2.7 Anticoagulation and bypass

Question 1

A 67-year-old patient is to undergo coronary artery surgery on cardiopulmonary bypass (CPB).
a) What dose of heparin is used to achieve full anticoagulation for CPB and how is it given? (2 marks)

Answer 1

> > Check patient’s
baseline activated clotting time (ACT)
via arterial sample.

> > Give 300–400 iu/kg via a
central venous cannula (CVC)
(check line patency first).

> > Take another arterial sample
after three to five minutes.

> > Ensure ACT is 3× baseline or
greater than 480 before initiating CPB.

> > Recheck every 30 minutes during CPB.

Question 2

b) Which laboratory and ‘point-of-care’ tests determine the effectiveness of heparin anticoagulation in
CPB patients?

Give the advantages and/or disadvantages of each test. (10 marks)

Answer 2

1&raquo_space; Activated partial thromboplastin time (APTT),
lab test:
Cheap test.
Slow turnaround time,
which may result in less-well-directed management.

2&raquo_space; Anti-Xa Assay, lab test:
Not widely used for this purpose,
poor inter-laboratory correlation.

3 &raquo_space; ACT, point-of-care (POC):
Rapid information, cheap, familiar.
Thrombocytopenia, antiplatelet agents, hypothermia,
haemodilution, aprotinin may all prolong ACT.

ACT has poor correlation with clinical anti-Xa activity.

4&raquo_space; Heparin concentration monitoring, POC:
Measuring of heparin concentration
once haemodilution has occurred with CPB
may be more appropriate to
direct heparin administration than ACT,
which is prolonged by commencing CPB.

Higher doses of heparin will therefore
be required when using heparin concentration measurement.

Expensive, not widely available.

Question 3

c) What are the causes of inadequate anticoagulation in a patient whom it is believed has already
received heparin? (5 marks)

Answer 3

Error:
» Wrong drug administered.
» Drug not given.
» CVC not patent.
» CVC not flushed after dose given.

Pharmacokinetic factors:
Heparin is highly protein bound, so an increase in the presence of certain proteins reduces free and therefore active amount:
» Acutely ill patients.
» Malignancy.
» Peri- or post-partum.

Lack of antithrombin III:
» Drug induced: recent heparin use.
» Accelerated consumption: DIC, sepsis.
» Dilution: CPB.
» Decreased synthesis: liver cirrhosis.
» Increased excretion: protein-losing states.
» Familial: 1/2,000–20,000.

Question 4

d) Describe the possible adverse reactions to protamine. (3 marks)

Answer 4

> > Arterial hypotension.

> > Reduced CO.*

> > Pulmonary vasoconstriction.*

> > Anaphylaxis.*

> > Unbound protamine inhibits
platelet reactivity,
adhesion and aggregation.
An excessive dose therefore promotes bleeding.