Significance of retrograde diastolic uterine artery blood flow during regional anesthesia in instrumented pregnant sheep T. KAVASMAA 1 , K. MA ¨ KIKALLIO 2 , T. ERKINARO 1 , G. ACHARYA 3 , M. HAAPSAMO 2 , S. ALAHUHTA 1 and J. RA ¨ SA ¨ NEN 2 1 Departments of Anaesthesiology and 2 Obstetrics and Gynaecology, University of Oulu, Oulu, Finland and 3 Department of Obstetrics and Gynaecology, Faculty of Medicine, Institute of Clinical Medicine, University of Tromsø, University Hospital of Northern Norway, Tromsø, Norway Background: We studied the interactions between uterine and placental hemodynamics during maternal hypoten- sion in chronically instrumented fetal sheep. In addition, we investigated maternal hemodynamic characteristics, fetoplacental hemodynamics and fetal acid–base status when a retrograde diastolic uterine artery blood flow pattern is present during maternal hypotension. Methods: Invasive maternal and fetal hemodynamic para- meters, uterine (Q UtA ) and placental (Q UA ) volume blood flows and acid–base values were examined in 24 chroni- cally instrumented sheep at baseline and during epidural- induced maternal hypotension at 117–132 (term 145) days of gestation. Uterine artery blood flow velocity waveforms were obtained by Doppler ultrasonography. Results: Maternal hypotension decreased Q UtA without affecting Q UA . During hypotension, eight out of 24 sheep demonstrated a retrograde diastolic blood flow velocity waveform pattern in the uterine artery. Maternal systolic, diastolic and mean arterial blood pressures were signifi- cantly lower in the retrograde group than in the antegrade group. No statistically significant differences in Q UtA ,Q UA and fetal blood gas values were detected between the two groups during hypotension. Conclusions: An acute decrease in uterine artery volume blood flow during maternal hypotension is not compen- sated by increased placental volume blood flow. A retro- grade diastolic blood flow pattern in the uterine artery is related to lower maternal arterial pressures, especially during diastole. A uterine artery retrograde diastolic blood flow pattern does not have any additional detrimental short-term effects on fetal acid–base status. Accepted for publication 2 July 2008 r 2008 The Authors Journal compilation r 2008 The Acta Anaesthesiologica Scandinavica Foundation I N uncomplicated human pregnancies, Doppler ultrasonography has demonstrated a retrograde diastolic blood flow velocity waveform pattern in the uterine artery during regional anesthesia for Cesarean section, despite nearly normal maternal heart rate (HR), systolic arterial pressure and cen- tral venous pressure. 1 A similar biphasic uterine artery blood flow pattern has also been described during uterine contractions in active labor. 2 Blood enters the uterine circulation during systole, but then escapes during diastole. The retrograde dia- stolic blood flow velocity waveform could repre- sent a steal syndrome in which blood flow in diastole is directed towards the vascular bed with the lowest resistance. 3 This blood flow pattern is distinct and different from the uterine artery blood velocity waveform abnormalities associated with placental insufficiency. 4 The maternal hemody- namic characteristics leading to a retrograde dia- stolic uterine artery blood flow velocity waveform pattern during regional anesthesia are not fully understood. In addition, the effects of diastolic uterine artery blood flow reversal on the fetus are unknown. We have developed a chronic fetal sheep model in which we can investigate the effects of maternal hypotension on uterine and placental hemody- namics and fetal acid–base status. In this study, we asked the following questions: (1) does placen- tal (Q UA ) volume blood flow increase when uterine artery (Q UtA ) volume blood flow decreases during maternal hypotension; (2) what are the maternal hemodynamic characteristics associated with a retrograde diastolic uterine artery blood flow pat- tern during maternal hypotension; and (3) does a retrograde diastolic uterine artery waveform 1291 Acta Anaesthesiol Scand 2008; 52: 1291–1297 Printed in Singapore. All rights reserved r 2008 The Authors Journal compilation r 2008 The Acta Anaesthesiologica Scandinavica Foundation ACTA ANAESTHESIOLOGICA SCANDINAVICA doi: 10.1111/j.1399-6576.2008.01764.x