Oxytocin
Oxytocin is a nonapeptide hypothalamic hormone synthesized in the paraventricular and supraoptic nuclei and stored in the posterior pituitary gland. In veterinary medicine, it is primarily utilized as a uterotonic agent to induce or enhance uterine contractions during parturition, treat postpartum retained placenta and metritis, promote uterine involution, and facilitate milk let-down in cases of agalactia. **Clinical Pearls:** * **Mini-dosing is preferred:** Modern veterinary obstetrics strongly advocates for "mini-doses" (e.g., 0.25-4 Units total in dogs) rather than historical high doses. High doses can cause tetanic, ineffective uterine contractions that compromise fetal oxygen supply via placental compression and increase the risk of uterine rupture. * **Calcium dependency:** Oxytocin's efficacy is highly dependent on extracellular calcium. Hypocalcemia or hypoglycemia must be corrected prior to administration. Calcium increases the *strength* of contractions, while oxytocin increases the *frequency*. * **Cervical status:** Oxytocin should never be administered if the cervix is closed or if there is an uncorrected obstructive dystocia.
Mecanismo: Oxytocin exerts its effects by binding to specific **G-protein coupled oxytocin receptors (OXTR)** on the cell membrane of uterine myofibrils and myoepithelial cells of the mammary gland. * **Uterine Contraction:** Binding to OXTR activates the **Gq/11 pathway** → stimulates **Phospholipase C (PLC)** → increases inositol triphosphate (**IP3**) and diacylglycerol (**DAG**) → triggers the release of intracellular calcium from the sarcoplasmic reticulum. Calcium binds to calmodulin, activating **Myosin Light Chain Kinase (MLCK)**, leading to smooth muscle contraction. The threshold for oxytocin-induced contraction is significantly lowered late in pregnancy due to upregulation of oxytocin receptors by high estrogen levels. * **Milk Let-down:** Oxytocin stimulates the contraction of myoepithelial cells surrounding the mammary alveoli, forcing milk into the larger ducts and cisterns. It does *not* have galactopoietic (milk-producing) properties.
Dosificación por especie
- For retained placenta in patients with uterine atony · 10-20 Units oxytocin. · IM/IV · Once · Limited value after 48 hours postpartum. Note for CAMELIDS (NW): 5-10 Units IM at 10-minute intervals for retained placenta (Adams 2008).
- For mild to moderate cases of acute post-partum metritis · 5-10 Units IM 3-4 times a day for 2-3 days · IM · TID to QID · 2-3 days
- For retained placenta in patients with uterine atony · 10-20 Units oxytocin. · IM/IV · Once · Limited value after 48 hours postpartum.
- For mild to moderate cases of acute post-partum metritis · 5-10 Units IM 3-4 times a day for 2-3 days · IM · TID to QID · 2-3 days
- To control post-extraction cervical and uterine bleeding after internal manipulations · 10-20 Units IV, may repeat SC in 2 hours · IV/SC · Once, may repeat in 2h
- To promote uterine involution after uterine prolapse manual reduction · 0.5-5 Units IM OR 5-10 Units (total dose) IM · IM · Once · Extrapolated from dog doses.
- To treat primary uterine inertia · 0.25 Units (total dose) SC or IM to a maximum dose of 4 Units per queen. Generally not given more frequently than hourly. · SC/IM · q1h · Given 15 minutes after calcium gluconate.
- For adjunctive treatment of metritis · 0.5-5 Units IM · IM · Once · May be used if birth occurred less than 24 hours prior.
Vías de administración
Contraindicaciones
- Dystocia due to abnormal fetal presentation (unless corrected)
- Closed or unrelaxed cervix
- Known hypersensitivity to oxytocin
- Significant cephalopelvic disproportion
- Obstetrical emergencies where surgical intervention (C-section) is warranted
- Pregnancy (for nasally administered oxytocin)
Efectos adversos
- Uterine cramping and discomfort
- Tetanic uterine contractions (at high doses)
- Uterine rupture
- Fetal compromise or death (due to placental compression)
- Water intoxication (with prolonged high-dose infusions)
- Hypersensitivity reactions (more common with non-synthetic products)
Interacciones farmacológicas
- Thiopental · May delay thiopental anesthesia (reported in humans; clinical significance in animals is not firmly established).
- Vasoconstrictors (Sympathomimetics) · Concurrent use post-partum may result in severe hypertension. Monitor closely.
Monitorización
- Uterine contractions (frequency, duration, and strength via tokodynamometry if available)
- Status of the cervix (must be dilated)
- Fetal monitoring (heart rate/distress)
- Maternal calcium and glucose levels
Sobredosis
Effects of overdosage on the uterus depend on the stage of the uterus and the position of the fetus(es). * **Uterine Effects:** Hypertonic or tetanic contractions can occur, leading to tumultuous labor, uterine rupture, fetal injury, or fetal death due to compromised placental blood flow. * **Water Intoxication:** Can occur if large doses are infused for a long period, especially with large volumes of electrolyte-free IV fluids. * *Early signs:* Listlessness, depression. * *Severe signs:* Coma, seizures, death. * *Treatment:* Stop oxytocin therapy, restrict water access. Severe cases may require osmotic diuretics (mannitol, urea, dextrose) with or without furosemide.
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