You want to learn how to interpret an ECG? Great, you’re in the right place, but before you do that, make sure that you have understood the Anatomy and Physiology of the Heart.
Introduction to ECG
As you should know by now, the heart functions as a pump. It has a regular rate and rhythm of how many times it pumps; otherwise, blood will not circulate properly around the body. Typically, in a healthy adult, the heart pumps around 60 – 100 times per minute with regular intervals.
You might start to wonder, though, what causes the heart to beat? And the answer is that a heartbeat is triggered by an electrical impulse coming from the Sinus Node (SA Node).
The SA Node is an area close to the superior vena cava on the right atrium. It sends an electrical impulse that rapidly travels through the atria to the Atrioventricular Node (AV Node). This flow of electrical impulses is known as Conduction, and it is why the heart contracts.
When the atria are contracted, the AV Node slows down the electrical impulse to give time for the ventricles to fill with blood – this is known as the Atrial Kick. From there, the electrical impulse rapidly travels through the bundle of His into both the Right and Left Bundle Branches, and the Purkinje Fibers which are in the ventricular muscle.
As soon as the muscle cells of the ventricles receive the electrical stimulation, they contract, causing what is known as Systole. Then while the cells repolarise, the ventricles let go and relax, and this causes Diastole.
So to Recap:
Electrical Stimulation is Depolarization which causes Contraction aka. Systole
Electrical Relaxation is Repolarization which causes Relaxation aka. Diastole
The Electrocardiogram – ECG
Essentially, an ECG reflects the heart’s electrical impulse as it goes through the cardiac cycle. In fact, every stage of the cardiac cycle produces a different wave, which can be seen on a cardiac monitor or a strip of paper.
To obtain the ECG, you will need to stick electrodes (little stickers, with a metal piece in the middle) on your patient’s body, and attach them to a monitor. Because the electrodes are conductors, electricity travels through them, and it translates into a graph.
In simple words: The electrical impulse caused by the heart can be felt by the electrodes, and this creates a wave on the monitor.
The number of electrodes and placement will vary depending on why you want the ECG. If it is for continuous monitoring, a 5-lead ECG is used, while if it is for a quick diagnostic look, a 12-lead ECG is done.
Ideally, you should avoid sticking the electrodes on bony areas, as these can create interference from the electrical activity of the skeletal muscles. Now let’s move on to interpreting it, which is the whole reason why you’re here.
Interpreting an ECG
When looking at an ECG, you’ll notice that it is shown on a graph:
The horizontal axis measures the time and rate, while the vertical axis measures the amplitude or voltage. A wave facing upwards is called a Positive Deflection, and a wave facing downwards is called a Negative Deflection.
The next thing you should know is that you can refer to an ECG either by the waves:
P Wave
QRS Complex
T Wave
U Wave
Or by the segments:
PR Interval
ST Segment
QT Interval
Let’s start to understand the waves…
As the electrical impulse starts in the SA Node and spreads through the Atria, it causes Atrial Depolarization (Contraction of the Atria). This creates the P Wave, which is typically not more than 2.5mm tall and 0.11 seconds long.
Next comes Ventricular Depolarization (Contraction of the ventricle), and this creates the QRS Complex. The first negative deflection (drop) in the graph is the Q Wave, followed by a Positive Deflection known as the R Wave, and another negative deflection called the S Wave. The whole complex takes around 0.12 seconds to complete.
After depolarizing (contracting) both the atria and the ventricles, it is now time to repolarize (relax) them. So, as Ventricular Repolarization occurs, a bump known as the T Wave is created on the graph.
Atrial Repolarization occurs at the same time as Ventricular Repolarization, but there is no wave to represent it as the T Wave covers it.
In standard cases, those are all the waves that you’ll see P, QRS, T. But in rare cases like patients with Hypokalemia, you will also see a U Wave.
The U Wave is the Repolarization of Purkinje Fibers, and it is shown as another ‘bump’ after the T Wave.
Ok so those are all the waves, the next thing to know is the intervals…
The PR interval is from the start of the P Wave to the beginning of the QRS complex, and it averages between 0.12 seconds to 0.20 seconds long. It basically reflects how long the impulse takes from the SA Node to reach the start of Ventricular Depolarization.
The time between the end of the QRS complex and the beginning of the T Wave is the Early Ventricular Repolarization period, and it is called the ST Segment.
The QT interval is the total time taken for ventricular depolarisation and repolarisation to occur, which is approximately 0.32 seconds to 0.40 seconds long. To calculate it you need to find the beginning of the QRS and the end of the T Wave.
There is one point at which the ECG does not register any electrical activity, so it draws an Isoelectric line (a flat line). This is usually between the end of the T wave and the start of the next P wave, and it is called the TP interval.
Lastly, if you want to measure the Atrial rate and rhythm, you need to calculate the PP interval so from one P Wave to the next P wave. And if you want the Ventricular rate and rhythm, you need the RR interval which is from one R wave to the next R wave.
And those are all the waves and segments that you need to learn! Not so bad no?
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