Assessment of Fetal
Lung Maturity
Dr. Ashraf Fawzy Nabhan
Assistant Professor of Obstetrics & Gynecology
Ain Shams University, Cairo, Egypt
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Vision Statement
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An obstetric practice where the
unnecessary tragedy of iatrogenic
prematurity no longer exists
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Objective
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This presentation reviews those
techniques that enable the obstetrician
to predict accurately the risks of
respiratory distress syndrome (RDS) for
the infant requiring premature delivery
and to avoid the unnecessary tragedy of
iatrogenic prematurity
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How Did We Get Here?
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Prior to the now common practice of
using ultrasound to establish
gestational age and amniotic fluid
studies to assess fetal pulmonary
maturation, iatrogenic prematurity was
an important clinical problem.
Untimely or unwarranted intervention
was responsible for 15 percent of cases
of RDS.
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Today’s Situation
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Modern Obstetric practice has shown a
decline in iatrogenic prematurity and RDS.
Several changes in clinical practice appear to
have decreased the incidence of RDS due to
iatrogenic prematurity
The corner stone of those changes appear to
be the increased availability of ultrasound
and fetal lung maturity studies and advances
in the application and interpretation of these
diagnostic procedures
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Available Options
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Quantitation of Pulmonary Surfactant
Measurement of Surfactant Function
Evaluation of Amniotic Fluid Turbidity
Appropriate use of Ultrasonography
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Quantitation of Pulmonary
Surfactant: L/S Ratio
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It is the most valuable assay for the assessment of
fetal pulmonary maturity.
At 32 weeks the L/S ratio reaches 1. Lecithin then
rises rapidly, and an L/S ratio of 2.0 is observed at
35 weeks.
A ratio of 2.0 or greater has repeatedly been
associated with pulmonary maturity.
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Quantitation of Pulmonary
Surfactant: L/S Ratio
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A mature L/S ratio predicted the absence of RDS in
98 percent of neonates. With a ratio of 1.5 to 1.9,
approximately 50 percent of infants will develop
RDS. Below 1.5, the risk of subsequent RDS
increases to 73 percent.
Thus, the L/S ratio, like most indices of fetal
pulmonary maturation, rarely errs when predicting
fetal pulmonary maturity, but is frequently incorrect
when predicting subsequent RDS. Many neonates
with an immature L/S ratio will not develop RDS.
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Quantitation of Pulmonary
Surfactant: Test for PG
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A rapid immunologic semiquantitative agglutination
test (Amniostat-FLM) can be used to determine the
presence of PG.
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It can detect PG at a concentration >0.5 μg/ml. It takes 20 to
30 minutes to perform and requires only 1.5 ml of amniotic
fluid.
It is highly sensitive.
•
A positive Amniostat-FLM correlates well with the presence
of PG by thin-layer chromatography and the absence of
subsequent RDS.
It can be applied to samples contaminated by blood
and meconium.
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Quantitation of Pulmonary
Surfactant: Microviscosimeter
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The relative lipid content of amniotic fluid may be
evaluated by fluorescence depolarization analysis.
It is an expensive test.
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Quantitation of Pulmonary
Surfactant: TDx Test
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The TDx analyzer is an automated fluorescence
polarimeter to determine surfactant albumin ratio
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The test requires 1 ml of amniotic fluid and can be run in
less than 1 hour.
The surfactant albumin ratio (SAR) is determined with
amniotic fluid albumin used as an internal reference.
A ratio of 50 to 70 mg surfactant per gram of albumin is
considered mature.
The TDx test correlates well with the L/S ratio and
has few falsely mature results, making it an excellent
screening test.
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Measurement of Surfactant
Function: Shake Test
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It evaluates the ability of pulmonary surfactant to generate a
stable foam in the presence of ethanol.
Ethanol, a nonfoaming competitive surfactant, eliminates the
contributions of protein, bile salts, and salts of free fatty acids to
the formation of a stable foam.
At an ethanol concentration of 47.5 percent, stable bubbles that
form after shaking are due to amniotic fluid lecithin.
Positive tests, a complete ring of bubbles at the meniscus with
a 1:2 dilution of amniotic fluid, are rarely associated with
neonatal RDS.
It is a screening test that gives useful information if mature.
