Determinants of haemostasis in CPB

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Given differences in bleeding rates and definitions of what is normal, excess bleeding can probably be defined as > 1 litre per procedure. There is twice the risk of bleeding after valve surgery than after CABG, and repeat operations redouble this risk; significantly, 19% of CABG patients need re-grafting within 10 years. Bleeding is usually manifest postoperatively, after protamine reversal ofheparin, and shed from the operative field into mediastinal and pleural drains. If aspirin or other anti-platelet therapy has been given, the operation may be ''wet'' from the start.191--193

Critical rates of blood loss, formulated by Kirklin and Barrett-Boyes,194 are:

500 ml in the first postoperative hour, 400 ml/h in the first 2 hours,

300 ml/h in the first 3 hours, 1 litre in 4 hours, or 1.2 litre in 5 hours.

If bleeding attains these rates, is acute and massive, or begins again after ceasing, resternotomy becomes unavoidable. Volume perse can mislead if there is progression from blood to serosanguinous drainage; the haematocrit of the fluid in the drain tubing (not the reservoir) may indicate that blood loss is being overestimated if the patient is otherwise stable.

The need for re-sternotomy entails 30% increase in perioperative mortality and, therefore, is a crucial endpoint in CPB studies. In 67% of cases bleeding vessels are found, often small medistinal arteries or the aortotomy incision; such vessels might not bleed if haemosta-

sis were normal. In the remainder, general microvascular oozing is

seen.195

Allogeneic RBC transfusion post-CPB shows wide, apparently irrational, variance between centres. Some centres transfuse RBC to fewer than 1/3 of standard risk CPB patients, others to more than 2/3 despite access to evidence-based guidelines, with similar inconsistency in platelet and fresh frozen plasma use.196--201

Withholding RBC unless the systemic haematocrit fell to < 25% (Hb < 8g/dl), post-CPB, had no adverse clinical or physiological impact in standard-risk patients. Observing this threshold is likely to reduce allogeneic transfusion. The role of cardiac surgeons and anaesthetists in UK hospital transfusion committees will be vital as a means of audit and effective guidelines in this area.196--201 Preoperative determinants of haemostasis in CPB:191--193

1. Bleeding increases if aspirin continues up to surgery. This effect is eliminated if aspirin is stopped 7 days before and restarted 1 to 6 hours after surgery. If aspirin cannot be stopped, haemostasis should be enhanced by anti-fibrinolytic therapy.

2. The calcium antagonist nimodipine was associated with excess post-CPB bleeding in one report.

3. Coumarin anti-coagulants (e.g., in transplantation when a donor heart arrives too suddenly to omit warfarin) require replacement therapy with prothrombin complex concentrate (PCC) containing factor VII if INR > 1.7.

4. Coronary angioplasty/stenting with the hybrid anti-GpIIb/IIIa monoclonal agent abciximab (c7E3, ReoPro) may need urgent conversion to CABG. Intra- and postoperative bleeding occurs, particularly if the interval between abciximab and CPB is < 12 hours or if standard doses of heparin are used for CPB. Reducing heparin (to ACT 400s) with postoperative (± preoperative) platelets is one approach. Others suggest aprotonin with platelet transfusion (6 units) given at the end of the bypass. True and pseudo-thrombocytopenia can occur after abciximab therapy and must be distinguished from heparin-induced thrombocytopenia.

5. Patients with congenital heart disease may acquire a deficiency of high-molecular weight von Willebrand's factor, which rarely poses a problem, as it corrects immediately post-surgery. Right-to-left shunts can lead to increased platelet size with misleading automated counts (check the blood film). Children with Noonan syndrome may have coagulation factor deficiencies and can bleed during surgery for heart defects; they need a haemostatic work-up before cardiac surgery.191--193

6. Patients with haemophilia may require CPB; the safest operative cover in all cases is high-purity or recombinant factor VIII:C or IX:C. In von Willebrand's disease a product with a reliably high content of high-molecular-weight multimers should be used. All such patients should be managed at centres with comprehensive expertise in haemophilia.191--193

7. Preoperative thrombocytopenia compounds the bypass-induced platelet function defect. The minimum acceptable preoperative platelet count is 100 x 109/l. In patients with ITP, administration of steroids and or IVIG might be required.191--193

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