Standardized evaluation and management of fetal heart monitoring abnormalities in the second stage of labor
Electronic fetal heart monitoring has become the most widely used means of fetal monitoring in obstetrics because of its simplicity, non-invasiveness and accurate results in real time. However, there are still many controversies in the interpretation of electronic fetal heart monitoring graphics, and there is no consensus on how to make correct clinical decisions based on the monitoring results. The article introduces the latest standardized definition of electronic fetal heart monitoring graphs, and explains the principles and standardized methods of treatment.
Electronic fetal heart monitoring is mostly used for prenatal and intrapartum monitoring of the fetus. Compared with manual listening to the fetal heart, electronic fetal heart monitoring can continuously monitor the dynamic changes of fetal heart rate and pressure in the uterine cavity and the relationship between them, which is a more objective means to monitor and understand the intrauterine condition of the fetus and the fetal reserve capacity in the uterus. The second stage of labor refers to the period from the opening of the uterus to the delivery of the fetus. During this stage, the frequent contractions of the uterus, the pressure or pulling of the umbilical cord and the decrease of amniotic fluid reduce the amount of blood circulating in the uteroplacenta and affect the blood and gas exchange between mother and child, which is the most dangerous period for the occurrence of fetal distress and can endanger the life of the fetus in serious cases. Therefore, electronic fetal heart monitoring in the second stage of labor will help to reduce the occurrence of adverse neonatal outcomes. However, there are still many controversies about electronic fetal heart monitoring and its interpretation in China and abroad. The most important of them is the individual differences in the interpretation of various graphical findings, and the complexity and diversity of clinical manifestations make it impossible for clinicians to simply classify and interpret a large number of monitoring graphs, which leads to uncertainty in the prediction of adverse neonatal outcomes and increases the rate of false-positive monitoring results, which increases unnecessary clinical interventions and leads to an increase in the rate of vaginal assisted labor and cesarean delivery. Therefore, standardized interpretation of electronic fetal heart monitoring results is extremely important to ensure maternal and child safety by eliminating unnecessary human interventions and determining standardized management. In this paper, we will explain the definition, interpretation and treatment principles of standardized electronic fetal heart monitoring.
I. Definition of electronic fetal heart monitoring standardization
In 1997, the National Institute of Child Health and Human Development (NICHD) standardized and defined electronic fetal heart monitoring graphics in a research planning workshop.1 In 2008, NICHD, the American College of Obstetricians and Gynecologists (ACOG), and the American Academy of Maternal-Fetal Medicine reaffirmed and revised this definition and established criteria for normal and abnormal contraction frequency.2]. . Normal contraction frequency is defined as no more than 5 contractions in 10 min in an average 30 min observation window. Contractions are considered too frequent if they are greater than 5 in 10 min. This definition applies to both spontaneous and artificially induced contractions. The important changes of fetal heart rate (FHR) are as follows: (1) FHR baseline: it is the average FHR level within 10 min excluding fetal heart rate acceleration, deceleration and significant variation, which is observed for at least 2 min. 110~160 beats/min is normal; below 110 beats/min is fetal bradycardia; above 160 beats/min is fetal tachycardia. min is fetal tachycardia. (2) Baseline variation: It refers to the irregular fluctuation of the baseline frequency and amplitude of FHR. The baseline variation is divided into 4 types: vanishing type is the lack of variation; small variation is the variation amplitude less than 5 beats/min; normal condition is the medium variation, i.e. the variation amplitude is 6~25 beats/min; significant variation is the variation amplitude more than 25 beats/min. The concept of difference between long variation and short variation is eliminated. (3) Normal FHR acceleration: For gestation greater than 32 gestational weeks, normal acceleration refers to a maximum increase of 15 times/min in FHR from baseline with a duration greater than 15s and less than 2min. For gestation less than 32 gestational weeks, a maximum increase of 10 times/min in FHR from baseline with a duration greater than 10s and less than 2min. a duration of 2~10min is extended acceleration. Acceleration time greater than 10min should be considered as FHR baseline variation. (4) Early deceleration: It is related to contractions, and the time from the beginning of deceleration to the lowest point of FHR is not less than 30s, and then it slowly returns to the baseline level. In general, the deceleration to the nadir is synchronized with the strongest uterine contractions. (5) Late deceleration: associated with contractions, the time from the beginning of deceleration to the point of decreasing to the lowest point of FHR is not less than 30s, and then slowly returns to the baseline level. In general, the start of deceleration, reduction to the nadir, and return to baseline level occur after the beginning of uterine contraction, the strongest contraction, and the end of contraction, respectively. (6) Variable deceleration: The time from the start of deceleration to the reduction to the nadir is less than 30s, the decline is not less than 15 times/min, and the duration is more than 15s but not more than 2min.(7) Prolonged deceleration: The decline is not less than 15 times/min, and it takes more than 2min to return to the baseline level from the start of deceleration but not more than 10min.(8) Sinusoidal change: That means that the FHR baseline oscillates in a smooth sinusoidal wave with a fixed frequency of 2~5 times/min and a duration of more than 20min.
