Review of the first 1502 cases of ECG-ST waveform analysis during labour in a teaching hospital

Authors

  • V Doria,

    1. Clinical Developmental Sciences, Section of Obstetrics and Gynaecology, St George’s University of London, London, UK
    Search for more papers by this author
  • AT Papageorghiou,

    Corresponding author
    1. Clinical Developmental Sciences, Section of Obstetrics and Gynaecology, St George’s University of London, London, UK
      Dr AT Papageorghiou, Division of Obstetrics and Gynaecology, St George’s University of London, Cranmer Terrace, London SW17 0RE, UK. Email a.papageorghiou@sgul.ac.uk
    Search for more papers by this author
  • A Gustafsson,

    1. Clinical Developmental Sciences, Section of Obstetrics and Gynaecology, St George’s University of London, London, UK
    Search for more papers by this author
  • A Ugwumadu,

    1. Clinical Developmental Sciences, Section of Obstetrics and Gynaecology, St George’s University of London, London, UK
    Search for more papers by this author
  • K Farrer,

    1. Department of Newborn Services, St George’s Healthcare NHS Trust, London, UK
    Search for more papers by this author
  • S Arulkumaran

    1. Clinical Developmental Sciences, Section of Obstetrics and Gynaecology, St George’s University of London, London, UK
    Search for more papers by this author

Dr AT Papageorghiou, Division of Obstetrics and Gynaecology, St George’s University of London, Cranmer Terrace, London SW17 0RE, UK. Email a.papageorghiou@sgul.ac.uk

Abstract

Objective  To assess the impact of introduction of the STAN monitoring system.

Study design  Prospective observational study.

Setting  Tertiary referral labour ward, St George’s Hospital, London.

Population  High-risk term pregnancies.

Methods  We report all consecutive cases of intrapartum monitoring using the STAN S 21 fetal heart monitor. Cases with adverse neonatal outcome were evaluated in relation to the ST waveform analysis and cardiotocography (CTG).

Main outcome measures  Cord artery metabolic acidosis, neonatal encephalopathy (NNE) and reasons behind cases with poor outcome.

Results  Between 2002 and 2005, there were 1502 women monitored by STAN. Based on combined STAN analysis in the 1502 women, action was indicated in 358 women (23.8%), while in 1108 women (73.8%) no action was indicated. Traces were not interpretable in 36 women (2.4%). Of the 836 cases (55.7%) where cord blood gases were available, there were 23 cases (2.8%) of metabolic acidosis and 16 of these (70%) were identified by STAN. Overall, there were 14 cases of NNE monitored by STAN. Retrospective analysis of these highlights human errors, such as poor CTG interpretation, delay in taking appropriate action and not following the guidelines.

Conclusions  Our experience suggests the need for more intense training on interpretation of CTG and strict adherence to guidelines.

Introduction

Although continuous electronic fetal monitoring has led to a reduction in intrapartum stillbirth rates, intrapartum asphyxia remains an important cause of long-term deficits in survivors, and through litigation is a considerable drain on healthcare resources.1–3 Actions aimed at improving the outcome include education about the best use of existing technology. In addition, new technologies have been introduced with the same aim. A recent Cochrane review,4 following a large randomised controlled trial on cardiotocography (CTG) with ST waveform analysis,5 suggested a significant reduction in metabolic acidosis, incidence of fetal scalp blood sampling and operative delivery by using this new technology. Most importantly, use of CTG-ST waveform analysis has been associated with a significant reduction in neonatal encephalopathy (NNE).6

CTG-ST waveform analysis was introduced to St George’s Hospital, London, a busy inner city teaching hospital, in June 2002. The aim of this paper was to evaluate and report the strengths and weaknesses of introducing CTG-ST waveform analysis during labour in a teaching hospital set up.

Methodology

St George’s Hospital, London, has over 4000 deliveries per year. In addition to providing services to the local population of about 250 000 people, it is a regional tertiary referral centre for fetal medicine, neonatology, paediatric surgery and high-risk obstetrics. STAN™ (ST segment analysis, Neoventa Medical AB, Mölndal, Sweden) was introduced to our unit in June 2002. Numbers of trained staff and available STAN monitors were restricted during the first 12 months, thus initially not all high-risk women were monitored using this method. During the subsequent 3 years, more monitors were available and increased numbers of staff were trained, thus increasing the use of the technology. From the time of introduction, a database was maintained prospectively with relevant features of antenatal and intrapartum factors, events in labour and neonatal outcomes.

