At some point or another, we have all heard someone mumble the terms “ECG axis”, “cardiac axis” or “cardiac axis deviation” when looking at a 12 lead ECG. But what does axis mean? I’ll let you in on a little secret, but you have to promise to tell EVERYONE! Here goes: cardiac axis is just a fancy way to discuss the flow of electrical conduction within the heart. And the kicker? It’s actually a relatively easy concept to understand.
If you haven’t already read the article that helps with demystifying the 12 lead ECG, I strongly suggest that you do that first! Have you done it? Really? Ok…I believe you! As you have read, the 12 leads of an ECG view 12 different areas of the heart. Therefore, each lead is going to generate a slightly different recording of the PQRST complex as they are all viewing the flow of electrical energy within the heart from different vantage points.
If the electrical conduction within the heart flows towards a positive pole (where the lead is looking at the heart from), there will predominantly be a positive inflection on the ECG.If electrical conduction within the heart flows away from a positive pole (where the lead is looking at the heart from), there will predominantly be a negative inflection on the ECG. If electrical conduction within the heart starts to flow towards and then starts to flow away from a positive pole (where the lead is looking at the heart from), there will be a biphasic inflection on the ECG (relatively equal positive and negative inflections on the ECG).
It is very important to understand this relationship between what is recorded on an ECG to the electrical conduction within the heart as flow of energy moves from the sinoatrial (SA) node, to the atrioventricular (AV) node down through the bundle of His and purkinje fibres.
If our electrical conduction pathway stays constant and our lead viewpoint changes (comparing one lead to another lead), each lead will reflect the electrical flow in relation to itself thereby showing different waveforms. If our viewpoint stays constant (same lead) and the electrical conduction within the heart changes it’s pathway, the lead waveform will change to reflect this. This is where cardiac axis +/- deviation come into play.
Understanding the Cardiac Axis
So if we think about the flow of electrical conduction through the heart, the majority of electrical conduction will head towards the left ventricle as there is more muscle to depolarise. Think of the image below as if we have superimposed a graph over the top of the heart, thereby allowing us to see a 2D representation of where the majority of electrical flow is heading as it depolarises the ventricles. Therefore, we say that normal electrical conduction flow within the ventricles (normal cardiac QRS axis) is anywhere between -30 degrees to +90 degrees.
If something within the heart causes the electrical conduction to deviate from its normal path, we refer to this as a cardiac axis deviation. A cardiac axis deviation is not normal and usually prompts the clinician analysing the ECG to have a closer look.
- Left axis deviation = QRS axis between -30 to -90 degrees
- Causes can include left ventricular hypertrophy, left bundle branch block, left anterior fascicular block, inferior myocardial infarction, Wolff-Parkinson-White syndrome, ventricular tachycardia and/or ventricular paced rhythm
- Right axis deviation = QRS axis between +90 to +180 degrees
- Causes can include right ventricular hypertrophy, right bundle branch block, left posterior fascicular block, anterolateral myocardial infarction, Wolff-Parkinson-White syndrome, ventricular tachycardia and/or dextrocardia
- Extreme axis deviation = QRS axis between -90 and +180 degrees
Calculating the Cardiac Axis
So how do we calculate the cardiac axis when looking at a 12 lead ECG? There is a slow way, fast way and really fast way!
The Slow Way
Lead I and lead aVF look at the heart in a way that gives us a horizontal and vertical axis, respectively, that we can create a 2D graph from.
Once you have used leads I and aVF to plot a graph, you need to do the following:
- In lead I, count how many little boxes there are from baseline to the lowest point of the negative inflection of the QRS complex – remember this number!
- In lead I, count how many little boxes there are from baseline to the highest point of the positive inflection of the SAME QRS complex – remember this number!
- Now deduct the negative inflection number from the positive inflection number, and plot this on the horizontal axis of your graph
- IMPORTANT: a positive number is to be plotted on the horizontal axis heading towards 0 degrees; a negative number is to be plotted on the horizontal axis heading towards 180 degrees.
