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[Effect of cardiac rate on the index of feto-placental circulatory resistance].

Variations of the fetal heart rate in normal or pathological fetuses may induce significant changes of the Doppler indices widely used for the evaluation and the assessment of the placental or cerebral resistances. In case of a low heart rate (120/mn for example), the end diastolic amplitude will be weaker than in case of a heart rate of high frequency (160/mn). This phenomena may explain the great scattering of the normal values of the Doppler indices and the relatively poor sensitivity of this parameter. We tried to minimize the effect of the heart rate on the evaluation of the doppler indices by two methods. Firstly by measuring on the maximum frequency curve the end diastolic frequency for a standard heart rate of 140/mn and the corresponding Doppler index (R'140). Secondly by using a formula giving directly (from the R value on the trace) the value of the index (R"140) for a heart rate of 140/mn. The diagrams of the indices (R'140 R"140) calculated for a standard frequency of 140/mn (on normal and pathological fetuses) show a greater dispersion than the R indices directly measured on the Doppler waveform. Therefore, the sensitivity and the specificity of the Doppler method are reduced. Finally, we conclude that the haemodynamic parameters in the different vascular areas of the fetus can adapt to the heart rate except for very strong abnormalities of the heart rate as it is the case in atrio ventricular blocks for instance. Out of these cases, it seems that the normalization of the index by the heart rate does not improve the accuracy of the doppler indices.