A comparative study of three indices of umbilical blood flow in relation to prediction of growth retardation.

The literature reporting Doppler ultrasound flow velocity waveform analysis of the umbilical artery and its applications for detecting intrauterine growth retardation (IUGR) is growing rapidly. Several indices originally introduced to evaluate different vascular beds in adults, are used to assess the vascular bed resistance downstream the umbilical artery (i.e. the placental bed). A higher resistance in this placental bed causes a decreased end-diastolic flow, which is described in combination with IUGR. The three most commonly used flow indices are: A/B ratio, Resistance Index (RI = (A-B)/A) and Pulsatility Index (PI = (A-B)/mean). In this study we evaluated which of these indices correlated best with IUGR. We calculated the three above mentioned indices from the same recordings of the blood flow (at a gestational age of 34 +/- 2 weeks) and correlated the results with the subsequent birthweight of the child. In the studied group (n = 48) 15 children were small for gestational age (SGA = less than P 10) and 33 children were appropriate for gestational age (AGA = greater than or equal to P 10). All three indices showed a significant difference in value between AGA and SGA children (see table III). The Spearman correlation-coefficients were calculated and were approximately one. So there was a high correlation between all three indices. This indicates that the different indices have the same relation to IUGR. Thus, no arguments were found to prefer one index above the other. In the second part of the study we investigated the validity of these indices to predict IUGR. The validity can be estimated from two components: the sensitivity and specificity. Both sensitivity and specificity are determined by the value of the index which discriminates between AGA and SGA. Changes in this discriminating value (the so-called cut-off point) will change both sensitivity and specificity. This last effect can be visualized in the Receiver-Operating-Characteristic (ROC) curve, which plots sensitivity against specificity for different cut-off points (see figures 4 and 5). Arbitrarily sensitivity and specificity were calculated for the cut-off points PI = 1.1 and A/B ratio = 3.0. Choosing these cut-off points, the same specificity of 87.9% with a sensitivity of 53.3% were found (table IV). It is concluded, that the sensitivity of the PI of the umbilical artery in predicting growth retardation with acceptable specificity is rather disappointing. A possible reason for this is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)