The role of Doppler studies in diagnosing foetal compromise.

The advent   of Doppler     ultrasound   has permitted

non invasive assessment   of the fetal maternal and placental   circulations. With   Doppler    ultrasound    we can obtain   information    about uteroplacental   blood flow and resistance,     which may be markers of fetal adaptation   and reserve. This   method of fetal assessment has only been demonstrated    to be of value in reducing   perinatal mortality and unnecessary obstetrical   interventions in fetuses   with suspected    IUGR   and possibly other disorders of   uteroplacental    blood flow. A detailed    description    of the underlying   principles   and use of Doppler    ultrasound for fetal assessment is available   

Antenatal   fetal assessment .We the obstetricians woud like to know the  fetal reserve    in cases   of suspected    FGR: But how to assess that ??  Out of so many modalities of foetal wellbeing studies like 1) progressive growth of fundus  2)  DFMC,  3) CTG, 4) amount of liquor as assessed by clinical & sonological methods , 5)  foetal growth curve as done in serial  USG   the last verdict however comes from   Doppler  studies of fetoplacental vessels.  However, many believe that antenatal      fetal surveillance can be best done by Doppler which conceptualized   from   fetal a) vascular   flow and b) vascular  resistance ,These parameters can be used as  a follow up  test to determine   fetal reserve    in cases   of suspected    FGR, and    not as a primary    method of antenatal      fetal  surveillance for either   high risk or low risk    pregnancies.

Will low risk women benefited by Doppler?  A  2010   Cochrane review   of 18 randomized trials including   more  than 10,000    high risk   women , in which   the use of Doppler   ultrasound    was associated   with decreased    perinatal   deaths as well as significantly   fewer    inductions   of labor   and cesarean delivers . By that we mean we can prolong the pregancy for 1-2 weeks without much apprehension or risk. .  .Studies   of low risk pregnancies     have not shown   a benefit  from the   use of  Doppler   ultrasound   as has been most   recently  described   in a 2010   systematic    review   of five studies   including more than  14,000 women.

The utility    of Doppler   ultrasound in the assessment    of fetal   well being    is based on the   relationship    between Doppler    parameters    with metabolic    status   , rate of   disease progression    and the risk   for stillbirth  . Doppler parameters   are influenced by several variables including vascular    histology, vascular tone and fetal blood pressure.  By contrast the placental   respiratory function is related to the   integrity     of the villous vasculature. A decrease in arterial p O2 can trigger    autoregulatory    adjustments of vascular smooth muscle   tone. As     diagnostic tools   1) elevated umbilical artery   brain sparing provide   evidence of    placental   dysfunction.    .

 In this   context   distinction between   early and late fetal   vascular   responses   to placental    insufficiency provides a useful   framework within   which   to   estimate these risks. Stage I:--in mild   placental   vascular   disease -à early   responses to placental   insufficiency are observed in mild   placental   vascular   disease when umbilical   artery end diastolic velocity is still present.

CPR= A decrease    in the  cerebral /   placental Doppler    ratio provides   an early and sensitive marker of redistribution of cardiac    output often  preceding   overt  growth delay   by up to  2 weeks . As there is    1) elevation   in umbilical    artery   blood flow    resistance à there will be 2) reduction of fetal growth   velocity which is almost pari passu and  is followed  by 3) decreasing  middle   cerebral  artery    impedance  . The  nadir   of cerebral   blood  flow resistance    is typically   reached after   a median of 2 weeks   and is followed    by 4) an  increase in aortic blood  flow  impedance.

Early   cardiovascular responses   are considered   compensatory   because they   occur at a time when cardiac function is normal   and are typically   accompanied   by preferential    perfusion of   vital organs   and the placenta. While the fetus   may be hypoxemic, the    risk for acidemia is low . When   fetal compromise   accelerates there  is  a further   steady  rise in umbilical blood flow   resistance   venous Doppler  indices escalate  over a wide  range,  and the development of oligohydramnios   and metabolic   acidemia   is  characteristic    of  ineffective    downstream   delivery   of cardiac  output

Step II:-Late  responses   to placental  insufficiency . Such are    observed   when  accelerating      placental disease results  in loss  or reversal of umbilical   artery  end   diastolic    velocity and    when fetal   deterioration   becomes   evident   through   parallel elevations in placental  blood flow   resistance   and  venous   Doppler  indices.

 Although   the development   of abnormal    venous blood    flows has been documented   in many veins   the precordial veins,   including    the ductus venosus, the inferior   vena cava,   and the umbilical    vein, are typically  utilized  in clinical   practice  .

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Stage III:-- final  stages  of compromise:-- In  the  final  stages  of compromise , cardiac  dilatation   with holosystolic tricuspid insufficiency, complete  fetal   inactivity, short term  variation   below  3.5  msec,   and spontaneous   cardiac   late decelerations of the fetal   heart   rate can be   observed    as preterminal events.

