23
ST-Analyser
◈
Case Examples and Pitfalls
Ana Piñas Carrillo and Edwin Chandraharan
Handbook of CTG Interpretation: From Patterns to Physiology, ed. Edwin Chandraharan.
Published by Cambridge University Press. © Cambridge University Press 2017.
Key Principles
ST-Analyser (STAN) is only validated for monitoring fetuses >36 + 0 weeks of
gestation and should not be used in fetuses <35 weeks + 6 days of gestation.
Contraindications to the application of fetal scalp electrode (e.g. any maternal
infection with increased risk of vertical transmission through a defect in the fetal
skin and fetal haemorrhagic disorders which may increase the risk of scalp
haemorrhage) should be excluded.
Fetus should retain the capacity to respond to evolving intrapartum hypoxic
stress (i.e. should have a stable baseline fetal heart rate (FHR) and a reassuring
variability indicative of good oxygenation of the central organs).
STAN should not be commenced in the presence of an unstable baseline FHR
(i.e. myocardial hypoxia), reduced baseline variability with preceding
decelerations (hypoxia to the central nervous system) or in the presence of a
preterminal trace indicative of total loss of the ability to respond to hypoxia.Table 23.1 STAN guidelines: classification of the CTG trace
Cardiotocographic
classification
Baseline heart
frequency
Variability
reactivity
Decelerations
Normal 110–150 bpm 5–25 bpm
accelerations
Early decelerations
Uncomplicated
variable
decelerations with
a duration of
<60 seconds and a
beat loss of
<60 bpm
Intermediary* 100–110 bpm
150–170 bpm
Short bradycardia
episode
>25 bpm
without
accelerations
<5 bpm for
>40
min
Uncomplicated
variable
decelerations with
a duration of
<60 seconds and a
beat loss of
>60 bpm
Abnormal 150–170 bpm
and reduced
variability
>170 bpm
<5 bpm for
>60
minutes
Sinusoidal
pattern
Repeated late
decelerations
Complicated variable
decelerations with
a duration of
>60 seconds
Preterminal Total lack of variability and reactivity with or without
decelerations or bradycardia
* Combination of several intermediary observations will result in an abnormal CTG.
STAN is a test to detect intrapartum hypoxia. However, other pathways of fetal
neurological damage (e.g. meconium aspiration syndrome, chorioamnionitis and
inflammatory brain damage) should be considered while using STAN.
CTG should be classified (Table 23.1) and STAN guidelines (Table 23.2) should
be used while using STAN in clinical practice. Algorithms such as the ‘6C’
approach (Figure 23.1) may aid in management.Table 23.2 STAN guidelines: interpretation of ST events
ST events Episodic T/QRS
rise
Baseline T/QRS
rise
Biphasic ST
Normal CTG Expectant management and continued observation
Intermediary
CTG
>0.15 >0.10 3 biphasic log
messages
Abnormal CTG >0.10 >0.05 2 biphasic log
messages
Preterminal CTG Immediate deliveryFigure 23.1 Suggested algorithm: 6C approach for use of STAN in clinical practice.
Case 1. Commencement of STAN
The CTG trace shows a stable baseline FHR and a reassuring baseline variability prior
to commencement of STAN. Appearance of ‘crosses’ or ‘x’ below the tocograph
indicates that the analysis of fetal ECG has commenced (Figure 23.2). The ‘event log’
indicates that the ‘baseline T/QRS was determined at 18:36 hours’. Therefore, it is
appropriate to rely on STAN from this time onwards.Each ‘x’ represents an average of 30 ECG complexes, and therefore, 4 ‘x’s are
seen in a box (1 minute) because baseline FHR is 140 bpm (30 × 4 = 120).
STAN requires approximately 120 ‘x’s (i.e. approximately 4 minutes if baseline
FHR is 120 bpm) to calculate a baseline T/QRS ratio for an individual fetus.
Subsequently, it compares the average of 30 ECG complexes with this original baseline
T/QRS ratio as well as the ST segment of the fetal ECG to determine whether there is a
significant change in the T/QRS ratio or the ST Segment.
