The electrocardiogram (ECG) is a continuous record of cardiac electrical activity obtained by placing sensing electrodes on the surface of the body and recording the voltage differences generated by the heart. The equipment amplifies these voltages and causes a pen to deflect proportionally on a paper moving under it. This gives a plot of voltage as a function of time.
The ECG Records the Dipoles Produced by the Electrical Activity of the Heart
To understand the ECG, it is necessary to understand the behavior of electrical potentials in a three-dimensional conductor of electricity. Consider what happens when wires are run from the positive and negative terminals of a battery into a dish containing salt solution. Positively charged ions flow toward the negative wire (negative pole) and negatively charged ions simultaneously flow in the opposite direction toward the positive wire (positive pole).
The combination of two poles that are equal in magnitude and opposite in charge and located close to one another, is called a dipole. The flow of ions (current) is greatest in the region between the two poles, but some current flows at every point surrounding the dipole, reflecting the fact that voltage differences exist everywhere in the solution.
Measurement of the Voltage Associated With a Dipole.
What points encircling the dipole in Figure 13.7 have the greatest voltage difference between them? Points A and B do because A is closest to the positive pole and B is closest to the negative pole. Positive charges are drawn from the area around point B by the negative end of the dipole, which is relatively near. The positive end of the dipole is relatively distant and, therefore, has little ability to attract negative charges from point B (although it can draw negative charges from point A). As positive charges are drawn away, point B is left with a negative charge (or negative voltage). The opposite happens between the positive end of the dipole and point A, leaving A with a net positive charge (or voltage). Points C and D have no voltage difference between them because they are equally distant from both poles and are, therefore, equally influenced by positive and negative charges. Any other two points on the circle, E and F, for example, have a voltage difference between them that is less than that between A and B and greater than that between C and D. This is also true of other combinations of points, such as A and C, B and D, and D and F. Voltage differences exist in all cases and are determined by the relative influences of the positive and negative ends of the dipole.
Changes in Dipole Magnitude and Direction. What would happen if the dipole were to change its orientation relative to points C and D? Figure 13.8 diagrams an apparatus in which electrodes from a voltmeter are placed at the
Creating a dipole in a tub of salt solution.
The dashed lines indicate current flow,- the current flows from the positive to the negative poles (See text for details.).
Effect of dipole position and magnitude on recorded voltage. In a salt solution, the dipole can be represented as a vector having a length and direction determined by the dipole magnitude and position, respectively. In this example, electrodes for the voltmeter are at points C and D. When a vector is directed parallel to a line between C and D, the voltage is maximum. If the magnitude of the vector is decreased, the voltage decreases.
edges of a dish of salt solution in which the dipole can be rotated. This solution is analogous to that depicted in Figure 13.7, except the dipole position is changed relative to the electrodes instead of the electrode being changed relative to the dipole. Figure 13.8 shows the changes in measured voltage that occur if the dipole is rotated 90 degrees. The measured voltage increases slowly as the dipole is turned and is maximal when the positive end of the dipole points to C and the negative end points to D. In each position, the dipole sets up current fields similar to those shown in Figure 13.7. The voltage measured depends on how the electrodes are positioned relative to those currents. Figure 13.8 also shows that the voltage between C and D will decrease to a new steady-state level as the voltage applied to the wires by the battery is decreased. These imaginary experiments illustrate two characteristics of a dipole that determine the voltage measured at distant points in a volume conductor: direction of the dipole relative to the measuring points and magnitude (voltage) of the dipole,- this is another way of saying that a dipole is a vector.
Portions of the ECG Are Associated With Electrical Activity in Specific Cardiac Regions
We can use this analysis of a dipole in a volume conductor to rationalize the waveforms of the ECG. Of course, the actual case of the heart located in the chest is not as simple as the dipole in the tub of salt solution for two main reasons. First, excitation of the heart does not create one dipole, instead, there are many simultaneous dipoles. We will focus with the net dipole emerging as an average of all the individual dipoles. Second, the body is not a homogeneous vol ume conductor. The most significant problem is that the lungs are full of air, not salt solution. Despite these problems, the model is useful in an initial understanding of the generation of the ECG.