Second, the principle of standardized interpretation of electronic fetal heart monitoring
The purpose of electronic fetal heart monitoring during labor is to assess the adequacy of fetal oxygenation during labor. The process of fetal oxygenation involves the transfer of oxygen from the surrounding environment to the fetus and the corresponding physiological changes that occur in the fetus when the oxygen supply is interrupted. A normal type of FHR (normal baseline fetal heart rate, normal variability, presence of acceleration without deceleration) predicts that fetal oxygenation is largely normal; repeated late decelerations or variable decelerations or severe bradycardia without FHR variability predicts that fetal oxygenation is severely impaired and that severe respiratory distress is occurring or will occur, which may lead to fetal neurological or other damage or even death [3]. The interpretation of electronic fetal heart monitoring during labor can be simplified to the following 3 basic principles: (1) Oxygen is transferred to the fetus along the pathway of the maternal lungs, heart, blood vessels, uterus, placenta, and umbilical cord. Interruption of oxygen supply at any point or points along that pathway results in a deceleration of the fetal heart rate. For example, interruption of oxygen supply due to compression of the umbilical cord can result in a variable deceleration. Inadequate placental perfusion during uterine contractions can lead to late decelerations. Although there are minor differences in the physiological mechanisms for the occurrence of variable deceleration, late deceleration and prolonged deceleration, they all share a common initiating factor, i.e., interruption of oxygen supply. Therefore, the first rationale for standardized interpretation of electronic fetal heart monitoring during labor is that all clinically significant decelerations (variable decelerations, late decelerations, and prolonged decelerations) reflect an interruption of the fetal oxygen supply pathway at one or more points. (2) Fetal oxygenation disorders have the potential to lead to hypoxic neurological injury. The injury also involves a series of sequential physiological steps. It begins with hypoxemia, a decrease in the oxygen level in the blood, which leads to tissue hypoxia. Tissue hypoxia can in turn stimulate anaerobic metabolism, lactic acid buildup and tissue metabolic acidosis. Ultimately, blood pH drops, causing metabolic acidosis. the 2nd rationale for standardized interpretation of FHR is that moderate variability and/or decelerations reliably predict the presence of fetal metabolic acidosis. (3) Significant fetal metabolic acidosis (cord arterial blood pH 7.0; base deficit, 12 mmol/L) is a necessary prerequisite for acute intrapartum hypoxic neurological injury (e.g., cerebral palsy). Therefore, the 3rd rationale for standardized interpretation of FHR is that disruption of fetal oxygenation during acute labor does not lead to neurological injury when it does not cause significant fetal metabolic acidosis.
3. Standardized classification of electronic fetal heart monitoring during labor
The 2008 edition of NICHD guidelines proposes that the analysis of fetal heart monitoring graphics should take into account the baseline, variability, acceleration and deceleration of its embodiment and classify different fetal heart curves into 3 categories, namely normal (category I), intermediate (category II) and abnormal (category III).
Fetal heart monitoring graphics grading monitoring graphics characteristics Class I (monitoring graphics that do not suggest the manifestation of fetal acidosis) FHR variation: moderate late or variation deceleration: no early deceleration: may exist FHR acceleration: exists Class II (all monitoring graphics between Class I and Class III) FHR baseline fetal heart rate tachycardia not accompanied by fetal heart variation disappearance of fetal heart rate bradycardia FHR baseline variation variation decrease variation increase not accompanied by FHR acceleration: lack of effective acceleration after stimulation of the fetus Periodic or episodic decelerations Periodic variant decelerations accompanied by moderate or decreased baseline variant prolonged decelerations more than 2 min shorter than 10 min Periodic late decelerations accompanied by moderate baseline variant variant decelerations followed by some specific graphics, such as single/double shoulder sign, deceleration followed by acceleration, slow FHR recovery, etc. Category III (more clearly reflecting the presence of fetal acidosis in the monitoring FHR baseline disappearance accompanied by one of the following points graphics, suggesting the need for further treatment) cyclic late deceleration cyclic variant deceleration FHR over-slow sine wave graphics.
