Reassuring and Nonreassuring FHR Patterns


There are 2 types of fetal heart rate (FHR) patterns, heart rate patterns that are either reassuring or nonreassuring.

As the name implies, reassuring fetal heart rate patterns are those which we want to see and that are positive to see and indicate good health and lack of distress in a fetus.

Nonreassuring fetal heart rate patterns are those which we do not want to see and that may show that the fetus is experiencing some type of distress.

That's the broad view of it.

Reassuring FHR Patterns

Now we will go specifically into the components that make up reassuring FHR patterns.

So FHR patterns that are reassuring are characterized by the following parameters.

  • A baseline FHR in the normal range of 110-160 bpm
  • Moderate variability
  • Accelerations with fetal movement
  • Absence of nonreassuring signs

So all of these factors are things that we want to see in the heart rate of a fetus.

The normal baseline value that a fetus's heart rate should be is 110-160 bpm. Below 110 bpm prolonged for 10 minutes or more would be a bradycardic heart rate. Above 160 bpm prolonged for 10 minutes or more would be a tachycardic heart rate. We do not want to see values prolonged either below or above this baseline. So if it's anywhere from 110-160 bpm, then this is a reassuring heart rate.

Next, we want to see moderate variability in the heart rate. Variability is the amount of beats that a heart rate varies by in a certain period of time. We don't want to see a straight line, meaning the heart rate isn't changing at all, which is called absent variability. We don't want to see this constant, unchanging heart rate because if the fetus is moving around and is engaging in activity, this should cause increases in the heart rate. Just like with adults, if we exercise, our heart rate accelerates to make up for the amount of oxygen we need due to this activity. With a fetus, it's the same exact thing. Movement should cause a rise in heart rate. Except in the case where a fetus is sleeping, if the fetus is moving, an absent variability could be an ominous sign. So while the fetus is moving, the heart rate should be varying. Moderate variability causes a change in heart rate between 6 and 25 bpm from the baseline. So, if the baseline is, for example, 140 bpm, we want to see the heart rate varying at least 6 bpm from this baseline. Any variability is good because it shows an intact autonomic system, in which there is variability in the heart rate based on whatever conditions a fetus is going through.

The next reassuring sign, which ties in with what we just talked about, is accelerations with fetal movements. As we just mentioned, heart rate should increase with movement. So, if a fetus is moving around, we should see a corresponding rise in heart rate with these movements. During late in pregnancy, a woman may go for a test called a nonstress test. This nonstress test measures the fetal heart rate with fetal movements. How the system works is any time a woman feels movement of the fetus in her uterus, she should press a button that she is provided with. The healthcare professional will then examine the heart rate as the mother claims she senses movement. Seeing accelerations in heart rate is a reassuring sign. It shows the baby's autonomic nervous system is functioning well and compensating for increased oxygen demand caused by the movements. The classifications of a nonstress test are either reactive or nonreactive. If a nonstress test is reactive, there will be 2 fetal accelerations in a 20-minute period. A fetal acceleration is defined as an increase of at least 15 bpm from the baseline heart rate for at least 15 seconds. Again, this is a good, reassuring sign of fetal health.

The last reassuring sign is the absence of nonreassuring signs, which indicate possible fetal distress, which we talk about below.

Nonreassuring FHR Patterns

We now go into nonreassuring signs, which are negative in nature and could indicate possible fetal distress.

So FHR patterns that are nonreassuring are characterized by the following parameters.

  • Tachycardia
  • Bradycardia
  • Absent variability
  • Late decelerations
  • Variable decelerations (falling to less than 70 bpm for longer than 60 seconds)
  • Prolonged decelerations

Again, we want to see a baseline FHR of 110-160 bpm. A heart rate elevated above 160 bpm for 10 minutes or more is defined as tachycardia. Heart rate below 110 bpm for 10 minutes or more is defined as bradycardia. Both of these could be ominous signs. Numerous things can cause tachycardia. If a fetus is not getting enough oxygen, the fetus may become tachycardic in order to pump blood faster throughout the body as a form of compensation. Thus, hypoxia can cause a state of tachycardia. But it can be caused by other things as well such as maternal fever, maternal dehydration, fetal acidosis, maternal or fetal hypovolemia, fetal cardiac disrhythmias, severe maternal anemia, maternal hyperthyroidism, and different types of drugs. So there's no definitive cause that can be found immediately. Just the fact that tachycardia is there can indicate some anomaly such as these. So once tachycardia is found out, then a healthcare professional would have to investigate the cause of it.

The same thing is the case for bradycardia. It's a sign that something is wrong and needs to be found and fixed. Things that can cause bradycardia are hypoxia, fetal head compression, fetal acidosis, fetal heart block, umbilical cord compression, and late second-stage maternal pushing. Once bradycardia is assessed, a cause needs to be found.

So tachycardia and bradycardia are both nonreassuring signs.

Absent variability also can be an ominous sign. When the heart rate isn't varying, it may be that the fetus is not moving, which is a cause for concern, or that the fetus's autonomic nervous system isn't functioning properly to adjust the heart rate in accordance with activity. Except in the case of sleep, where heart rate is fairly constant, the heart rate should be varying somewhat. Absent variability means the heart rate is essentially unchanged over a period of time. What we want to see is moderate variability, where the heart rate changes 6 to 25 bpm from baseline. If the fetus is asleep, an acoustic vibrator may be placed on the woman's abdomen to awaken the fetus. When awake, the fetus should show signs of moderate variablity. Thus, absent variability is a nonreassuring sign for an awake fetus.

Late decelerations are decreases in a fetus's heart rate below baseline usually after the peak of a contraction. It is a serious condition that signifies impaired placental exchange of blood flow and oxygen to the fetus. Seeing late decelerations is a nonreassuring sign, because the health status of the fetus is being compromised. When late decelerations are seen, especially constantly over a period of time, medical intervention needs to be done in order to improve the blood flow and oxygenation to the fetus. These interventions include giving fluid to the fetus, sitting the mother up in a semi-Fowlers to high-Fowlers position to ease lung expansion, or giving oxygen to the mother.

Most variable decelerations aren't necessarily a cause for concern unless the heart rate falls to less than 70 bpm for at least 60 seconds. In this case with such a dramatic drop in heart rate, the status of the fetus can be compromised. With such a low heart rate, oxygenation to the fetus may be compromised. Thus, this is a nonreassuring sign when the heart drops this low for at least a minute.

Prolonged decelerations are decelerations that drop at least 15 bpm for at least 2 minutes or greater but lasting less than 10 minutes. There can be a number of causes including maternal hypotension, uterine hyperactivity, cord compression, and cord prolapse. If the heart rate decelerates for more than 2 minutes from baseline, then this needs to be investigated by a healthcare professional to improve the fetal status.

So reassuring FHR patterns include a baseline FHR in the normal range of 110-160 bpm, moderate variability, accelerations with fetal movement, and the absence of nonreassuring signs. These are things that are good to see in a fetus's heart rate pattern and are positive signs of good health.

Nonreassuring FHR patterns include tachycardia, bradycardia, absent variability, late decelerations, variable decelerations falling to less than 70 bpm for longer than 60 seconds, and prolonged decelerations. These are things we do not want to see and are signs of fetal distress and health compromise.

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