In addition, to ensure adequate ECG signals, crosses (‘x’) should not be absent
continuously for >4 minutes and there should be a minimum of 10 ‘x’s in any 10-minute
period.
Figure 23.2 Stable baseline and reassuring baseline variability at the commencement of
STAN monitoring. Note the event log.
Case 2. STAN Events on a Normal CTG Trace
Repeated fetal movements can result in fetal catecholamine surge as a part of ‘startle
response’ leading to glycogenolysis in the myocardium to release extra energy. This
results in the release of potassium ions stored within glycogen into the myocardial cells
as glycogen is broken down to glucose to provide extra energy substrate for the
myocardial cell. Increased intracellular myocardial potassium level leads to tall ‘T’
waves on the ECG complexes and the generation of ST event (Figure 23.3).
Such false-positive ST events are common with repeated fetal movements,
especially if there is a confluence of accelerations (Figure 23.3). No intervention isrequired as the CTG trace is entirely normal with no evidence of ongoing hypoxia
(Table 23.2).
Figure 23.3 ‘Episodic’ ST events secondary to repeated fetal movements. Note the
abrupt increase in ‘x’s coinciding with the ST events confirming that there was an abrupt
in
crease in the height of ‘T’ wave of fetal ECG due to intracellular release of potassium
secondary to catecholamine-mediated glycogenolysis.
Case 3. Nonsignificant ‘Episodic’ STAN Events
Even if the STAN generates ST events, no action is needed if the magnitude of the event
does not meet the threshold for intervention (Table 23.2). Therefore, every ST event that
is noted on the monitor does not require an intervention.
The onset of maternal active pushing may result in the development of hypoxia
within 10 minutes, leading to the generation of an ST event (Figure 23.4). However,
even if the magnitude of episodic T/QRS event is 0.10, if the CTG trace is not abnormal,
no intervention is required. In an intermediary CTG, a higher threshold (>0.15) would
be required to warrant an intervention (Table 23.2).Figure 23.4 Worsening decelerations with the onset of maternal active pushing and
transient increase in the ‘x’ in response to the release of intracellular potassium ions
secondary to catecholamine-mediated myocardial glycogenolysis. The ECG complex also
shows an elevation of the ST segment. However, no intervention is required as the type and
magnitude of ST event is not significant for the observed CTG changes.
Case 4. Abnormal CTG without Significant ST
Events
If the CTG remains abnormal despite absence of any significant ST events, a careful
assessment should be made to ensure that the signal quality is adequate. In the presence
of meconium staining of amniotic fluid and/or ongoing infection (i.e. chorioamnionitis),
coexisting hypoxia can significantly worsen perinatal outcomes. Therefore, in the
presence of deep prolonged decelerations and baseline tachycardia (Figure 23.5) and/or
loss of baseline FHR variability, asphyxia-mediated intrapartum meconium aspiration
syndrome and fetal inflammatory brain damage may occur, despite the absence of ST
events.
Clinicians should appreciate the fact that STAN is a test of hypoxia and would not
predict or diagnose meconium aspiration syndrome or diagnose ongoing inflammatory
brain damage secondary to clinical or subclinical chorioamnionitis. Therefore,
management decisions should be made based on the degree of CTG abnormalities noted,parity, cervical dilatation, progress of labour, need for augmentation with oxytocin and
fetal reserve.
If other risk factors are absent and the CTG is abnormal, it is appropriate to
continue labour in the absence of ST events, and conservative measures such as
changing maternal position to avoid umbilical cord compression may be attempted to
improve the CTG. However, in the presence of other risk factors such as thick
meconium staining of amniotic fluid or clinical chorioamnionitis, intrapartum
management should not depend on the absence of significant ST event alone. It is
recommended that in the presence of clinical chorioamnionitis, an intermediary CTG
should be upgraded to an abnormal CTG while interpreting ST events to recognize the
fact that coexisting inflammation lowers the threshold at which hypoxia can damage the
brain cells.
