Cardiopulmonary Bypass

OVERVIEW

• Cardiopulmonary bypass (CPB) replaces the function of the heart and lungs while the heart is arrested to provide a bloodless, stable surgical field

COMPONENTS OF CPB

• Cannula is inserted into right atrium to drain venous return
• Venous blood passes into venous reservoir under gravity
• Oxygenated (and CO₂) removed usually by membrane oxygenator
• Heat exchanger controls blood temperature
• Surgery often performed with cooling to ~28-34C
• A 40 mm filter removes air bubbles
• Pump (non-pulsatile flow of 2.4L/min/m2) returns blood into aorta distal to a cross clamp
• Suction used to remove blood from operative field
• Returned to patient via cardiotomy reservoir

ACTIONS

• oxygenation via increasing FiO2
• removal of CO2 via increasing gas flow
• non-pulsatile circulation of blood

BYPASS PROCEDURE

Preparation for bypass

• full anticoagulation (heparin 300IU/kg, ACT >400s)
• bypass circuit primed with crystalloid, heparin +/- mannitol
• just before aortic cannulation -> reduce SBP to 80-100mmHg
• prepare and pressurize cardioplegia to 300mmHg (bubble free circuit if cold 4C crystalloid used) -> e.g. K 20mmol/L, Mg 16mmol/L and procaine, can be administered anterograde or retrograde

During bypass

• turn off ventilator (allow lungs to deflate and clear surgical field)
• propofol 6mg/kg/hr OR midazolam OR volatile agent on bypass machine
• MAP maintained at 50-70mmHg by altering SVR
• volume can be added or removed by ultrafiltration
• pressure maintained with vasopressors and vasodilators
• ABG + ACT Q30min
• hypothermia to 28-34 C used

Coming off bypass

• warm to 37 C
• K 4.5-5.0
• HCT >20%
• normal acid-base status
• HR 70-100/min (ideally SR)
• 100% O2
• venous line progressively clamped and heart gradually fills
• -> start inotropes if inadequate cardiac output
• protamine once surgeon happy (3mg/kg)
• restart volatile and opioid

ADVERSE EFFECTS OF BYPASS

Adverse effects result from:

• cannulation and cross-clamping
• exposure to the CPB circuit
• ischemia, emboli and hypoperfusion
• accessing the chest and collapsing the left lung
• exposure to drugs and blood products
• hypothermia

HAEMATOLOGICAL

• activation of coagulation, complement, kallikrein system
  -> fibrinolysis, decreased platelet function
  -> bleeding
  -> transfusion risks
• platelet aggregation and thrombocytopaenia
• coagulopathy due to dilution, hypothermia, acidosis, drugs and platelet dysfunction)
• anaemia (blood loss from circuit, hemolysis)
• systemic heparinisation and exposure to protamine

CENTRAL NERVOUS SYSTEM

• emboli (gas, debris, calcium)
• poor cerebral perfusion
  -> ischaemia, CVA (watershed or focal), seizures, spine ischemia, long-term cognitive impairment (1-5%)
• phrenic nerve palsy

RENAL

• AKI -> oliguria, elevated creatinine
• renin-angiotensin-aldosterone activation and vasopressin release -> oliguria
• post-bypass diuresis (also hypothermia)

HYPOTHERMIA

• myocardial depression
• arrhythmias
• diuresis
• altered pharmacokinetics

CARDIOVASCULAR

• SIRS response involving activation of neutrophils, complement, kallikrein system and release of TNF -> vasodilation, hypotension and tachycardia
• RV dysfunction due to protamine induced pulmonary hypertension
• myocardial stunning
• myocardial ischemia due to graft failure, air embolus, non-pulsatile flow or hypotension
• injury or dissection of vessels from cannulation (aorta, femoral)
• leukocyte release -> capillary leakage
• systemic microemboli -> can affect any organ (ischemia, MODS)

RESPIRATORY

• ALI/ ARDS secondary to SIRS, neutrophil activation and decreased surfactant
• left lower lobe collapse due to failed lung re-expansion and/or phrenic nerve palsy
• increased PVR due to protamine
• neutrophil activation -> pulmonary dysfunction

GASTROINTESTINAL

• ischemic gut, acalculous cholecystitis, hepatitis, or pancreatitis

METABOLIC

• hypothermia and glucagon release -> unsulin resistance and hyperglycaemia
• electrolyte disturbance

IMMUNE

• allergy (e.g. to protamine)
• SIRS response due to exposure to bypass circuit leading activation of WBCs, complement, the clotting cascade, cytokines and cell adhesion molecules

OTHER

• usual complications of surgery and anaesthesia
• bypass catastrophes

BYPASS CATASTROPHES

• supply failure -> pump stops working
• inadequate anticoagulation -> circuit clots
• oxygenation failure -> hypoxaemia and ischemia
• disconnection, empty reservoir -> RV distension, increased PAP -> cardiovascular collapse
• gas emboli: into system circulation
• aortic dissection: renal failure, bowel ischaemia, paraplegia, cardiac tamponade, limb ischemia, stroke
• dislodgement of cannulae: unable to get appropriate flow, increased circuit pressures
• occlusion: high pressures/low flows

References and Links

• Favaloro RG. Landmarks in the development of coronary artery bypass surgery. Circulation. 1998 Aug 4;98(5):466-78. Review. PubMed PMID: 9714098. [Free Full Text]
• Machin D, Allsager C. Principles of cardiopulmonary bypass. Contin Educ Anaesth Crit Care Pain (2006) 6 (5): 176-181. doi: 10.1093/bjaceaccp/mkl043 [Free Full Text]
• Mota AL, Rodrigues AJ, Evora PR. Adult cardiopulmonary bypass in the twentieth century: science, art or empiricism?. Rev Bras Cir Cardiovasc. 2008 Jan-Mar;23(1):78-92. PMID: 18719832.
• Murphy GS, Hessel EA 2nd, Groom RC. Optimal perfusion during cardiopulmonary bypass: an evidence-based approach. Anesth Analg. 2009 May;108(5):1394-417. PMID: 19372313.
• Sniecinski RM, Chandler WL. Activation of the hemostatic system during cardiopulmonary bypass. Anesth Analg. 2011 Dec;113(6):1319-33. PMID: 22003219.