# Step 7: Measuring the QT Interval (Nursing)

by Rhonda Lawes, PhD, RN

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00:01 The final step in our list is to evaluate the QT interval.

00:05 Now, as you can see, the QT interval extends from the beginning of the QRS complex through the ST segment and to the end of the T wave.

00:15 In our cardiac cycle, that represents the entire process of ventricular depolarization, and repolarization.

00:25 Now how to calculate the QT interval is not typically a part of your regular shift looking at an ECG.

00:34 I'm going to walk you through how we do it but I don't expect you to memorize this math or to know how to do this.

00:41 But I just want you to know that because the QT interval is due to heart rate, we use the QTc to normalize a person's heart rate.

00:50 So let's take a look at some corrected QT intervals just for you to get a feel.

00:55 Now hang on because here comes the math.

00:58 First you measure the QT in seconds, then you measure the R to R interval in seconds, then you get the square root of the RR interval, and then you divide the QT seconds by the square root of the RR.

01:13 So if you went through that, it's gonna be not that difficult to do but it's not something that's easy to just remember off the top of your head.

01:22 So, when a QTc is calculated, then we can use it for some comparison.

01:28 A normal QTc is less than 0.46 seconds in women and less than 0.45 seconds in men.

01:38 When we get really excited is when a QTc is greater than .5 seconds.

01:44 This is dangerously prolonged, and it really puts your patient at risk.

01:49 So we're not gonna walk through how to calculate this today.

01:54 I just wanted you to be aware that this exists.

01:57 And to know that greater than .5 seconds is dangerously prolonged for either men or women.

02:04 Now, what we call this abnormal QT interval is a prolonged QT interval.

02:10 So that makes sense, right? A prolonged QT interval is also called a long QT syndrome.

02:17 We use a lot of alphabet soup when it comes to ECGs.

02:20 And you'll see it abbreviated LQTS.

02:23 Now, Long QT Syndrome can cause rapid and chaotic heartbeats.

02:30 This is not an effective heartbeat to pump blood through your body.

02:34 Mild cases can lead to dizziness or seizures, but severe cases can lead to sudden death.

02:39 So this is a significant clinical problem.

02:44 Let's take a look at the causes of LQTS.

02:47 Well, they're grouped into two categories: non-modifiable risk factors and modifiable risk factors.

02:55 Well, the non-modifiable risk factor for LQTS are genetic mutation.

03:01 So there's nothing a patient can do about that, which is why it's called a non-modifiable risk factor.

03:09 Now the other category is modifiable risk factors, we can intervene with those.

03:15 So that can include certain medications, significant mineral imbalances, or some other underlying medical condition.

03:23 So when you're thinking about who might end up in this trouble, LQTS can be caused by a genetic mutation, we can't do much to fix that, or we have medications, mineral imbalances or an underlying medical condition.

03:38 So what do we do to treat this? Well, one part of the treatment could be to avoid those medications that gives the patient LQTS - that would be a good move.

03:49 But if that's not enough, and we're still having trouble, the patient can have an implantable defibrillator device.

03:56 Now my mom got one of these and it has been life changing for her.

04:00 It's given her so much energy, because she has a defibrillator, which has never gone off, thank you.

04:06 But she has a pacemaker, and that has kept her heart beating at a consistent rate and she has much more energy for activities of daily life.

04:16 Now LQTS, modifiable what was the risk factor? Ah, see, I went with the second piece first.

04:24 What I did, there was what I want you to do when you study.

04:28 Don't keep going in the same exact order.

04:31 When you're studying with a group, ask questions like that.

04:34 See if you can pause and recall.

04:36 Quiz each other to see if that is making it into your long term memory.

04:42 Now, I have a list here of things that can cause LQTS and when you go through pharmacology, you'll probably learn about this but take a look at this list.

04:53 There's some really common antibiotics like erythromycin, azithromycin and there's even others, some antifungal medications that are taken by mouth and used to treat yeast infections.

05:04 Diuretics that causes a significant electrolyte imbalance, like low potassium can cause LQTS.

05:11 Heart rhythm drugs, especially anti-arrhythmic medications can lengthen the QT interval to a point that it becomes a problem.

05:19 There's also certain antidepressants and antipsychotic medications.

05:22 The list is too long for you to memorize but you'll know when you have patients on these medications when you review that possible adverse effects that this is an option.

05:35 Now we talked about the mineral imbalances you'll see here, this could be low potassium, could be low calcium, low magnesium, and even COVID-19 (SARS-CoV2) infection can cause LQTS.

05:51 So let's wrap this up.

05:52 We learned how to assess and measure each of the waveforms on an ECG.

05:57 And we know it's the first step in accurately identifying cardiac dysrhythmias.

06:02 Now, we encourage you to watch the other videos in this series to learn how to apply these new skills and identifying normal, abnormal and life threatening cardiac rhythms.

06:13 Thank you for watching our video today.

The lecture Step 7: Measuring the QT Interval (Nursing) by Rhonda Lawes, PhD, RN is from the course The Basics of ECG Strips (Nursing).

### Included Quiz Questions

1. It is considered dangerously prolonged for both men and women.
2. It is considered normal for both men and women.
3. It is considered normal for women but dangerously prolonged for men.
4. It is considered normal for men but dangerously prolonged for women.
1. Sudden death
2. Dizziness
3. Seizures
4. Tachycardia
1. Hypomagnesemia
2. Covid-19 infection
3. Azithromycin
4. Hypokalemia
5. Hypercalcemia

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