Postpartum hemorrhage pathophysiology

At term, the uterus and placenta receive 500-800 mL of blood per minute through their low resistance network of vessels. The high circulatory exchange predisposes a gravid uterus to significant bleeding if not well physiologically or medically controlled. 

Biologic hemorrhage protection:

Maternal blood volume increases by 50%  the third trimester (increases the body’s tolerance of blood loss during delivery).

Biological protection of the maternal body against postpartum hemorrhage:

Reduction in uterine size:

The gravid uterus contracts down significantly after delivery given the reduction in volume. This allows the placenta to separate from the uterine interface, exposing maternal blood vessels that interface with the placental surface. 

Constriction of vasculature

After separation and delivery of the placenta, the uterus initiates a process of contraction and retraction, shortening its fiber and kinking the supplying blood vessels, like physiologic sutures or “living ligatures.”

Complications:
If the uterus fails to contract, or the placenta fails to separate or deliver, then significant hemorrhage may ensue. 

Risk factors for complications:

  1. Uterine atony, or diminished myometrial contractility, accounts for 80% of postpartum hemorrhage. 
  2. Abnormal placental attachment
  3. Retained placental tissue
  4. Laceration of tissues or blood vessels in the pelvis and genital tract
  5. Maternal coagulopathies. 
  6. Uterine inversion during placental delivery (rare). 

Etiology Pneumonic:

 
The traditional pneumonic “4Ts: 

  1. tone
  2. tissue
  3. trauma
  4. thrombosis

Carusi, Daniela,Yaa, Maame,  and Yiadom, A B. 2010. Pregnancy, Postpartum Hemorrhage. Emedicine. Retrieved October 24, 2010 from http://emedicine.medscape.com/article/796785-overview

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