Question 2

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

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College Answer

Effects related to blood contact with non-biologic surfaces and blood-gas interfaces

Activation of coagulation cascade- consumptive coagulopathy, thromboembolic phenomena, haemolysis,  rarely TTP.
Systemic inflammatory response syndrome due to leucocyte and complement activation, cytokine release  and expression of adhesion molecules- vasodilatory shock, fever, acute lung injury, liver dysfunction,  multiorgan dysfunction.
Platelet dysfunction

Effects related to non-pulsatile flow

Renal dysfunction
Cerebrovascular events, watershed infarcts, neurocognitive dysfunction
Splanchnic ischaemia

Effects related to haemodilution

Dilutional coagulopathy, anaemia.
Electrolyte abnormalities

Effects of hypothermia

Coagulopathy
Decreased tissue oxygen delivery
Insulin resistance and hyperglycaemia

Effects of heparin and protamine

Residual heparinisation leading to bleeding
Increased pulmonary vascular resistance and right ventricular dysfunction from protamine, allergic  reactions to protamine

Effects related to aortic manipulation (cross-clamping and proximal grafts)

Systemic embolisation with potential for neurologic, mesenteric and renal dysfunction.
Difficulty with myocardial protection resulting in postoperative myocardial dysfunction (especially right-sided) due to stunning or infarction

Other

Left phrenic nerve palsy (surgical injury, use of cold cardioplegia “slush”)
Left lower lobe collapse (poor re-inflation post bypass, phrenic nerve injury)

Additional comments:

Candidates who failed did not address complications specific to CP bypass and/or did not describe the underlying mechanisms. Some answers were poorly structured with a tendency to repeat points. The above answer template is not the only way to structure the answer, for example the complications could be classified by body system affected.

Discussion

There are several different ways to answer ths, as has been pointed out by the examiners. The previous (identical) incarnation of this SAQ was Question 1 from the second paper of 2011; in their model answer the college had classified the complications by organ system. Both approaches are available for revision, presented as tables in the chapter on complications attributed to the cardiopulmonary bypass circuit. Below, the revising candidate can see a tabulated form of the alternative college answer from 2011.

Complications of Cardiopulmonary Bypass
Organised According to Organ Systems
Organ System	Complication	Aetiology
Respiratory	Left lower lobe collapse	Phrenic nerve neuropraxia, due to cold slush cardioplegia
	Left lower lobe collapse	Poor reinflation following restoration of circulation
	Pulmonary hypertension	Due to increased pulmonary vascular resistance (protamine)
	Acute Lung Injury	SIRS due to bypass circuit-associated complement activation
Cardiovascular	Myocardial stunning	Due to direct effects of cardiotomy and cardioplegia
	Myocardial infarction	Coronary graft ischaemia (air embolism)
	RV dysfunction	Due to pulmonary hypertension related to protamine
	Arrhythmias	Due to electrolyte disturbances and hypothermia
	Heart block	Due to hypothermia or direct conduction system trauma
	Systemic MODS	Hypoperfusion and end-organ ischaemia related to non-pulsatile flow and/or air/atheroma embolism
Neurological	Stroke	All thought to be due to the sluggish low-flow state following the recommencement of bypass, as well as due to air emboli microemboli and possibly microemboli from the bypass circuit itself
	Watershed infarcts
	Neurocognitive impairment
Electrolytes and Endocrine	Hypothermia	Due to intra-operative cooling and delayed re-warming
	Hyperglycaemia	Due to hypothermia-related insulin resistance
	Hyperglycaemia	Due to circulating endogenous catecholamines
	Electrolyte derangement	Haemodilution
Renal	Post-op diuresis	"Cold diuresis" due to intra-operative cooling and delayed re-warming
	Post-op renal failure	Low flow, and thromboembolic events
	Electrolyte derangement	Haemodilution
Gastrointestinal	Splanchnic ischaemia	Low flow, and thromboembolic events
	Hepatic dysfunction
	Pancreatitis
Haematological	Coagulopathy	Due to consumption of clotting factors by the bypass circuit
		Due to residual anticoagulation
		Due to dilutional coagulopathy
	Platelet dysfunction	Due to antiplatelet agents, and due to the SIRS response
	Anaemia	Due to haemodilution and haemolysis
	Haemolysis	Due to mechanical destruction by the bypass pump, as well as due to MAHA and SIRS
Metabolic	Hypothermia	Due to intra-operative cooling and delayed re-warming
	Hyperglycaemia	Due to hypothermia-related insulin resistance
	Hyperglycaemia	Due to circulating endogenous catecholamines
Immune	Coagulation cascade activation	Due to blood contact with non-biological surfaces and blood-gas interface
	SIRS	Due to complement activation by circuit components
	Anaphylaxis	A reaction to protamine

References

Frederick A. Hensley, Jr., M.D., Donald E. Martin, M.D., Glenn P. Gravlee, M.D. A Practical Approach to Cardiac Anaesthesia, 3rd ed. Sibylle A. Ruesch and Jerrold H. Levy. CHAPTER 9. The Postcardiopulmonary Bypass Period: A Systems Approach. 2003 by LIPPINCOTT WILLIAMS & WILKINS

André, Arthur C. St, and Anthony DelRossi. "Hemodynamic management of patients in the first 24 hours after cardiac surgery." Critical care medicine 33.9 (2005): 2082-2093.

Ray Raper's chapter in Oh's manual (pp.285)

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.)

UpToDate contains a couple of excellent summary pieces on cardiac and non-cardiac complications of CABG.

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