The introduction of the system was performed in a systematic fashion, with theoretical and practical training, assessment and certification of all staff involved in the care of women in labour on the delivery suite. Quality control was performed on a day-to-day basis by a specialist dedicated senior midwife recruited for teaching and implementing STAN (A.G.), as well as the labour ward consultants. In addition, weekly multidisciplinary meetings were used to reinforce these skills and to ensure continuing assessment. While it would have been more than a little naïve to have expected no guideline violations to occur despite these mechanisms, it is important to realise that hundreds of cases were managed appropriately.

Cases that had a suspicious or abnormal CTG and women with high-risk pregnancies (e.g. fetal growth restriction, postterm pregnancy) were monitored with STAN technology when the equipment was available. Only those who had received training in this technology were allowed to use the equipment, unless adequate close supervision by another trained person was available.

STAN monitoring involves application of a fetal scalp electrode, which provides continuous CTG and fetal electrocardiogram (ECG) analysis. The ST waveform reflects events during ventricular repolarisation. The ability of the fetal heart to pump blood is dependent on the equilibrium between the available energy and energy consumed. If this is compromised, for example in the hypoxic state, an imbalance (i.e. a negative energy balance) will occur. Fetal adaptation to this situation occurs through processes of glycogenolysis and anaerobic metabolism. As the rate of glycogenolysis increases, the T-wave amplitude will rise. It is this quantitative measurement (the height of the T wave and the height of the QRS waveform or T/QRS ratio) as well as the presence of ST segment depression that is monitored using STAN analysis. Thus, STAN analysis differs from fetal blood sampling, as the former informs us about the ability of the fetal heart to adapt to hypoxia, while the latter informs us about acidosis. The STAN guidelines are applicable to pregnancies of at least 36 completed weeks of gestation. Significant ST changes are defined as a baseline rise of the ratio of the T/QRS complex of the fetal ECG or the presence of biphasic ST segment changes. These ECG changes are electronically analysed, and any consistent rise in the T/QRS ratio, or presence of biphasic ST segments appears in the event log of the computer screen. The significance of these ST changes depends on the concurrent CTG. The significant changes that indicate situations in which intervention is required are shown in Table 1.7 Appropriate intervention is either delivery, or alleviation of a cause of fetal distress, such as uterine overstimulation or maternal hypotension. If intervention is required during active pushing in the second stage of labour, immediate delivery is recommended. If there is an abnormal CTG and a normal ST during second stage of labour, delivery is indicated by 90 minutes. The event log requires 20 minutes before automatic ST analysis can begin. At start-up, and when there is a decrease in signal quality with discontinuous T/QRS ratios, manual data analysis is required. The STAN guidelines of how to interpret the CTG were used (Table 2), as these had minor differences in interpretation compared with guidelines from the National Institute of Clinical Excellence.

Table 1.  Management guidelines for STAN: ST changes indicating clinical intervention depending on whether CTG was intermediary or abnormal. With a normal CTG intervention is not necessary, regardless of ST changes7
STANCTG
IntermediaryAbnormalPreterminal
Episodic T/QRS rise (duration <10 minutes)Increase >0.15 from baselineIncrease >0.10 from baselineImmediate delivery
Baseline T/QRS rise (duration ≥10 minutes)Increase >0.10 from baselineIncrease >0.05 from baseline
Biphasic ST: a component of the ST segment below the baselineContinuous >5 minutes or >2 episodes of coupled biphasic ST type 2 or 3Continuous >2 minutes or >1 episode of coupled biphasic ST type 2 or 3
Table 2.  Classification of the CTG pattern used for STAN monitoring7
CTG classificationBaseline heart rateVariability reactivityDecelerations
Normal110–150 beats/minute5–25 beats/minute; accelerationsEarly decelerations; uncomplicated variable decelerations with a duration <60 seconds and beat loss <60 beats/minute
Intermediary100–110 beats/minute>25 beats/minute without accelerationsUncomplicated variable decelerations with a duration <60 seconds and beat loss >60 beats/minute
150–170 beats/minute; short bradycardia episode; a combination of several intermediary observations will constitute an abnormal CTG<5 beats/minute for > 40 minutes
Abnormal150–170 beats/minute and reduced variability; >170 beats/minute; persistent bradycardia<5 beats/minute for >60 minutes; sinusoidal patternRepeated late decelerations; complicated variable decelerations with a duration >60 seconds
PreterminalTotal lack of variability and reactivity with or without decelerations or bradycardia 