How important is Umb a Doppler ?  Ans:-In the past  , the major  focus   of Doppler   studies for the    assessment  of fetal   health has been the umbilical    circulation. The association   between an elevation in Doppler   blood flow   indices in the umbilical   artery, increased   disturbance  of placental   perfusion,  and the deterioration   of fetal   acid- base   status that is proportional to the degree   of the  Doppler   abnormality   has been demonstrated by several   investigators. In the  fetal compartment,  elevation of the   umbilical    artery  Doppler   index    is observed   when  approximately   30%   of the fetal villous vessels    are abnormal.  Absence   or even    reversal    of umbilical   artery end- diastolic velocity   can occur   when 60%  to 70 % of the villous   vascular   tree is damaged. Incidences of   intrauterine    hypoxia   ranging from  50%    to 80%     in fetuses   with absent   end diastolic  flow have been   reported. 

The benefit of umbilical   artery   Doppler   in management has been documented in randomized  controlled   trials   and meta analysis . In these   studies umbilical    artery  Doppler   , when  used in conjunction with standard antepartum testing  was associated with a decrease of  up to 38%  in perinatal   mortality   antenatal   admissions , inductions  of labor,  and cesarean   deliveries   for fetal distress  in labor   in women    considered at high risk. However    several studies   that have   examined the cerebral and especially the venous circulation   have provided   greater   insight into   the relationships    between Doppler    abnormality  and outcome .The  development of   umbilical  venous pulsations in fetuses with absent  end diastolic velocities    in the umbilical artery  was   associated   with a  fivefold increase  in mortality . Arduini and colleagues demonstrated   that gestational age  at onset,   maternal hypertension, and  the development   of pulsations in the umbilical venous  velocities   were significantly correlated  with the interval  of time between diagnosis and delivery  for late decelerations of the FHR . Subsequently   several studies   have confirmed that fetuses    with abnormal   precordial    venous   velocities    had a higher   morbidity   and mortality    than fetuses    without   abnormal  venous flow. These   studies and subsequent analyses    confirm that fetal    Doppler    assessment  that is based on the umbilical   artery   alone  is no longer   appropriate , particularly in the setting   of early  onset  IUGR   prior  to 34 weeks. Incorporation   of middle   cerebral artery   and venous   Doppler   provide   the best   prediction of acid base status,   risk of stillbirth, and  the anticipated   rate of progression.

Hypoxemia but  normal pH :--In growth   restricted  fetuses  with an  elevated  Doppler index   in the umbilical    artery,   brain sparing   in the presence  of normal  venous Doppler   parameters   is typically   associated  with hypoxemia but a normal pH. Elevation  of venous   Doppler    indices,  either alone or in combination   with umbilical venous pulsations, increases the risk for  fetal acidemia. This  association  is strengthened   by serial elevations  of the ductus venosus  Doppler  index. Dependent  on the cut – off and the combination  of veins  examined  , sensitivity  for prediction of acidemia  ranges  from 70%  to 90%   and specificity   from 70%  to 80%.  Abnormal venous  Doppler  parameters    are the strongest   Doppler  predictors of stillbirth. Even   among fetuses   with severe  arterial Doppler   abnormalities   , the   risk of   stillbirth    is largely  confined   to those   fetuses  that have   abnormal  venous  Dopplers . The likelihood of stillbirth increases  with  the degree  of  venous Doppler   abnormality.

Venous   Doppler findings   that are particularly   ominous are absence  or reversal   of the ductus   venous   a –wave   and  biphasic/ triphasic   umbilical      venous pulsations. In  the setting  of a 25%   stillbirth   rate  in a preterm   severe IUGR   population , these Doppler   findings   have a 65%    predictive sensitivity   and 95%   specificity.

Although   neonatal   morbidity     is primarily determined by   gestational age   at delivery   and neonatal   mortality   is the product of several factors, both   of these outcomes are also related   to fetal Doppler   studies. Arterial redistribution and brain   sparing   are not   associated   with a significant rise in major neonatal    complications. In contrast, a  2SD elevation of the ductus   venosus Doppler index  is associated  with a 3- fold  increase  in neonatal  complications , and further escalation of ductus  venosus Doppler   indices   leads to an 11-fold  increase  in this relative  risk. The neonatal mortality rate in fetuses  with absent   or reverse   umbilical   artery   end – diastolic velocity   ranges from  5%  to 18%  when the  venous  Doppler  indices    are normal . Elevation  of the  ductus venosus Doppler index   greater   than 2 SDs   doubles  this mortality   rate,   although   predictive   sensitivity  is only  38%  with   a specificity  of 98% .

 In summary, Doppler   evaluation    of the umbilical, cerebral, and   precordial vessels    of the growth    restricted    fetus provides   important     diagnostic   and prognostic   information. Fetal acidemia and the risk  of stillbirth  are high    with progressive   elevation of venous Doppler   indices. Advancing   Doppler   abnormalities    indicate   acceleration    of  disease  and require    increased   frequency  of fetal monitoring . In growth restricted fetuses, Doppler   evaluation is complementary  to all other   surveillance    modalities.

Ultrasound can be    a valuable tool in  evaluating  fetal growth   , estimating fetal  weight , and    detecting  hydramnios   and malformations. Maternal serum a- fetoprotein  determination  at 16 weeks  gestation is often employed in association  with a detailed ultrasound  study  during  the midtrimester in an attempt  to detect   neural   tube defects    and other  anomalies.