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Dr. Chapa’s OBGYN Clinical Pearls
FHT Baseline Change (110-160) in Labor: Danger, or Disregard?
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Back in 2002, the National Institute of Child Health and Human Development called the nichd, proposed the three tier FHT classification system that was subsequently adopted by many organizations, including acog, for intrapartum fetal heart rate interpretation. Category one, of course, is normal, Category two is indeterminate, and something had to be done to see if it resolved. And then category three, of course was abnormal and demanded some kind of action. Well, recently we received another question through our social media channel that had to do with the fetal heart rate baseline. And her question was great. It was something that actually you have to look in between the three tier FHT system to find an answer because it's not all the way crystal clear until you look at the ancillary data. And it has to do with the fetal heart rate baseline. All right, so remember, we got three tiers, 1, 2 and 3. One is normal, three is abnormal, two is something in the middle. But what do you do when the FHT baseline changes? Well, I'll tell you what, Emily, why don't you play that question and then we'll, we'll end the intro. But let's hear this question directly. If there is a change in the fetal heart rate tracing intrapartum, but it is still in the normal range, like 120 going to 150 beats per minute, and variability is normal, is that a worrisome finding? And what is meant by a zigzag FHT pattern different than marked variability? You see, that's actually a pretty good question because there's two things in there. One is if you're intrapartum, things are going good, but the baseline starting off in labor was 120, and then lo and behold, it changes to 150. Is that weird? Now, most people worry about it going up, but the reverse is also true. If it's 150 and it goes down to the 120s, is that worrisome? In other words, if there's a change in baseline but still within the normal levels, and here, of course, we're talking about 110 to 160, which is normal, less than 110, of course, is bradycardia. More than or at 160 is tachycardia. But what do you do when it's still in the normal range but the baseline has changed? Now, our podcast family member brought up something very, very specific here with regard to baseline fetal heart rate change, which is the zigzag pattern and that is correct. That is different than marked variability. So let's answer these two questions. Is it okay intrapartum if the fetal heart rate baseline goes from say 120 to 150? In other words, if there's a change but still within the normal levels with normal variability and what is meant by a zigzag pattern that's also called a wandering pattern baseline. Let's get into that when we come back. This is Dr. Chapa's obgyn no spin podcasts.
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Well, it's a real question. So what do we do? What do we do when the fetal heart rate is changes its baseline but it's still within the normal tiers, the normal levels that's considered acceptable between 110 and 160. Is this an issue if the baseline changes and is sustained as a continuum? Now I'll be very clear. That's different than what's called a zigzag baseline, AKA a wandering baseline. But we'll talk about that at the end of the episode. So to put this into perspective again, the question was if I start labor and the heart rates. Baby's heart rate is 110 to 120. It's normal. Looks great, it's category one. But then intrapartum, wow, the baseline is now like 150. Is this an issue? So we're gonna answer this and then answer the zigzag pattern, AKA wandering baseline here in a minute. So in order for us to answer this, we gotta set the standards here of what is normal and what is not normal. First of not normal, of course, is at or above 160 with 160 to 180 called malate tachycardia. And then above. Wait, 160 to 180, mild tachycardia. I want to make sure I said that right. And then above 180 is called severe tachycardia. That has a higher chance of baby decompensation. That's just going really fast. Right. Baby can have a type of high output cardiac failure, so to speak, with sustained duration. So we're going to get into this. If there's a change in baseline, but it is sustained and normal. It just continues now it's just 150, 50s. It's not fluctuating. Is that okay? And why does that happen? Does that. Is there data to say that that's okay for the baseline to change in that pattern? Okay, now, and at the end, we'll talk about the zigzag pattern here in a minute. But this is also loosely associated with what we covered in the last podcast in the last episode. Was that the last episode? Yeah, that was the last episode about fetal tachycardia. Remember, the most common cause of fetal tachycardia, outside of, you know, a pharmacological issue or a supraventricular tachycardia, is maternal infection. Okay? So intra amniotic infection, the baby is the first thing to go. Hey, something's off here. Because of the fetal inflammatory response syndrome called firs. So baby may get tachycardia first before maternal fever shows up. Okay? And we covered that in the last