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.

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Umbilical artery (UA)Umbilical vein (UV)

Uterine arteriesUterine veinsMom

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Archer TL 2006 unpublished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

www.siumed.edu/~dking2/erg/images/placenta.jpgfrom Google images

www.childbirths.com/.../fetalmonitoringetc.htm

Colman-Brochu S 2004

http://www.manbit.com/OA/f28-1.htm

Manbit imageshttp://www.manbit.com/OA/f28-1.htm

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 30:98-102 © 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 30:98-102 © 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)– 1st line drug

• Oxytocin antagonists (atosiban)—1st line drug

Hauth JC Semin Perinatol 30:98-102 © 2006