ST Segment Analysisas an Adjunct toElectronic FetalMonitoring,Part I : Background,Physiology, andInterpretation

Michael A. Belfort, MBBCH, MD, PhDa,*, George R. Saade, MDb

KEYWORDS

� ST segment analysis � Fetal heart rate � Electrocardiogram� T/QRS ratio

Fetal electrocardiogram (ECG) ST segment analysis (STAN) was approved in 2005 inthe United States as an adjunct to electronic fetal heart rate (FHR) monitoring todetermine whether obstetrical intervention is warranted when there is an increasedrisk for developing metabolic acidosis.1 This modality has been available in Europefor some time, and there have been several randomized controlled trials (RCTs) andobservational studies performed in Europe1–9 suggesting its utility in the reductionof fetal acidosis at birth, decreased need for fetal scalp blood sampling during labor,and decreased need for operative vaginal delivery and emergency cesarean deliveryfor fetal indications. The Cochrane Library10 endorsed the use of STAN “when a deci-sion has been made to undertake continuous electronic FHR monitoring duringlabour.” However, despite these endorsements, enthusiasm for this technology inthe United States has been muted, and there are a few concerns that need to beaddressed before widespread adoption of another electronic fetal monitoringmodality. None of the randomized trials published thus far have been performed inthe United States, and given the different patient case mix, different health caredelivery models, and different obstetrical practices in the United States and Europe,

a Department of Obstetrics and Gynecology, Baylor College of Medicine, Texas Children’sHospital, Houston, TX, USAb Division of Maternal-Fetal Medicine, University of Texas Medical Branch, Route 1062,Galveston, TX 77555, USA* Corresponding author.E-mail address: michael.belfort@hcahealthcare.com

Clin Perinatol 38 (2011) 143–157doi:10.1016/j.clp.2010.12.009 perinatology.theclinics.com0095-5108/11/$ – see front matter � 2011 Elsevier Inc. All rights reserved.

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direct extrapolation of the European data to the US population may not be appro-priate. There have also been different interpretations of the published data(which are discussed in greater detail later in the article) that have resulted fromdifferent definitions of standard terms such as metabolic acidosis and neonatalencephalopathy (European vs United States). Based on these concerns, a largeRCT addressing the use of STAN in a North American population is currentlyunderway. The primary aim of this study is to address neonatal outcome, in theform of a composite perinatal morbidity/mortality score, and compare it with that ofpregnancies managed under US conditions with standard electronic fetal monitoringalone. The results of this RCT are expected in 2013. Further detail of the study protocolis provided later in the article.The fetal STAN system operates using established principles of ECG analysis

(Fig. 1) and select components of the fetal ECG signal specifically to determine thepresence of myocardial ischemia.11–14 A detailed description of the ECG physiologyis beyond the scope of this article, and only selected points are covered. In adultECG analysis, a large component of the interpretation is governed by the comparisonof different leads and the differential views of the heart that they give. Using theseviews, regions of the heart affected by abnormal electrical wave propagation(such as infarction or ischemia) and the axis (general direction of depolarization) canbe determined. Fetal ECG does not provide this perspective, and only 1 lead isused (scalp lead) to provide a global electrical picture of what is happening withinthe fetal heart as a whole.In general, an ECG is a surface representation of changes in action potentials within

the myocardium. In the normal resting-state myocardium, the myocytes are polarizedduring diastole with an accumulation of positive charges, balanced by an equalnumber of negatively charged extracellular electrons. When depolarization occurs,as triggered by the sinoatrial (SA) node and distributed through the atria and Hisbundle to the ventricles, the distribution of positive and negative charges is reversedin a self-propagating wave from cell to cell, which produces the different waves in theECG. During normal atrial depolarization, the P wave is a reflection of the mainelectrical vector traveling from the SA node to the atrioventricular (AV) node and

Fig. 1. Typical P, QRS, and Twaves of an ECG waveform. Calculation of the T/QRS ratio is alsoshown.

