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.

CCC 700 6

Critical Care

Compendium

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