The reliability and validity of three dimensional ultrasound volumetric measurements using an in vitro balloon and in vivo uterine model.

OBJECTIVE: To evaluate the reliability and validity of two and three dimensional ultrasound volumetric measurements using balloon and uterine models. DESIGN: Prospetive observational study. SETTING: Obstetric ultrasound department at a university teaching hospital. METHOD: Two and three dimensional ultrasound volumetric measurements (with 5, 10 and 15 ultrasonic slices) were performed on 30 different sets of ultrasound images obtained from 15 water filled balloons with volumes ranging from 19 to 697mL. The measurements were performed independently by two observers who were blinded to the true volumes of the balloons. For the uterine model, only three dimensional ultrasonic volume measurements were performed independently on 16 uteri by two observers who were again unaware of the definitive uterine volumes. OUTCOME MEASURE: For the assessment of intra-and inter-rater reliability, the intraclass correlation coefficient was used. The index of concordance between the ultrasonic volumes and those obtained by the reference standard (validity) was assessed with the conventional Pearson's correlation coefficient, limits of agreement method and the intra-class correlation coefficient. RESULTS: High levels of reliability and validity were obtained for both two and three dimensional ultrasound balloon volume measurements. For two dimensional ultrasonic volume measurements, the intra-class correlation coefficient ranged from 0.992 to 0.998 for reliability and validity whereas the Pearson's correlation coefficient for validity was 0.996. With three dimensional ultrasonic volume measurements, the intra-class correlation coefficient ranged from 0.991 to 0.999 for reliability and validity whereas the Pearson's correlation coefficient for validity was 0.999. Both two and three dimensional ultrasonic measurements tended to underestimate the true balloon volume with the largest observed mean difference obtained with three dimensional ultrasound measurements using five ultrasonic slices and the smallest value obtained with three dimensional ultrasound measurements employing 15 ultrasonic slices. The mean difference in volume measurement for two dimensional ultrasound was intermediate between these two values. However, two dimensional ultrasound volume measurement generated the largest range between the limits of agreement whereas the smallest range was obtained with three dimensional ultrasound using 10 ultrasonic slices. The intra-class correlation coefficient for reliability and validity with three dimensional ultrasonic uterine volume estimation ranged from 0.956 to 0.996 whereas the Pearson's correlation coefficient for validity ranged from 0.993 to 0.999). The use of three dimensional ultrasound also consistently under-estimated the actual uterine volumes. The larger the number of ultrasonic slices employed for three dimensional ultrasound, the smaller was the mean difference between the ultrasonic and true uterine volume measurements and the smaller the limits of agreement. CONCLUSIONS: The reliability and validity of balloon and uterine volume measurement by three dimensional ultrasound is high. This allows further research on three dimensional ultrasound for measuring pelvic organ volumes in the prediction of pelvic pathology.

Cerebral pathology in the very low birthweight infant: predictive value of peripartum metabolic acidosis.

One-hundred-and-three infants with a birthweight of less than 1500 g and delivering at a gestation of less than 32 weeks were examined by serial cranial ultrasound scans. A capillary blood sample was drawn for blood gas analysis within 1 h of birth in all cases. The subsequent development of intracranial pathology was found to be significantly associated with gestation at delivery (P less than 0.01), birthweight (P less than 0.01) and base deficit within 1 h of birth (P less than 0.001). For infants with a base deficit of greater than 5.0 mmol/l within the first hour of life, the sensitivity for predicting the subsequent development of cerebral pathology was 51.5% with a specificity of 97.3%, and a positive predictive value of 97.1%. This relationship between a metabolic acidosis within 1 h of birth and the subsequent development of cerebral pathology held for both major and minor degrees of pathology, but was stronger in those infants developing major cerebral pathology. The study suggests that improved surveillance of the very preterm infant during labour and at birth with the aim of reducing the incidence of metabolic acidosis at birth, may help to reduce subsequent intracranial pathology, and thereby perinatal and long-term morbidity.

The predictive value of fetal breathing movements in the diagnosis of preterm labour.

A real time ultrasound investigation was performed on 64 patients who presented in threatened preterm labour. Fetal breathing movements (FBM) were absent in 17 patients, all of whom were delivered within 56 h. FBM were present in 47 patients of whom five were delivered within 56 h from examination and four of these had spontaneous rupture of membranes and/or chorioamnionitis. In 33 the pregnancy continued for at least 1 week. Two of the fetuses with false negative results showed a distinct breathing pattern different from the rest of the study group. This suggests that pattern recognition of fetal breathing might improve the specificity of this investigation.

Total intra-uterine volume and symphysis fundus height. Is there a relation?

The concordance between measurements of total intra-uterine volume (parallel planimetric area method) and symphysis fundus height and the efficacy of these measurements in predicting birthweight by centile range was prospectively investigated in a blind study of 81 measurement sets in 74 women in the third trimester of pregnancy. Gestational length was determined by early ultrasound scans in all cases. Reference curves were from separate populations. Correlation between the measurements was significant, both at 32 weeks (p = 0.015) and 36 weeks (p less than 0.001), but only 21% of the variation in intra-uterine volume at 32 weeks and 63% at 36 weeks' gestation was attributable to changes in fundal height. Intra-uterine volume was therefore not accurately reflected by fundal height. Fundal height and intra-uterine volume were in the same centile range in 34% of cases. Maternal height and weight did not correlate with intrauterine volume, but there were significant relations with fundal height, particularly at 36 weeks. Birthweight correlated better with volume (r = 0.665) than with fundal height (r = 0.461) at 32 weeks, while at 36 weeks there was little difference (r = 0.691 and 0.794). In 54% of volume and 47% of symphysis fundus measurements there was agreement with the same centile range of birthweight. Predictive capacity of volume measurements appeared better for the lower birthweight ranges.