Content Reviewers:Rishi Desai, MD, MPH
An electrocardiogram is also known as an ECG; the Dutch and German version of the word, elektrokardiogram, is shortened to EKG. It is a tool used to visualize, or “gram,” the electricity, or “electro,” that flows through the heart, or “cardio.” Specifically, an ECG tracing shows how the depolarization wave, which is a wave of positive charge, moves during each heartbeat, by providing the perspectives of different sets of electrodes. This particular set of electrodes is called lead II; one electrode is placed on the right arm and the other on the left leg. Essentially, when the wave’s moving toward the left leg electrode, you get a positive deflection. This big, positive deflection corresponds to the wave moving down the septum. To read an ECG, there are a few key elements to keep in mind; one is to figure out the axis.
The axis of an ECG is the average direction of electrical movement through the heart during a depolarization. Specifically, axis usually refers to the mean QRS vector, which is the size and direction of the depolarization wave as it moves through the ventricles. Normally, the QRS axis aims downward and to the left in relation to the body.
So, if we simplify this heart a bit, normally, the sinoatrial, or SA, node sends an electrical signal that propagates outward through the walls of the heart and contracts both upper chambers. Then, that signal moves to the atrioventricular, or AV, node, where the signal is delayed for a split second, and then goes down into the ventricles, or lower chambers. Here, it moves down the bundle of His into the left and right bundle branches, and into each ventricle’s Purkinje fibers, which causes them to contract as well.
On an ECG, the atrial depolarization is seen as a P wave, the ventricular contraction is seen as a QRS complex, and the ventricular repolarization, a period of relaxation, is seen as a T wave. Keep in mind that a depolarization is caused by the movement of positive charge, so if this movement of positive charge is going toward the positive electrode, then it’s captured as a positive deflection on an ECG.
With this in mind, let’s take a closer look at the mean, or average, QRS vector. After the depolarization wave arrives at the AV node, it travels down the interventricular septum and begins depolarizing the ventricles. The purkinje fibers sit just below the endocardium, which is the innermost layer of the heart. After the endocardium, there’s the myocardium, the cardiac muscle cells, and finally, the epicardium, which is the outer layer. Therefore, each purkinje fiber initiates a depolarization vector that travels directly outward; it starts in the endocardium, goes through the myocardium, and ends in the epicardium. Because they transmit a depolarization wave so quickly, they all fire off pretty much in unison. The more muscle tissue in the myocardial layer that a vector has to travel through, the large the size of the vector. So, as with hypertrophic cardiomyopathy, where the heart muscle gets thicker, you get bigger vectors.
However, if the heart tissue has been damaged, such as from a heart attack, then you have smaller vectors, because the heart cells can’t depolarize anymore. The position of the diaphragm can also affect vectors, because it’s usually sort of sitting right up against the heart. In obese people, the diaphragm gets pushed upwards, rotating the heart further to the left. In thin people, the diaphragm lowers, rotating the heart a bit the other way.