Wednesday 13 October 2010
OC18.04: Direct measurement of blood flow into the intervillous space and its relationship with uterine artery blood flow
Lack of anatomical landmarks means that the spiral arteries are often identified by their waveform alone; this could lead to a failure to recognise abnormal flow. We investigated whether the jet of blood from the spiral arteries entering the intervillous space (IVS) can be identified at a consistent anatomical site and quantified. As uterine artery (UA) flow may reflect downstream changes, we also compared the UA indices with those of the jets.
With ethical approval, we prospectively recruited women with a singleton pregnancy for scans at 12, 14, 16, 18, 23, 28 & 33 weeks. Using transabdominal 2D colour Doppler and pre-determined anatomical criteria, the jets were identified entering the IVS. Their pulsatility index (PI) and resistance index (RI) were measured. To assess intra-observer variability, 1 jet was measured 3 times by the same observer; for inter-observer variability, 3 jets were measured by 2 operators unaware of the other's results. UA indices were also measured. Statistical analysis was performed with SAS.
65 women were recruited. Intra-observer variability was minimal (intra-class correlation coefficient for jet PI and RI = 0.92) as was inter-observer variation (ANOVA; P < 0.001). The PI and RI for both the jets and UA were negatively correlated with gestation (mixed model analysis P < 0.001) and after controlling for gestation the PI and RI for the jets and UA were seen to be correlated (regression analysis P < 0.001).
To our knowledge this is the first study to measure the spiral artery jets into the IVS at a consistent anatomical site using transabdominal ultrasound. Intra- and inter-observer studies show the measure is precise and reproducible. The jet PI and RI decreases with advancing gestation as predicted by changes from trophoblast invasion. The correlation between jet and UA indices supports the theory that flow in the uterine arteries reflects spiral artery changes. This technique might allow observation of the pathology underlying impaired placentation.