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then spreading from the right atrium to the left atrium. The cells in the SA node aremodulated by vagal and sympathetic tone as well as by hormones such asepinephrine. The P wave shape and amplitude are affected by that part of the auto-nomic nervous system that controls cardiac pump function. Once the signalreaches the right and left ventricles, there is rapid depolarization of a much largermuscle mass than in the atria, causing the higher-amplitude QRS complex. The PRinterval, measured from the beginning of the P wave to the beginning of the QRScomplex, reflects the time the electrical impulse takes to travel from the sinus nodethrough the AV node and gives an estimate of the AV node function. The T wave repre-sents the repolarization (or recovery) of the ventricles and is divided into 2 portions.The interval from the beginning of the QRS complex to the apex of the T wave isreferred to as the absolute refractory period, and the interval from the apex to theend of the T wave is called the relative refractory period (or vulnerable period).The ST segment connects the QRS complex and the T wave and is an isoelectricperiod during which the ventricles are depolarized. The J point is the point at whichthe QRS complex finishes and the ST segment begins, and it can be used to measurethe degree of ST elevation or depression. The QT interval is the period from thebeginning of the QRS complex to the end of the T wave. A prolonged QT interval isa risk factor for ventricular tachyarrhythmias and varies with the heart rate.

SPECIFIC FETAL ECG CHANGES AND THEIR SIGNIFICANCEP Waves

The P wave of the fetal ECG, obtained by application of a fetal scalp electrode toa fetus in a cephalic presentation, has a positive deflection. If the electrode is appliedto a fetus in a breech presentation, the entire fetal ECG waveform is inverted anda negative P wave ensues. Very occasionally, in extreme bradycardia, a negative Pwave is seen from an appropriately placed fetal scalp electrode.

PR Interval

Hypoxia can cause a shortening of the PR time interval, which can occur despitea lengthening of the RR interval (bradycardia). This shortening occurs as the fetal heartattempts to preserve optimal filling of the atria during periods of decreased venousreturn15,16 and usually represents a situation of vagal dominance and not a directdepressive hypoxic effect.

QRS Complex

The QRS complex is a robust and well-defined electrical signal in the fetal ECG and isindicative of ventricular depolarization. In addition, the interval between successive Rwaves (RR interval) can be used to determine FHR. The adult ECG displays the QRScomplex in 3 consecutive peaks: a negative Q peak, a positive R peak, and a negativeS peak. The R peak is the dominant peak and the tallest component of the QRScomplex. In the fetal ECG, however, there are significant variations in the shape andconfiguration of the QRS complex. Sometimes the R peak is of low amplitude orappears as a split R peak. Occasionally an entire component of the QRS complex ismissing or of such low amplitude that it is undetectable.

QT Interval

When intrapartum hypoxia leads to metabolic acidosis, there is a significantshortening of the QT interval regardless of correction for the heart rate. A shortenedQT interval reflects the effect of the ionized calcium fraction on the inotropic capabilityof the fetal myocardium.17

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ST Interval and ST Changes

A normal fetal ST interval is made up of an ST segment and the T wave. A normal fetalST interval is characterized by a horizontal or upward-sloping ST segment and a Twavewith a constant and stable amplitude. A normal ST interval usually indicates a positiveenergy balance and aerobic myocardial function.Dawes and colleagues18 showed that during hypoxia, fetal myocardial function and

survival depends on myocardial glycogenolysis. As glycogenolysis increases, so doesthe amplitude of the T wave in the fetal ECG,12 and this relationship has been demon-strated to be linear.13 The amplitude of the QRS complex remains relatively stable untilquite late in the hypoxia/acidosis process, providing a metric against which thechange in the height of the T wave is standardized—the T/QRS ratio (see Fig. 1).Fig. 2 shows the effects of progressive hypoxia on the ECG of the fetal guinea pig.The ST segment and T-wave changes are clearly seen and occur within a few minutesof the initiation of hypoxia. Myocardial glycogenolysis can be pharmacologicallyenhanced in the presence of hypoxia by increasing workload with b-adrenergic drugs,and in fetal sheep the T/QRS ratio increases linearly during an infusion of terbutaline tothe ewe.19 The fetus responds to moderate hypoxemia with a catecholamine surge,b-adrenergic activation and myocardial glycogenolysis, all of which stimulate anincrease in the T-wave amplitude.Repolarization of the myocardium (as reflected by the ST segment and T wave) is an

energy-consuming process. When there is hypoxia, the energy balance within themyocytes becomes negative and the cells resort to b-adrenoceptor–mediatedanaerobic glycolysis. This pathway produces both lactate and potassium ions, andthese ions affect the myocyte cell membrane potential and cause an elevation in theT-wave amplitude.20 When energy balance cannot be maintained by the compensa-tory mechanisms (vasodilatation and anaerobic metabolism), the endocardiumbecomes ischemic, altering the sequence of repolarization and direction of electrical

