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3 Reasons You Should Know Cardiac Axis

When I was in school, the professor who taught my cardiology class admitted that he didn't really understand cardiac axis. He told us that hemiblocks were not really that important, and they we were not going to be tested on it anyway. I carried that attitude towards cardiac axis for years until I started getting more involved in ECG reading (the correct way). I then came to realize that cardiac axis is the most basic and fundamental thing you should probably know when reading an ECG. Without knowing the direction of electricity in the heart, it's hard to really say you fully understand the interpretation you're coming up with. Here are my top three reasons why you need to know cardiac axis!

-Here is a quick reference that I made a while back. You might want to keep something like this saved in your phone for refrence. Ideally, you should be able to draw this from memory.

1. Subtle Inferior STEMI

If you have paid any attention to Dr. Smith's ECG blog, subtle inferior STEMI is probably on your radar. If not, be sure to check this out:

How does cardiac axis play into finding an inferior STEMI? Leads II, III, and AVF usually apply to the 'inferior' portion of the heart. It's easy to recognize when a patient has STE over 1mm. However, let's say that the patient has a left anterior hemiblock, with an axis of -45. This would be that the perpendicular lead to that axis is lead II. This axis will cause the STE to be very small in leads II and AVF because those leads are perpendicular to the mean axis. Still with me? Any lead perpendicular to the mean axis will be small in amplitude, so you will not see large, 'textbook' STE. Check out these illustrations:

When you have your axis coming straight at the lead you're looking though (let's say the axis is +60 and you're looking at lead II) it will be really easy to spot STE in that lead, because the complex will be large and well defined. Think of it this way... When a car comes straight at you and pulls right up in front of you, you can see all kinds of details about the car. You can see what the driver looks like, and any little scratches or imperfections.

However, if the STE occurs in a lead that is perpendicular to the mean axis (let's say the STE is in lead II and the mean axis is -30), the complex will usually be small in lead II, perhaps biphasic, and the STE will usually be very small as well. If you only go by the standard AHA measurements, you will miss the subtle inferior STEMI. When you have this biphasic or very small lead, it's like watching a car drive by on a road in front of you. It's kind of far away (so it looks small), you can't really get a good look at the driver, you can't read the license plate, you can't tell how dirty it is, etc. That's exactly how the perpendicular lead is. You don't have a very good look at what is going on in that lead (so you must look extra close and be extra suspicious of it).

STE can be very small in leads that are perpendicular to the mean axis. If lead II is your mean point of STE, the STE may be 1mm if the axis is pointing that way, but perhaps only 0.25mm if the axis is perpendicular! Remember the old saying: 'you can't squeeze blood out of a turnip?' You just won't get large STE in a lead with a tiny QRS complex. But, that doesn't make the STE any less significant (just harder to find). Tyler did a podcast about this quite a few episodes ago. Check it out here:

2. Hemiblocks

Hemiblocks might not seem like the most exciting topic, but they can point to larger issues. Hemiblocks can occur in coronary artery occlusion. The branches of the left bundle are fed by the coronary arteries. Low blood flow to these electrical branches may cause them to 'short out.' The left anterior superior fascicle has only one blood supply (the LAD). The Left Posterior Inferior Fascicle has two blood supplies (the LAD and RCA), which may be why it is much less common to see a LPFB. Besides possibly indicating low flow, what else can a hemiblock warn of?

Bifascicular blocks can also occur, where there is a RBBB present, along with either a LAFB (more common) or a LPFB (less common). Bifascicular block patients are only relying on one branch* of their left bundle to lead the depolarization of their ventricles. These patients are at high risk for developing a third degree heart block (a TRIfascicular block).

*It is possible that some patients may also have what is called a middle, or septal fascicle as well (not present in every patient).

3. Rhythm Interpretation

Axis can help you determine if your patient may be in VTACH, or in a paced rhythm. It is important to remember that these rules are not absolute, but they can help guide your interpretation. In general, VTACH presents with 3 characteristics:

1. Extreme Right Axis Deviation

2. Upright AVR

3. Backwards R Wave Progression.

This is an oversimplification, but it can help guide your diagnosis of the rhythm. Many people have difficulty distinguishing between a pacer malfunction and VTACH. In general (here comes another over simplification), the pacemaker should generate a pathological left axis deviation, since the electrode is generally placed into the right ventricle (the opposite of the right ventricle is the left atria, which is where pathological left axis points).

Can I Just Read The Number?

If you're only reading the number that shows your the axis on the ECG printout, you're missing the point of checking your perpendicular leads. You must be able to picture the direction of the electricity in your head, and then evaluate your ECG from your new perspective. It's more than a number, it's a looking glass.

Cardiac axis can help you with finding a hard-to-spot inferior STEMI, interpret hemiblocks that may indicate other problems, and help differentiate between certain rhythms. Cardiac axis is the general direction of the electricity in the heart. It should be at the base of your understanding when it comes to reading an ECG.


Here are a few resources to check out of you want to know more about hemiblocks and axis!

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