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Questions and Answers
What percentage of maternal CO is uterine blood flow in the 3rd trimester?
What is the primary source of blood flow to the gravid uterus?
What is the most common cause of postpartum hemorrhage?
What is the effect of oxytocin on uterine receptors?
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What is the mechanism of action of Mg in uterine relaxation?
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What is the effect of β2 agonist activation on uterine smooth muscle?
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Which of the following is a high-risk factor for postpartum hemorrhage?
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What is the action of methylergonovine (methergine)?
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What is the primary mechanism of action of tocolytics?
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What is the main goal of therapy when using tocolytics for preterm labor?
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What is the effect of magnesium on seizure threshold in pre-eclampsia?
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What is the primary mechanism of action of nifedipine?
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What is the risk of using magnesium in patients with preterm labor?
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What is the effect of ritodrine and terbutaline on smooth muscle?
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What is the formula for uterine perfusion?
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What is the effect of magnesium on the fetus when used in preterm labor?
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What is the effect of stimulation on glucose metabolism?
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What is a pulmonary implication of terbutaline use on the maternal side?
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What is the mechanism of action of indomethacin in the context of tocolysis?
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What is a potential side effect of indomethacin use in the fetus?
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What is the mechanism of action of nitroglycerin in the context of uterine relaxation?
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What is oxytocin primarily used for?
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What is the 'Rule of 3s' in the context of oxytocin administration?
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What is the mechanism of action of uterotonics?
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What is the primary indication for the use of Hemabate in obstetrics?
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What is the primary concern when administering misoprostol to a patient with reactive airway disease?
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What is the mechanism of action of ephedrine?
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What is the primary use of vasopressors in obstetrics?
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What is the effect of PGE2 on hemodynamics?
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What is the primary indication for the use of phenylephrine in obstetrics?
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What is the primary effect of hydralazine on the cardiovascular system?
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What is the primary indication for the use of epinephrine in obstetrics?
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What is the primary effect of oxytocin on blood vessels?
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What is the recommended dose of oxytocin for laboring patients requiring a cesarean section?
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What is the effect of continuous exposure of myometrium cells to oxytocin?
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What is the mechanism of action of methergine (methylergonovine)?
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Why should methergine (methylergonovine) be avoided in certain patients?
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What is the effect of oxytocin on systemic vascular resistance?
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What is the role of prostaglandins in uterine tone and contractions?
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What is the mechanism of action of prostaglandins?
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What percentage of maternal cardiac output is uterine blood flow in the third trimester?
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Which of the following is a low-risk factor for postpartum hemorrhage?
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What is the action of β2 agonist activation on uterine smooth muscle?
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What is the primary source of blood flow to the gravid uterus?
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What is the effect of magnesium on uterine smooth muscle?
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What is the middle layer of the uterus?
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What is the most common complication of postpartum hemorrhage?
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What is the effect of prostaglandins on uterine smooth muscle?
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What is the primary goal of administering steroids to the mother in preterm labor?
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What is the effect of magnesium on uterine contractions in preterm labor?
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What is the mechanism of action of calcium channel blockers in tocolysis?
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What is the risk of using β-mimetics in preterm labor?
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What is the indication for administering antibiotics to the mother in preterm labor?
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What is the effect of nifedipine on uterine smooth muscle?
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What is the effect of magnesium on the fetus when used in preterm labor?
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What is the indication for using magnesium in preterm labor?
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What is the primary effect of indomethacin on fetal development?
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Why is terbutaline used for external cephalic version?
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What is the mechanism of action of nitroglycerin in uterine relaxation?
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What is the primary indication for the use of uterotonics?
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How does oxytocin stimulate uterine contractions?
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What is a potential side effect of indomethacin use in the fetus?
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What is the recommended dose of terbutaline for tocolysis?
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Why is uterine hypertonicity a concern during labor?
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What happens to oxytocin receptors in active labor compared to non-laboring patients?
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What is the effect of oxytocin on systemic vascular resistance?
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What is the mechanism of action of methergine (methylergonovine)?
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What is the primary use of prostaglandins in obstetrics?
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What is the effect of continuous exposure of myometrium cells to oxytocin?
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Why should methergine (methylergonovine) be avoided in certain patients?
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What is the recommended dose of oxytocin for laboring patients requiring a cesarean section?
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What is the effect of oxytocin on cardiac output?