Manufacturer’s guidelines were used for the combined use of CTG and ST waveform analysis to inform decision-making (Table 1). Decision-making is influenced by considering the type and magnitude of the ECG change interpreted by the computer and flagged up as a ST event in association with the CTG interpreted by the caregiver as normal, suspicious or abnormal and the clinical situation.

We prospectively collected all consecutive cases managed with STAN over the first 3.5 years. All traces were categorised into (a) those where action was indicated based on the ST analysis and CTG interpretation; (b) those where ST events occurred, but combined analysis did not indicate the need for action; (c) those with no ST event (i.e. where ST analysis neither indicate a rise in T/QRS nor the presence of biphasic ST changes); (d) and those with uninterpretable CTG/ECG recording. Umbilical artery and vein blood gas analysis was not available in all cases; cases that did are reported separately from those that did not have blood gas analysis. The neonatal outcomes of metabolic acidosis (pH < 7.0 or base excess (BE) < −12), low Apgar score (<7 at 5 minutes), admission to the neonatal unit and NNE were evaluated in relation to the ST waveform analysis in conjunction with the CTG. The BE from umbilical cord blood is reported with no correction to calculate extracellular fluid BE. For comparison of outcomes over the study period, chi-square test was used to examine for a linear trend across the groups.

Results

All women monitored by STAN from the 1 July 2002 to 31 December 2005 (42 months) were considered for the study. During this time, there were 14 890 deliveries, and of these, 1502 women had intrapartum monitoring with STAN. These women were categorised as those where action was indicated based on the ST analysis and CTG interpretation (n = 358, 23.8%), those where ST events occurred, but combined analysis did not indicate the need for action (n = 532, 35.4%), those with no ST event (n = 576, 38.3%) and those with uninterpretable CTG/ECG recording including poor quality traces or the absence of recording for > 20 minutes prior to the time of delivery (n = 36, 2.4%). These categories were analysed based on the availability, or not, of cord pH blood gases; these were available in 836 cases (55.7%), while in 666 cases (44.3%) no umbilical cord blood gas was recorded (Table 3). Neonatal outcomes based on the availability of pH and blood gases are given in Tables 4 and 5.

Table 3.  Cases based on the availability or not of cord blood gases grouped according to STAN information
 STAN+CTG indicated actionST event but STAN+CTG indicated no actionNo ST eventNot interpretableTotal
  1. UA, umbilical artery.

UA pH available, n (%)251 (30.0)304 (36.4)258 (30.9)23 (2.8)836 (56)
UA pH not available, n (%)107 (16.1)228 (34.2)318 (47.7)13 (2.0)666 (44)
Total, n (%)358 (24)532 (35.5)576 (38.5)36 (2.0)1502 (100)
Table 4.  Adverse neonatal outcomes in cases with umbilical blood gas analysis, which occurred in 97 of the 836 cases (11.6%)
 STAN+CTG indicated actionST event but STAN+CTG indicated no actionNo ST eventNot interpretable
  1. NNU, neonatal unit.

Metabolic acidosis (pH < 7 and BE > −12) (n = 23)16142
Apgar score < 7 at 5 minutes (n = 23)16331
Apgar score < 4 at 5 minutes (n = 3)1101
Metabolic acidosis and Apgar score < 7 (n = 5)4001
Admission to NNU (n = 87)5216181
NNE (n = 12)7221
Total adverse outcomes (n = 97)5617222
Table 5.  Adverse neonatal outcomes in cases without umbilical blood gas analysis, which occurred in 22 of the 666 cases (3.3%)
 STAN+CTG indicated actionST event but STAN+CTG indicated no actionNo ST eventNot interpretable
  1. NNU, neonatal unit.

