# The Isoelectric Line of an ECG Strip (Nursing)

by Rhonda Lawes, PhD, RN

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00:01 So let's start digging into that ISO electric line.

00:04 So we're gonna ask you a question, what exactly is the ISO electric line? Well, the answer is, it's nothing.

00:15 The ISO electric line is the flat line of the ECG strip, where the electrical activity is absent.

00:23 There's no or nothing going on there, there's no electrical activity.

00:29 So take a look at our strip.

00:31 We've outlined the ISO electric line for you.

00:34 See it there? Yeah, there's nothing going on during that time.

00:40 Now, I know when you see P wave, and then the sharp QRS.

00:44 Remember, the isoelectric line is the average space where nothing is going on.

00:49 You draw a line to connect all those spots where nothing is happening.

00:54 So between cardiac cycles, the ECG recorder returns to the ISO electric line.

01:00 So it goes to the P wave isoelectric line, QRS, then back to the isoelectric line before the T wave.

01:08 And then after the T wave, back to the ISO electric line.

01:12 So between those cardiac cycles, that's when the ECG recorder or stylus returns to that line.

01:19 This is kind of our landmark. This is where we measure everything.

01:24 The last piece I want you to know about that isoelectric line, or no electricity is going on, is that a positive wave is the ones that go above the isoelectric line.

01:35 We call that a positive deflection.

01:38 Now, negative waves are below the line.

01:41 So take a minute, circle or mark the waves, you know as positive by just putting a little plus sign in your notes.

01:48 The ones that you see as negative or a negative deflection, mark those with a minus sign.

02:01 So on this one, draw in the isoelectric line.

02:05 Follow it all the way through.

02:07 Does that make sense that it would be placed there? Well, right. It's the spot that everything returns to, and it means there's no electrical activity.

02:16 But I want to talk about three key points.

02:19 So we've laid arrow one, two, and three there.

02:22 Let's look at the line this way.

02:25 Arrow one is the time before the SA node sends the signal through the left and right atriums.

02:33 Okay, so arrow one, that part of the isoelectric line before the P wave that represents the time before the SA node sends the signal through the left and right atrium.

02:44 Now, between cardiac cycles, it keeps returning to that ISO electric line.

02:51 Second arrow. This is the AV nodal delay, which is the time between the atrium and ventricle depolarizing.

02:59 This is what separates the contraction of the atria from the ventricles to allow the ventricles to fill, receive blood from the atrium.

03:08 Okay, that was a lot of words.

03:10 Let's pause and kind of walk through this again.

03:13 We know the isoelectric line is the flattest part, right? That's where everything returns back to.

03:20 Now, arrow two is telling us, "Wow, it is made it through the SA node.

03:25 Arrow two is the flat part of the isoelectric line that lets us know it's the AV nodal delay.

03:34 Now, why do we need an AV nodal delay? Because we want the atrium and the ventricles to work together.

03:41 So when the atriums contract, right, when they depolarize, and they contract, they squish blood into the ventricle.

03:49 We don't want the atrium and the ventricles contracting at exactly the same time.

03:55 That's just going to make chaos.

03:57 What we need is atrium, small delay, ventricles.

04:02 Atrium, ventricles; atrium, ventricles; atrium, ventricles.

04:06 See what's happening here? The atrium is being able to increase the pressure, right? So it contracts.

04:14 We need time for that blood to make it through the valve into the ventricle and for the ventricle to fill up before it contracts or depolarizes, and sends blood on to the next destination.

04:28 So that little AV nodal delay is really important to an efficient heart.

04:34 So when you see that right after the P wave where we've marked it with arrow two that's exactly what's going on in the heart.

04:43 Now, the third arrow. This is the ST segment.

04:47 And you see those letters there S and T.

04:50 This represents the interval between the ventricular depolarization and repolarization.

04:56 So when the ventricle is depolarize, and it contracts then it repolarizes or relaxes.

05:04 And that's what you'll see on an ECG right there at arrow three.

05:08 Now, here's a note.

05:10 I want you to kind of file away for later.

05:12 But if the ST segment is elevated, now that means in comparison to the isoelectric line.

05:19 If the ST segment is elevated, or it's depressed, compared to that isoelectric line, this can let us know, "Hey, something serious could be going on with this patient's heart." It might mean myocardial ischemia, like chest pain, where the tissue is still alive, but it's really in trouble.

05:37 Or myocardial infarction, which is dead tissue.

05:42 And remember, dead tissue is stiff tissue, and it's not able to contract, and expand, or relax, like we need it to to be efficient.

05:52 So let's wrap up this part of the video series.

05:56 Now, just as a gentle reminder, each small box is one millimeter on each side.

06:01 So, it's a one millimeter high, one millimeter wide, and that represents 0.04 seconds.

06:08 Each large box contains five small boxes, and represents 0.20 seconds.

06:14 Now, high amplitude and the QRS may be seen in patients with cardiac hypertrophy.

06:21 Low amplitude or voltage may be seen in patients with increased resistance to current flow.

06:27 It could be obesity, COPD, or pericardial effusion.

06:32 The isoelectric line is the flatline of the ECG strip where electrical activity is absent.

06:40 The ECG recorder returns to the isoelectric line between cardiac cycles in the AV nodal delay and after ventricular depolarization.

06:51 By using the isoelectric line as a landmark, important ECG strip intervals can be accurately measured as one part of the process to diagnose the health of the patient's heart.

07:05 Thank you for watching this part of our video series.

The lecture The Isoelectric Line of an ECG Strip (Nursing) by Rhonda Lawes, PhD, RN is from the course The Basics of ECG Strips (Nursing).

### Included Quiz Questions

1. The time between cardiac cycles
2. The repolarization of the ventricles
3. The stimulation of the AV node
4. The stimulation of the SA node
1. Positive deflection
2. Upward deflection
3. Downward deflection
4. Negative deflection
1. ST-segment
2. PR segment
3. T wave
4. QRS complex
5. P wave

### Customer reviews

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very informative, professional language in a way is implified to get the information short.
By Glaucia G. on 28. April 2023 for The Isoelectric Line of an ECG Strip (Nursing)

Very simple language to give us the simple guide that sometimes even as medical students we need from the nurses. | am particularly like to review nurses' lectures. She just make simple what is so complicated in our medical books