Changes in myometrial 'perfusion' during normal labor as visualized by three-dimensional power Doppler angiography

Myometrial contractions are one of the most important aspects of effective labor. For cells within the myometrium to work efficiently they need to be well oxygenated and this requires an adequate blood supply. This study used quantitative three-dimensional (3D) power Doppler angiography to calculate the percentage change in myometrial blood flow during a relaxation-contraction-relaxation cycle of active labor.

Transabdominal 3D power Doppler ultrasound imaging was used to acquire volumetric data during the first stage of spontaneous labor in 20 term, nulliparous women. 3D datasets were acquired during a single cycle of uterine relaxation, contraction and subsequent relaxation for each subject. The resultant datasets were analyzed independently by two investigators on two occasions using Virtual Organ Computer-aided AnaLysis to define a volume of interest within the myometrium; the power Doppler signal within this volume was quantified to provide 3D indices of vascularity: vascularization index (VI), flow index (FI) and vascularization flow index (VFI). The percentage change in these indices, during a uterine contraction, was calculated from the baseline value during the initial uterine relaxation phase (taken as a maximum of 100%).

Myometrial blood flow fell significantly during the uterine contraction and returned during the subsequent relaxation phase of the cycle (P < 0.001 for VI and VFI, P = 0.002 for FI). From the initial baseline relaxation value, VI dropped to 43.9%, FI to 85.5% and VFI to 40.8% during uterine contraction, and returned to 86.7%, 98.1% and 89.1%, respectively, during the subsequent relaxation. The intraclass correlation coefficients in blood flow measurements of 0.982-0.999 between the two investigators were indicative of good interobserver reliability.

This study confirms that myometrial perfusion, as measured by quantitative 3D power Doppler angiography, significantly falls during uterine contractions, returns during the subsequent relaxation phase, and can be quantified reliably from stored datasets. Further work is now required to establish clinical applicability for this non-invasive investigation.