List the complications and their likely underlying mechanisms specifically related to cardiopulmonary bypass that may be seen in the Intensive Care Unit following cardiac surgery.

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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

References

References

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, 2nd ed. Chapter 4 by Creswell and Karis.

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