IV. Principles of standardized treatment
The goal of standardized intrapartum electronic fetal heart monitoring processing is to identify and minimize potential, preventable sources of error. The first step is to ensure that the information displayed on the electronic fetal heart monitor is reliable. Therefore, the first step is to confirm that the monitor is accurately recording fetal heart rate and uterine activity. A thorough assessment of electronic fetal heart monitoring includes uterine contractions accompanied by an assessment of the 5 basic elements of FHR: i.e., baseline rate, variability, acceleration, deceleration, and change or trend over time. If the monitoring results meet the inclusion criteria of category I, it is considered normal, and for this type of fetal heart monitoring only subsequent routine monitoring is required, which should be performed every 30 min in the first stage of labor or fetal heart auscultation, and every 15 min in the second stage of labor. If a category I fetal heart monitor pattern appears as a category II or III monitor pattern during the follow-up monitoring, the appropriate clinical management is required. If the Class III pattern does not improve in the short term, labor must be terminated as soon as possible. If the monitoring results do not meet the standard classification I, a systematic ABCD approach can help clinicians avoid overlooking important considerations and make appropriate decisions in a timely manner [4].The ABCD approach is as follows.A(assess): Evaluate the oxygen supply pathway and look for other factors that cause FHR changes. If the results of FHR monitoring do not meet the criteria of category I, it is necessary to systematically assess the oxygen supply pathways, i.e., to search for factors causing oxygen supply disorders from maternal lungs, heart, blood vessels, uterus, umbilical cord and placenta in order. Lungs: check the respiratory rate, whether the airway is open, and whether there are combined lung diseases; heart: check whether the heart rate and rhythm are normal, and whether there are combined heart diseases; blood vessels: assess the blood pressure and blood volume status; uterus: check the uterine contraction force, contraction frequency, and uterine tension to exclude uterine rupture; placenta: check whether there is placental abruption, placenta praevia bleeding, etc.; umbilical cord: vaginal examination is feasible to exclude the umbilical cord prolapse, etc. In addition, other factors that may cause fetal oxygen supply disorders should be checked, such as whether the mother has a history of fever, infection, use of drugs and hyperthyroidism, or whether the fetus is in a sleep cycle, infection, anemia, cardiac arrhythmia, heart block, congenital developmental abnormalities, etc. B(begin): start taking appropriate corrective measures. Interruption of the oxygen supply pathway needs to be corrected with appropriate conservative measures. For example, oxygen should be administered to improve maternal respiratory status; position should be changed and fluids should be given to correct hypotension; and uterine stimulation should be minimized or uterine contraction inhibitors should be used when contractions are too strong. Taking these standard conservative measures in an orderly fashion helps to ensure that important considerations are not overlooked. Electronic fetal heart monitoring should be reassessed within a reasonable time after the use of the conservative corrective measures described above. If the monitoring results return to category I, routine monitoring can be resumed. If the results progress to category III, consideration should be given to accelerating the labor process. If the monitoring results remain in category II, further evaluation is required. If moderate variability and/or acceleration is not accompanied by significant decelerations, continued observation and monitoring is recommended. If a category II FHR does not show moderate variability and acceleration, but instead shows persistent late decelerations or significant variability decelerations, metabolic acidosis cannot be ruled out at this time. Moreover, these types of decelerations imply the presence of physiologic stress, which increases the risk of metabolic acidosis. Therefore, accelerated delivery is recommended [6]. However, some category II monitoring findings are difficult to interpret and the clinical care team may not always agree on the risk assessment. For example, one type II fetal heart monitoring graph shows a normal baseline fetal heart rate with minor variability and no acceleration or deceleration. Some physicians may consider accelerating the labor process because they see a lack of normal variability or acceleration in the FHR; others may place more weight on the absence of decelerations in the FHR and decide to continue observation. Thus, a standardized approach minimizes these arguments over confusing category II monitoring results at this time. If any member of the medical team has any doubt about the significance of the moderate variability, the presence of acceleration, or the observed decelerations, the safest and easiest way to proceed is to proceed to the next step C. C(clear): clear the obstacle and prepare for accelerated delivery. If conservative correction methods do not work, it is prudent to prepare in advance for an accelerated delivery, including all aspects of equipment, personnel, labor, fetal and delivery. Because the considerations summarized in this article are considered common sense by many clinicians, they are not given the attention they deserve and are often overlooked, resulting in delays that can