Fetal hemodynamic study is routine in clinical practice since 1977, with the
velocymetric analysis of UmA and the MCA and the capacity to diagnose fetal
perfusion abnormities(6,20). Therefore, evaluation of the perfusion of intra uterine
organs and it correlation with the well – being of the fetus become possible (12),
giving a reliable and useful method for pregnancy follow –up. During this study, we
determined the Cameroonian reference values of Doppler Resistive index of 4 arteries
(UmA, McA, LUt and RUt) on which the fetal hemodynamic evaluation lie on and
elaborate the reference curve at 5th and 95th percentiles.
This study had shown that these curves have a parabolic pattern and that they have the
same shape of others curves found elsewhere in the world (3,6,14,21–26).
Resistive index of umbilical artery.
The same general findings concerning the RI value of UmA had been made in this
study. First we found a decrease of RI UmA value according to the evolution of GA.
The fall in RI UmA during the progress of pregnancy reveals a progressive increase of
fetal cardiac ejection volume and the decrease of placental vessel resistivity giving to
the fetus enought blood supply for its growth. This decrease of the resistivity of UmA
is the respond mechanism for a progressive fetal need during growth. The moderate
inverse relation found (r = - 0.445, p≤0.001) between RI UmA and EFW confirm ours
assertions. The slightly lower RI values on normogram UmA curves (5th and 95th
percentiles) found in this studied population may be partly due to the difference of fetal characteristics in general and in EFW in particular. Kehila et al. in tinusia have demonstrated that the RI UmA values are linearly (r= -0.67) linked to the fetal growth (here the gain of fetal weight)(27). When we focus on their data like EFW (mean =2291 g vs 2337+/- 734 g in ours), RI UmA (mean = 0.63 vs 0.55) and the extremes (0.51 vs 0.51 et 0.77 vs 0.74), we remark that ours data seem to follow the same gradient which link the fetal growth but with a deeper decrease of UmA RI. Other Sub Saharan author like Adekanmi et al. in 2016 in Nigeria found the same lower RI tendency in a group of singleton black women with normal pregnancy at the second and the third trimester respectively 0.60 +/- 0.11 et 0.53 +/- 0.11(25). At this point, man could question on either the racial specificity or ethnical tendency of this findings that Misra and al found (18) in USA. They found that the magnitude of change in the Umb A RI predicted the EFW in African American women. This suggests a careful interpretation of a RI UmA during pregnancy particularly in African women. The relative lack of study in Sub Saharan black African women in general, and in Cameroonian in particular, to the best of our knowledge reveals the interest of the question. Thus, these low normal values tendancy of Umb A RI than those generally describe may justify the utility and the need to elaborated national reference curves of RI during pregnancy. Underlying the fact that for UmA RI, any increase of RI value up to two standard deviation is considered as abnormal and which needs an intervention (23).
Resistive Index of Mean cerebral artery
Doppler study of the McA have a primordial place in the evaluation of the well- being
of the fetus during pregnancy because it directly reveals fetal suffering by evaluating
the diastolic flux disturbance in brain. This due to embryologic ranking of blood
supply during organogenesis starting first in the cephalic than the caudal portion (28).
In this study, we found a cubic pattern of the normogram curves of RI McA and no
correlation with GA nor than others fetal characteristics. This contrast with others
findings elsewhere concerning only GA(3,10,21,24). The absence of correlation between RI McA relation with GA found in this study could be explained to fetal characteristics. In fact, our studied population is mostly made up of third trimester pregnant women. During this period, the variation of McA RI does not depend on the fetus himself but depends on the blood flow through placental villi in general, and on the oxygen perfusion in particular (6,14,24). Thus the lack of correlation between McA RI and GA could simply reflect the good staus of placental exchanges and/or the absence of hypoxie or anemia in studied pregnant women that are the direct reflection of the exclusion criterias (Cf materials and methods). It is clearly prove that abnormal oxygen perfusion through placental villi is associated with to a reduction of the brain
vessel resistivity and that Doppler analysis of blood flux in the MCA is a substantial tools in the prediction of the worse neurological outcomes after birth(6,12,29). This is why in clinical situations where UmA RI is normal, only McA RI value less than the 5th percentile is considered abnormal (6). We found 5th percentile McA RI values superior to those generally used (3,10,21,22). These high McA RI values in our studied population support ours previous assertions on the good fetal well – being of pregnant women in this study. Moreover, the absence of correlation between EWF and MCA RI values highlight the absence of a direct link between the fetal growth (pregnancy progress) and the vascular resistance of fetal cerebral vessel generally observed and reported at the third trimester during pregnanct (1,10,20,29,30).
Rigth and Left Uterine arteries Resistive index
The vascularization of materno –fetal complex is done by uterine arteries. This study
did not find any correlation between RI values of both LUt and RUt and the materno –
fetal characteristics. There were no differences between RI values of the LUt and the
RUt. Like others normogram curves obtained, RUt and LUt curves had a similar cubic
pattern seen elsewere in Africa (25) and in the world (3,30,31) but with the RI values
slightly lower in our setting. This could be due to sample size, difference in
methodology or may be to some racial factors mentioned earlier.
Cerebro – placental RI ratio
This ratio is often used to clarify clinical situation in which there is opposite variation
of UmA, MCA or UmA RI values are normal or low. Our study revealed that the RCP is not constant through the pregnancy after 28 weeks of GA and that its values were correlated either to fetal characteristics (EFW and GA) or Maternal age (Table 6). Many cut of point are proposed in the literature (<1; <1.05 ; <1.08) for fetal prognostic prediction during pregnancy (32,33). This study underline, like elsewhere, interest of analysis CPR values according to GA especially after 34 week (3,21,34,35). Then, the decrease of CPR value under the 5th percentile reflect a fetal blood flow redistribution in favor of brain due to decrease of placental perfusion or hypoxia. This phenomenom often observed in pathological situation during pregnancy had been described as the brain sparing effect phenomenom (36).
Study limitation and perspectives
This study is the first study in Cameroon on resistive index on low risk singleton
pregnant women. It provide to physician a national normogram of RI values of UmA,
McA, both LUt and RUt that could help for clinical situation after been validate in
general mass population. But for us to generalize our result, we should carefully take
in consideration the reduced sample size of our studied population limited by the will
of strictly fulfill the selection criterias of subject. We used only one experienced
ultrasonologist to avoid inter-observer variation and only one high resolution
ultrasonography machine and one trans-abdominal transducer to avoid equipment's
variation. As well as, our data had higher reliability based on this fact that all
gestational age was established by careful history to identifying only patients with
accurate date and confirmation by early ultrasound examination. Furthermore, all
newborns were proved to have normal growth and having no structural abnormality.
The choice of RI was guided by the aim to adapt the normogram to physician
aptitudes and competences in one hand, and to physiological mechanisms of fetal
hemodynamic in other hand.