The incidence of PPH ranges between 5% and 8% in places where some form of prophylaxis is practised, but may be as high as 18% when a physiological approach is the norm3. Physiological control of post-partum bleeding occurs by contraction and retraction of the interlacing myometrial fibres surrounding maternal spiral arteries in the placental bed. Myometrial contraction compresses the spiral arteries and veins, thereby obliterating their lumina. Haemostasis following placental separation is thereby initially a mechanical process not primarily dependent upon an intact coagulation system. Development of this mechanism was a crucial aspect of viviparity without which mammals would not have evolved. However it is flawed: Primary PPH due to uterine atony occurs when the relaxed myometrium fails to constrict these blood vessels, thereby allowing haemorrhage. Since up to one-fifth of maternal cardiac output, which is in excess of 600 ml/min, enters the uteroplacental circulation at term, it is understandable that primary atonic PPH can be catastrophic—capable of exsanguinating the mother within minutes. Whilst uterine atony is responsible for the majority of primary PPH, the surgical obstetrical causes such as injury of the cervix, vagina, paravaginal spaces, perineum and episiotomy comprise about 20% of all primary PPH.
Posted on: Fri, 02 Jan 2015 22:06:14 +0000