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What is the primary concern when administering Hemabate to a patient with reactive airway disease?
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What is the effect of PGE2 on hemodynamics?
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What is the primary indication for the use of ephedrine in obstetrics?
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What is the effect of hydralazine on the cardiovascular system?
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What is the primary mechanism of action of vasopressors in obstetrics?
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What is the main goal of therapy when using misoprostol in postpartum hemorrhage?
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What is the primary indication for the use of phenylephrine in obstetrics?
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What is the effect of misoprostol on the fetus?
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Study Notes
Uterine Anatomy and Physiology
- Uterine arteries (branches of internal iliac arteries) are the primary source of blood flow to the gravid uterus
- Uterine blood flow is 500-900 ml/min, accounting for 20% of maternal cardiac output in the 3rd trimester
- The uterus is composed of three layers: perimetrium (outer), myometrium (middle, muscular), and endometrium (inner)
Uterine Atony and Postpartum Hemorrhage
- Uterine atony is the most common cause of postpartum hemorrhage, complicating up to 10% of deliveries
- Risk factors for uterine atony include: low risk (singleton pregnancy, <4 previous deliveries, unscarred uterus, absence of history of postpartum hemorrhage), medium risk (prior cesarean or uterine surgery, >4 previous deliveries, multiple gestation, large uterine fibroids, chorioamnionitis, Mg sulfate use, prolonged oxytocin use), and high risk (previa, accreta, increta, percreta; Hct <30%; bleeding at admission; known coagulation defect; history of postpartum hemorrhage; abnormal vital signs)
Uterine Receptors
- The uterus is made up of smooth muscle with multiple receptors, including oxytocin receptors, prostaglandins, α1, and β2
- Oxytocin acts directly on oxytocin G-protein receptors to stimulate calcium channels and increase prostaglandin production, leading to smooth muscle contraction
- α1 agonists, such as methylergonovine, stimulate smooth muscle contraction
- β2 agonists, such as terbutaline, relax smooth muscle
- Magnesium competes with calcium receptors, leading to uterine relaxation
Tocolytics
- Tocolytics are medications used to slow or stop contractions
- Mechanisms of action include decreasing calcium influx or blocking prostaglandin synthesis
- Indications include preterm labor, uterine tetany, retained placenta, and fetal head entrapment
- Goals of therapy include transferring the mother to a tertiary care facility, administering antibiotics to prevent neonatal group-B strep infection, administering steroids to decrease neonatal morbidity associated with immature lungs, and treating any underlying cause of preterm labor
Medications for Uterine Relaxation
- Magnesium IV bolus (4-6 g) over 20-30 minutes followed by 1-2 g/hr infusion: blocks calcium at binding sites on sarcoplasmic reticulum, used for seizure prophylaxis with pre-eclampsia
- Nifedipine: antagonizes calcium channels at sarcoplasmic reticulum, has a greater effect on the uterus compared to other calcium channel blockers
- Ritodrine and terbutaline: selective β2 agonists, stimulate gluconeogenesis, and decrease blood glucose and insulin secretion
- Indomethacin: inhibits cyclooxygenase-1 and -2, preventing prostaglandin synthesis, prolongs pregnancy by 2-7 days in preterm labor
- Nitroglycerin: inhibits guanylate cyclase, increases cGMP, and inhibits calcium influx, leading to smooth muscle relaxation, used for rapid uterine relaxation in cases of head entrapment or fetal distress
Uterotonics
- Oxytocin: stimulates calcium influx, increases prostaglandin production, and causes smooth muscle contraction
- Methylergonovine: α1 agonist, stimulates smooth muscle contraction, used for postpartum hemorrhage
- Prostaglandins: naturally occurring hormones, stimulate smooth muscle contraction, used for postpartum hemorrhage and induction of labor
Vasopressors
- Epinephrine: α1 and β1 agonist, used in epidural "test dose" to rule out epidural catheter misplacement
- Ephedrine: direct α and β agonist, indirect inhibitor of norepinephrine reuptake, used for hypotension following neuraxial placement
- Phenylephrine: α1 agonist, vasopressor of choice in maternal cocaine use, maintains cardiac output even with a decrease in heart rate
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Description
Test your knowledge of uterine anatomy, blood flow, and drug interactions during pregnancy. Learn about the risks of uterine atony and postpartum hemorrhage, and how to manage them.