episode where traditionally you needed temperature elevation either 39 degrees Celsius as an isolated maternal temperature, or 38 degrees Celsius or 38.9 that is sustained with some other clinical factor. However, remember that we covered this in the last episode in July of 2024. The update to that says no, that that left a gap. It is okay to say, I suspect intra amniotic infection even without fever. If there's other things going on. If there's foul smelling amniotic fluid, the baby has tachycardia, there's uterine tenderness. Please call her IAI regardless of whether there's temperature or not, because fever will lag in some cases. All right. Actually, about 50% of the cases are based on one report. And if you don't treat them quickly, they run the chance of both fetal and maternal sepsis. So you do not need fever to be present to diagnose IAI if there is fetal tachycardia. Now remember that fetal tachycardia is just like fetal bradycardia. If there is good variability is category two. Okay. Bradycardia with absent variability is category three as an intrapartum trace. All right, so just quickly, even though we're going to answer this very fast, is it okay if the baseline changes from say 110 to 120 up to 150? Is that okay? And why does it do that? We're going to answer that in a minute, but let's just quickly remind ourselves what category one is. So that's a baseline rate of 110 to 160. Anything in that range is okay because it's going to fluctuate. The baseline FHT variability is moderate, not marked. It's moderate. Accelerations may or may not be present. Later variable D cells are absent and early D cells may or may not be present as well. Okay, so remember, you don't need A cells to be called category one and you don't need early D cells to be Category 1, although they may be present, but the normal rate is 110 to 100. The person who sent me this message is a senior resident and said, we had this discussion. Hey, the baseline is 150. Should we suspect infection? This was all based on the previous podcast that was just out. These are questions that people are thinking and that's good. We want our podcast to change the way we think and to challenge us into practice. That's why we do the show. And this was a brilliant, brilliant follow up question to that, which is, well, if we don't need fever to diagnose IAI, if the heart rate changes this baseline to say 150, that's a rise of, let's say in this case, 30 beats per minute. Is that enough to say something is off? Brilliant questions. We had this text communication back and forth. Short answer is if there's nothing else going on, if there's normal variability and the heart rate just changes between 110 to 150, if it doesn't cross the threshold of tachycardia, yes, that is absolutely normal. And I'm going to give you data that says that that is different than, again, a wandering baseline, which typically follows repetitive period of D cells, which is a little bit more ominous. Okay, but in general, as long as the baby's heart rate stays between 110 to 160, it is totally normal. And that does not necessarily imply iai. IAI can be marked by fetal tachycardia. Not a change in baseline alone. It has to cross that 160 threshold. So 160s, now that's tachycard. Now you can say, now that's abnormal. We've left the zone of normality. As if there is a zone of normality. But it now in tachycardia, now you gotta look for cause. All right? And remember that baseline is defined as more than 10 minutes duration. Otherwise it's just a prolonged acell. Okay, so we gotta make, we've gotta clarify some things here. Baby's heartbeat goes up and then comes back down. Last two minutes. That's just an acell. It's not a change in baseline until it is 10 minutes in duration, which is considered the definition of fetal heart rate baseline. It is a change in the norm in the average heart rate beats per minute for longer than 10 minutes that is sustained. Okay, so category one is normal. Category two includes bradycardia and tachycardia when there's still normal variability. But if there is bradycardia with absent variability, then that is category three. And, or you know, if there's severe bradycardia, like, you know, you're in the 60s, obviously do something. I mean, that goes without saying. All right, but baseline heart rate issues are typically, they fall into category two when it's bradycardia not accompanied by absent variability or when there's tachycardia. Of course, with the caveat that category 3 fetal heart rate trace includes bradycardia when it is severe and or absent variability with pathological D cells. Okay, so this is one of the issues here of the three tier system. Three tier system is great. When it works, it's super simple. But there's been criticisms of this because things like this, hey, what about baseline change? Can we talk about this? Is this normal or not? That's not really in the actual criteria. You got to look at the outside data that went into that. And very briefly, we're going to go into this here very briefly. That's why some have said, hey, maybe there's a five tier system. We've covered that in the past, but it never really took off. The five tier system is color cod and basically breaks up category two, which is a big chunk of things, into