22 ST-Analyser (STAN). Handbook CTG

ST-Analyser (STAN)

◈

Principles and Physiology

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 Facts

A recent systematic review of randomized controlled trials has concluded that

the use of STAN reduced the incidence of use of fetal blood sampling and

metabolic acidosis.

STAN has been shown to reduce the interobserver variation when indicating

interventions in the presence of intermediate or abnormal CTG traces.

The principle of STAN is to assess the oxygenation of a central organ (the fetal

heart). It helps to differentiate between a fetus that is exposed to hypoxic stress

but compensating well and maintaining a good oxygenation in the myocardium

from the fetus that switches to anaerobic metabolism in the myocardium and

depends on catecholamine-mediated glycogenolysis to respond to negative

energy balance in the myocardium.

The fetal ECG is monitored through a fetal scalp electrode. As soon as it is

applied, the STAN machine calculates (over 4–5 minutes) the normal T/QRS

ratio for that particular fetus and establishes it as the ‘baseline value’. From thisKey Features on the STAN

Please see Figure 22.1.

moment, the machine analyses every 30 ECG complexes (i.e. if baseline fetal

heart rate (FHR) is 150 bpm, there would be five analyses in 1 minute) and

compares them with the original ‘baseline value’. Each of them is recorded on

the CTG trace as a cross (‘X’). When the analysed ECG complexes differ

significantly from ‘baseline value’, it will be flagged up as an ‘ST event’.

In the presence of an ‘ST event’, it is necessary to classify the CTG trace

according to STAN guidelines (normal, intermediate or abnormal) first and then

to determine if the ‘ST event’ is significant and requires any action. Conversely,

if the ST event is not significant, no further action is required at this time.

There are three types of ‘ST events’: episodic T/QRS rise, baseline T/QRS rise

and biphasic events.

‘Episodic T/QRS rise’ event appears when the T/QRS ratio rises in response to

a short-lasting hypoxia for <10 minutes. The significance depends on the

magnitude of rise and classification of the CTG trace. If the CTG trace is

classified as normal, the ST event is not significant as it can be secondary to

fetal movements and resultant release of catecholamine-mediated myocardial

glycogenolysis; if the CTG trace is intermediate, a greater increase in the T/QRS

ratio is allowed before it becomes significant than if the CTG trace is classified

as abnormal.

‘Baseline T/QRS rise’ indicates a longer-lasting hypoxia (>10 minutes) with the

resultant increase in the T/QRS ratio persisting for >10 minutes. The magnitude

of rise is shown on the screen (event log), and the significance depends again on

magnitude and classification of the CTG trace.

‘Biphasic ST’ appears when there is a shift in the ST segment. There are three

degrees of biphasic ST events (grades 1–3). Repetitive grade 2 and 3 biphasic

events are significant in the presence of an intermediate or abnormal trace asFigure 22.1 STAN trace showing ST event (black box), event log (left-hand column) and

crosses (‘x’) – each ‘x’ (oval) represents 30 fetal ECG complexes.

Key Pathophysiology behind Patterns Seen on

the CTG Trace

they may reflect instability of the myocardial membrane secondary to hypoxia

and resultant changes in the morphology of ST segment of the fetal ECG

complex.

Event log indicates documentation by clinicians (e.g. vaginal examination, blood

pressure monitoring, administration of oxytocin) as well as by the computer (e.g.

type and magnitude of ST events, loss of contact).

The fetal ECG reflects the oxygenation of a central organ, the myocardium,

which is the last organ to fail when a fetus is exposed to hypoxia.

Physiology behind ‘T/QRS ST events’: The fetus releases catecholamines

(emergency hormone) that increase the FHR and also activate ‘glycogenolysis’

in the myocardium to increase the glucose available for the heart to function. The

process of ‘glycogenolysis’ results in a release of potassium ions which have

been stored within glycogen, and the resultant ‘hyperkalemia’ produces a rise in

‘T waves’ and an increase on ‘T/QRS ratio’. This phenomenon results in

‘T/QRS ST events’.Recommended Management

Physiology behind ‘biphasic ST events’: The ST segment reflects the refractory

period (isoelectric) after depolarization (myocardial contraction) and before

repolarization when there is no transfer of ions through the myocardial cells. In

the presence of a disturbance to the myocardial pump function (secondary to

hypoxia, infection, prematurity or cardiac defects), the ST segment shifts

upwards or downwards resulting on a biphasic ‘ST event’, reflecting

myocardial membrane dysfunction.

