oxytocin
Pitocin, Syntocinon

Available forms
Available by prescription only
Injection: 10-units/ml ampules, vials, and closed injection system

Indications and dosages
 Induction of labor. Adults: Initially, no more than 0.5 to 1 milliunits/minute I.V. infusion. Rate of infusion may be increased slowly (1 to 2 milliunits/minute at 30- to 60-minute intervals until response occurs). Decrease rate when labor is firmly established.
 Augmentation of labor. Adults: Initially, 2 milliunits/minute I.V. infusion. Rate of infusion may be increased slowly to maximum of 20 milliunits/minute.
 Reduction of postpartum bleeding after expulsion of placenta. Adults: 20 to 40 milliunits/ minute by I.V. infusion to total of 10 units (or 10 units I.M.) after delivery of the placenta.
 To induce abortion. Adults: 10 units mixed in 500 ml D₅W or normal saline solution I.V. at 10 to 100 milliunits/minute (not to exceed 30 units in 12 hours).
 Oxytocin challenge test to assess fetal distress in high-risk pregnancies of greater than 31 weeks’ gestation. Adults: Prepare solution by adding 5 to 10 units oxytocin to 1 L of D₅W injection, yielding 5 to 10 milliunits/ml. Infuse 0.5 milliunits/minute, gradually increasing at 15-minute intervals to maximum of 20 milliunits/minute. Stop infusion when three moderate uterine contractions occur in a 10-minute interval. Response of fetal heart rate to test may be used to evaluate prognosis.

Pharmacodynamics
Oxytocic action: Oxytocin increases the sodium permeability of uterine myofibrils, indirectly stimulating the contraction of uterine smooth muscle. The threshold for response is lowered when estrogen levels are high. Uterine response increases with the length of the pregnancy and increases further during active labor. Response mimics labor contractions.

Pharmacokinetics
Absorption: Administered I.V. and I.M.
Distribution: Distributed throughout the extracellular fluid; small amounts may enter the fetal circulation.
Metabolism: Metabolized rapidly in the kidneys and liver. In early pregnancy, a circulating enzyme, oxytocinase, can inactivate the drug.
Excretion: Only small amounts are excreted in the urine as oxytocin. Half-life is 3 to 5 minutes.

Contraindications and precautions
Contraindicated in patients hypersensitive to drug and in those with severe toxemia, hypertonic uterine patterns, total placenta previa, and vasoprevia. Also contraindicated when cephalopelvic disproportion is present; when delivery requires conversion, as in transverse lie; in fetal distress when delivery isn’t imminent; and in prematurity and other obstetric emergencies.
 Use cautiously during first and second stages of labor and in patients with history of cervical or uterine surgery (including cesarean section), grand multiparity, uterine sepsis, traumatic delivery, overdistended uterus, or invasive cervical cancer.

Interactions
Drug-drug. Cyclopropane anesthesia: May modify CV effects of oxytocin. Use together cautiously.
Sympathomimetics: May increase pressor effects, possibly resulting in postpartum hypertension. Avoid use together.
Thiopental anesthesia: Delays induction of anesthesia. Use together cautiously.

Adverse reactions
Maternal
CNS: subarachnoid hemorrhage (from hypertension), seizures or coma (from water intoxication).
CV: hypertension; increased heart rate, systemic venous return, and cardiac output; arrhythmias.
GI: nausea, vomiting.
GU: tetanic uterine contractions, abruptio placentae, impaired uterine blood flow, pelvic hematoma, increased uterine motility, uterine rupture, postpartum hemorrhage.
Hematologic: afibrinogenemia (may be related to postpartum bleeding).
Other: hypersensitivity reactions (anaphylaxis), water retention.
Fetal
CNS: infant brain damage.
EENT: retinal hemorrhage.
CV: bradycardia, PVCs, arrhythmias.
Hepatic: jaundice.
Respiratory: anoxia, asphyxia.
Other: low Apgar scores.

Effects on lab test results
None reported.

Overdose and treatment
Signs and symptoms of overdose include hyperstimulation of the uterus, causing tetanic contractions and possible uterine rupture, cervical laceration, abruptio placentae, impaired uterine blood flow, amniotic fluid embolism, and fetal trauma.
 Drug has a very short half-life; halt therapy and start supportive care.

Special considerations
• Drug must be given by I.V. infusion, not I.V. bolus injection. Use an infusion device.
• Have magnesium sulfate (20% solution) available for relaxation of the myometrium.
• Drug isn’t recommended for routine I.M. use. However, 10 units may be given I.M. after delivery of the placenta to control postpartum uterine bleeding.
• When giving the oxytocin challenge test, monitor fetal heart rate and uterine contractions immediately before and during infusion. If fetal heart rate doesn’t change during test, repeat in 1 week. If late deceleration in fetal heart rate is noted, consider terminating pregnancy.
• I.V. infusion rates up to 6 milliunits/minute produce maternal plasma levels similar to spontaneous labor. Rates exceeding 9 to 10 milliunits/minute are rarely needed.
• Solution containing 10 milliunits/ml may be prepared by adding 10 units of oxytocin to 1 L of normal saline solution or D₅W. Solution containing 20 milliunits/ml may be prepared by adding 10 units of oxytocin to 500 ml of normal saline solution or D₅W.
• During long infusions, watch for signs of water intoxication.
• Oxytocin injection is incompatible with fibrinolysin, norepinephrine bitartrate, prochlorperazine edisylate, and warfarin sodium.
• Record uterine contractions, heart rate, blood pressure, intrauterine pressure, fetal heart rate, and character of blood loss every 15 minutes.
• Discontinue drug if uterine contractions are prolonged (more than 90 seconds), intrauterine pressure rises, or uterine motility interferes with fetal heart rate.
Breast-feeding patients
• Minimal amounts of drug appear in breast milk. Evaluate risks.

Patient education
• Explain possible adverse effects of drug.

Reactions may be common, uncommon, life-threatening, or COMMON AND LIFE THREATENING.
◆ Canada only
◇ Unlabeled clinical use