Fig. 2. Fetal ECG signal from guinea pig fetuses showing the effect of progressive hypoxiaon the T wave, ST segment, and T/QRS ratio. PaCO2, partial pressure of carbon dioxide,arterial; PaO2, partial pressure of oxygen, arterial. (Data from Towell ME. Catecholaminedepletion and the response to asphyxia in the fetal guinea pig. Biol Neonate1971;18:212–24.)

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flow. This imbalance between the endocardium and epicardium causes depression ofthe ST segment, with or without inversion of the T wave.21 In some fetuses, certainconditions prevent the usual myocardial response of ST segment elevation andT-wave amplitude increase, and there is the development of a biphasic shape tothe ST segment, with progressive depression of this biphasic ST segment as thehypoxia worsens (Fig. 3). Because the perfusion pressure in the endocardium isalways lowest when the mechanical strain is highest and because the response ofthe myocardium (b-receptor activation and enhanced Frank-Starling curve) to anincreased volume load is not instantaneous, there can be delays in the repolarization,which is manifested as ST changes. Thus, any stimulus that substantially alters thebalance and performance characteristics of the myocardium may result in ST depres-sion. Conditions that may be associated with ST depression and biphasic fetal ECGwaveforms include prematurity, infection, maternal fever, myocardial dystrophy,cardiac malformations,22 chronic hypoxia, and the initial phase of acute hypoxia(when the fetus has not had enough time to develop the classic response). In addition,Yli and colleagues23 recently showed that ST depression occurs more frequently infetuses of mothers with diabetes mellitus (possibly related to the higher prevalenceof myocardial dystrophy in such babies).STAN is based on the ability to detect changes in the ST interval when the fetus

mobilizes its compensatory mechanisms against hypoxia. A fetus that is alreadyhypoxic, or one that has a significantly decreased capability to mount a response tohypoxia, may not show a change in the T-wave amplitude with further hypoxia. In thiscase, even progressive metabolic acidosis does not elicit a T-wave response. This facthighlights the importance of using STAN only in a fetus that is initially deemed to becapable of mounting a hypoxic response and underlines the adjunct nature of thissystem, that is, reliable and accurate interpretation of the standard electronic fetalheart pattern is essential to the correct use of STAN.

Maternal ECG

If the STAN electrode is attached to the maternal cervix or to a dead fetus, thematernal ECG may be recorded, resulting in a different averaged ECG complex.

Fig. 3. Grade 1, grade 2, and grade 3 biphasic ST segment depression indicating that gradedepends on the amount of the ST segment below the isoelectric line.

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Typically, there is no P wave, the QRS complex is much wider, and the displayed heartrate correlates with the maternal rate.

Development of STAN for Clinical Use

Clinical application of STAN to fetal monitoring began in 1979, and significantadvances occurred between 1979 and 1989.24 Signal processing capability improvedto the point that clinical trials became possible.

USE OF THE STAN SYSTEM

The STAN system includes a fetal ECG electrode, a maternal skin reference electrode,and a microprocessor-based monitor that identifies the fetal ST segment and T-wavechanges and compares them with the normal baseline established for that patient.This is an important distinction between this technology and others that have beenused to evaluate fetal condition in labor. The STAN monitor requires that all fetusesmonitored have had a normal heart rate pattern for at least 500 consecutive heart-beats, which equates to approximately 4 to 5 minutes of monitoring. This initial periodof signal acquisition is used to generate the baseline parameters against which thealgorithms will compare all subsequent changes. This stipulation means that, despiteits capability of discerning ST segment and T-wave changes, the STAN technologydepends completely on the initial decision as to whether a heart rate tracing is normalor not.Each fetal heartbeat generates a single ECG complex, which is recorded,

processed, and analyzed by the monitor. The monitor records 30 consecutive fetalECG complexes and creates an average complex. The average ECG complex isanalyzed in 2 ways: (1) a T/QRS ratio is computed (ratio between the amplitude ofthe T wave and the amplitude of the QRS complex) and (2) biphasic changes are deter-mined by the analysis of the slope of the ST segment.The monitor then plots the T/QRS ratios along the lower section of the tracing. If

