Obgyno Wino Podcast Episode 80 - Antepartum Fetal Surveillance
“Must I accept the barren Gift?
-learn death, and lose my Mastery?
Then let them know whose blood and breath
will take the Gift and set them free:
whose is the voice and whose the mind
to set at naught the well-sung Game-
when finned Finality arrives
and calls me by my secret Name.
Not old enough to love as yet,
but old enough to die, indeed-
-the death-fear bites my throat and heart,
fanged cousin to the Pale One's breed.
But past the fear lies life for all-
perhaps for me: and, past my dread,
past loss of Mastery and life,
the Sea shall yet give up Her dead!
Lone Power, I accept your Gift!
Freely I make death a part of me;
By my accept it is bound
into the lives of all the Sea-
yet what I do now binds to it
a gift I feel of equal worth:
I take Death with me, out of Time,
and make of it a path, a birth!
Let the teeth come! As they tear me,
they tear Your ancient hate for aye-
-so rage, proud Power! Fail again,
and see my blood teach Death to die!”
― Diane Duane, Deep Wizardry
PB#145 - Published July 2014 (Reaffirmed 2016)
Changes in fetal activity over serial assessments may reflect uteroplacental compromise
NST reflects short-term fetal acid-base status; amniotic fluid assessment is a long-term indicator of placental function
Variable decels are present in up to 50% of NSTs. If they aren't repetitive and last for <30 sec, then they are not indicative of fetal acidemia or compromise (no interventions needed)
If the fetus is growth-restricted, it's likely due to placental dysfunction, which may be detected as the stopping of umbilical artery blood flow or even its reversal during diastole on umbilical artery Doppler velocimetry.
Generally speaking, surveillance should begin no sooner than 32 wga and should be repeated no more frequently than weekly
Antepartum fetal surveillance provides insight into fetal acid/base status
- FHR pattern, fetal activity levels, and muscular tone reflect acid/base status of the fetus
- when the fetus is hypoxemic, blood is redistributed in their little body to the brain and adrenals such that renal blood flow is relatively diminished --> oligohydramnios
- early detection of hypoxemia can help to avoid fetal acidemia
- when oxygen is present, aerobic respiration drives energy production; in its absence, lactic fermentation takes over --> acid production!
- surveillance implies more than one assessment: changes in fetal activity over serial assessments may reflect uteroplacental compromise
- we know that low pH found from cord blood specimens in newborns suggests in-utero compromise prior to the birth
- several studies were conducted a while back to correlate fetal umbilical vein pH with various activities noted on US and fetal cardiotocography (FHR patterns)
- this was done by drawing blood from the fetal umbilical vein transabdominally under ultrasound guidance ("cordocentesis")
- mean umbilical vein pH is 7.28; cessation of movement begins to appear at a pH of 7.16
- so...less movement noted by the mom ("kick counts"), by ultrasound, or in the FHR pattern may suggest uteroplacental dysfunction as this is associated with lower fetal pH
Recall: Hypoxemia refers to low partial pressure of oxygen in the blood. Hypoxia refers to oxygen deprivation at the tissue level. The latter may be due to poor delivery or poor uptake/utilization by the tissues of oxygen circulating in the blood.
Antepartum fetal surveillance CANNOT tell you the severity of the acidemia or hypoxemia
- nor does it tell you anything about the duration of the compromise
- abnormal surveillance findings don't always reflect fetal acidemia or hypoxemia
- fetal sleep cycles, prematurity, maternal medications, maternal smoking, and fetal CNS abnormalities can all produce abnormalities on fetal surveillance
- antepartum fetal surveillance methods are not designed to detect abrupt changes in fetal status such as those that you see with placental abruption or a cord prolapse --> rapid onset of acidosis --> may be catastrophic (fetal demise)
- maternal perception of decreased fetal movement may predict fetal death in the coming days
- 10 distinct movements over a 2-hour period, including kicks, punches, and turns
- sensing hiccups? very good sign that everything is peachy
Contraction stress test
- let's see how the FHR pattern changes in response to uterine contractions!
- through oxytocin infusion or nipple stimulation, a uterine contraction pattern is produced such that 3 contractions are occurring over a 10-minute period, each lasting at least 40 sec
- look for decelerations: late decels suggest uteroplacental insufficiency; variable decels suggest cord compression perhaps due to oligohydramnios
- no absolute contraindications
negative: no decels or significant variable decels
positive: late decels w/ ≥ 50% of contractions (even if <3 contractions over 10 minutes)
equivocal-suspicious: late decels w/ <50% of contractions or significant variable decelerations
equivocal: decels that occur with contractions BUT the contractions are coming way too fast or lasting too long in the test! (more frequently than q2 min or lasting longer than 90 sec)
unsatisfactory: <3 contractions per 10-minutes; or if tracing sucks
Note: Late decels are defined as a decrease in FHR below baseline in which the nadir of this drop occurs at least 30 seconds from onset. Onset of the decel occurs after the onset of the contraction.
