Before I understood the concepts behind electrocardiography, the lines on the paper were just a bunch of squiggles with no rhyme or reason. I was then introduced to the concept of vectors. Suddenly things began to make sense and I started to look at the electrocardiogram (ECG) and the myocytes as a unit. I hope this chapter will be as eye opening to you as it was to me in opening that door.
At this point, there are two paths that you could follow: pattern recognition or vector analysis. Pattern recognition is based on pure memory of what certain patterns look like. Many people learn by this method and can function well with it under most circumstances. For example, they know what a strip showing atrial fibrillation looks like and when confronted by it simply make the call. If only life were that simple. The problem comes in when other confounding variables enter into the equation. What if the patient’s potassium is high? Low? What if the patient has a preexisting bundle branch block? What if there is aberrancy? What if there are two processes running at the same time? There are countless roadblocks that can interfere with the picture of the variant that you are familiar with. After 20 or so years, you’ll think you’ve seen it all. But, you haven’t. There is always something new or different present.
Vector analysis is the other method at our disposal. It is based on the simple fact that each myocyte’s bioelectrical activity creates a single vector. That vector is oriented by magnitude, direction, and time. We can see these smaller vectors formed by the myocytes merging to form larger vectors. The summation and subtraction of these vectors eventually lead to the formation of one main regional vector per area, including the left atrial vector, the right atrial vector, the left ventricular vector, and the right ventricular vector. These regional ventricular vectors eventually unite with their regional counterpart, leading to the formation of the main cardiac vector, the electrical axis of the heart.
Now, a positive vector heading toward a particular lead causes a positive or upward deflection on the tracing. A positive vector heading away from a lead creates a negative or downward deflection. Let’s see how this knowledge can help us interpret a strip.
A P wave is formed by atrial activity. Typically, where does a P wave start? The sinoatrial (SA) node is the main pacemaker of the heart and is located near the posterosuperior portion of the right atrium. When the SA depolarizes, it causes a depolarization wave to travel down the atria, heading inferiorly toward the ventricles. As you will see in this chapter, leads II, III, and aVF have their positive poles facing the inferior area. If you have a normal SA node pacing, and if that wave moves inferiorly toward the ventricles, which direction would the atrial vector face? It travels inferiorly. A positive wave traveling toward an electrode gives you a positive wave. Therefore, a normal P wave will always be upright in leads II, III, and aVF.
Note that any wave that starts in the SA node will be positive in leads II, III, and aVF. Any and all of them. Period. What if the P wave is negative in leads II, III, and aVF? In that case, we know that it did not start in the SA node and that it is, therefore, an ectopic wave. This is because all or part of that wave had to have traveled in a superior direction away from the electrode. Which would you prefer, to memorize the morphology of thousands to millions of P waves or to understand that simple principle?
When you understand vectors, electrocardiography becomes simpler. You just have to be open to the concepts and ideas. I leave you with this allegory from ancient China. A very arrogant prince wanted to personally draw a calligraphy scroll for his great father, the king. The king’s name was Five Thousand, and the prince wanted to honor that name. He hired the greatest calligrapher in the land to teach him to make a scroll as a gift to honor his father. On the first lesson, the calligraphy master started by teaching him to draw the number one by placing a single line. Then he drew two lines to represent the number two. Three lines were three, and four lines were four. At this point, the prince lost his patience and said, “Enough, I will take it from here.” For two weeks the prince worked night and day. Finally, on the great day, the prince had not finished since he had drawn only 4,459 lines. If he had shown patience, he would have seen that from five on, each number is represented by a different single image. Moral of the story: Understanding the mechanisms is always the shortest route.
—Daniel J. Garcia