P Wave in ECG [Explained]

This is a micro lecture on the basics of the Electrocardiograph (ECG) and P wave.

In today’s session, we’re going to continue our discussion on the basics of the ECG. In our previous discussion, we talked briefly about the nature of voltage and time and how when the atria contract, the top chambers of the heart contract on the ECG you see the P wave. So every time you see the P wave, you know that there’s been atrial contraction.

When you see the QRS complex, you know that there’s been ventricular contraction. And when you see the T wave, you know that there’s been ventricular relaxation. So let’s take that conversation a little bit deeper. Now the P wave serves a purpose of showing you, as the paramedic looking at it, that not only is there a contraction in the atrial, but also that the contraction is doing exactly what we think it should be doing.

Now the way that we see any abnormalities is in the shape of the P wave and also in the duration. The amount of time it takes to get from here to here. So this line here really is what we call the PR interval. Now all P waves should be what we call popping. So they should be nice and round. So they shouldn’t be triangle. They shouldn’t be big. They shouldn’t be more than one. It should be nice and round. That’s what we call a popping P. And that’s what your P wave should look like.

Now the PR interval is the time it takes for the P wave to actually occur in the first place and just to the bottom of this Q wave here, just before the Q wave actually goes down, the Q wave is that first negative deflection. Remembering negative means below the line. That there is the PR interval.

Now that PR interval should not be more than five small squares. And as you can see here, it’s one, two, three. So the PR interval here is between 12 and 20 small squares. So look at this time scale here. One small square on an ECG represents 0.04 of a second, so that’s really fast.

Whereas if you take five of them, and put them together, 0.04 times five is 0.2 of a second, so again really, really fast. What you have here is 0.04, 0.08, 0.12, so that’s absolutely fine. That’s 0.12. So according to this it’s perfect timing. If you had four, five, six, if this line here was six long, that means that there’s a block somewhere, and they actually call that first degree heart block.

Now on it’s own, it’s not a life threatening emergency, but in the presence of signs and symptoms and other things on the EGC, it can cause the patient some problems. So that’s my collection number two, with a basic recap of what we talked about in number one. This time we’ve added round first degree heart block, and the characteristics of the P wave.

I hope you’ve found that useful. My name’s Sam Willis from the Australian Paramedical College. I look forward to speaking to you again in another lecture.

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