32. Blood Flashcards
Intro
Blood transfusion may be life-saving, but it is not without risk. Many issues may be covered in an examination question including composition and storage of blood, risks and benefits of transfusion, effects of massive blood transfusion, infective and immunological effects of blood transfusion, and blood conservation strategies.
Describe the composition and storage of blood
plasma
rbc
plt
what is done next
what about immunocomp patients
changes in stored
Whole blood is separated into red cells, plasma and platelets via centrifugation.
Once separated plasma
must be frozen as soon as
possible to under –18 °C,
red cells must be stored at 1–6 °C
and platelets must be stored at room temperature 20–24 °C) on shaking platforms (lifespan 3–5 days).
Leucodepletion is then performed in order
to remove the white blood cells via filtration.
Leucodeplete blood minimises the
chances of alloimmunisation,
reduces the incidence of
febrile transfusion reactions
and reduces the chances of
cytomegalovirus (CMV) transmission.
Severely immunosuppressed
patients require irradiated blood
in order to prevent donor T-lymphocytes dividing in the recipient, which can result in transfusion and associated graft-versus-host disease.
Stem cell transplant (immunosuppressed)
patients will also require CMV-negative blood.
The following changes occur in stored blood:
> Fall in 2,3 DPG levels
> Left shift of the oxyhaemoglobin dissociation curve
> Fall in pH (to approximately 7.0)
> Rise in potassium concentration
Storage solutions include:
> Acid-citrate-dextrose: Citrate acts as an anticoagulant by binding calcium while dextrose is an energy source for glycolysis. Red cell survival is 21 days.
> Citrate-phosphate-dextrose:
Increases red cell survival to 28 days.
> Citrate-phosphate-dextrose-adenine:
The addition of adenine
increases red cell AT P thereby
increasing red cell survival to 35 days.
> Saline (140 mmol/L)-adenine (1.5 mmol/L)-glucose (50 mmol/L)- mannitol (30 mmol/L):
Allows a greater volume of plasma to be
removed from blood in order to use plasma for coagulation factors.
Discuss the risks and benefits
of transfusion.
The principal benefit of red cell transfusion
is the improvement in oxygencarrying
capacity by increasing haemoglobin levels.
Raising the haematocrit
has also been shown to
improve microcirculation in
trauma and septic patients.
However, there are multiple risks and complications:
1 > Haemolytic transfusion reaction, e.g. life-threatening ABO incompatibility versus delayed haemolysis to minor red cell antigens
2
> Febrile reactions
3 > Infection transmission: HIV (1 in 4 000 000), hepatitis A, hepatitis B, hepatitis C, malaria, variant CJD, CMV, syphilis, HTLV
4
> Metabolic:
hyperkalaemia and hypocalcaemia
5
> Hypothermia:
transfusion of cold stored blood
6
> Circulatory overload:
cardiac failure
7 > Immune: graft-versus-host disease and possible increase in colorectal tumour recurrence rates
8
> Transfusion-related acute lung injury (TRALI):
incidence has fallen since the introduction of leucocyte-deplete blood
9
> Iron overload in chronic transfusion
10
> Septic shock from
bacterial contamination of donor blood
11
> Impaired coagulation:
If red cells are transfused rapidly in large
volumes the liver’s ability to clear the citrate may be overwhelmed and coagulation may become impaired.
What constitutes a massive
transfusion?
> There are many different definitions
of what constitutes a massive transfusion,
but they all define a volume of
transfusion and a time period,
e.g. 10 units of blood within 6 hours
or replacement of entire circulating
volume with transfused blood within 24 hours.
> The aim of treatment is the
rapid and effective restoration of an adequate
blood volume to maintain blood composition within safe limits with regards to haemostasis, oxygen-carrying capacity, oncotic pressure and
biochemistry.
What are the problems
encountered during massive
transfusion?
Massive blood transfusion encompasses all of the risks of individual blood unit transfusion listed above but also has other specific problems:
1
> Thrombocytopenia:
2
> Coagulation factor depletion:
3
> Oxygen affinity changes:
4.