- reflects short-term fetal acid-base status
- the hallmark of the NST is the presence of absence of accelerations
- an accel is defined as an increase in FHR above baseline by ≥15 bpm for ≥15 sec (≥10 bpm for ≥10 sec if <32 wga)
- a fetus that is not acidotic will show accels on an NST
- FHR pattern must be observed for at least 20 minutes, but it may take much longer given the normal fetal sleep-wake cycle
- no accels? try vibroacoustic stimulation
reactive: ≥2 accels over a 20-minute period
nonreactive: <2 accles over a 40-minute period
- NST is not as useful w/ preterm infants
- 50% of fetuses at 24-28 wga may have nonreactive NSTs
- 15% at 28-32 wga
- variable decels are present in up to 50% of NSTs
- if they aren't repetitive and last for <30 sec, then they are not indicative of fetal acidemia or compromise (no interventions needed)
- any deceleration (variable or late) that lasts for ≥60 sec is associated with fetal demise...sooo...definitely watch out for that
- 10 possible points, 8 of which come from ultrasound evaluation while the remaining two come from NST
- regardless of composite score, oligohydramnios (1 of the 5 components) should prompt further investigation
- amniotic fluid assessment is a long-term indicator of placental function
- stop using AFI, start using deepest vertical pocket (DVP)
Per Cochrane: "The single deepest vertical pocket measurement in the assessment of amniotic fluid volume during fetal surveillance seems a better choice since the use of the amniotic fluid index increases the rate of diagnosis of oligohydramnios and the rate of induction of labor without improvement in peripartum outcomes."
Modified biophysical profile
- amniotic fluid assessment + NST
- far better false negative rate than the full BPP or CST
Umbilical artery Doppler velocimetry
- validated as a surveillance tool for monitoring vascular resistance only in pregnancies affected by fetal growth restriction
- in normally-growing fetuses, blood flow at end of diastole is still very high
- if the fetus is growth-restricted, it's likely due to placental dysfunction, which may be detected as the stopping of umbilical artery blood flow or even its reversal during diastole
- Doppler velocimetry of other vessels like the middle cerebral artery and ductus venosus hasn't been found to improve perinatal outcomes; utility is unclear
Is there evidence that these methods actually improve outcomes?
- data is available from large observational studies on NST, CST, BPP, and mBPP
- very low false negative for stillbirth within 1 week (>99.8% negative predictive value for NST, CST, BPP, and mBPP); relatively high false positive rates
- stillbirth rate is 2 per 1,000 births after a reactive NST
- 0.3 per 1,000 after a negative CST
- 0.8 per 1,000 after a normal BPP/mBPP
- these numbers do not hold in the presence of congenital anomalies or for other, unpredictable causes of fetal demise (e.g. cord prolapse, acute placental abruption)
- similar large studies haven't been conducted for Doppler velocimetry; however, the available evidence suggests that it has a comparable negative predictive value
- this DOES NOT mean that antepartum fetal surveillance prevents poor fetal outcomes, as no RCTs have been completed
- a reassuring result from any of these surveillance methods does not preclude the use of intrapartum fetal heart rate monitoring
When should surveillance be initiated (if indicated at all)? And how often?
- in the premature period, utility of surveillance varies as false positive rate is higher (remember that 50% of NSTs at 24-28 wga and 15% of NSTs from 28 - 32 wga are non-reactive!), so the risks/benefits of surveillance is an important part of the conversation
- even at term timing and frequency should be considered carefully, taking into consideration prognosis of neonatal survival, patient's attitudes about intervention, risk of fetal death, risk of prematurity, and severity of maternal disease
- generally not recommended to begin surveillance before 32 wga (but exceptions can always be made, eg. fetal growth restriction in the setting of chronic hypertension)
- if the clinical condition that prompted an initial assessment persists, then repeating periodically (generally no more often than weekly) may be necessary until birth
- serial growth assessment DEFINITElY shouldn't be performed more frequently than every 2 weeks
What do I do if the assessment is abnormal?
- false positive rates are high, so you need to take into consideration the whole clinical picture
- consider the etiologies, and try to correct any potential maternal conditions that may be resulting in the abnormal test (e.g. hypoglycemia in a diabetic patient)
- if etiology is not thought to be reversible (e.g. placental dysfunction): repeat testing or delivery may be indicated
- decreased fetal movement? --> NST, CST, BPP, or mBPP
- abnormal NST or mBPP? --> CST or BPP
- if absent end-diastolic flow is noted on umbilical artery Doppler velocimetry at ≥ 34 wga, delivery should be considered (same for reversed end-diastolic flow at ≥ 32 wga)
- if S/D ratio is >95th percentile but diastolic flow is still present, delivery should be considered at 37 wga
What's the significance of oligohydramnios (low fluid)?
- first make sure membranes are intact (to the best of your ability)
- ACOG feels that delivery should be considered between 36w0d and 37w0d gestation if persistent isolated oligohydramnios is detected in the absence of other risk factors for placental dysfunction
- if <36 wga, risks of prematurity need to be weighed against risks of expectant management