Okay, so you’ve decided that you want to learn how to interpret an ECG…
Interpreting an ECG as a paramedic is just that, an interpretation of the ECG. If you google steps in interpreting an ECG you will get thousands of different steps (all fairly similar, some very different)… but most will give you a basic interpretation of an ECG. In my careers as a paramedic I have often seen cardiologist review and debate the interpretation of the very same ECG. These are the steps that I use… if they work for you that’s great… if your ambulance service requires you to utilise a different technique than that is what you should use. This is just a guide.
But first… a little bit about ECGs and what you’re actually looking at.
At a very basic level, an ECG is just a graph representing the basic electrical flow of the heart, as seen by small (snap-shot) views of the heart. If you think of an ECG lead as a camera looking at the heart, each separate lead is a different camera angle of the heart. Therefore, multple leads will allow a better overall view of the heart. For the purpose of this ECG interpretation webpage, I will only discuss lead II.
So what normally happens with the heart? In a normal person there is an electrical impulse generated by the sino-atrial (SA) node in the right atrium of the heart. This impuse is conducted through the cardiac muslce fibre to the atrial-ventricular (AV) node. From here the impulse conducts down the right and left bundle branches and finally disipates in the purkinje fibers causing the ventrical to contract and thus creating the pumping action of the heart. These electrical waves can be measured by the ECG electrodes which are placed on the skin and record minute levels of electrical activity.
- Here is an example of an ECG
The first small spike you will see is called the P wave. It represents the firing of the impulse from the sino-atrial node and the subsequent contraction of the atria or the upper chambers of the heart. The space between the P wave and another wave called the R wave is known as the PR interval. This represents the time between the electrical signal of atria contraction and the signal of ventricle contraction. The R wave (represented by tall spikes) is in between a small dip called the Q wave and another small dip called the S wave. Together, the QRS complex (Q, R, and S waves) demonstrates the activation of the left and right ventricles of the heart.
On the other hand, the T wave represents the resetting (or repolarization) of electrically charged cells in the ventricles. This resetting occurs during the contraction of the ventricles, masking the signals so they will not appear on the waveform. When resetting finishes, the entire cycle repeats.
Similar to that of the PR interval, the waveform from the end of the QRS complex to the starting point of the T wave is called the ST segment. The waveform from the starting point of the QRS complex to the end of the T wave is called the QT interval.
When reading the ECG, the height of the waves denotes electrical signal strength and the overlap of the opposing signals. The space between the waves represents the time it takes for the signal to pass through your heart.
Okay… so you know a little bit about the the hear and the electrical activities now… how do you interpret the ECG?
Follow these steps:
1. Look at the QRS Complexes – Are they regular or regular? You can feel the patient’s pulse while you do this and see that the patient has a regular or irregular pulse and that this matches up with what you are seeing on the ECG or EKG if you’re in the US.
2. Determine the rate by counting the QRS Complexes in a 10 second strip. Is it fast (greater than 100 beats per minute), Normal (60-100 beats per minute) or slow (less than 60 beats per minute)?
3. Look at the P- Waves. Are they normal? A normal P wave should be upright in lead II and should be identical to all the other P waves in the ECG. A P wave should also precede a QRS Complex. If you can’t see a P-wave, the person most likely has Atrial Fibrillation – (see if this is normal for the patient, or new).
4. Look at the PR interval and the relationship between the P-Waves and the QRS Complexes. A PR interval should normally be between 0.12 and 0.20 seconds in length. If it is longer than this, there is a conduction problem between the SA node and the AV node. If it is shorter than this, the SA node is firing too quickly as is the case in Atrial Tachycardia.
5. Look at the RR intervals – are they regular? Are they identical in timings? In a normal sinus rhythym an RR wave should be consistent.
6. Look at the QRS Complexes – It is normally less than 0.1 seconds in length. If this is longer, there is a conduction problem between the AV Node and the Perkinje Fibres (this indicates a Bundle Branch Block).