> Hypocalcaemia:
5
> Hyperkalaemia:
6
> Acid–base disturbances
7
> Hypothermia
8.
> ARDS
> Thrombocytopenia:
> Thrombocytopenia: Dilutional thrombocytopenia is inevitable following massive transfusion as platelet function declines to zero after only a few days of storage.
It has been shown that at least 1.5 times
blood volume must be replaced
for this to become a clinical problem.
However, thrombocytopenia can occur following smaller transfusions if disseminated intravascular coagulation (DIC) occurs or there is preexisting thrombocytopenia.
> Coagulation factor depletion:
> Coagulation factor depletion:
Stored blood contains all coagulation
factors except V and VIII.
Production of these factors is increased
by the stress response to trauma.
Therefore, only mild changes in coagulation
are due to the transfusion per se,
and supervening DIC is more likely
to be responsible for disordered haemostasis.
DIC is a consequence of delayed
or inadequate resuscitation,
and the usual explanation
for abnormal coagulation indices
out of proportion to the volume of blood
transfused.
> Oxygen affinity changes:
> Oxygen affinity changes:
Massive transfusion of stored blood
with high oxygen affinity adversely
affects oxygen delivery to the tissues.
However, 2,3 DPG levels rise rapidly following transfusion and normal oxygen affinity
is usually restored in a few hours.
> Hypocalcaemia:
> Hypocalcaemia:
Each unit of blood contains approximately
3 g citrate, which binds ionised calcium.
The healthy adult liver will metabolise 3 g
citrate every 5 minutes.
Transfusion at rates higher than one unit every
5 minutes
or impaired liver function may thus lead to citrate toxicity and hypocalcaemia.
> Hyperkalaemia:
> Hyperkalaemia:
The plasma potassium concentration of stored blood
increases during storage and
may be over 30 mmol/L.
Hyperkalaemia is generally not a problem
unless very large amounts of blood are given
quickly.
On the contrary,
hypokalaemia is more common as red cells begin active metabolism and intracellular uptake of potassium restarts.
> Acid–base disturbances
> Acid–base disturbances:
Lactic acid levels in the blood pack give
stored blood an acid load of up to 30–40 mmol/L.
This, along with citric acid,
is usually metabolised rapidly.
Indeed, citrate is metabolised to bicarbonate,
and a profound metabolic alkalosis may ensue.
The acid–base status of the recipient is
usually of more importance,
final acid–base status being
dependent on tissue perfusion,
rate of administration and
citrate metabolism.
> Hypothermia
> Hypothermia:
Leads to reduction in citrate
and lactate metabolism
(leading to hypocalcaemia
and metabolic acidosis),
increase in affinity of haemoglobin for oxygen,
impairment of red cell deformability,
platelet dysfunction
and an increased
tendency to cardiac dysrhythmias.
> Acute respiratory distress syndrome (ARDS):
> Acute respiratory distress syndrome (ARDS):
The aetiology of ARDS is as
yet not fully understood,
but various risk factors have been identified.
Both under– and over-transfusion are associated with an increased risk of ARDS,
as is albumin <30 g/L.
Microaggregate filters should be
used during massive transfusion
except when giving fresh
whole blood or platelets.
During massive blood transfusion, specific component therapy is required,
During massive blood transfusion,
specific component therapy is required,
namely FFP 12 mL/kg,
to provide coagulation factors,
platelets (keep
count >50 000),
cryoprecipitate for fibrinogen replacement
(if fibrinogen <1.0 g/L).
In major haemorrhage blood products are generally given in a 1:1:1 ratio.
That is one unit packed
red cells:one unit of FFP :one unit of platelets.
It should be noted that one
pool of platelets is made up
of the platelets taken
from five donated units of blood.
Hence, platelets should be given after every
10 units of blood products
(i.e. five units red cells + five units FFP + one pool
platelets).
Calcium levels are checked after every
10 units and levels should
be kept in the normal range.
If blood products are given in the
ratios above and patients are kept warm
coagulation normally remains effective.
In some cases of major haemorrhage recombinant factor (rF) VII may be required to boost coagulation.
