In Chapter 3 we discussed the electrophysiology of a single cell. We learned how an individual cardiomyocyte becomes polarized, depolarized, and repolarized. Now let’s see how those electrical events apply to multiple cells, complete muscle fibers, and the entire myocardium. The graphical representation of this process is what we have come to know as the electrocardiogram (ECG).
The ECG does not actually directly measure the electrophysiologic activity of the heart. It gives us no information about anything we learned about membrane potentials in the previous chapter. What it does represent is a graph of the differences in electrical potentials as each wave of depolarization travels throughout the myocardium as recorded by electrodes (leads) on the body surface. Depolarization and repolarization of millions of myocardial cells create local regions with various potential differences. As portions of the heart become alternately depolarized and repolarized, the wave of depolarization moves through the myocardium. Because the body contains fluids and electrolytes that can conduct electricity, we can place electrodes on the skin surface and make a recording of this depolarization wave with a galvanometer. The modern electrocardiograph machine is today’s evolution of the galvanometer that Einthoven invented more than a century ago.
A source of confusion is that cellular electrophysiologists view the cell from inside and consider the resting potential as negative. Clinical electrocardiographers view the cardiomyocyte from the outside and consider resting cells as positively charged.
The concept of measuring potential differences is illustrated in Figure 4-1. Think of a galvanometer as a form of balance scale that measures the difference between weights placed on the right and left sides of the apparatus. (Don’t forget that in medicine, we refer to the right and left sides of an image as if the patient were facing us, so we’ll use the same approach here). In this example, we will define the left side of the scale as positive and the right side as negative. At baseline there are three, 1-kg weights placed on each side of the scale. There is no difference between the two sides; therefore, the measurement is recorded as zero. If we now take away two of the weights from the right side of the scale, the measured difference has a value of +2 kg (3 minus 1). Using the same convention, if we had initially taken away two weights from the left side leaving three on the right, the measurement would have been recorded as −2 kg (1 minus 3).
A galvanometer is like a balance scale that measures potential differences. Here the recordings are (a) zero, (b) +2, and (c) −2. The right and left sides of the image are depicted from the perspective of a patient facing you.