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.

So to figure out the direction of the axis in the frontal plane, we can start by looking at lead 1 and aVF. Lead 1 moves from right to left across the heart, so anything pointing to the left will be positive in lead 1.

aVF, on the other hand, points downward, so any vector pointing downward will be positive in aVF. Looking at our overall vector from before, it’s pointing both down and to the left, both positive, so it’s in the bottom left quadrant between 0 and +90 degrees and that’s a normal axis.

If it’s positive, or up, on an ECG in 1 and aVF you can imagine that’s like seeing two thumbs up, meaning that everything is okay!

If the vector is positive in lead 1 and negative in aVF, then it’s in this quadrant, and it could also be normal if it’s between 0 and -30 degrees.

If it’s between -30 and -90 degrees, though, it could be considered left axis deviation. Left axis deviation can happen when the left ventricle hypertrophies or when the right ventricle gets damaged and loses healthy tissue.

If the opposite happens, and the vector is negative in lead 1 and positive in aVF, then it’s in this quadrant between +90 and +180 degrees and we call that right axis deviation.

That can happen when the right ventricle hypertrophies or when the left ventricle is damaged and loses healthy tissue. Finally, if the vector is negative in both lead 1 and aVF, then that’s a super rare case called extreme right axis deviation, which is between -90 and +180 degrees.

This can sometimes happen when there’s an ectopic focus that causes depolarization to start from the bottom of the ventricles and travel in the reverse direction.

It’s also a good idea in this case to double check that the leads are placed correctly. Now, you can figure out the QRS vector, within about 30 degrees, by looking at someone’s ECGs, specifically the 6 leads - I, II, III, aVR, aVL, and aVF, corresponding to 0 degrees, +60 degrees, +120 degrees, -150 degrees, -30 degrees, and +90 degrees, respectively.

Lead I here is clearly positive, which means that the axis is somewhere over here, and lead aVF is positive as well, putting the axis in the lower left quadrant between 0 and +90 degrees which is a normal axis.

To be a bit more precise, it looks like lead III is the closest to being isoelectric, which means it has equal positive and negative deflections.

Since it’s not pointing directly at or away from any vectors, it must actually run perpendicular to the QRS vector, so it must be 120 degrees minus 90 degrees from lead III, which is at the +30 degree line.

Notice that this happens to sit on the same line as aVR, but in the opposite direction, so we’d guess that this vector should look negative in aVR, and in fact it does!Now, let’s switch gears and talk about the chest leads, which essentially view the heart in a horizontal plane.

Now looking down at the heart we have leads V1, V2, V3, V4, V5, and V6. Normally, the QRS complex is negative in leads V1 and V2, it’s isoelectric in lead V3 and V4, which is called the transition zone, and positive in leads V5 and V6.

Here, the horizontal axis of the heart lies along the isoelectric chest lead. If the transition zone shifts toward V1 or V2, then it suggests that the heart might be rotated to the person’s right which can happen if the right ventricle has hypertrophied.

On the flip side, if leads V5 or V6 look isoelectric instead of positive, it suggests that the heart might be rotated to the person’s left which can happen if the left ventricle has hypertrophied.

Alright, as a quick recap - a normal heart axis is between -30 and +90 degrees. Right ventricular hypertrophy can cause the axis to be between +90 and +180 degrees.

And can sometimes cause the transition zone to shift towards the V1 and V2 chest leads. Left ventricular hypertrophy can cause the axis to be between -30 and -90 degrees.

And can sometimes cause the transition zone to shift towards the V5 and V6 chest leads that transition zone shifts toward V one or V two then it suggests that the heart might be rotated to the person's right which can happen if the right ventricle is hypertrophied on the flipside If Leeds V five or V six look I so electric instead of positive it suggests that the heart might be rotated to the person's left which can happen if the left ventricle is hypertrophied All right As a quick recap a normal heart axis is between negative 30 and plus 90 degrees Right ventricular hypertrophy can cause the access to be between plus 90 and plus 100 and 80 degrees and can sometimes cause the transition zone to shift towards the V one and V two Chest leads left ventricular hypertrophy can cause the access to be between negative 30 and negative 90 degrees and can sometimes cause a transition zone to shift towards the V five