80 Questions
What percentage of maternal CO is uterine blood flow in the 3rd trimester?
20%
What is the primary source of blood flow to the gravid uterus?
Uterine arteries
What is the most common cause of postpartum hemorrhage?
Uterine atony
What is the effect of oxytocin on uterine receptors?
Stimulation of calcium channels and prostaglandin production
What is the mechanism of action of Mg in uterine relaxation?
Blockade of calcium receptors
What is the effect of β2 agonist activation on uterine smooth muscle?
Relaxation
Which of the following is a high-risk factor for postpartum hemorrhage?
Abnormal VS
What is the action of methylergonovine (methergine)?
α1 agonist
What is the primary mechanism of action of tocolytics?
Blocking calcium or prostaglandin synthesis
What is the main goal of therapy when using tocolytics for preterm labor?
To transfer the mother to a tertiary care facility
What is the effect of magnesium on seizure threshold in pre-eclampsia?
It increases seizure threshold
What is the primary mechanism of action of nifedipine?
Antagonism of calcium channels at the sarcoplasmic reticulum
What is the risk of using magnesium in patients with preterm labor?
Pulmonary edema
What is the effect of ritodrine and terbutaline on smooth muscle?
It causes relaxation
What is the formula for uterine perfusion?
(Uterine arterial pressure - uterine venous pressure)/uterine vascular resistance
What is the effect of magnesium on the fetus when used in preterm labor?
It decreases fetal heart rate variability
What is the effect of stimulation on glucose metabolism?
It increases blood glucose levels
What is a pulmonary implication of terbutaline use on the maternal side?
Pulmonary edema
What is the mechanism of action of indomethacin in the context of tocolysis?
Inhibition of prostaglandin synthetase
What is a potential side effect of indomethacin use in the fetus?
Premature closure of the ductus arteriosus
What is the mechanism of action of nitroglycerin in the context of uterine relaxation?
Increase in cGMP levels
What is oxytocin primarily used for?
Postpartum hemorrhage
What is the 'Rule of 3s' in the context of oxytocin administration?
3 units of Pit in 3 ml over 3 minutes
What is the mechanism of action of uterotonics?
Increase in calcium levels
What is the primary indication for the use of Hemabate in obstetrics?
Treatment of postpartum hemorrhage
What is the primary concern when administering misoprostol to a patient with reactive airway disease?
Precipitating severe bronchospasm
What is the mechanism of action of ephedrine?
Indirect α and β agonist
What is the primary use of vasopressors in obstetrics?
Emergent situations
What is the effect of PGE2 on hemodynamics?
Increases systolic volume and pulmonary vascular resistance
What is the primary indication for the use of phenylephrine in obstetrics?
Maternal cocaine use
What is the primary effect of hydralazine on the cardiovascular system?
Decreases peripheral vascular resistance
What is the primary indication for the use of epinephrine in obstetrics?
Epidural test dose
What is the primary effect of oxytocin on blood vessels?
Vasodilation
What is the recommended dose of oxytocin for laboring patients requiring a cesarean section?
3IU bolus followed by 5-10 U infusion
What is the effect of continuous exposure of myometrium cells to oxytocin?
Significant loss in capacity to respond to oxytocin
What is the mechanism of action of methergine (methylergonovine)?
α1 activation
Why should methergine (methylergonovine) be avoided in certain patients?
Due to its risk of retinal damage, pulmonary edema, and cerebral hemorrhage in hypertensive patients
What is the effect of oxytocin on systemic vascular resistance?
Decreases it
What is the role of prostaglandins in uterine tone and contractions?
Essential for uterine tone and contractions
What is the mechanism of action of prostaglandins?
Increases Ca2+ leading to uterine contraction
What percentage of maternal cardiac output is uterine blood flow in the third trimester?
20%
Which of the following is a low-risk factor for postpartum hemorrhage?
Singleton pregnancy
What is the action of β2 agonist activation on uterine smooth muscle?
Decreases calcium levels
What is the primary source of blood flow to the gravid uterus?
Uterine arteries
What is the effect of magnesium on uterine smooth muscle?
Relaxes smooth muscle
What is the middle layer of the uterus?
Myometrium
What is the most common complication of postpartum hemorrhage?
Uterine atony
What is the effect of prostaglandins on uterine smooth muscle?
Stimulates smooth muscle contraction
What is the primary goal of administering steroids to the mother in preterm labor?
To reduce neonatal morbidity associated with immature lungs
What is the effect of magnesium on uterine contractions in preterm labor?
It relaxes uterine smooth muscle
What is the mechanism of action of calcium channel blockers in tocolysis?
They block calcium at binding sites on the sarcoplasmic reticulum
What is the risk of using β-mimetics in preterm labor?
Pulmonary edema
What is the indication for administering antibiotics to the mother in preterm labor?
To prevent neonatal group-B strep infection
What is the effect of nifedipine on uterine smooth muscle?
It relaxes uterine smooth muscle more than other CCBs
What is the effect of magnesium on the fetus when used in preterm labor?
It decreases fetal heart rate variability
What is the indication for using magnesium in preterm labor?
To prevent seizures in pre-eclampsia
What is the primary effect of indomethacin on fetal development?
Premature closure of ductus arteriosus
Why is terbutaline used for external cephalic version?
To relax the uterus and prevent distress
What is the mechanism of action of nitroglycerin in uterine relaxation?
Inhibition of GC leading to increased cGMP
What is the primary indication for the use of uterotonics?
Postpartum hemorrhage
How does oxytocin stimulate uterine contractions?
By increasing calcium influx
What is a potential side effect of indomethacin use in the fetus?
All of the above
What is the recommended dose of terbutaline for tocolysis?
25 mg IV/SQ
Why is uterine hypertonicity a concern during labor?
It can lead to fetal distress
What happens to oxytocin receptors in active labor compared to non-laboring patients?
They decrease
What is the effect of oxytocin on systemic vascular resistance?
It decreases it
What is the mechanism of action of methergine (methylergonovine)?
α1 activation → uterine contraction
What is the primary use of prostaglandins in obstetrics?
Uterine contraction
What is the effect of continuous exposure of myometrium cells to oxytocin?
Decreased oxytocin receptor expression
Why should methergine (methylergonovine) be avoided in certain patients?
It can cause hypertension and peripheral vasoconstriction
What is the recommended dose of oxytocin for laboring patients requiring a cesarean section?
3 IU bolus followed by 5-10 U infusion
What is the effect of oxytocin on cardiac output?
It increases cardiac output
What is the primary concern when administering Hemabate to a patient with reactive airway disease?
Precipitating severe bronchospasm
What is the effect of PGE2 on hemodynamics?
Increases SVR and PVR
What is the primary indication for the use of ephedrine in obstetrics?
Hypotension following neuraxial placement
What is the effect of hydralazine on the cardiovascular system?
It is a direct arteriole dilator
What is the primary mechanism of action of vasopressors in obstetrics?
Alpha-1 agonist
What is the main goal of therapy when using misoprostol in postpartum hemorrhage?
To treat postpartum hemorrhage
What is the primary indication for the use of phenylephrine in obstetrics?
Maternal cocaine use
What is the effect of misoprostol on the fetus?
It increases fetal heart rate
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
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.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
