Comparison of v a r i o u s methods
r i p e n i n g a n d l a b o u r , i n c l u d i n g of i n d u c t i o n of p r o s t a g l a n d i n s ,
9 cervical oxytocin, oestradiol and mechanical means
Calder, A . A .
I n d u c t i o n of l a b o u r has been practiced for c e n t u r i e s . C e r v i c a l r i p e n i n g is a more recent i n n o v a t i o n . B o t h , h o w e v e r , have the object, to deliver the infant. The recent focus of attention on cervical ripening shows that clinicians have come to recognise the d e f i c i e n c i e s in the methods hitherto available f o r l a b o u r i n d u c t i o n . To achieve s a f e d e l i v e r y it is necessary to make a c a r e f u l s t u d y of each patient and to plan the method of i n t e r v e n t i o n a c c o r d i n g l y . If the f e t u s is already seriously compromised and likely to become even more so d u r i n g l a b o u r , d e l i v e r y will be best a c h i e v e d by elective Caesarean section; if not thought must be g i v e n to the l i k e l y response to d i f f e r e n t methods of induction of labour, including, if necessary, a procedure for cervical ripening.
U n l i k e other f o r m s of medical i n t e r v e n t i o n , induction of l a b o u r aims to p r o d u c e an e f f e c t w h i c h w o u l d o c c u r s p o n t a n e o u s l y in the n a t u r a l course of e v e n t s . H u m a n p a r t u r i t i o n is not the r a t h e r s u d d e n p h e n o m e n o n it may sometimes a p p e a r . R a t h e r , it is a g r a d u a l e v o l u t i o n of uterine contractility and cervical r i p e n i n g , o c c u r r i n g in concert over several weeks.
One f a c t o r above all o t h e r s d e t e r m i n e s the response to induction of labour, namely - How close is the patient to the s p o n t a n e o u s onset of l a b o u r ? A patient who w o u l d be in s p o n t a n e o u s l a b o u r t o m o r r o w w i l l respond well to l a b o u r i n d u c t i o n t o d a y , almost regardless of the method employed.
C o n v e r s e l y , if s p o n t a n e o u s labour is a d i s t a n t prospect a t t e m p t s at i n d u c t i o n , especially by inappropriate methods, may fail and expose both mother and f e t u s to much g r e a t e r d a n g e r s . For the m o t h e r these are the risks of prolonged l a b o u r , sepsis and damage to the g e n i t a l t r a c t w i t h the l i k e l y need f o r Caesarean section. For the fetus the risks are i n f e c t i o n , h y p o x i a a n d t r a u m a i n c l u d i n g i n t r a c r a n i a l haemorrhage.
The c l i n i c i a n should t h e r e f o r e tread cautiously and should direct his a t t e n t i o n to the c e r v i x . T h e r e is no b e t t e r i n d i c a t o r of the likely response to induction of labour If the cervix is ripe (soft, compliant, effaced and beginning to d i l a t e ) a n d t h e p r e s e n t i n g part well i n t o t h e p e l v i s , induction should be trouble free. If the c e r v i x is not r i p e i n d u c t i o n of labour w i t h o u t f i r s t ripening the cervix may lead the patient, fetus and obstetrician into trouble. 4
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The i n d i v i d u a l t e c h n i q u e s of labour induction and cervical ripening will now be considered against this background.
(1) Mechanical Methods; In the p a s t , a v a r i e t y of m a n i p u l a t i o n s and stimuli were applied to the genital tract t o i n t e r r u p t p r e g n a n c y . B i z a r r e f o r e i g n m a t e r i a l s w e r e i n t r o d u c e d into the v a g i n a and cervix and for a time rigid bougies were popular. These are now of h i s t o r i c a l i n t e r e s t only except intracervical balloons, laminaria, sweeping the m e m b r a n e s a n d a m n i o t o m y . W e n o w k n o w t h a t a l l these i n f l u e n c e s release p r o s t a g l a n d i n s w i t h i n t h e u t e r i n e compartment.
(2) G x y t o c i n : In the 75 y e a r s since t h i s p e p t i d e was i s o l a t e d a n d made a v a i l a b l e as a t h e r a p e u t i c agent much experience has accrued about its benefits and dangers. It is the most c o m m o n l y used agent for induction of labour and is best given by i n t r a v e n o u s i n f u s i o n . Its e f f e c t i v e n e s s is g r e a t l y e n h a n c e d by a m n i o t o m y which increases the oxytocin sensitivity of the myometrium, probably as the result of an increase in prostaglandin production. It is also much more effective if the cervix is r i p e . The p r i m i g r a v i d a w i t h an unripe cervix will respond poorly to oxytocin infusion. This does not p r o d u c e e f f e c t i v e u t e r i n e a c t i v i t y or c e r v i c a l r i p e n i n g and if amniotomy is performed with the the cervix still unripe the complication rate is high.
(3) Prostaglandins; These so called "local hormones1 1
( e s p e c i a l l y P G E 2 ) h a v e l a r g e l y compensated f o r t h e deficiencies of oxytocin. Experience has taught us that t h e y are most e f f e c t i v e g i v e n locally in the genital tract and that they have a specific effect in ripening the cervix. They
also stimulate effective uterine contractions.
If the cervix is unripe PCs should be d i r e c t e d t o w a r d s the c e r v i x (extra-amniotically, endocervically or vaginally in a gel or p e s s a r y ) to p r o d u c e r i p e n i n g . A m n i o t o m y should be d e f e r r e d u n t i l the cervix is ripe and thereafter myometrial contractions may be e n h a n c e d if r e q u i r e d w i t h i n t r a v e n o u s oxytocin or further prostaglandin therapy. This dramatically reduces the incidence of complications f r o m i n d u c t i o n of labour in patients with an unripe cervix.
(4) Other Hormones; O e s t r o g e n s and relaxin have also been i n v e s t i g a t e d f o r c e r v i c a l r i p e n i n g . T h e i r effect is less dramatic than that of the p r o s t a g l a n d i n s but t h e y h a v e the p o t e n t i a l b e n e f i t of c a u s i n g less m y o m e t r i a l stimulation.
This may allow the processes of ripening and induction t.o be a c c o m p l i s h e d s e p a r a t e l y . E x p e r i e n c e w i t h these agents is limited, however, and further study and knowledge is required before they can have a clinical impact.