Apgar score < 7 at 5 minutes (n = 9)2340
Apgar score < 4 at 5 minutes (n = 1)0010
Admission to NNU (n = 16)9250
NNE (n = 2)0101
Total adverse outcomes (n = 22)9571

Of the 836 cases that had umbilical cord blood gases available, there were 23 cases (2.8%) of metabolic acidosis (Table 4). Sixteen of these (70%) were identified by STAN and CTG analysis. In one, there was ST event, but combined analysis did not warrant action. In four cases, there were no ST events, and in two cases, the STAN trace was not interpretable.

In the group of 836 cases with cord blood gas analysis, there were 23 neonates (2.8%) with an Apgar score of less than 7 at 5 minutes, and of these 16 (70%) were identified by combined CTG and ST waveform analysis (Table 4). Three cases had an Apgar score of less than 4 at 5 minutes; one of these was identified by STAN, another had an ST event which was thought not to be significant when combined with CTG analysis, while in the third case the ST was not interpretable because of a very short period of ST analysis. There were five cases with metabolic acidosis and an Apgar score of less than 7 at 5 minutes. Four of these were identified by STAN. In the one case that was not identified, no ST trace was available for 4 hours prior to delivery.

In 666 cases, cord blood gases were not available (Table 5). In this group, there were nine cases with an Apgar score of less than 7 at 5 minutes. Of these, two were identified by ST analysis, three had nonsignificant ST events and four had no ST event. There was only one case with Apgar score <4 at 5 minutes, and this case did not show any ST event.

Out of the 14 890 deliveries during the study period, there were 35 (0.27%) cases of NNE. Of these, 14 cases were monitored by STAN. There were two cases of NNE where no cord sample was available (Table 5), while in 12 cases cord blood analysis was available (Table 4). Of these 12 cases of NNE with cord blood gases, only 7 had significant ST events, two had ST events thought not to be significant by combined CTG-STAN analysis, two did not have any ST event and in one the recording was uninterpretable (Table 4). Of the two cases without cord blood gases, one had an ST event interpreted as not significant on combined analysis, while in the other the trace was not interpretable. All 14 cases resulting in NNE were reviewed independently by two of the authors (V.D. and S.A.) retrospectively, and the findings are summarised in Table 6. The details of gestation, mode of delivery, 5 minutes Apgar score, cord arterial and vein blood gases and grade of NNE were reviewed. The likely reasons for the poor outcome were identified and fall into categories A–F as described below.

Table 6.  Reasons for poor outcome in the 14 cases of NNE. Categories A–F are the likely reasons for the poor outcome identified on retrospective review (see text for details)
 Cat.GA (weeks)Mode of deliveryASArtery pH/BEVein pH/BENNE gradeDecision made*Comments based on retrospective review
  • AS, Apgar score at 5 minutes; Cat., category; EM LSCS, emergency lower segment caesarean section; GA, gestational age; IUGR, intrauterine growth restriction; SVD, spontaneous vaginal delivery.

  • *

    Decision made by the obstetrician attending.

  • **

    Extremely low pH: probably due to error in the equipment.

1A38Ventouse97.14/−107.28/−51ActionDelay in delivery 110 minutes after STAN event indicating need for action
2A/C38EM LSCS56.90/−187.21/−121ActionDelay in delivery 43 minutes after bradycardia and STAN event
3A42EM LSCS96.23/−22**6.24/−17**1ActionDelay in delivery 80 minutes after STAN event indicating need for action
4A41EM LSCS87.14/−9.57.18/−9.42ActionDelay in delivery 70 minutes after STAN event indicating need for action
5A/C41EM LSCS76.90/−186.98/−161ActionDelay in delivery 54 minutes after STAN event indicating need for action, abnormal CTG and terminal bradycardia
6A/C35Ventouse67.09/−97.14/−71ActionDelay in delivery 60 minutes after repeated STAN events indicating need for action, abnormal CTG and terminal bradycardia
7B42Ventouse86.90/−187.06/−152ExpectantSTAN connected when CTG was grossly abnormal; 155 minutes of active pushing with abnormal CTG; no STAN change but guidelines dictated delivery
8B41Ventouse57.00/−137.08/−123Expectant90 minutes of abnormal CTG followed by 35 minutes of ventouse delivery
9B41Ventouse8Not known7.27/−62ExpectantSTAN connected when CTG was grossly abnormal; 160 minutes of active pushing with abnormal CTG; STAN changes not significant but guidelines dictated delivery
10C41EM LSCS76.96/−177.10/−121ExpectantDelay in delivery; 35 minutes of no STAN analysis, including 28 minutes of profound bradycardia
11D42EM LSCS66.90/−167.00/−141ActionPostdates, unrecognised IUGR, abnormal CTG, with STAN applied too late; STAN event to delivery: 19 minutes
12E/C39Ventouse10Not known7.17/−101ExpectantNo STAN analysis for 40 minutes; poor quality trace, but suggestive of bradycardia with doubling of FHR
13E/C42Forceps87.16/−87.25/−91ExpectantAbnormal CTG, 27 minutes of no STAN analysis prior to delivery, with terminal bradycardia of 13 minutes
14F41SVD47.15/−127.25/−92ExpectantIntermediary CTG, STAN event suggesting action at the time of delivery