three other subdivisions. So you get five total categories and it's kind of nice. There's actually an app for it, the five tier FHT system, where it's like, you know, total green is fine, reds do something and then other others are color changes in the middle it's fine. It's a little bit more complicated, but most people without a doubt adhere to the three tier system. Okay, so let's go back to the FHT baseline change within the normal brackets. Okay, so if it starts at 110 or 120, goes up to 150, but doesn't yet cross to 160, is that normal? Assuming that everything else is fine, you'll have your occasional variables, you'll have your earlys. I get that. You know, it's all right to have one or two lates every once in a while, as long as they're not repetitive and recurrent. That's going to happen. That's okay. If the baseline changes and everything else is all right, that is completely okay. That was published back in the American Journal of perinatology in 2017, which was itself a piggyback of an earlier study that showed, hey, following a normal labor where there was no neonatal morbidity. And what they actually found is 42% of term labors. Now that's the catch. Preterm is a little different because there's immaturity of the CNS, SA node, cardiovascular communication. But at term, 42% of term labors will have normal baseline change throughout labor. As long as there's normal variability. That's the catch, guys. And as long as it's not a wandering baseline, if there is normal variability, but the heart rate has gone from 110 to 120 at the start of labor and now she's 9cm in its 150s. As long as everything else is cool. Hey, you're all right. Just follow the main categories of category 1, 2 or 3. But a change in baseline rate, if it's still within the normal brackets, is still absolutely fine and is acceptable. It could be close to 50% of term labors will do that. Okay, and so this is why we call this FHT baseline change 110 to 160 in labor. Is this dangerous or do we just disregard it? Now, it's very hard to say disregard changes in the baby's heart rate because we always need to be aware of what the baby's heartbeat is doing. However, just by changing that heart rate baseline, assuming variability is intact and there's no other, you know, weird clinical issues going on, that does not portend adverse neonatal issues. Okay? And there's a lot of reasons for this. As long as, once again, variability is intact, the baby's heart rate can change baseline, typically because of catecholamine stress intrapartum, which is a normal physical physiologic response to being squished as your home is trying to evict you. Okay, so there's no problem with that. That actually is a compensatory mechanism. It's fine. There was a nice review on this also in the gray journal AJOG in 2023 and that title was Pathophysiological Interpretation of Fetal Heart Rate Tracings in clinical practice. Again, AJOG 2023. I'll put that link in our show notes. So as long as it has not crossed below 110 where it's true bradycardia, or over 160, which is tachycardia, which may signal intra amniotic infection even without fever, a change in fetal heart rate baseline, as long as there is normal variability, as long as variability is intact is not a concerning issue. But it's a good question because our resident who sent in the message again was, you know, pretty concerned that potentially they were missing something and or missing iai. What do I do? So in that case, as long as it is not past the 160, you keep on going and just report the fetal heart rate tracing in the categories 1, 2 or 3, as we've already discussed, knowing of course that you should document, hey, there's a normal change in baseline. We're still in the normal levels. There's good variability. I'm considering that a physiological stress response to the child. We keep on going and then once you hit 160 or above, you go, hey, now we're tachycardic, potentially. This is iai and you do a full history including uterine tenderness. Check, make sure there's no foul smelling amniotic fluid. Potentially. Get a CBC with DIF to look for leukocytosis and a dif, put it together with a clinical context. If she's been ruptured for 85 hours, I'm just throwing that out there for emphasis for a long time. Then say, hey, now we've crossed tachycardia. We've not just done a baseline shift, we've now left the zone of normality and we now have tachycardia. I'm going to assume that's aai. Wait a minute. Iai? Intramniotic infection. Good Lord. This is what happens, guys. First podcast after Christmas. My brain is still on holiday mode. Okay, but we're trying to knock this out quickly. So the short answer is if you have a change in baseline but you're still in the normal levels, that is totally okay as long as that heart Rate is sustained. Now, what is not normal is if the heart rate goes up for a while, gets sustained for about 30 minutes, drops, and then changes again. That's called a wandering baseline. Typically, wandering baselines have a decreased variability, either minimal or absent. That's worrisome. So if you get a call from labor and delivery going, hey, her baseline was 120. It's okay now. Now it's 150. Oh, it's weird. Oh, now it's back to, you know, one trend again. If it's doing