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Both phenylephrine and hypercapnia cause acute placental and fetal organ hypoperfusion with fetal brain sparing in rat pregnancy: BOLD functional MRI changes mirrored by Doppler ultrasound.
Abstract Number: BP-01
Abstract Type: Original Research
The fetal circulatory changes that follow acute placental hypoperfusion are not fully understood. Most available data comes from Doppler ultrasound but is limited as it cannot assess placental and fetal vessels simultaneously and cannot make repeated measurements at short fixed time intervals. We have used blood oxygen-level dependent functional magnetic resonance imaging (BOLD-fMRI) as a novel fetal imaging strategy to identify acute changes in placental and fetal organ perfusion. We used this technique to demonstrate acute placental hypoperfusion (and fetal brain sparing) in response to both acute normoxic hypercapnia and intravenous maternal phenylephrine administration. In this study we hypothesize that regional changes in BOLD-fMRI will be accompanied by similar changes in Doppler ultrasound pulsatility index.
Methods: All experiments were performed on spontaneously breathing adult female Wistar rats (on E20), anesthetized with 30 mg/kg intraperitoneal pentobarbital. For normoxic hypercapnia (n=7), animals were attached by a loose-fitting facemask to a breathing system with 4 l/min fresh gas flow; for three consecutive 4 min periods, animals breathed the following gases in sequence: medical air, air-carbon dioxide (5% CO2, 21% O2, 74% N2) and carbogen (5% CO2, 95% O2). In the phenylephrine group (n=3), animals received drug (10microgram/kg) in tail vein as a bolus. Doppler ultrasound pulsatility index and fetal heart rate were assessed in umbilical arteries and ductus venosus. Abdominal fur was removed; gel applied to the abdomen; Doppler performed using high-resolution ultrasound imaging system Vevo 770, VisualSonics, equipped with a 40mHz mechanical transducer (RMV707B) focal length of 6 mm, a frame rate of 30 Hz, and an 8 × 8 mm field of view with spatial resolution of 30 µm. Doppler indices of pulsatility index were obtained from at least three similar consecutive waveforms. Decreased values indicate decreased vascular resistance and correlated with increased flow.
There was a marked increase in umbilical artery PI following hypercapnia 27.4% (2.7) and phenylephrine 28.7% (9.7), accompanied by fetal bradycardia -22.5% (5.9) and -20.6% (3.8) respectively. These results are represented in Fig G-J and may be compared with earlier data in Fig A-F.
The changes in Doppler flows support our previous observations of using BOLD-fMRI as a non-invasive monitor of fetal hemodynamic responses to acute intrauterine asphyxia