction 0o. To determine the potential of vector A recorded in lead I, draw a line perpendicular to the axis of lead I, from the apex of vector A to the axis of lead I, called vector B, is drawn. along the axis of lead I. The arrow of vector B indicates the positive direction of lead I, which means that the instantaneous potentials on the electrocardiogram of lead I are positive. The instantaneous potential is recorded as the length of B divided by the length of A, 2 or 1 mV.
The figure is another example of vector analysis. Where, vector A represents the potential and its axis during cardiac ventricular depolarization, where depolarization on the left side of the heart is faster than on the right. In this case, the instantaneous vector has a direction of 100o, and its potential is 2mV. To determine the resting potential in lead I, draw a line perpendicular from the top of vector A to the axis of lead I and find vector B. Vector B is very short and this time it is only negative, so there are times point it is negative, that is, go below the isoelectric line of the electrocardiogram, the potential is very small, about -0.3mV. This figure shows that, when the heart vector is oriented roughly perpendicular to the lead axis, the voltage recorded in the lead is very low. Conversely, when the vector of has an axis coincident with the axis of the lead, the entire potential of the vector is recorded.
Figure. Identify vector B along the axis of lead I when vector A represents the instantaneous potentials of the ventricles.
Vector analysis of resting potentials in the three bipolar limb leads
Figure. Determine the projection vector on leads I, II, and III when vector A represents the instantaneous potentials of the ventricles.
In the figure, vector A depicts the instantaneous potential of the partial depolarization of the heart. To determine the instantaneous electrocardiogram potential in one of the three bipolar limb leads, a perpendicular line is drawn from the apex of vector A to the axes of the three different leads. Vector B describes the potential at an instant in lead I, vector C describes the potential in lead II, and vector D describes the potential in lead III. The potential recorded in these cases is positive, i.e. above the isoelectric line in the electrocardiogram, because the projection vectors (A, B, C) point in the positive direction along the axes of all leads. is shown on the figure. The potential in lead I (vector B) accounts for only about half of the true potential of the heart (vector A); in lead II, vector C has a potential equivalent to that of the heart; and in lead III (vector D), the potential is 1/3 of the potential of the heart.
The same analysis can be used to determine the potentials recorded in the limb leads, except for the corresponding axis of the limb leads and used instead of the bipolar limb axis used. for picture.