Oxytocics

Oxytocin is a peptide hormone of the posterior pituitary gland. It stimulates the contractions of the pregnant uterus, which becomes much more sensitive to it at term. Patients with posterior pituitary disease (diabetes insipidus) can, however, go into labour normally.

Oxytocin is reflexly released from the pituitary following suckling (also by manual stimulation of the nipple) and causes almost immediate contraction of the myoepithelium of the breast; it can be used to enhance milk ejection (nasal spray). The only other clinically important effect is on the blood pressure, which may fall if an overdose is given.

Synthetic oxytocin (Syntocinon) is pure and is not contaminated with vasopressin as is the natural product, which is obsolete.

Oxytocin is used i.v. in the induction of labour and sometimes for uterine inertia, haemorrhage or during abortion. It produces, almost immediately, rhythmic contractions with relaxation between, i.e. it mimics normal uterine activity.

The decision to use oxytocin requires special skill. It has a t1/, of 6 min and is given by i.v. infusion using a pump (see below); it must be closely supervised; the dose is adjusted by results; overdose can cause uterine tetany and even rupture. The utmost care is required.

Oxytocin is structurally close to vasopression and it is no surprise that it also has antidiuretic activity (p. 711). Serious water intoxication can occur with prolonged i.v. infusions, especially where accompanied by large volumes of fluid. The association of oxytocin with neonatal jaundice appears to be due to increased erythrocyte fragility causing haemolysis.

Oxytocin has been supplanted by the ergot alkaloid, ergometrine, as prime treatment of postpartum haemorrhage.

Ergometrine is used to contract the uterus. It is an a-adrenoceptor and dopamine receptor agonist and acts almost immediately when injected i.v. The uterus is stimulated at all times, but is much more sensitive in late pregnancy (see also ergotamine, p. 327).

Ergometrine and oxytocin differ in their actions on the uterus. In moderate doses oxytocin produces slow generalised contractions with full relaxation in between; ergometrine produces faster contractions superimposed on a tonic contraction. High doses of both substances produce sustained tonic contraction. It will be seen, therefore, that oxytocin is more suited to induction of labour and ergometrine to the prevention and treatment of postpartum haemorrhage, the incidence of which is reduced by its routine prophylactic use (generally i.m.).

There are advantages in a mixture of oxytocin and ergometrine (Syntometrine).

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