Mean Electrical Axis and Determining Axis on ECG – Electrocardiogram

00:01 Okay.

00:02 Let's combine our augmented leads – aVR, aVL, aVF – with our standard limb leads, 1, 2, and 3.

00:11 So, now, we have six different pictures of the heart around one single plane.

00:18 So, this is along the frontal plane here.

00:22 And each of the arrow tips represent where the positive pole is.

00:28 So, the negative pole is on the opposite side of the arrow.

00:33 A depolarization traveling towards a positive pole is going to give you a positive deflection.

00:40 And again, the maximal positive deflection will be closest to that mean vector.

00:48 And then, you need to look at what is the wave perpendicular, meaning that it would be a 90° angle from the line that you drew from that particular axis that had the highest deflection point.

01:02 Okay, we’ll come back to this and revisit it a couple of times, but just have that in your mind as we move forward.

01:11 One plane, this plane right here; six different views, you have them.

01:17 And we can all put them around one circle.

01:20 And algebraically, one circle is 360°.

01:24 So, we're going to give certain leads a degree.

01:30 And this is going to be a helpful way for us to assess if someone's heart is normal or not based upon what degree their mean electrical axis might be.

01:41 Okay.

01:42 We need to build on some of that process and that is we need to talk about what really is a mean electrical axis.

01:49 A mean electrical axis should be somewhere in between about negative 30 to 100.

01:56 That's the normal range for mean electrical axis.

02:00 Some people will say that normal is in between 0 and 90, and that's okay too.

02:06 I'm just giving you a little bit wider range.

02:08 Not everybody always agrees in medicine what is normal and what's abnormal.

02:14 So, it's either 0 to 90 or negative 30 to 100.

02:18 Two good ways to think about it.

02:21 Your mean electrical axis should lie somewhere in between that range.

02:29 Two pathologies will affect this.

02:32 If you have hypertrophy, the mean electrical axis will move towards a hypertrophy.

02:40 If you have a myocardial infarction – that is a heart attack – the mean electrical axis will move away from the heart attack.

02:48 So, you move towards hypertrophy or away from heart attack.

02:54 So, knowing that information, let’s go calculate a couple.

03:01 So, let’s take this example here and where we are looking first for which particular wave is the most biphasic.

03:10 What do we mean by biphasic? Can't we just use simple words? I know it's frustrating, but let's work through it.

03:18 Biphasic means that your positive deflection equals your negative deflection.

03:25 So, whichever electrode lead shows an equal positive deflection versus a negative deflection, okay? So, it has to have a blip up and a blip down and those two amplitudes need to be very similar.

03:42 You're looking for that lead.

03:44 Then, you want to look for the lead that’s 90° from it.

03:50 And you want to see which one is 90° from it, closest to 90° from it, has a positive electrode deflection.

04:01 Okay.

04:01 Let's go through two examples.

04:03 Everybody needs examples.

04:05 When you first learn this, it’s very hard.

04:07 So, you just need an example.

04:08 You need to walk through a couple.

04:09 So, let’s take one.

04:12 If standard limb lead 1 is biphasic and aVF is the most positive, the mean electrical axis is 90.

04:23 So, if you traced this out, you could see standard limb lead 1.

04:28 That’s that arrow that's going towards zero here, right there.

04:33 That's going towards zero.

04:36 That would be your most biphasic.

04:41 aVF is located perpendicular to standard limb lead one, and that is a direct downward movement.

04:52 If that is a positive deflection, that gives you your mean electrical axis a positive 90.

05:00 If we take a second example, let's say aVL, so here we have aVL.

05:06 If that is the most biphasic, you'd want to take what is 90° from that, and so that would be standard limb lead 2.

05:14 That’s standard limb lead 2, therefore, you would have the most positive deflection that would give you a mean electrical axis of 60.

05:26 Those two examples are normal mean electrical axes.

05:30 Why? Because mean electrical axes should be somewhere between negative 30 and 100.

05:36 Both 90 and 60 fall within those means, and therefore, that is a normal ECG.

05:44 Remember our two pathologies.

05:46 If you have hypertrophy, it will cause a shift towards the hypertrophy.

05:50 If you have an infarction, it will shift it away from the infarction.