Uterine blood flow and tocolysis Tom Archer, MD, MBA UCSD Anesthesia

Uterine blood flow (UBF) Fetal O2 supply depends on adequate perfusion of placental lacunae. Adequate perfusion requires high inflow pressure and low outflow pressure  avoid aorto-caval compression with LUD. UBF stops during uterine contraction  need to avoid hyperstimulation from too much oxytocin.

) Umbilical artery (UA) Umbilical vein (UV) Uterine arteriesUterine veins Mom Fetus Normal placental function: fetal and maternal circulations separated by thin membrane (syncytiotrophoblast). “Lakes” of maternal blood Archer TL 2006 unpublished Fetal capillaries in chorionic villi Precariously oxygenated environment

from Google images

Colman-Brochu S 2004

Manbit images

Chestnut chap. 2

Short term: Why increase uterine tone? Stop placental implantation site from bleeding. –Let baby breast feed  nipple stimulation causes oxytocin release from posterior pituitary. –Exogenous oxytocin (causes hypotension) –Methylergonovine (Methergine). No in HBP. –Carboprost (Hemabate). No in asthma / COPD. May cause diarrhea.

Short term: Why decrease uterine tone? Entrapped placenta (uterus has contracted with placenta or fragments inside). Retained placenta will not allow uterus to fully contract  continued bleeding. Methods to relax uterus (for manual removal): –Traditional: halothane anesthesia (+ETT) –Probably better: IV or SL NTG. –NTG also helps placenta to separate –How about a spinal or epidural? What will they do? Not do?

Tocolysis Current OB practice: –no tocolytics after 34 weeks (because 34 weekers do very well) –If membranes are ruptured, don’t delay delivery (chorioamnionitis  neurological injury to fetus). –Does tocolysis improve outcomes before 34 weeks? You can delay delivery, but are you accomplishing anything? We don’t know. Hauth JC Semin Perinatol 30: © 2006

Tocolysis: Why decrease uterine tone? Allow time for betamethasone to promote lung maturation (before 33 weeks). Does tocolysis before 34 weeks improve outcomes? Maybe not. If membranes are ruptured, delaying delivery may allow chorioamnionitis and fetal damage

Management of spontaneous preterm labor < 33 weeks, steroids. < 34 weeks consider tocolysis < 37 weeks, group B strep prophylaxis Hauth JC Semin Perinatol 30: © 2006

Tocolytics Ethanol (historical interest). MgSO4– NOT! And, >50 gm MgSO4 associated with neonatal brain damage (IVH) (Mittendorf R Journal of Perinatology (2006) 26, 57–63). Beta agonists (terbutaline, ritodrine). Pulmonary edema, tachycardia, hypotension, anxiety Cyclooxygenase inhibitors (indomethacin) Ca++ channel antagonists (nifedipine)– 1 st line drug Oxytocin antagonists (atosiban)—1 st line drug

Hauth JC Semin Perinatol 30: © 2006