Primary and Secondary Powers and Cardinal Mechanisms of Labor (Nursing)

by Jacquelyn McMillian-Bohler

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    00:00 Okay, now let's talk about the powers of labor. So these are the forces that actually move Clitus out of the uterus and brings him into the world. The powers are the contractions of the uterus and the maternal effort or the pregnant patient as they help to move the baby out of the uterus. Let's first look at the uterus and the contractions. So if you remember from a few lectures ago, we talked about Braxton-Hicks contractions. And Braxton-Hicks contractions are tightening of the uterus, but they're not painful, they're not regular, and they don't lead to any type of cervical change and that's kind of what you're looking at in this graphic here. You can see that represented by the hills at the bottom. As we move towards labor, you'll find that the contractions get closer together, they're a little bit more regular, and they're stronger. And these begin to actually cause cervical change and you can see that and you can feel that and definitely the pregnant patient can. As we move to labor, we meet that 5-1-1 criteria. This is a test to see if you remember it from before.

    01:08 Contractions are 5 minutes apart, they last at least 1 minute, and they've been that way for at least 1 hour. And that's what you can see in this very last graphic. These contractions are regular and they are strong and they actually will lead to cervical change. So, when we think about contractions being close together and regular, there is such a thing as too much of a good thing. And that's what we see here with what we call tachysystole. So again, looking at this graphic, what you'll see is these contractions are almost on top of each other.

    01:41 They're about a minute apart. So we want the contractions to last for a minute, but we don't want them be a minute apart because that's too close together and it makes it hard for the uterus to relax in between, the baby can't be oxygenated and it's just not functional. This is too close together and it's called tachysystole. So, let's think about the powers in another way in terms of what they actually do to the cervix or the lower part of the uterus, because the primary powers are responsible for effacement, dilation, and descent of the fetus.

    02:19 So, this is a graphic representation of cervical effacement and dilation. Now, when you see on the unit, you'll see someone's report listed as 0–10, and what they mean is the openness of the cervix. It starts as 0 and moves all the way to 10 cm. When we talk about effacement, we're talking about the thinness or the length of the cervix. And that goes from 0 indicating that the cervix is not effaced and it's thick to 100% that's completely thinned out. And so the range of 0–10 or 0–100% is part of the progression of labor and it's caused by the powers of the contraction. So let's take a look at what that looks like. So in the first graphic, what you see is that there is no dilation, the cervix is closed or almost closed and there's no effacement, the cervix is long. For a first time pregnant patient, what we find is that effacement happens first so the cervix gets shorter before the cervix begins to dilate and open. Okay? So that's what you see in this second representation. The cervix is now almost 50% of the way thinned out, so this would be 50% but the cervix is still closed, not particularly unusual. As we move to the next graph, the cervix is completely thin, so 100% and now we have the beginning of a little bit of dilation so maybe the cervix is 3 cm or 4 cm dilated. As we get to the last graphic, we see that the cervix is completely gone away and it's completely effaced and completely dilated, and so now we're ready for descent. Now when we think about what descent and how it's measured, we want to think about our landmarks.

    04:04 So we determined station, that's the last part, by the relationship of the presenting part to the ischial spine. So how close is it? Before the fetus actually comes into contact with the ischial spines though, it's floating above the pelvis. And we actually call that ballotable. So to understand what that term means, think about a balloon. If I would touch an inflated balloon, if I touch it it would float away. That's called ballotable. If I were to touch the baby's presenting part by digital exam, if it's ballotable it will float away from my fingers. So it's not engaged into the pelvis. So, that's what ballotable means. But when you think about station, we're going to break it down a little bit more. So you can see on this graphic, this presenting part we have luckily an occiput presentation, we're looking at the relationship of that occiput to the ischial spines. Now in this graphic, the fetus is above the level of the ischial spine.

    05:03 So it's represented by a negative number. Now in this case, it's about 3 cm above the ischial spines, so that's -3. As it progresses down, it's going to go -2, now it's closer to the ischial spine -1, 1 cm above the ischial spine. 0 at the level of the ischial spine, also known as engagement. Then the numbers changed to plus, +1, now it's below the ischial spine, +2, +3, +4, +5 is pretty much on its way out. Okay? So that is what station looks like. I want to show you one more time with the pelvis to make sure you have it. Okay, ischial spines are here. So when we talk about station we mean the level of the presenting part in relationship to the ischial spines. Ballotable is going to be up here. As the baby moves down, we go 5, 4, 3, 2, 1, 0 at the level of the ischial spines. And then as the baby begins to rotate out, +1, +2, +3, +4, and so on. That is station. So those were primary powers. But what about the pregnant patient, they absolutely have a role in this and secondary powers actually refer to that pushing effort that the pregnant patient makes. So I want you to pay close attention to the fact that this patient is upright and using gravity to actually help push that baby out and that's what we want to do, if at all possible, is try to get the patient in an upright position so that gravity can actually help them push the baby through the pelvis. So, the more upright, the better. Patients that are pushing flat on their back like you often see on TV, that goes against the gravity. So only in the case when the patient needs a rest or maybe there is something about their medication that requires for them to be in that position should they be. Otherwise, the more upright and the more movement that we can get going, the easier it's going to be for that baby to come through the pelvis. The other thing is that you'll notice in this picture is that the patient is in a squatting position. So, the more we squat, what happens is the space actually narrow, so it makes the vaginal canal shorter.

    07:31 So the more the legs are pulled back and the more we're in that squatting position whether it's upright like this or on your back, it shortens the space between the inside and the outside. So that's better. So now that we've done all of the powers, let's really think about what that looks like in relationship to the mechanisms of labor. So this is how the baby comes out. We have engagement. Now we know what that means. The presenting part is at the level of the ischial spines. Then we have descent because the fetal head hits that pelvic floor and flexes into a flexed attitude. Again, did you see how that changed the diameter of the fetal head. We have internal rotation and we're going to go through a couple clicks of the slide so you can see that. Watch. And we've rotated to an OA position, so now we're in a great position for birth. And then as we move through station, we're going to have extension. And now the head is out. We have one more rotation, external rotation, and expulsion. So that happens as a direct result of the powers of both the uterus and the pregnant patient.

    About the Lecture

    The lecture Primary and Secondary Powers and Cardinal Mechanisms of Labor (Nursing) by Jacquelyn McMillian-Bohler is from the course The 5 Ps of Labor (Nursing).

    Included Quiz Questions

    1. Uterine tachysystole
    2. Uterine contractions, during labor, that are 5 minutes apart, last at least 1 minute, and have been the same way for at least one hour.
    3. Regular uterine contractions
    4. Strong uterine contractions
    1. Braxton Hicks contractions
    2. Eccentric contractions
    3. Concentric contraction
    4. Isotonic contractions
    1. Contractions that are a minute apart and may cause fetal oxygen deprivation
    2. Contractions that are five minutes apart that last at least 1 minute and remain so for at least an hour
    3. The body's way of preparing for true labor, which is also called false labor pains
    4. A strong contraction, which leads to cervical change and is a sign of good fetal oxygen supply
    1. All answers are correct.
    2. The primary powers of the uterus are responsible for effacement, dilation, and descent of the fetus.
    3. Regular and strong contractions lead to cervical change.
    4. Effacement is the thinness or length of the cervix.

    Author of lecture Primary and Secondary Powers and Cardinal Mechanisms of Labor (Nursing)

     Jacquelyn McMillian-Bohler

    Jacquelyn McMillian-Bohler

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