Question 1

College Answer

Respiratory complications

• Left lower lobe collapse (poor re-inflation post bypass, phrenic nerve injury)
• Increased pulmonary vascular resistance (protamine)
• Acute lung injury (SIRS)

Cardiovascular complications

• Myocardial stunning or infarction (inadequate myocardial protection)
• Coronary graft ischaemia (air embolism)
• Right ventricular dysfunction (pulmonary hypertension related to protamine)

Hypoperfusion and end-organ ischaemia related to non-pulsatile flow and/or air/atheroma embolism

Neurological complications

• Cerebrovascular events, watershed infarcts, neurocognitive dysfunction (low flow, thromboembolism)
• Phrenic nerve palsy (use of cold cardioplegia ‘slush’)

Renal complications

• Dysfunction related to ischaemia and SIRS

 

Gastro-intestinal complications

• Splanchnic ischaemia (low flow, thromboembolism)
• Hepatitic dysfunction, acalculous / gangrenous cholecystitis, pancreatitis (hypoperfusion, SIRS)

Haematological complications

• Coagulopathy (effects of hypothermia and dilutional coagulopathy, residual heparinisation, activation of coagulation cascade during bypass)
• Anaemia (haemodilution, blood loss in the circuit)
• Platelet dysfunction (bypass circuit)
• Haemolysis (bypass circuit)

Metabolic complications

• Hypothermia (intra-operative cooling and delayed re-warming)
• Insulin resistance and hyperglycaemia (hypothermia)
• Electrolyte abnormalities (haemodilution, post-pump diuresis)

Immune-mediated complications

• Activation of coagulation cascade (blood contact with non-biological surfaces and blood-gas interface)
• SIRS (leucocyte and complement activation, cytokine release and expression of adhesion molecules stimulated by contact with bypass circuit)
• Allergic reactions to protamine

Discussion

There is no more complicated discussion here, other than a rearrangement of the above answer into a different set of headings. The question was relatively straightfrward. The references below illuminate this topic adequately, and if I were pressed for time I would use the UpToDate articles. The poor pass rate suggests to me that many candidates have not had the pleasure of working in a post-op cardiothoracic setting.

In addition to the lazy effort below, I have written my own notes on the complications one might anticipate after cardiac surgery.

Complications due to mechanics of the bypass circuit

• Hemodilution
• Haemolysis
• Blood loss into the circuit

Complications due to SIRS (complement and coagulation cascade activation caused by exposure of blood to circuit components and open surgical wounds)

• Coagulopathy due to coagulation cascade activation and clotting factor depletion
• Platelet dysfunction and platelet depletion
• Renal failure due to SIRS
• Acute lung injury due to SIRS, “pump lung”

Complications due to circuit anticoagulation and its reversal:

• Bleeding from residual heparin
• Heparin-induced thrombocytopenia
• Anaphylaxis to protamine
• Increased pulmonary resistance due to protamine
• Right ventricular failure due to protamine-induced pulmonary hypertension

Complications due to prolonged hypothermia:

• Heart block
• Ventricular and atrial arrhythmias
• Hyperglycaemia (decreased insulin production as well as insulin insensitivity)
• Decreased tubular resorption in the kidney (thus diuresis)
• Phrenic nerve palsy (“cold slush cardioplegia”)

Complications due to prolonged ischaemia

• Renal failure due to ATN
• Myocardial infarction
• Encephalopathy and neurocognitive deterioration
• Hepatic dysfunction and delayed drug clearance
• Pancreatitis
• Splanchnic ischaemia

Complications due to mechanics of the surgery

• Atelectasis (especially of the  LLL) due to mechanical compression

Complications due to intraoperative embolism of clot, air or atheroma

• Myocardial infarction
• Stroke
• Splanchnic ischaemia
• Limb ischaemia

Ray Raper's chapter in Oh's manual (pp.285) had surprisingly little about the complications of bypass. Thankfully, there are exhaustingly complete reference books:

Complications in cardiothoracic surgery: avoidance and treatment, Little A.G, Merril W.H. - 2007, 2^nd ed. Chapter 4 by Creswell and Karis.

Cardiopulmonary bypass- Principles and Practice, Gravlee G.P. and Davis R.F -2007 (3^rd ed.)

These are very thick books to read for just one past paper question.

Thus, and most importantly for the time-poor but dollar-rich ICU public, UpToDate contains a couple of excellent summary pieces on cardiac and non-cardiac complications of CABG.