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.
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
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.