Table of Contents
Oxytocics Definition and Meaning
Oxytocics are a group of drugs that have uterotonic effect on the uterus, this means that their effect on the uterine myometrium is to cause contraction of the fibres. Some Oxytocics are naturally produced in the body while others are synthetic agents.
Uses of Oxytocics in Obstetrics
- Induction of Labour: oxytocics are used for induction of labour to initiate the contraction of the uterus
- Augmentation of Labour: sometimes labour can begin but may not progress well as a result of irregular uterine contractions or weak contractions and will need to be augmented; this is when oxytocics are used to augment labour.
- Stimulation of Labour
- Ripening of the Cervix
- Prevention of Postpartum Haemorrhage
- Treatment of Postpartum Haemorrhage
Names of Oxytocic drugs
- Prostaglandin E2
Types of Oxytocics and Uses (Classification of Oxytocics)
Oxytocin is a hormone produced naturally by the hypothalamus. There are oxytocin receptors in the myometrium whose concentration and sensitivity to the hormone increase as the pregnancy tends towards term. Oxytocin stimulates uterine activity which is sufficient to produce cervical changes and fetal descent without causing hyperstimulation. It stimulates rhythmic uterine contractions similar in frequency, force and duration as in labour. Oxytocin acts rapidly with a latency period of <1 min with IV administration but acts 2-4 min with IM. Continuous IV infusion gives a gradual response reaching steady state within 20-40 mins. Oxytocin is metabolised and removed by the liver and kidneys and about 1% is excreted unchanged in the urine.
Complications of oxytocin could be hypercontraction and hyperstimulation leading to uterine rupture and/or fetal distress/death. Another complication could be water intoxication because oxytocin is structurally and functionally related to antidiuretic hormone (ADH).
Syntocinon is a cyclic nonapeptide that is obtained by chemical analysis and is identical to the natural form oxytocin. This is a synthetic oxytocin and can be used like the naturally produced oxytocin in the induction, stimulation, augmentation of labour or the prevention and treatment of postpartum haemorrhage. It is very stable even when stored under tropical conditions. Complications of syntocinon are same as oxytocin.
Ergometrine is a naturally occuring ergot alkaloid which stimulates contractions of the uterine and vascular smooth muscle. It increases the amplitude and frequency of uterine contractions and tone andthus impedes uterine blood flow. Uterine contractions are initiated within 1 min of IV administration, and last for 45 minutes. With IM administration, contractions are initiated within 2-3 minutes and last for 3 hours or more.
Ergometrine is an oxytocic that produces tetanic contractions of the uterine myometrial fibres and therefore very useful for the prevention and treatment of postpartum haemorrhage. It is not used in labour, it is only used when the placenta has been delivered after labour and only given when there is no elevated blood pressure as it increased the blood pressure. When stored for prolonged periods of time, it must be kept at between 2-8oC and protected from light otherwise it losses about 20% of its potency when stored in tropical conditions. 0.5 mg is usually given intramuscularly.
This is a combination of 5 I.U of syntocinon and 0.5mg of Ergometrine. Syntometrine was found to be a better drug in the prevention of PPH than syntocinon. However, because of its stringent storage requirement as a result of its ergometrine constituent, it has not found widespread use in the developing countries as syntocinon.
Prostaglandin E2 (PGE2)
Prostaglandin E2 is used either as the pessary or as the gel for cervical ripening. The gel is applied locally on the cervix while the pessary is inserted into the posterior fornix and the cervix is assessed after 6 hours. The mechanism of action of prostaglandin is to cause dissolution of collagen bundles with increase in submucosal oedema. It does this by altering the extracellular ground substance, by increasing the activity of collagenase and levels of elastase and hyaluronic acid in the cervix. Prostaglandins relax the cervical smooth muscle and increase intracellular calcium leading to the contraction of the myometrium.
Misoprostol (Cytotec PG E1)
Misoprostol is an analogue of PGE1 and is the most common agent used in induction of labour in many places; it is metabolised in the liver and <1% of active metabolised excreted in the urine. It is absorbed across the vaginal mucosa in pregnancy. 50 micrograms is inserted into the posterior fornix every 4 hours and the cervix assessed for ripening. About 2-4 doses are required for cervical ripening and in some cases labour might be initiated without further intervention.
It must be noted that prostaglandins mimic spontaneous labour and enhance myometrial sensitivity to oxytocin. Caution must therefore be exercised in the dose of oxytocin to be administered after priming with prostaglandins.