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Measurement of Surfactant
Function: Foam Stability Index
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The test is based on the manual foam stability index (FSI), a
variation of the shake test.
The kit currently available contains test wells with a
predispensed volume of ethanol. The addition of 0.5-ml
amniotic fluid to each test well in the kit produces final ethanol
volumes of 44 to 50 percent. A control well contains sufficient
surfactant in 50 percent ethanol to produce an example of the
stable foam end point.
The amniotic fluid:ethanol mixture is first shaken, and the FSI
value is read as the highest value well in which a ring of stable
foam persists.
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Measurement of Surfactant
Function: Foam Stability Index
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This test appears to be a reliable predictor of fetal lung maturity.
Subsequent RDS is very unlikely with an FSI value of 47 or
higher.
The methodology is simple, and the test can be performed at
any time of day by persons who have had only minimal
instruction.
The assay appears to be extremely sensitive, with a high
proportion of immature results being associated with RDS, as
well as moderately specific, with a high proportion of mature
results predicting the absence of RDS.
Contamination of the amniotic fluid specimen by blood or
meconium invalidates the FSI results. The FSI can function well
as a screening test.
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Measurement of Surfactant
Function: Tap Test
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It is a rapid semiquantitative measurement of
surfactant function.
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In amniotic fluid from the mature fetus, the bubbles quickly
rise from the bottom layer of the amniotic fluid to the surface
and break down, while in amniotic fluid from an immature
fetus the bubbles are stable or break down slowly.
Note that these end points are opposite those used in the
FSI or shake test.
The cut-off for maturity is five bubbles. If no more
than five bubbles persist in the ether layer, the test is
considered mature. The test is read at 2, 5, and 10
minutes.
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Measurement of Surfactant
Function: Tap Test
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Fluid obtained from both amniocentesis or a freely
flowing vaginal pool may be used.
Amniotic fluid contaminated by blood, meconium, or
vaginal mucus should be centrifuged before the
assay is performed.
Fluid contaminated by blood or meconium or
obtained from the vaginal pool did not demonstrate
an increased incidence of falsely mature tests.
The tap test may be a valuable screening test,
particularly if a phospholipid profile is not available.
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Evaluation of Amniotic Fluid
Turbidity: Visual Inspection
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During the first and second trimesters, amniotic fluid
is yellow and clear. It becomes colorless in the third
trimester. By 33 to 34 weeks' gestation, cloudiness
and flocculation are noted, and, as term approaches,
vernix appears.
Amniotic fluid with obvious vernix or fluid so turbid
will usually have a mature L/S ratio.
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Evaluation of Amniotic Fluid
Turbidity: Optical Density
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This method is thought to evaluate the turbidity
changes in amniotic fluid that are dependent on the
total amniotic fluid phospholipid concentration.
An OD of 0.15 or greater at wavelength at 650 nm
correlates extremely well with a mature L/S ratio and
the absence of RDS.
Contamination with blood or meconium invalidates
the results.
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Evaluation of Amniotic Fluid
Turbidity: Lamellar Body Counts
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Lamellar bodies are the storage form of surfactant.
The test requires <1 ml of amniotic fluid and takes 15
minutes to perform.
A lamellar body count >30,000/μl is highly predictive
of pulmonary maturity, while a count <10,000/μl
suggests a risk for RDS.
Neither meconium nor lysed blood has an effect on
the lamellar body count.
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Appropriate use of ultrasound
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Grade 3 placenta in an uncomplicated pregnancy at
term suggests fetal pulmonary maturation. This
approach is not reliable in pregnancies complicated
by hypertension, DM, IUGR, and Rh isoimmunization
BPD of at least 9.2 cm will reliably predict the
absence of RDS in uncomplicated pregnancies. This
approach should not be used for patients with DM.
The most appropriate use of ultrasound in predicting
fetal lung maturity is early documentation of
gestational age so that elective delivery later in
pregnancy can be safely undertaken.
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Recommendation
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An accurate assessment of gestational
age and fetal maturity is essential
• before an elective induction of labor or
•
cesarean delivery
before the delivery of a patient whose fetus
may not have matured normally such as a
growth-restricted fetus or the fetus of a poorly
controlled diabetic mother.
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