- Do the exact same thing with the QRS complex in lead aVF
- IMPORTANT: a positive number is to be plotted on the vertical axis heading towards +90 degrees; a negative number is to be plotted on the horizontal axis heading towards -90 degrees.
- Mark the point on the graph where these two points intersect and draw a line from the centre – this is your cardiac axis degree!
So let’s use the following ECG as an example:
- Lead I has 3 little boxes of negative inflection from baseline and 7 little boxes of positive inflection from baseline
- 7 – 3 = +4
- Therefore, I will plot approximately 4 along the horizontal axis heading towards 0 degrees
- Lead aVF has 0 little boxes of negative inflection from baseline and 7 little boxes of positive inflection from baseline
- 7 – 0 = +7
- Therefore, I will plot approximately 7 along the vertical axis heading towards +90 degrees
- When I find the point that these two dots intersect, it is somewhere within -30 degrees to +90 degrees and therefore a normal cardiac axis!
The Fast Way
Based on the theory above, we can quicken the process a fair amount by understanding the following:
- If a lead has more positive inflection than negative inflection (predominantly positive), you will only be able to plot on the positive end of the graph
- If a lead has more negative inflection than positive inflection (predominantly negative), you will only be able to plot on the negative end of the graph
- If lead I and lead aVF are BOTH predominantly POSITIVE, the only area that their joining dot can fall is within 0 degrees to +90 degrees = NORMAL AXIS
- If lead I and lead aVF are BOTH predominantly NEGATIVE, the only areas that their joining dot can fall is within -90 degrees to 180 degrees = EXTREME AXIS DEVIATION
- If lead I is predominantly NEGATIVE and lead aVF is predominantly POSITIVE, the only area that their joining dot can fall is within +90 degrees to 180 degrees = RIGHT AXIS DEVIATION
- If lead I is predominantly POSITIVE and lead aVF is predominantly NEGATIVE, the only area that their joining dot can fall is within 0 degrees to -90 degrees
- This could mean it is either a LEFT AXIS DEVIATION or it could be a NORMAL AXIS if between 0 degrees to -30 degrees
So you have to ask yourself one last question if you have a positive lead I and a negative lead aVF: what is lead II doing? Lead II views the heart directly in line with the physiological movement of electrical energy during depolarisation. Therefore, if lead II is predominantly positive it will drag the cardiac axis back into the normal area thereby meaning that there is a normal cardiac axis. If lead II it is anything else (negative or biphasic), it does not have enough pull to drag the cardiac axis back into the normal area thereby meaning that there is a left axis deviation.
Let’s use the following ECG’s as practice:
The Really Fast Way
Now that you understand the theory behind how a cardiac axis is calculated, you understand that a QRS complex with an axis of 54 degrees means that it is a NORMAL CARDIAC AXIS!
The take home point? When you look at an 12 lead ECG and recognise that you do not have a normal cardiac axis, look a little closer. Right, left and extreme deviations are generally caused by an abnormal flow of electrical energy within the heart; upon closer inspection you might discover something that you didn’t realise was there before!
- Chung, D. C., & Nelson, H. M. (2008). ECG: A pictorial primer. Retrieved from: http://www.medicine-on-line.com/html/ecg/e0001en.htm
- Life in the Fast Lane. (2016). ECG axis interpretation. Retrieved from: http://lifeinthefastlane.com/ecg-library/basics/axis/
- Wagner, G. S., & Strauss, D. G. (2014). Marriott’s practical electrocardiography (12th ed.). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
- Wesley, K. (2017). Huszar’s ECG and 12 lead interpretation (2nd ed.). Missouri: Elsevier.
If you liked this post, why not subscribe to Blogging for your Noggin? Be one of the first to know when new content becomes available! I promise there will be no annoying spam!