A: Delay in taking action when ST event and CTG indicated delivery (suggested delivery time is 20 minutes) (n = 6, cases 1–6).

B: Nonintervention after 90 minutes of active second stage in the presence of an abnormal CTG trace, despite normal ST trace (n = 3, cases 7–9). The STAN guidelines suggest delivery with an abnormal CTG in the active second stage after 90 minutes of bearing down, despite the absence of ST events.

C: Prolonged bradycardia is a preterminal trace and warrants action based on the guidelines. Delay in action led to poor outcome (n = 1, case 10), cases 2, 5, 12 and 13 also had terminal bradycardia.

D: Initiating ST analysis with a preterminal CTG, which is contraindicated according to STAN protocol (n = 1, case 11).

E: Improper use of STAN (disconnected >20 minutes before delivery or poor quality trace) (n = 2, cases 12 and 13).

F: In one case, there was fetal tachycardia with variable decelerations but good short-term variability on CTG; a STAN event occurred at the time of normal vaginal delivery. This was an unexpected outcome (n = 1, case 14).

Discussion

The majority of adverse outcomes, among the cases that had cord arterial blood gases, were detected by the combined analysis of the CTG and ST traces. Thus, 16 of the 23 cases with metabolic acidosis, 16 of the 23 with low Apgar score and 4 of the 5 with metabolic acidosis combined with low Apgar score were preceded by a significant STAN event (Table 4). One of the limitations of the study was that cord blood gases were not available for the entire population. Hence, we have discussed in detail about the cases affected by NNE.

Our retrospective analysis of the 14 cases of NNE (Table 6) highlights the continuing problems of poor CTG interpretation, delay in taking action and nonadherence to guidelines, despite adding a new parameter of ST analysis, e.g. delivery after 90 minutes of abnormal CTG, despite no ST changes in the second stage. These mistakes are not new and were highlighted by a previous Confidential Enquiry into Stillbirths and Deaths in Infancy report.8 However, it is important to note that not all cases of NNE had evidence of metabolic acidosis or low Apgar scores at birth, suggesting other mechanisms could be responsible for the outcome.

Previous randomised controlled trials have shown a trend in reduction in caesarean section rates with the use of STAN monitoring.5,9,10 The Swedish trial showed a 26% reduction in caesarean section for fetal distress (P = 0.047),5 while the Plymouth trial showed a 46% reduction (P < 0.001).10 In the randomised control trial conducted by Ojala et al., the authors reported that ST analysis did not decrease the caesarean section rate.11 Even though this study was at a tertiary level university hospital, their emergency caesarean section rate was 6.4%, which may indicate a low-risk population. Hospitals with higher emergency caesarean section rate have indicated the reduction of caesarean section rate with the introduction of STAN technology.12

Conclusion

In our unit, the introduction of STAN technology has not changed the incidence of emergency operative delivery or NNE. Further strategies to improve our obstetric outcomes are needed. Our experience suggests the need for more intensive training and assessment of the users regarding the use of the CTG and ST analysis, with emphasis on the need to take prompt action when significant ST and CTG changes are present. Better training, assessment and supervision of users may help improve outcome.

Acknowledgements

We would like to thank Dr Ravindra Bhat for his help in obtaining the neonatal outcomes.

Ancillary