biphasic changes are detected, they are depicted as numerals (1, 2, or 3) beneaththe respective T/QRS ratio.The baseline T/QRS ratio is determined at the beginning of each recording and is

different for every fetus. The STAN monitor continuously calculates new T/QRS ratios(every 30 heart beats, eg, every 15 seconds in a baby that has a heart rate of 120 beatsper minute) and detects changes over time from the individualized baseline T/QRSratio. The fetus normally retains a stable T/QRS throughout labor, without showingsignificant changes in T-wave amplitude or ST segment slope. If this is the case, therewill not be any ST events detected (or displayed). Absence of ST events usually indi-cates a fetus that is in positive myocardial energy balance. However, under certaincircumstances, ST events may not be displayed in a fetus experiencing myocardialglycogenolysis. These events include insufficient time to obtain a baseline T/QRSratio, poor ECG signal quality, underlying hypoxia at the time STAN was initiated, fetalcardiac malformations, split R waves, and recording of maternal ECG signal.

ST Events

When the monitor detects a significant change in the ST interval (as compared with thebaseline T/QRS ratio), it displays an “ST event” on the main screen and in an event log.There are 3 types of ST events identified by STAN: episodic T/QRS ratio increase,baseline T/QRS ratio increase, and biphasic ST.

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Episodic T/QRS Ratio Increase

This increase in the T/QRS ratio lasts less than 10 minutes and is greater than 0.11above baseline. This episodic increase indicates a short-lived period of hypoxia duringwhich time the fetus has had to utilize anaerobic metabolism to sustain normalmyocardial function.

Baseline T/QRS Ratio Increase

This increase in the T/QRS ratio lasts longer than 10 minutes and is greater than 0.06above baseline, which indicates a fetal response to hypoxia with anaerobic metabo-lism that may take place over hours or may progress rapidly.

Reference T/QRS Baseline

The STAN system uses a 20-minute rolling window to determine the long-term T/QRSbaseline and a rolling 10-minute window to determine the short-term T/QRS baseline.The reference T/QRSbaseline is the lowest long-termbaseline calculatedwithin the last3 hours. Each short-term baseline value is compared with the reference T/QRS base-line. If there is a significant difference between the reference T/QRS baseline and theshort-term baseline, it is flagged as a baseline T/QRS increase. Each T/QRS averageis compared with the latest short-term baseline. If there is a significant differencebetween a T/QRS average and the latest short-term T/QRS baseline, it is flagged asan episodic T/QRS increase. Thus, the baseline increase and episodic increase repre-sent changes in the T/QRS ratio and not the absolute T/QRS ratio values.

Biphasic ST Segment

This segment is a downward sloping ST segment and is the result of fetal myocardialstress, during which time pumping ability is decreased (see earlier in the discussion).Biphasic ST segments are more commonly seen in fetuses with infection, immaturity,cardiac malformations, or a mother with diabetes mellitus. Biphasic ST segments areclassified into 3 different grades (see Fig. 3): grade 1, downward sloping ST segmentwith the entire ST segment above the fetal ECG baseline; grade 2, downward slopingST segment that crosses the fetal ECG baseline; and grade 3, downward sloping STsegment with the entire ST segment below the fetal ECG baseline.The monitor displays an ST event warning after 2 biphasic ST grade 2 or 3 fetal ECG

averages, either consecutive or separated by no more than 1 nonbiphasic fetal ECGaverage.

Clinical Use of the STAN System

During labor, fetuses essentially fall into 3 categories25,26: those who are toleratinglabor without any issue and the monitoring strip is strongly predictive of normalacid-base balance (National Institute of Child Health and Human Development[NICHD] category I), those who are clearly in trouble, with a strip predictive ofabnormal fetal acid-base balance with a need for urgent intervention of some sort, ifnot urgent delivery (category III), and those who are neither a category I nor categoryIII (category II). The category II monitoring strip is regarded as indeterminate, notpredictive of an acidotic fetus but without adequate evidence to classify the strip ascategory I or category III. The recommendation for the management of a category IIstrip is to evaluate, continue surveillance, and reevaluate, taking into account theentire clinical circumstances. Category II tracings are clearly the arena in whichmost of the more difficult decision making occurs. This area is also where STAN is

