As with the interpretation of any test, having a systematic approach to ECG interpretation can make a daunting task more manageable while minimizing the likelihood of missing important details. Most nurses will perform thousands of these tests through the course of their career, giving us a great opportunity to refine our ECG literacy and potentially identify trouble early in patients with cardiac trouble.
If you talk to any ECG boffin you’ll typically find they have a specific approach to breaking down ECGs into their vital elements to ensure their assessment is thorough and reproducible. I try to look at ECG’s the same way I look at physical assessments; you follow certain steps to gather information that will come together to show the bigger picture and reveal the diagnosis. While there are lots of different ways of breaking down ECGs, I use the following steps to assess the important elements of every ECG I perform:
- The first step is to identify the rate.
- Determine whether the rate is normal (60-100bpm), fast (>100bpm) or slow (<60bpm).
- Most ECG machines will automatically interpret the rate for you (this is perfectly acceptable – the machines are clever).
- Alternatively, just count the number of QRS complexes on the rhythm strip and multiply this by 6 (a 12 lead ECG records for 10 seconds – 6 lots of 10 seconds in a minute).
- The second step is to determine if the rhythm is regular or irregular.
- The easiest way to do this is to run a sheet of paper across the top of the QRS complexes on your ECG, using a pen mark the location of the first and second QRS complexes.
- Now move the paper to the right so that the mark from the first complex is over the second complex. If the rhythm is regular the mark from the second complex will now be over the third. This should apply for any complex you move your marker to.
- The third step is to identify whether all features of the waveform are present
- Is there a P wave?
- Is there a QRS complex? If there is not, immediately recheck your patient – They are probably in trouble!
- Is there a T wave?
- Are there any additional elements i.e. U waves, J waves etc.?
- The fourth step involves identifying whether different intervals and features have a normal appearance. This step is often the most difficult but helps us to identify and differentiate between various rhythms and disease states in a more definitive manner. I find if you work through the different features in the order they appear you are less likely to miss steps or get fixated on particular points.
- P waves – Are they upright and symmetrical or do they exhibit abnormal morphology (i.e. fibrillation waves, flutter waves, inversion, asymmetry)
- PR interval – Is it fixed or does it change from beat to beat?
- QRS complex – Does it follow every P wave? Is it wide or narrow?
- ST segment – Is there ST elevation or depression?
- T waves – Are they upright or inverted? Is the amplitude (size) normal?
- QT interval – Is it abnormally long?
- Other – Are there any other features that are not usual i.e. abnormal Q waves, U waves, J waves etc.?
If you identify any abnormal features, it is also important to identify if they are generalized (present in all/most leads) or localized (present in only certain leads).
While breakdown process gives you a lot of information, it doesn’t tell you whether there is a problem or directly establish a ‘diagnosis’. To ‘diagnose’ an ECG, you need a bit more information on how this data fits together.
By far the most important aspect of understanding ECGs is having the ability to identify a ‘normal’ ECG. This means, confidently identifying an ECG in sinus rhythm with no evidence of acute ischemia or pathology. If you’re comfortable using a systematic approach to interpret an ECG of normal sinus rhythm, you will be able to determine when something is NOT normal sinus rhythm and escalate appropriately.
Normal sinus rhythm represents the rhythmic contraction of the heart muscle when stimulated by an electrical impulse generated in the SA node that moves through the atrium, AV node and ventricular conduction system without interruption.
An ECG of normal sinus rhythm contains the following features:
- A regular rhythm with a rate of 60-100 beats per minute.
- A single P wave which is upright in leads I, II and is inverted in aVR,
- A single narrow QRS complex
- A T wave that is upright (inverted in aVR)
- Fixed PR interval (less than 200ms)
- Isoelectric PR and ST segments.
When you come across something that is NOT normal sinus rhythm, it can initially help to use a tool to assist in determining the nature of the rhythm. There are loads of great charts and info-graphics around designed to take you through the steps of pulling apart different rhythms. Joanne Reading has a great flowchart for rhythm interpretation on her blog that you can find at:
The way I break ECGs down is a little different so I took some time to try and map my thought process. Using the steps from above, you should have all the data you need to follow this chart through to a potential diagnosis. You should always be mindful that while these types of charts will get you close to a definitive answer, there are occasionally confounding factors like funky conduction and anatomical disturbance that can send you off track. Also worth noting that if you reach a diagnosis of VF by taking a 12 lead ECG and following a flow chart – something has gone very wrong! If you are concerned about either the appearance of the patient or the appearance of their ECG – you have all the information you need, go get help.
But for what it’s worth, here’s my chart…
Once you have determined the rhythm, it is important to check for the telltale signs of cardiac disease or dysfunction. Primarily this will involve looking for signs of ischemia associated with myocardial infarction, however, it can involve looking for signs of electrolyte imbalance or even secondary organ dysfunction (i.e. raised ICP). While there are too many individual syndromes and diseases to fit into one blog post, essentially you can cover most bases by looking for any of the following abnormalities:
1 – ST elevation or depression: suggestive of abnormal ventricular depolarization / repolarization that may be caused by myocardial ischemia.
2 – T wave abnormalities: suggestive of abnormal ventricular repolarization that may be caused by myocardial ischemia. Most importantly look for T waves that are inverted or disproportionately large in more than one lead.
3 – A really wide QRS: suggestive of poor ventricular conduction that may indicate serious conduction problems or electrolyte abnormalities.
Using this information, try to ‘diagnose’ the ECGs you perform in your workplace. Think critically about whether each ECG is normal; if it isn’t, think about why it isn’t. When getting abnormal ECG’s checked, talk to your colleagues about what you think is wrong and ask them what they see. Having these discussions with your workmates is one of the best ways of developing not only skills in ECG reading but also more broadly developing skills in critical thinking and analysis.
If you want more comprehensive resources exploring different arrhythmias and abnormalities, I would encourage you to take a look at both ‘Life in the fast lane’ http://lifeinthefastlane.com/ecg-library/ and Dr. Smith’s ECG blog at https://hqmeded-ecg.blogspot.ae. Both of these sites are awesome free resources with a huge supply of knowledge about all things ECG.