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Pharmacology

Oxytocin: Its Pharmacology and Use in Obstetrics and Gynaecology

Introduction

Oxytocin is one the commonly used drugs in obstetrics and gynaecology as it can be prepared synthetically. It is a peptide hormone secreted by the posterior pituitary gland. It’s also called cuddle or love hormone. It plays an important role in uterine contractions and milk ejection.

Pharmacokinetics

Oxytocin is a 9-amino acid peptide with an intrapeptide disulfide cross-link . Its amino acid sequence differs from that of vasopressin at position 3 and 8.
It is administered intravenously for initiation and augmentation of labour. The onset of action is 1-2 minutes and the duration is 3-5 minutes. It can be administered intramuscularly when the onset of action is 5-10 minutes and the duration of about 2 hours. Also, it can be given intranasally as a spray when the absorption may be erratic.
Oxytocin does not bound to plasma protein. It is inactivated in the stomach, intestine and the liver by the enzyme oxytocinase and therefore inactive orally. It is eliminated by the kidneys, liver and functional mammary gland, with half life of 5 minutes.

Pharmcodynamics

Oxytocin acts through G-protein- coupled receptors and the phosphoinositide-calcium second messenger system to contract uterine smooth muscle. Oxytocin also stimulates the release of prostaglandins and leukotrienes that augment uterine contractions. Oxytocin in small doses increases both the frequency and force of uterine contractions. At higher doses, it produces sustained contractions.
Oxytocin also causes contractions of myoepithelial cells surrounding mammary alveoli, which leads to milk ejection. Without oxytocin-induced contraction, normal lactation cannot occur. At high concentration, oxytocin has weak antidiuretic and pressor activity due to activation of vasopressin receptors.

Indications

Oxytocin has both diagnostic and therapeutic applications.
The fetal heart rate response to standardized oxytocin challenge test provides information about placenta circulatory reserve. An abnormal response suggest intrauterine growth restriction which may warrant immediate cesarean delivery.
Therapeutically, oxytocin is a drug of choice to induce labour, and augment dysfunctional labour in women requiring vaginal delivery. Also, use in incomplete abortion and to prevent or control post partum haemorrhage and intranasally to promote lactation.

Contraindication

Oxytocin is contraindicated in fetal distress, and some conditions where vaginal delivery is contraindicated such as abnormal fetal presentation, cephalopelvic disproportion, cord presentation or prolapse, major placental praevia, vasa praevia, previous classical cesarean section, two previous cesarean section, previous uterine rupture, previous vesicovaginal and rectovaginal fistulae repair.

Precautions

When oxytocin is judiciously used, toxicity and its complications rarely occur. But when used in high doses, it can lead to excessive stimulation of uterine contractions before delivery which can cause fetal distress, abruptio placenta and uterine rupture. It should be carefully used.
Also, it can inadvertently activate the vasopressin receptors which can cause excessive fluid retention or water intoxication, leading to hyponatraemia, heart failures, seizures, and death.
Since bolus injections of oxytocin causes hypotension, it should be prevented by administering it intravenously as a dilute solutions at a controlled rate.
Oxytocin should only be used when it is indicated.

Side effects

Oxytocin side effects includes, headache, nausea, vomiting, tachycardia, bradycardia, uterine rupture, fetal distress, abruptio placenta, water intoxication, hypotension, both maternal and fetal hyponatraemia, premature ventricular contractions, neonatal jaundice, seizures, neonatal retinal haemorrhage, anaphylactic reactions and shock.

Conclusion

The use of oxytocin in obstetrics and gynaecology has played a significant role in reducing maternal morbidity and mortality due to post partum haemorrhage, which is why it’s one of the drugs in WHO list of essential drugs.