What Has Blood Got To Do With Babies?

21.3.06

Normal haemostasis
• 3 players:
1. Blood vessel.
2. Platelets - form initial plug.
3. Coagulation factors - form fibrin clot.

Bleeding from injured vessel will cease if
1. Internal pressure = external pressure
eg. increased blood surrounding tissue → increased vasoconstriction → increased hypotension (blood pressure decreased).
2. Hole blocked by solid material
ie. platelet plug → fibrin clot.

Coagulation of blood
Essential reaction: Soluble fibringogen I ==> [Prothrombin IIa] ==> Fibrin clot Ia

Physiological changes in coagulation in pregnant women
Increased clotting factors
• Including Factor VIII.

Reduced coagulation inhibitors
• Protein S.
• Activated protein C resistance.

Factors unchanged
• Antithrombin.
• Protein C levels (functional impairment).

Blood transfusion - haemolytic disease of the newborn.

Blood group systems
• ABO antigens.
-Glycoproteins.
-Same protein backbone (15 amino acids).
-Variable terminal sugars:
§L-fucose "H."
§L-fucose + N'acetyl galactosamine "A."
§L-focose + D galactose "B."
• A and B genes code transferases for sugar addition.
• ABH antigens increase in strength from 0-3 years.
• Isoagglutinins (anti A, anti B) present from 6 months.

Mortality and morbidity of ABO incompatibility - intravascular haemolysis
• Death ~ 10%.
• Irreversible renal failure ~ 30%.
• Renal failure ~ 30%.
• Asymptomatic ~ 30%.

Rhesus system
• Rh antigens are proteins in red cell membranes. ? Function.
• 3 closely-linked genes.
-D (d).
-C (c).
-E (e).
• Most immunogenic and clinically significant = D.
• Haemolytic disease of newborn (HDN) - delayed transfusion reaction.
• Rh (D) group (anti-D and red cells) SPIN.
Positive }
} control
Negative }

Antenatal screening
• Booking clinic (12 weeks).
-ABO.
-Rh (D).
-Antibody screen (indirect).
• Third trimester (28 weeks).
-Repeat for all.
• More frequently for some.

Anti-D prophylaxis
• Offered to all Rh (D) negative women.
• 500 IU anti-D immunoglobulin at 28 weeks.
• 500 IU anti-D immunoglobulin at 34 weeks.
• 500IU anti-D immunoglobulin after delivery.

Red cell transfusion indications - avoid wherever possible
• Peri-operative loss >1,000ml.
• Depends on haemoglobin (Hb) pre-op.
• Depeneds on cardiovascular status.
• Tx Hb at 10G/dl - WRONG!

Pre-compatibility testing - electronic crossmatch
• ABO/Rh (D) group patient (recipient) red cells.
• Antibody screen patient plasma (IAT + enzyme).
• ABO/Rh (D) group red cells.
• Major crossmatch (IAT).