Assessment of Fetal Lung Maturity

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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

• An obstetric practice where the unnecessary
  tragedy of iatrogenic prematurity no longer exists

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Objective

• 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?

• 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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Todays Situation

• 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

• 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

• 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

• 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

• A rapid immunologic semiquantitative
  agglutination test (Amniostat-FLM) can be used to
  determine the presence of PG.
• It can detect PG at a concentration gt0.5 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

• 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

• The TDx analyzer is an automated fluorescence
  polarimeter to determine surfactant albumin ratio
  
• 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

• 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 12 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

• 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 fluidethanol 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

• 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

• It is a rapid semiquantitative measurement of
  surfactant function.
• 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

• 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

• 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

• 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

• Lamellar bodies are the storage form of
  surfactant. The test requires lt1 ml of amniotic
  fluid and takes 15 minutes to perform.
• A lamellar body count gt30,000/µl is highly
  predictive of pulmonary maturity, while a count
  lt10,000/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

• 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

• 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.