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believed to be potentially helpful and also where it requires evaluation in the UScontext.The STAN system is based on a combination of FHR interpretation and adjunctive

STAN. Because the utility of the system is based on an assurance that the fetus is notacidotic at the baseline, the ability to identify an abnormal FHR tracing is the rate-limiting step of this technology. Competence in the assessment and managementof FHR monitoring is an important component of any usage of the STAN system.Again, herein lies one of the important reasons why STAN must be studied in theUS context before any widespread use can be promulgated or endorsed. The systemhas not been tested using the NICHD26 FHR monitoring guidelines (as opposed toInternational Federation of Gynecology and Obstetrics (FIGO) and local Europeanguidelines) and has not been extensively studied using the recently US Food andDrug Administration (FDA)-approved STAN guidelines (see http://www.fda.gov/cdrh/pdf2/p020001a.pdf).27–34

The system requires an initial assessment of the FHR monitoring strip, with classi-fication into 1 of 3 zones (Fig. 4): green, yellow, or red. Heart rate tracings classified asgreen zone do not require any intervention and may be watched expectantly (Fig. 5)regardless of any ST change. Tracings classified as red zone need expeditiousdelivery regardless of ST changes. Yellow-zone tracings have a more complexmanagement schema (see Fig. 5) that relies on the presence or absence of STchanges. The color zones in the STAN system are roughly analogous to the NICHDcategories but with 2 significant differences. The green zone allows the presence ofvariable decelerations as long as they are less than 60 seconds in duration and lessthan 60 beats per minute in depth. NICHD category I does not allow any variabledecelerations, and as such, the STAN system may be slightly more lenient than theNICHD classification in terms of what is regarded as acceptable to continue to monitorwithout concern for acidosis. However, NICHD category II is less stringent than theSTAN system in what it regards as indeterminate in that category II tracings allowabsent variability without recurrent decelerations, whereas the STAN system classifiessuch a strip as red zone needing expeditious delivery.The real utility of the STAN system therefore comes in the management of those

patients who are classified in the yellow zone because ST changes in this zonebecome an important indicator of progressive hypoxia, and the initiation of anaerobicmetabolism, with its potential for metabolic acidosis.This article does not provide a handbook for STAN monitoring; only interesting

aspects of the process are covered. Approved training, certification, and credentialing(as mandated by the FDA) are needed before use of the system. In addition, as hasbeen noted in most publications, there is a definite learning curve to the use of thissystem and indiscriminate use without adequate supervision is not advised.The system is FDA approved only for use in singleton pregnancies in which the fetus

is more than 36-0/7 weeks, the membranes are ruptured, and the mother is in the firststage of labor without active or involuntary pushing. There should also be no contra-indication to a fetal scalp electrode analysis or STAN. Once the ST electrode (similar indesign to the scalp electrodes in common usage, except that it is gold plated forimproved signal fidelity) is placed, 2 actions should follow. The fetal monitoring stripshould be checked to ensure that the fetal condition remains stable (moderate vari-ability) and is not deteriorating and that the signal quality is adequate with a normalaverage fetal ECG waveform. Only when the system indicates that it has determineda baseline T/QRS ratio can it be used clinically for STAN. If a fetal ECG signal is notdetermined and no adequate baseline is detected, the system can be used as a stan-dard FHR monitor as long as the quality of the FHR tracing is satisfactory. If for some

Fig. 4. Classification for STAN using a 3-color zone system. (Data fromMacones GA, Hankins GD, Spong CY, et al. The 2008 National Institute of Child Healthand Human Development workshop report on electronic fetal monitoring: update on definitions, interpretations, and research guidelines. Obstet Gynecol2008;112(3):661–6 and American College of Obstetricians and Gynecologists. Intrapartum Fetal Heart Rate Monitoring: Nomenclature, Interpretation, andGeneral Management Principles. ACOG Practice Bulletin No. 106. American College of Obstetricians and Gynecologists. Obstet Gynecol 2009;114:192–202.)

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Fig. 5. Guidelines for the use of STAN in the management of a fetal heart rate tracing.

