Haematology and Immunology - Sickle Cell Disease

Question 1

What is sickle cell disease?

Answer 1

Hb has 4 polypeptide chaints, each with a covalently bound haem moiety, which has a porphyrin ring with a central iron atom, usually in ferrous state (Fe²⁺).
96% is HbA1 in adults, with 2 α and 2 β chains.

Sickle cell is genetic, switching an adenine for thymine, with a resultant amino acid substitution from glutamic acid to valine at position 6 on the β chain - HbS.

Homozygous (HbSS) causes sickle disease, Heterozygous (HbAS) causes sickle trait.

HBS polymerises at low oxygen tensions, making RBC stiffer & more fragile.
Increased viscosity, with microclots. Shortens RBC life to 5-15 days rather than 120, with a haemolytic anaemia.

Sickledex test exposes red cells to sodium metabisulphite - induces sickling in HBS, but doesn’t distinguish betweein trait/disease.
If +ve, then HPLC test needed, or electrophoresis (High Performance Liquid Chromatography)

Question 2

What factors can induce sickling?

Answer 2

Hypoxia
HbSS sickles at 85%
HbAS sickles at 40%
Hypothermia
Acidosis
Dehydration
Pooling of blood or stasis (Low CO or hypovolaemia)

Question 3

What are the anaesthetic considerations of sickle cell disease

Answer 3

Avoid:
Hypoxia
Preoxygenate & supplement O2 for minimum 24hrs post-op
Shift oxygen dissociation curve to the left (slight hyperventilation can help)

Dehydration
Aim for haematocrit < 0.35
Consider Transfusion to aim Hb >100. Crossmatch may take longer.

Hypothermia
Active warming

Infection
Functional hyposplenism - at risk from encapsulated bacteria, more likely to need surgical prophylaxis - already likely to be on Penicillin V long term.

If the patient has had transfusions in other countries, be aware of BBV