Thus far we’ve learned the genesis of electrical current in cardiac tissue and how to record their movements on an electrocardiograph. Now it’s time to understand why these funny little squiggles look the way they do in health and disease. To do so, we need to be familiar with the concept of vectors.
A wave of depolarization is the driving force behind every cardiac cycle. Over time, this electrical current moves through the entire heart, originating with the primary pacemaker in the SA node, traveling through the atria, the AV node, the specialized conduction system, and ultimately spreading throughout every ventricular muscle cell. At each point in time, we can illustrate the magnitude and direction of the electrical wave as it completes its journey from start to finish as an instantaneous cardiac vector. A vector is represented graphically as an arrow that has magnitude, direction, and polarity. The length of the arrow indicates the magnitude (voltage) of the electrical current, the orientation of the arrow indicates the direction of the force, and the tip of the arrowhead indicates electrical positivity (Figure 6-1). In electrocardiography, we assume that the galvanometer records the instantaneous cardiac vectors as if they were generated from a single point in the center of the chest.
Cardiac vectors have magnitude, direction, and polarity. The direction of vector (a) can be described as leftward, inferior, and anterior. The direction of vector (b) is leftward, inferior, and posterior.
The direction and magnitude of an instantaneous cardiac vector are not really a single measurement, but represent the mathematical resultant of all the electrical forces moving in every direction at that moment in time. You can understand the concept of combining vector forces by picturing the path of a football as the kicker boots the ball in the presence of a crosswind (Figure 6-2). The flight of the ball is the net effect of the combined forces of the “kicking vector” and the “wind vector.” Just like the fan whose concern is on the ultimate flight of the football, our interest is the net effect of multiple, simultaneous electrical waves. The same concept applies when we analyze cardiac vectors. Each single vector depicted represents the net resultant of multiple concurrent depolarizations (Figure 6-3).
The ultimate path of a kicked football (z) will result from combining the “kicking vector” (x) with the “wind vector” (y).
Vector (b) represents the net resultant of multiple, simultaneous depolarization waves.
Different Looks from Different Leads
The 12 leads of the electrocardiograph provide ...