Something I Learned Today: Fetal Heart Tones

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One of the things that obstetricians (doctors who deliver babies) use to decide whether or not a mother might need a C-section delivery (read my post yesterday for a bit of background on that) is by looking at a monitor that looks very much like the picture above. If you've ever been in a labor and delivery unit, you've probably seen something that looks very similar to this on some of the screens around the unit. These monitors measure what we in medicine call "Fetal Heart Tones", and it gives us an imperfect but useful perspective into how a potential newborn is handling the course of labor. In this post, I'll give you a brief overview of what this means, because I just learned about it formally for the first time this week.

In the picture above, you can appreciate two different "tracings" or graphs. The top graph is a tracing of the fetal heart rate as time goes along. On the units, we read this graph (and essentially every other graph in medicine with a few exceptions) from left-to-right, with the earlier values on the left and the newer values towards the right. The bottom graph (with the three bigger peaks) shows measurements from something called a tocometer. A tocometer measures the pressure generated by the mother's contractions as she progresses through labor. The peaks, unsurprisingly, correspond to increasing pressure generated by the contractions. Generally, these contractions that we measure tend to get stronger and more frequent as the mom is closer to delivering her child. Because these contractions are quite literally squeezing the baby inside the mother's abdomen, the baby can actually start to be "stressed". By "stressed", I mean that the baby isn't getting the proper amount of oxygen from the blood flow it's receiving from their mom.

As the baby gets more "stressed", its heart rate tends to slow down. This is actually a bit backwards compared to what happens to a people get stressed in real life, but it has more to do with complicated relationships between the baby's oxygen status and how the brain and heart respond to low-oxygen states. That's another topic for another day. For our purposes in this post, just knowing that the heart rate slows when the baby is squeezed helps us appreciate that being able to measure these contractions and monitor the fetus' heart rate is important. We can actually estimate which part of the baby is being squeezed too hard by how their heart rate changes in relation to when the contractions happen.

For example, if the heart rate slows down in a way that mirrors the contraction (see below), we call that an "Early Deceleration". That type of fetal heart tone suggests that the baby's head is being squeezed a bit, but THAT IS ACTUALLY OKAY! This is a completely normal response from the baby, and it's actually an unconcerning reading. The heart rate will drop briefly, but will return to a completely normal reading when the contraction stops.

A different type of reading is one where the heart rate slows down in a way that lags slightly behind the time when the contractions happen. We call these "Late Decelerations". Take a look below to see how this differs from the first type I described.

Looks pretty similar, but slightly different, right? From a doctor's perspective, this reading tells a completely different story than the previous one. This pattern corresponds to an issue we call "uteroplacental insufficiency". In short, uteroplacental insufficiency means that there is a major issue with blood flow through the placenta. The placenta is the site where blood flows from the mother to the baby, allowing for the baby to have enough oxygen to survive and grow even though it's sitting in a pool of liquid not breathing real air. If the placenta isn't working, the baby is in serious trouble. When we see late decelerations, we need to change something. Whether it's just repositioning how the mom is sitting/laying down or heading straight to the operating room for an emergent C-section delivery, we as L&D physicians/med students treat this reading as somewhat of an emergency.

There's more to talk about on this topic, but I won't bore you with a lengthy, lengthy post. I just wanted to illustrate that a really small difference seen on a monitor can make a HUGE difference in how we handle a patient's labor from an obstetric perspective. Now, since you've read this, you're essentially certified to help manage a baby's Fetal Heart Tones if you ever randomly find yourself waltzing through a Labor and Delivery department!

Well, maybe not, but you DO know more about this topic than I did before this week started!