100 Obstetrics and Gynecology Board Review •••
❍ With that in mind, after how many hours in the second stage has it been shown that the chance of vaginal
delivery significantly decreases while maternal morbidity increases?
4 hours. The incidence of vaginal delivery with a second stage <2 hours is 99%, 2 to 4 hours is 91%, and >4 hours
is only 66%.
❍ Treatment of active phase arrest includes?
Amniotomy and/or oxytocin augmentation.
❍ Minimally effective uterine activity is defined as?
Three contractions per 10 minutes of at least 25 mm of mercury above baseline or a contraction pattern exceeding
200 Montevideo units per 10-minute window without cervical change.
❍ Tachysystole is defined as?
More than 5 contractions in 10 minutes, averaged over a 30-minute window.
❍ What is oxytocin?
A peptide hormone made in the hypothalamus and released from the posterior pituitary in a pulsatile manner.
❍ What is the mechanism of action of oxytocin?
On binding to its receptor, phospholipase C is activated. This increases intracellular calcium by stimulating the
release of intracellular calcium and by initiating influx of extracellular calcium.
❍ Mean plasma half-life of oxytocin is?
3 to 4 minutes, but shorter when high doses are infused.
❍ The interval to reach a steady-state concentration of oxytocin in plasma is approximately?
24 minutes.
❍ True or False: X-ray pelvimetry is generally considered of little value in the treatment of active phase arrest.
True.
❍ Maximal dose of oxytocin is generally considered to be?
30 to 40 mU/min.
❍ In patients with documented disorders of labor, what percentage responds to oxytocin infusion resulting^
in a vaginal delivery?
80%.