compromise fetal safety. For example, the equipment should ensure that the operating room is available and the appropriate facilities are ready; the personnel should be prepared, including obstetricians, anesthesiologists, pediatricians, nurses, etc.; for the mother, informed consent should be prepared, appropriate anesthesia should be selected, necessary laboratory tests should be performed, blood products should be prepared, intravenous access should be established, urinary catheters should be placed, the abdomen should be prepared, transfer to the operating room, etc.; the number of fetuses, gestational weeks, estimated fetal weight, fetal position, fetal delivery pattern, and the fetal safety should be clearly defined. The number of fetuses, gestational weeks, estimation of fetal weight, fetal position, fetal delivery style and whether the fetus is developing normally should be considered beforehand; adequate monitoring during delivery should be ensured. After taking appropriate conservative measures, it is wise to estimate in advance how long it will take to complete the delivery in case of an emergency, a step that should be done by the clinician ultimately responsible for performing the cesarean section. The time between the decision and the final delivery is determined by several factors: equipment, staff, labor, fetus, and delivery. For example, equipment response time, location and availability of the operating room; staff availability, training and experience; maternal factors such as history of hypertension, diabetes, SLE, obstetric factors such as pelvic measurements, placental position, etc., should be considered to influence the procedure. On the fetal side, expected weight, gestational week, fetal orientation and fetal delivery style are considered; on the delivery side, such as delayed labor. The systematic ABCD approach to management is relatively uncontroversial and the vast majority of decisions must be made during labor and delivery. However, these steps are not a substitute for clinical judgment, but rather need to be supported by comprehensive and timely clinical judgment. Once the 4 ABCD steps are completed, the clinician must decide whether to continue to wait for spontaneous vaginal delivery or to take measures to speed up the labor process. This decision requires a trade-off between the time required for vaginal delivery and the time to develop metabolic acidosis and potential injury. The estimation of the duration of vaginal delivery should only take into account general obstetric factors, such as pelvic size, fetal orientation, and fetal delivery style. Estimates of the time to onset of metabolic acidosis, on the other hand, can only rely on limited data that suggest that metabolic acidosis does not occur suddenly, but gradually after approximately 60 min [7]. The inherent inaccuracy of these estimates can make decision making more difficult. One of the commonly avoided mistakes is to postpone making the necessary clinical decisions in the hope that the problem will resolve spontaneously. If the decision is to accelerate the labor process, the rationale should be documented and implemented immediately; if the decision is to wait and observe, both the rationale and the plan should be documented and the decision further revised after a reasonable period of time based on the actual situation. It is important to recognize that there is a fundamental difference between “decision to wait” and “decision to wait”, the former reflecting positive clinical decision making and the latter negative delay.
V. Assessment and Treatment of Several Special FHR II Patterns
5.1 Intermittent or Recurrent Variable Decelerations
Intermittent variable decelerations are the most common abnormal FHR pattern during labor and delivery, and most of them do not require any treatment and have a good prognosis. The frequency, amplitude, duration, type of contraction and other FHR characteristics such as FHR variability should be evaluated for recurrent variable decelerations. The longer the duration and the greater the magnitude of recurrent variant decelerations, the more likely it is that fetal acidosis will occur. The presence of moderate variability or spontaneous, induced acceleration in the repetitive variant FHR graph implies that the fetus has not yet developed metabolic acidosis. For recurrent variant decelerations focus on relieving the cord compression condition. A more reasonable first step in management is to change the maternal position. In addition, improving fetal oxygenation is an effective tool.
5.2 Repeated late decelerations
Recurrent late decelerations reflect transient or chronic uteroplacental insufficiency. Common causes include maternal hypotension, too frequent contractions and maternal hypoxia. Management is based on promoting uteroplacental perfusion, including lateral positioning, oxygenation, and assessment of the frequency of contractions. In category II fetal heart monitoring patterns with recurrent late decelerations, management includes intrauterine resuscitation and re-evaluation for improvement in fetal status. Considering the high false positive rate of late decelerations to predict acidosis and fetal neurological damage, it is important to assess whether the fetus will develop acidosis in conjunction with the occurrence of accelerations or severe FHR variability. If late decelerations persist after intrauterine resuscitation, it is important to consider that the fetus may have developed acidosis and measures need to be taken to accelerate delivery. If the FHR variation disappears, it means that the FHR has progressed to category III and needs to be treated accordingly.
