Question 2

Last updated 30/04/2017 - 18:21
 Topic: Cardiothoracic Intensive Care
Pass rate
63%
Other SAQs in this topic:
Other SAQs in this paper:

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

a) 

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

b) 

Effects related to non-pulsatile flow

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

c) 

Effects related to haemodilution

  • Dilutional coagulopathy, anaemia.
  • Electrolyte abnormalities

d) 

Effects of hypothermia

  • Coagulopathy
  • Decreased tissue oxygen delivery
  • Insulin resistance and hyperglycaemia

e)

Effects of heparin and protamine

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

f)

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

g) 

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

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

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

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