Fluid redistribution and hematological changes in pre-eclampsia
THE CONTEXT
Pre-eclampsia combines salt retention and extravascular fluid accumulation with intravascular volume contraction and hematological abnormalities.
Fluid balance and other organ changes
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Pulmonary edema is attributed to low oncotic pressure and leaky capillaries.
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Adrenal glands may show hemorrhage and necrosis, while the stomach may show hemorrhagic gastritis.
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Salt retention occurs to varying degrees. Its precise mechanism is uncertain, but renal vasoconstriction with reduced glomerular filtration rate is a likely explanation.
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Salt retention is not attributed to increased aldosterone; aldosterone is diminished in pre-eclampsia.
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Retained salt increases extracellular fluid volume and sensitivity to angiotensin-II. Arteriolar endothelial swelling may increase peripheral resistance.
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The net effect is intravascular volume contraction with extravascular fluid accumulation, producing hemoconcentration and a raised hematocrit.
Changes after delivery
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Immediately after delivery, sodium and water rapidly move from the extravascular compartment into the intravascular space.
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Failure of this mobilization may cause shock, described as postpartum vascular collapse.
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Plasma volume increases after delivery, with decreases in hemoglobin and hematocrit.
Blood and coagulation abnormalities
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The normal pregnancy-associated increase in blood volume is absent in severe pre-eclampsia. Thrombocytopenia is the most common hematological change, and antithrombin III activity is low.
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The passage calls for PT, PTT and fibrinogen testing when the platelet count falls below 100,000.
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Increased erythrocyte destruction after eclampsia produces hemoglobinemia and hemoglobinuria.
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Disseminated intravascular coagulopathy affects widespread organs and appears to begin with release of thromboplastin into the circulation.
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Platelets, fibrinogen, antithrombin III and plasminogen decrease. Microthrombi involve arterioles in vital organs as well as the placenta, producing organ-specific pathological changes.