In the presence of a normal CTG trace, any ‘ST events’ can be managed with

expectant management, as they are not significant at this stage. Most commonly,

they are secondary to fetal movements that also release catecholamines.

In the presence of a preterminal trace, delivery should be expedited regardless

of the presence or absence of ‘ST events’.

In the presence of a significant ST event during the first or passive second stage

of labour, interventions to improve utero-placental oxygenation need to be

instituted. These include stopping oxytocin infusion, administration of

intravenous fluids, postural changes and/or acute tocolysis (terbutaline) to

improve fetal oxygenation. If the changes observed on the CTG improve and/or

there are no further ST events, labour can be allowed to continue. If there are

further significant ST events, CTG should be reclassified, and if the ST-events

are significant and if no further conservative measures are possible, then

immediate delivery (within 20 minutes) is indicated.

During active second stage of labour, any significant ST event should be

managed with immediate operative delivery (by the safest and quickest mode of

birth) as soon as possible unless spontaneous vaginal delivery is expected in the

following 5 to 10 minutes.

CTG changes and ST events should always be correlated with the clinical

picture (presence of meconium, chorioamnionitis, vaginal bleeding, growthKey Tips to Optimize Outcome

Common Pitfalls

restriction). Immediate delivery may be indicated in the presence of any of these

risk factors regardless of the significance of ST events.

Remember STAN can only be used in fetuses >36 weeks of gestational age as the

endocardial–epicardial interphase may be underdeveloped and interfere with

signal conduction leading to multiple ST events (most commonly biphasic ST

events). For the same reason, it cannot be used in fetuses with structural cardiac

defects.

In the presence of infection, any ST event, even in the presence of an

intermediary trace, may be regarded as significant.

If, when applying the fetal scalp electrode, repetitive ST events appear, ensure

that fetal presentation is cephalic. If the fetus is presenting by breech, ECG

complexes will be inverted and the machine interprets this as repeated biphasic

ST events due to perceived inversion of ST-segment of the fetal ECG complex

(i.e. will be recorded as a negative wave). If a decision is made to continue

labour in anticipation of an assisted vaginal breech delivery, then the STAN

machine has a ‘breech mode’ to invert the ECG complex so that biphasic events

can be stopped and the fetus can be continuously monitored using the STAN

technology.

Relying on STAN in the presence of chorioamnionitis. The STAN is a test of

hypoxia and not for infection. In the presence of an infection, there may not be

any ST events until the very final stages when the infection is affecting the

oxygenation to central organs (i.e. fetal myocardium leading to myocarditis).

Chorioamnionitis may lead to repeated biphasic ST events due to the

inflammatory damage to myocardial membrane. In this case, the CTG may notConsequences of Mismanagement

Recent Developments

show significant decelerations, and a high index of clinical suspicion of ongoing

chorioamnionitis should be exercised and labour should be managed

accordingly.

Applying the STAN in the absence of a stable baseline heart rate and a

reassuring variability. The STAN device calculates the normal ECG complex for

each fetus during the first 4–5 minutes. It is not possible to rely on STAN when it

is applied during ongoing subacute hypoxia with a loss of a stable baseline

and/or reassuring variability

Erroneous monitoring of the maternal heart rate (MHR) as FHR. When MHR is

being monitored, the P-waves would be absent on the ECG complexes as the

signal (maternal P-wave) is not powerful enough to be transmitted to the fetal

scalp electrode.

Unnecessary interventions for a normal CTG trace when ST events are flagged

up or lack of intervention on a preterminal trace in the absence of ST events.

Unnecessary interventions (operative delivery) due to nonadherence to STAN

guidelines

Stillbirth

Neonatal death

Hypoxic-ischaemic encephalopathy and subsequent cerebral palsy

Neonatal sepsis – when relying on STAN and ignoring signs of ongoing clinical

chorioamnionitis

A large multicentre randomized controlled trial from the United States of

America in 2015 reported that the use of STAN did not reduce operativeTherefore, in the authors’ opinion based on published systematic evidence,

compared to other adjunctive tests (i.e. pulse oximetry, fetal scalp pH and lactate),

which have been shown to have no robust scientific evidence of benefit, STAN is the

only adjunctive test which has been shown to be beneficial in 2016 (i.e. a statistically

significant reduction in metabolic acidosois, fetal blood sampling and operative vaginal

births). However, training in fetal physiology prior to introducing STAN is essential to

maximize its benefits and to minimize harm.

ObGyncases (Healthcare Touch)

Tìm kiếm Blog này

22 ST-Analyser (STAN). Handbook CTG

Nhận xét

Đăng nhận xét