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

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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