ECG axis

An electrocardiogram - an ECG - or the Dutch and German version of the word - EKG, is a tool used to visualize the electricity that flows through the heart And the way it looks depends on the set of electrodes you’re using. This particular set of electrodes is called lead II, for example, with one electrode on the right arm and the other on the left leg, so essentially when the wave’s moving toward the left leg electrode, you get a positive deflection, like this big positive deflection corresponding to the wave moving down into the left and right ventricles. To read an ECG there are a few key elements to keep in mind, one of them includes figuring out the axis.

The axis of an ECG is the average direction of electrical movement through the heart during a depolarization. More 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 out 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 where it moves down the bundle of His into the left and right bundle branches and into each ventricle’s Purkinje fibers, causing the ventricles to contract as well. So, in a healthy heart, the upper chambers contract first, then shortly after, the lower chambers contract.

On an ECG, the atrial depolarization and contraction is seen as a p-wave, the ventricular depolarization and contraction is seen as a QRS complex, and the ventricular repolarization, and therefore its relaxation, is seen as a T-wave. A general principle to keep in mind is that a depolarization is caused by the movement of positive charge, so if that movement of positive charge is going toward the positive electrode, then it’s captured as a positive deflection on an ECG.

With that 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 starts depolarizing the ventricles. The Purkinje fibers sit just below the endocardium, which is the innermost layer of the heart. After the endocardium is the myocardium, the cardiac muscle cells, and then finally there’s the epicardium, which is the outer layer. So the Purkinje fibers initiate depolarization vectors that travel directly outward, starting in the endocardium, going through the myocardium, and ending in the epicardium. And because they transmit a depolarization wave so quickly, they all fire off pretty much in unison. Also, the more muscle tissue in the myocardial layer that a vector travels through, the larger the size of the vector. So like with hypertrophic cardiomyopathy, where the heart muscle gets thicker, you end up with bigger vectors. Although, if the heart muscle has been damaged, from something like a heart attack, then you get smaller vectors because the heart cells can’t depolarize anymore. Another thing that can affect vectors is the position of the diaphragm, which is usually sort of sitting right up against the heart. In obese people, the diaphragm gets pushed upwards, which rotates the heart further to the left, and in thin individuals, the diaphragm lowers, which rotates the heart a bit the other way. When everything is taken into consideration, and all of the individual vectors are added up, there is one overall representative vector arrow, which starts from the AV node, and points in one direction through the ventricles. Now, remember that vectors can be broken down into two perpendicular vector components, so if you look at the component that points at the positive lead electrode, that’s what gets recorded on the ECG, and therefore when you plot these vectors over the course of ventricular depolarization, you end up with the QRS complex.