At rest, myocardial cells have a negative charge inside and a positive charge outside the cell membrane. As cells depolarize, the depolarized cells become negative on the outside, whereas the cells in the region ahead of the depolarized cells remain positive on the outside (Fig. 13.9). When the entire myocardium is depolarized, no voltage differences exist between any regions of myocardium because all cells are negative on the outside. When the cells in a given region depolarize during normal excitation, that portion of the heart generates a dipole. The depolarized portion constitutes the negative side, and the yet-to-be-depolarized portion constitutes the positive side of the dipole. The tub of salt solution is analogous to the rest of the body in that the heart is a dipole in a volume conductor. With electrodes located at various points around the volume conductor (i.e., the body), the voltage resulting from the dipole generated by the electrical activity of the heart can be measured.
Depolarization in progress
Depolarization in progress
Repolarization in progress
Cardiac dipoles. Partially depolarized or re-polarized myocardium creates a dipole. Arrows show the direction of depolarization (or repolarization). Dipoles are present only when myocardium is undergoing depolarization or repolarization.
Consider the voltage changes produced by a two-dimensional model in which the body serves as a volume conductor and the heart generates a collection of changing dipoles (Fig. 13.10). An electrocardiographic recorder (a voltmeter) is connected between points A and B (lead I, see below). By convention, when point A is positive relative to point B, the ECG is deflected upward, and when B is positive relative to A, downward deflection results. The black arrows show (in two dimensions) the direction of the net dipole resulting from the many individual dipoles present at any one time. The lengths of the arrows are proportional to the magnitude (voltage) of the net dipole, which is related to the mass of myocardium generating the net dipole. The colored arrows show the magnitude of the dipole component that is parallel to the line between points A and B (the recorder electrodes); this component determines the voltage that will be recorded.
The P Wave and Atrial Depolarization. Atrial excitation results from a wave of depolarization that originates in the SA node and spreads over the atria, as indicated in panel 1 of Figure 13.10. The net dipole generated by this excitation has a magnitude proportional to the mass of the atrial muscle involved and a direction indicated by the solid arrow. The head of the arrow points toward the positive end of the dipole, where the atrial muscle is not yet depolarized. The negative end of the dipole is located at the tail of the arrow, where depolarization has already occurred. Point A is, therefore, positive relative to point B, and there will be an upward deflection of the ECG as determined by the magnitude and direction of the dipole. Once the atria are completely depolarized, no voltage difference exists between A and B, and the voltage recording returns to 0. The voltage change associated with atrial excitation appears on the ECG as the P wave.
The PR Segment and Atrioventricular Conduction. After the P wave, the ECG returns to the baseline present before the P wave. The ECG is said to be isoelectric when there is no deflection from the baseline established before the P wave. During this time, the wave of depolarization moves slowly through the AV node, the AV bundle, the bundle branches, and the Purkinje system. The dipoles created by depolarization of these structures are too small to produce a deflection on the ECG. The isoelectric period between the end of the P wave and the beginning of the QRS complex, which signals ventricular depolarization is called the PR segment. The P wave plus the PR segment is the PR interval. The duration of the PR interval is usually taken as an index of AV conduction time.
The QRS Complex and Ventricular Depolarization. The depolarization wave emerges from the AV node and travels along the AV bundle (bundle of His), bundle branches, and Purkinje system; these tracts extend down the interventricular septum. The net dipole that results from the initial depolarization of the septum is shown in panel 2 of Figure 13.10. Point B is positive relative to point A because the left side of the septum depolarizes before the right side. The small downward deflection produced on the ECG is the Q wave. The normal Q wave is often so small that it is not apparent.
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