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reason the monitor is stopped or if there is poor signal quality with gaps in the T/QRSratio documentation for more than 4 minutes, there is a risk of missing important STevent information, and in that case, management should revert to FHR pattern inter-pretation and clinical circumstances without regard to the STAN component of thesystem. If the labor continues and the tracing remains in, or returns to, the greenzone for a period of 10 minutes or more with good signal quality, monitoring withthe STAN component may be resumed. If the tracing remains in the yellow zone afterreturn of the T/QRS signal, STAN cannot be regarded as reliable and should not beused.Once a baseline T/QRS ratio has been determined, the STAN system alerts the

clinician when preset thresholds are exceeded and the risks of fetal myocardialhypoxia and anaerobic metabolism are increased. The monitor screen is shown inFig. 6. The T/QRS ratios are plotted as crosses on the scale in the lower sectionof the STAN recording (Fig. 7). If biphasic ST events are identified, they are shownas digits 1, 2, or 3 (see Fig. 7C), depending on the biphasic grade, beneath eachrespective T/QRS ratio. Any detected significant change in the ST interval, comparedwith the baseline, is displayed as an ST event. ST events are shown as flags in therecording on the main screen of STAN, in the “Event Log,” and in the printout.Green-zone tracings do not require any intervention, regardless of ST events. This

brings up an important aspect of the training because, for an as yet incompletelyunderstood reason, ST events (usually biphasic changes) are quite common in thegreen zone. The observer’s competence in classification is therefore important:if the strip is misclassified as green when it is actually yellow, a potentially hypoxicor acidotic fetus may be undertreated. Similarly, if the strip is misclassified as yellow

Fig. 6. STAN monitor screen showing fetal heart rate tracing, maternal heart rate (o),uterine activity tracing, and T/QRS ratios (x). The average fetal ECG tracing is shown inthe upper left panel, and the event log displays important information as the monitoringcontinues.

Fig. 7. (A) An episodic T/QRS elevation that has resulted in an ST event. (B) A baseline T/QRSelevation that has resulted in an ST event. (C) The system has detected several biphasic STevents that are displayed by grade number (in this case, 2 or 3) beneath the T/QRS ratiodepictions. The ST event flags above the T/QRS ratio depictions indicate significant changesin the ST interval and necessarily indicate the biphasic ST events. In the instance of an STevent flag, the provider should look to the event log to determine what type of ST eventis being flagged, and if it is noted as a biphasic ST event, then 2 such flags are needed tohave the same significance as a single T/QRS-stimulated ST event flag.

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when it is actually green, there is a significant potential for overtreatment (unnecessaryintervention or early delivery). Why some fetuses develop ST changes during a green-zone tracing is unclear, but these changes may be related to b-adrenergic stimulationin the first stage.Red-zone tracings require expeditious delivery regardless of the presence or

absence of ST events because these babies have such a poor reserve that theymay never generate an ST event and waiting for one to occur would place the fetusat an extreme risk.Yellow-zone tracings with a single ST event resulting from either type of T/QRS ratio

increase or 2 ST events generated by biphasic ST segment changes require interven-tion (see Fig. 5, Fig. 7C). The STAN guidelines suggest that in the second stage withactive pushing, a yellow-zone tracing with an ST event should precipitate operativedelivery unless spontaneous delivery is expected within 5 to 10 minutes. If a yellow-zone tracing persists for more than 60 minutes (or if it shows evidence of rapid

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deterioration), regardless of ST changes, it requires qualified assessment and closeobservation of the fetal status. Because an infected fetus may not be able to generatean ST event, STAN should not be initiated in a patient in whom chorioamnionitis is sus-pected or potentially present.The fetal ability to cope with hypoxia depends on several factors. A previously

healthy fetus that becomes hypoxic reacts with a significant increase in the T/QRSratio. The reaction is more pronounced initially and becomes progressively lessmarked as hypoxia/acidosis persists. In the case of a continually abnormal tracing,the disappearance of ST events should not be regarded as reassuring or as a signof recovery. It may well represent exhaustion of the fetal ability to respond. In thecase of initiating STAN late in the process of hypoxia, after the fetus has used up itsreserve, the T/QRS ratio may be stable and incapable of change. Fetuses exposedto longer-term stress (chronic or acute-on-chronic uteroplacental insufficiency) maynot develop T/QRS ratio changes and may only show a biphasic ST change or maynot generate any ST events at all.

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