Figure 23.5 Ongoing complicated (or atypical) variable decelerations and increasing
baseline FHR secondary to evolving hypoxia. Although the CTG is ‘abnormal’ according
to STAN guidelines, there are no ST events and the signal quality is good (‘x’s are absent
only for 2 minutes).
Case 5. Abnormal CTG with a Significant STAN
EventsAny significant ST event requires an immediate intervention to improve utero-placental
circulation (changing maternal position, stopping oxytocin infusion with or without
administration of tocolytics and/or administration of intravenous fluids,) and if this is
not possible, an urgent delivery should be accomplished within 20 minutes of the
significant ST event. This holds true during first stage of labour and passive second
stage of labour. A significant ST event on an abnormal CTG (Figure 23.6) requires an
urgent intervention. The ‘event log’ would highlight the type and magnitude of the ST
event (Figure 23.6).
During active second stage of labour (i.e. after the onset of active maternal
pushing), immediate operative delivery (within 20 minutes) should be carried out,
unless a spontaneous vaginal delivery is imminent within the next 10 minutes. This is
because the evolution of hypoxia could be very rapid during active second stage of
labour. If a delay in delivery is anticipated, maternal pushing should be stopped to
rapidly improve fetal oxygenation.
If CTG has improved with conservative measures, it is appropriate to continue
labour with close observation. If there are repeated ST events, the timing of the very
first ST event should be considered in formulating a management plan. Worsening
magnitude of ST events (e.g. baseline T/QRS ratios of 0.06, 0.09, 0.11, etc.) over time
would indicate a progressively worsening hypoxic insult to the fetus, and urgent action
should be taken to improve utero-placental oxygenation, and if this is not possible or
appropriate (e.g. placental abruption or uterine rupture), an immediate operative
delivery should be undertaken.
Figure 23.6 Abnormal CTG trace with repeated complicated (or atypical) variable
decelerations lasting >60 seconds. The event log highlights baseline T/QRS ratio of 0.06,
which is significant for an abnormal CTG according to STAN guidelines.Pitfalls with the Use of STAN
Human error with regard to CTG interpretation and appropriate classification remains
the main concern. In addition, failure to incorporate the wider clinical picture such as
presence of thick meconium staining of amniotic fluid or evidence of clinical
chorioamnionitis also may lead to poor outcomes.
Failure to adhere to STAN guidelines (commencing STAN monitoring in preterm
fetuses or fetuses with known cardiac malformations), failure to accomplish delivery
within 20 minutes in the presence of a significant ST event or commencement of STAN
monitoring in the presence of preterminal CTG or chronic hypoxia (i.e. loss of baseline
FHR variability) may also lead to poor outcomes.
Conclusions
STAN determines oxygenation of the fetal myocardium and the capacity of the
myocardium to deal with ongoing hypoxic stress via the onset of anaerobic metabolism,
catecholamine-mediated cardiac glycogenolysis and consequent release of potassium
ions within the myocardial cell. If the T/QRS ratio is significantly higher than the ratio
calculated within the first 4 minutes of commencement of STAN monitoring, an ST event
will be generated.
Intervention should be based on the type and magnitude of ST event as well as the
classification of the CTG. Additional risk factors such as the presence of meconium
staining of amniotic fluid, evidence of ongoing chorioamnionitis, lack of progress of
labour, reduced physiological reserve of the fetus etc., should also be considered, and
one should not merely rely on STAN alone.
It should be noted that if a fetus has exhausted all its reserves and does not have the
capacity to mount a compensatory response to ongoing hypoxic stress (i.e. preterminal
CTG trace), ST events may not be seen. This is because of depletion of myocardial
energy stores (i.e. glycogen) and the resultant absence of catecholamine-mediated
glycogenolysis and consequent absence of any further changes in ST segment or T/QRS
ratio.Further Reading
1. Chandraharan E. STAN: an introduction to its use, limitations and caveats. Obs Gyn
Midwifery Prod News; 2010.
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