Step 4: Measuring the QRS Duration (Nursing)

by Rhonda Lawes, PhD, RN

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    00:01 Now let's start with step four.

    00:03 I want you to measure the QRS duration.

    00:06 Now the QRS complex represents the depolarization of the left and right ventricles.

    00:12 Now it's made up of the Q wave, the R wave and the S wave.

    00:16 So see it, follow it with your finger.

    00:19 You've got the isoelectric line dips down, that's the Q wave, then it shoots up to the top, that's the R wave, and then back down again for the S wave, and you see the isoelectric line again.

    00:32 So the QRS for the left and right ventricles is made up of the Q, R and S waves.

    00:39 Now, ventricular depolarization is normally what triggers the contraction of the ventricles as we see that QRS happening on the ECG strip, it's the contraction of the ventricles that's actually happening in the heart.

    00:52 Have you ever wondered why the QRS is so much bigger than the P wave? Well, I want to see if you can pause and recall with what we've talked about each of these waves? Why would the QRS be so much bigger? Ready for the answer? Well, ventricular depolarization involves a greater muscle mass than that of the atria.

    01:17 Remember that ventricle is responsible for pushing blood into massive spaces, that's why it is so much bigger and stronger.

    01:26 The QRS is measured from the point where the complex begins to move from the baseline, and ends where the last wave of the complex begins to level out, or distinctly changes direction.

    01:38 So take a look at the complex we have there.

    01:41 Now we put two lines to give you an indication.

    01:43 Look how that first line, you see, it's measured where the complex begins to move from the baseline.

    01:50 That's where you would start the measurement.

    01:52 And then the second line we have in the spot where you want to measure at the end of it.

    01:57 That's where the last wave of the complex begins to level out.

    02:01 And we say level out meaning the isoelectric line.

    02:05 Now, we can get into a lot of arguments when we are interpreting strips.

    02:09 So as long as you systematically and consistently follow these guidelines, you're going to have a much better shot at getting something accurate when you're doing your own ECG interpretation.

    02:21 So you see where we start, you see where we end.

    02:24 The space in between that represents time should be the normal duration of the QRS complex.

    02:30 So it should be from about .06 seconds to 0.12 seconds, okay? So normal QRS is .06 to .12, and that stands for seconds.

    02:45 Or since you've already done the math .12 would be 3 small boxes.

    02:52 Now the QRS complex represents depolarization of both ventricles, but you only see one QRS, just like you only see one P wave.

    03:02 Even though both atria are contracting, you see one P wave because they're kind of right on top of each other.

    03:08 But when you talk about ventricles, the left ventricle pushes blood out all the way throughout the body, right? So that left ventricle is stronger than the right ventricle.

    03:21 The left ventricle has a greater muscle mass.

    03:24 It's because of its big job of shoving everything out through the aorta to the rest of the body.

    03:29 Now the deflection of the right ventricle is often hidden by the larger left ventricle.

    03:36 So one P wave, two atrium, one QRS complex, two ventricles, and likely what you're seeing is the left ventricle, because it is much stronger than the right ventricle.

    03:50 Whoa, I thought we said we weren't doing 12-leads here.

    03:54 Well, we just put this in here because I wanted you to take a look at how QRS complexes can look in different leads.

    04:02 Now, when we say different leads, you see where we have these 12-leads placed on our patient.

    04:07 Okay, each one of these little shots here I, right, You've got the lead I, AvR, you see the names of the different leads, those are just referring to the direction with which you're looking at the heart.

    04:21 You'll notice that in certain leads, the QRS is upright or positive, and in other leads, it's negative.

    04:29 Some of them it looks like wow, the amplitude is really small.

    04:32 That's all normal.

    04:35 So essentially, we'll be talking most about lead II, okay, so we'll be talking about lead II, So find lead II and our group of 12 there, and you'll see that oh, okay, that looks like we expect it to look.

    04:50 You've got the P wave, you got the QRS and you've got back to the isoelectric line.

    04:55 That's how we're teaching you.

    04:57 But know that there are 11 other leads where things might look a little different and still be normal.

    05:05 Now there can be abnormalities in a QRS waveform.

    05:09 A narrow QRS is normal as long as you're within the normal parameters.

    05:13 A wide QRS that's greater than .12 is abnormal.

    05:18 See, a wide QRS means there's some trouble.

    05:21 It could mean that the ventricles aren't all contracting together.

    05:24 And this might because you've got the disruption of the electrical conductivity, that is a bundle branch block.

    05:31 Okay, so that means you've got things moving through, but we've got a block and so the electrical conductivity, it's not able to move through as it would in a normal heart.

    05:41 So when you see a wide, kind of weird looking QRS, it might be a bundle branch block.

    05:48 We also see why QRS complexes when the electrical inputs for the heart originated in the ventricles instead of the SA node.

    05:56 Woah! We will show you some of those are called premature ventricular contractions is an example of a really weird looking QRS, that the heartbeat originated in the ventricles instead of the SA node.

    06:13 So remember, SA node - top and works down to the ventricles but if we have something like a premature ventricular contraction, that's the ventricles way of saying, 'I'm going to do it my own way', and the signal kind of goes backwards, so that's why it looks so weird and bizarre.

    About the Lecture

    The lecture Step 4: Measuring the QRS Duration (Nursing) by Rhonda Lawes, PhD, RN is from the course The Basics of ECG Strips (Nursing).

    Included Quiz Questions

    1. Ventricular depolarization involves a greater muscle mass than atrial depolarization.
    2. Ventricular depolarization involves less muscle mass than atrial depolarization.
    3. Atrial repolarization involves a greater muscle mass than ventricular repolarization.
    4. Atrial repolarization involves less muscle mass than ventricular repolarization.
    1. 0.06 – 0.12 seconds
    2. 0.1 – 0.2 seconds
    3. 0.01 – 0.09 seconds
    4. 1 – 2 seconds
    1. A QRS complex greater than 0.12 seconds
    2. A QRS complex less than 0.04 seconds
    3. An absent QRS complex
    4. An inverted QRS complex in lead II

    Author of lecture Step 4: Measuring the QRS Duration (Nursing)

     Rhonda Lawes, PhD, RN

    Rhonda Lawes, PhD, RN

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