weird fluctuations like that that are not related to periodic D cells or periodic A cells. If there's two changes in baseline, remember, that's at least 10 minutes sustained, where it's a baseline change that keeps. And you don't have a true baseline that you can look at to go. I. I don't even know where the average is. That is called a zigzag FHT pattern, otherwise known as a wandering baseline. That is concerning. Okay. In most of the data, a wandering baseline typically follows a repetitive intervals of recurrent D cells, either variables or lates, and can signal. I'm not saying that something's wrong. I'm saying you should get your attention, because I don't want to say that something's wrong. You know, I don't want to pigeonhole somebody into this, but I'm saying a wandering baseline, guys, especially with absent variab, is a flag. That is an issue because that, that is what you're looking at. There is autonomic instability where the sympathetic system is trying to fix things. Then the parasympathetic system says, wait, let me have a chance at it. Because the baby's system, the mechanics and the circuitry is on the fritz, AKA potentially some hypoxia going on. So if you see a wandering baseline together with decreasing variability, that is a flag. So again, I want to be. That's the same thing I communicated to our podcast family member when I said, hey, do you mean that the fetal heart rate is still within the normal brackets and it's sustained, or is it jumping around, like, for 30 minutes? It stays at 150, and then it kind of goes down to 110, and then maybe it chums up to, like 140 for a while? What are we talking about? No, no, no. It just. It just changes and it stays, like, at a plateau. All right, if it changes in a plateau, that's fine. If it keeps wandering, zigzagging, that potentially is a flag. Be worried about that one. Okay? And this is Again, one of the criticisms of the three tier system is that there's really nothing in there about a wandering baseline. You have to infer all of that and pick out the little, the, the data behind each category. So we know what category one is, we know what category two is, expand that into three in the five tier system and we know what category three is. A wandering baseline, typically because it's associated with D cells and minimal raps and variability, very easily crosses into category three. Okay, so we wanted to do this quickly as everyone's brains come back from, from Christmas. Welcome back, by the way. We're kind of a weird Christmas in between Christmas and New Year's is always a weird time. I'm still working, but people kind of holiday mode and unless they're on call, they unplug. But we want to stay true to our commitment and we have a fantastic team who's putting things together and we're thankful. Our team is always growing. Now we have somebody else, Emily, who's trying to help us out. Thank you, Emily, for your help. So we just wanted to knock something out quickly and be mindful of that question that came in because it was again, very loosely tied to this issue of fetal tachycardia without fever. So now let me set the stage here with one last thing and then we'll be done. We'll exit the stage. And if you have a change in baseline, if it goes from 110 to 150 and there's maternal temperature elevation, well, that's a whole different issue. Right, because now if you got a temperature elevation, then that is by diagnostics, she's got a fever from something else. You have to assume that's IAI and that's gonna make the heartbeat go up. But remember what we said at the beginning, we're assuming everything else is fine. There's no fever, nothing else weird has happened. She hasn't gotten some weird medication and the heart rate just has changed baseline from 110 or 120, say to 150s potentially that' an issue. Once you cross the 160 threshold, that's tachycardia and that requires its own evaluation because now that lives in category two. Okay, so you still, the three tier system still is in effect, is still valid. But a change in baseline, if everything else is cool, that's why the range is between 110 up to 160 in category one or in tier one, because the baby wolf actually fluctuated his heartbeat. And we've known that from previous, previous data that up to 42%. Some have said 50% will do that and sustain it as long as it's not fluctuating and or zigzagging. So, podcast family, again, welcome back from Christmas. Just wanted to knock this out as we want to be true to our podcast family member. I said I would knock this out for her and I did. So FHC baseline change 110 to 160 in labor danger or disregard? Well, as long as variability is okay and there's nothing else going on, it's okay to disregard. But it still requires documentation that, hey, we see this physiological stress response, we're good. And then once you hit the 160, then that potentially is an issue. Podcast family, as always, we're thankful for you. We're glad you're part of our podcast family. I think what we've done, we're supposed to do so now. Let's end this and let's take it home. This has been Dr. Chapa Zobichyn, no Spin podcast podcast family. Thank you for your support. Thank you for listening. And as always, we'll see you on another episode of the no Spin Podcast.