Doppler studies of foetal vessels in hypoxaemia , FGR , PIH

Broadly  speaking  there are two kinds of antenatal fetal monitoring:  A)  Direct fetal Doppler which is  used to quantify  fetal condition   and determine  the time  of delivery  This has become popular in last three decades and most of the machines are now are fitted with this technology.. B) Non Doppler tests    1) CTG :-- FHR   monitoring  relates  to CNS   control  of cardio regulation   and  2) BPP to CNS behavior determination . Cerebral   Doppler ( like MCA) however   defines the redistribution of cardiac output. Central venous Doppler defines changes in cardiac load  and contractility . but more evaluation  is needed particularly   with long  term neurological   performance  as an end  point.

Doppler Ultrasonography

Doppler ultrasonography   is a non invasive technique   used to assess the hemodynamic components   of vascular    impedance. Commonly  measured flow indices  based on the  characteristics of peak  systolic  frequency   shift  end diastolic   frequency  shift and mean   peak  frequency  shift   over the  cardiac  cycle  , include   the following :

· Systolic   to diastolic  ratio

· Resistance     index

· Pulsatility  index

Foetal Arterial Doppler

Umbilical   artery: The  normal Doppler flow pattern  of  umbilical   artery is a low  impedance circulation, with   an increase   in amount  of end diastolic flow with     advancing gestation. Thus  the S/D   ratio decreases  from about   4.0   at 20  weeks to 2.0  at term. The S/D   ratio is generally less than 3.0 after  30 weeks. Umbilical arterial    Doppler   waveforms reflect  the status  of the   placental   circulation and the increase   in end diastolic flow seen with advancing gestation   is a direct  result of    an  increase  in the number   of tertiary   stem villi.

What happens in PIH / other Placental diseases?? Diseases   that obliterate small muscular    arteries   in placental   tertiary     stem villi result in   a progressive   decrease in end diastolic   flow in umbilical arterial Doppler waveforms until absent and then the flow   reverses   during diastole. Reversed   diastolic flow   represents an advanced   stage of placental compromise. Absent or reversed flow is commonly associated   with severe  IUGR  and oligohydramnios .

What about Middle  cerebral artery ??   The normal Doppler flow pattern  of middle cerebral artery is a high   impedance    circulation  with continuous forward flow  throughout    the cardiac cycle.

What happens in diseases state?? In the presence of  fetal hypoxemia  , there is  central   redistribution   of blood flow  occurs  which results    in an increase  in blood flow to the brain, heart and adrenals   and reduction    in flow to the peripheral and placental   circulations. This is known as brain  sparing reflex   and has a   major role to play  in fetal   adaptation   to hypoxemia.  Sequential MCA-PSV is often  used to   assess fetal well being    in suspected IUGR. An increase   in flow in MCA indicates fetal compromise. Although   the MCA indices of resistance    are low in  association   with fetal compromise,   the values  overlap with  the  lower part of   normal range   in many small   fetuses .

How to know deteriorating situation??  What  is CPR or Cerebroplacental Ratio?? Ans:-To differentiate and recognize a deteriorating   situation cerebroplacental ratio has been suggested. It is   defined as middle cerebral artery pulsatility   index   value   divided by umbilical   artery pulsatility index value and is used to identify the fetuses in whom the placental    insufficiency is associated   with altered cerebral blood flow. Ratio of less than 1.0   indicates sever neonatal morbidity. The   measurement    of peak   systolic   velocity in the middle   cerebral artery has become the standard method for assessment of fetal anemia in rhesus   alloimmunization. In cases of fetal anemia values   are increased because of increased cardiac   output   and decreased blood viscosity.

Fetal  venous  Doppler

The flow velocity   waveforms are great veins like ductus venosus   inferior   vena cava and umbilical vein have been studied. . Why DV Doppler is preferred and considered as most representative??

The flow velocity    waveform in the central fetal veins is influenced by the central venous pressure   which is a reflection of cardiac function. In cases of fetal growth   restriction    the changes in arterial circulation    have a major    effect on after load preload and cardiac contractility. High    resistance in umbilical   circulation    leads to increased right ventricular after load. In the most compromised fetuses   low resistance in cerebral circulation   reduces the left ventricular    after load  with   further   hypoxemia   myocardial  contractility and cardiac  output  fall.

Fetal  Ductus Venosus  and  inferior Vena  cava

They   reflect the physiologic status of the right   ventricle. Specific   information with regard to right  ventricular  preload, myocardial   compliance   and right   ventricular   end diastolic pressure    can be derived from Doppler   flow studies  of the IVC  and ductus  venosus  in the fetus. A large number of   parameters have been calculated    from the Doppler   waveform   of these vessels. These parameters are related  to fetal  blood  gas  status  and   especially fetal   hypoxemia. Ductus venosus is used more widely because   of its   unique pattern.

Maternal  Uterine Circulation

With   advancing gestation a progressive decrease  in impedance  is noted in   uterine   circulation . The presence   of notch in the waveforms and an increase in resistance    index after 22 weeks is characteristic   of abnormal uterine   circulation. Pregnancies   that show   abnormal   uterine circulation in late second and third trimester have an increased risk of complications like IUGR, pre- eclampsia, preterm delivery .

Clinical   implications of Doppler in Fetal  Compromise

Doppler studies can be used at two points in the  evaluation of fetal compromise   in high  risk  pregnancy

1.      Umbilical Doppler   studies  are used  to identify high risk     pregnancy  and confirm  the potential for fetal compromise.

2.       MCA & DV : as direct  fetal Doppler test which are  used to quantity the fetal  risk.

How useful is  umbilical artery   Doppler?? U A Doppler    assumes that   placental  vascular   lesions underline all fetal  compromise    except in cases of  acute fetal  deterioration  e. g.  Placental abruption . The use of   umbilical   artery Doppler    flow studies    have been   evaluated by several  randomized  control trials. Metaanalysis has established   that women with high risk    pregnancies with    a compromised fetus should have access  to Doppler  studies  of the umbilical  artery. Metaanalysis   of these trials demonstrated a reduction in the perinatal deaths by 38%.