5.3 Fetal tachycardia during labor
Fetal tachycardia is defined as a baseline rate exceeding 160 beats/min for more than 10 min. Possible causes of fetal tachycardia include infection (e.g., chorioamnionitis, pyelonephritis, or other maternal infections), drugs (e.g., terbutaline, cocaine, and other stimulants), maternal disease such as hyperthyroidism, obstetric factors (e.g., ruptured placental vessels or fetal hemorrhage), fetal Tachyarrhythmias (often accompanied by FHR above 200 beats/min). When viewed in isolation, tachycardia is not a more accurate predictor of fetal hypoxemia or acidosis unless accompanied by small or no variation in FHR or repeated late decelerations. FHR class II graphs with tachycardia should be focused on finding the etiology. In addition, other graphic features that accompany it, especially baseline variants, should be evaluated in conjunction. If the FHR shows tachycardia with minimal variation and no acceleration, the possibility of fetal acidosis cannot be ruled out at this time.
5.4 Intrapartum bradycardia and prolonged decelerations
Fetal bradycardia is defined as a baseline heart rate below 110 beats/min that persists for more than 10 min. prolonged decelerations are defined as decreases of not less than 15 beats/min that take more than 2 min, but not more than 10 min, from the start of deceleration to return to baseline levels. it is recommended that clinical intervention be initiated before the FHR graphs for prolonged decelerations and fetal bradycardia are distinguishable, and the management of these two conditions is are similar. Possible causes of prolonged deceleration or fetal bradycardia are maternal hypotension, umbilical cord prolapse, too rapid fetal descent, frequent contractions, placental abruption, or uterine rupture. These causes of bradycardia often occur at the time of delivery and usually present initially with a normal FHR baseline. The management of FHR class II patterns with fetal bradycardia or prolonged decelerations should also focus on finding the cause. Assessment of baseline fetal heart rate variability allows for a better assessment of the risk of fetal acidosis. If bradycardia is accompanied by minimal or no variability or prolonged decelerations, the delivery process needs to be accelerated.
5.5 Minimal Variability
Like other characteristics of the FHR, baseline variability often varies with fetal sleep and arousal status and the progress of labor, and it may vary from moderate to small and then back to moderate variability. The assessment of small variances takes into account potential causes such as maternal medication (e.g., opioids, magnesium sulfate), fetal sleep cycle, or fetal acidosis. Minimal variability due to maternal opioid administration usually returns to moderate variability within 1-2h. Minimal variability due to fetal sleep generally lasts 20-60 min, but returns to moderate variability when the fetus awakens. Therefore, in these cases, only continuous observation and anticipatory therapy are required. If the minimal variability is suspected to be caused by reduced fetal oxygenation, maternal position changes and oxygen administration are required. If no improvement is seen after these measures and there is no acceleration of FHR, additional evaluation such as digital scalp stimulation or acoustic stimulation is required. Persistent unexplained microvariations suggest possible fetal acidosis and need to be treated accordingly.
5.6 Excessive contractions
Frequent contractions are defined as >5 contractions in 10 min. Detection of associated fetal heart rate abnormalities is the key to management. Women in spontaneous labor with frequent contractions with recurrent FHR decelerations should be evaluated and treated. For women receiving contractions, every effort is generally made to reduce contractions to reduce the risk of fetal hypoxemia and acidosis. For women with induced labor, reduce the dose of contractions if they occur too frequently during a Class I FHR graph; if a Class II or III FHR occurs, discontinue the contractions and perform uterine resuscitation. In addition, initiate multiple resuscitations simultaneously to improve fetal status more quickly. If FHR abnormalities induced by over-frequent contractions do not improve by the above means, the use of contraction inhibitors should be considered [8]. Electronic fetal heart monitor, as one of the most widely used devices in obstetric clinics, is the main test to correctly assess the intrauterine condition of the fetus. However, when it was introduced into clinical practice, there were no instructions, no pre-market testing as is now common, and no clearly defined parameters to use. Therefore, the serious challenge we, the majority of obstetricians, currently face is how to accurately and rapidly interpret fetal heart monitoring graphics and make timely and optimal clinical decisions based on them to maximize the usefulness of this monitoring tool. Therefore, standardized interpretation of fetal cardiac monitoring patterns and standardized clinical management opinions are important for improving pregnancy outcomes and reducing doctor-patient disputes.