Cardiopulmonary Bypass

This provides a broad overview of the process of CPB and the roles of the anaesthetist, surgeon, and perfusionist. Methods of myocardial preservation are covered elsewhere.

Use of an external pump and oxygenator to generate pulseless blood flow and facilitate gas exchange. CPB involves:

Blood Gases

Prior to CPB

Anaesthetist:

  • Ensure both a central and peripheral temperature monitor are in situ if deep hypothermic arrest is going to be performed
  • Prior to sternotomy, ensure:
    • Blood pressure is well controlled
      Reduces the chance of significant haemorrhage from vascular damage.
    • Cease ventilation prior to sternotomy
  • Measure baseline ACT
    Normal is 90-130.
  • Commence TXA infusion
    • Some evidence demonstrating reduced fibrinolysis, blood loss, ICC output, reoperation, and transfusion requirement
    • Many different protocols with wide variability in dosing
      One protocol: 1g bolus then 6.5mg/kg/hr.
  • Complete pre-bypass checklist:

HAD2SAVE: * Heparin given
Confirm administration by verifying back-bleeding from heparin delivery line. * ACT ⩾480
Threshold varies between centres. * Take sample from arterial line, or failing that PA distal on a PAC * Drugs
* Consider additional analgesia * Anaesthesia plan * Consider relaxant
Prevent shivering if cooling. * Vaporiser off * Drips off * Swan * Drugs not being infused down RV port during CPB
Will not be given during bypass, resulting in a bolus of these drugs into the systemic circulation when coming off CPB. * Alarms
Disable alarms. * Ventilator * Swap onto bag and open APL valve * Disable alarms * Emboli
Check arterial cannula for bubbles.

Perfusionist:

  • Prime circuit with bypass solution
    Combination of heparinised crystalloid and colloid.

Instituting CPB

Anaesthetist:

  • Prepare 300 Units/kg heparin
    • Give heparin when requested
      Close loop.
    • Check ACT 3 minutes post UFH
      Confirm ACT >480 prior to cannulation.
  • Reduce SBP to 80-100mmHg to reduce risk of aortic dissection during cannulation
    • Too low and there is ↑ risk of puncturing the posterior wall of the aorta, leading to dissection
    • Too high and there is ↑ risk of aortic dissection from the anterior wall
  • Connects coronary sinus cannula to PA transducer to monitor pressure during retrograde cardiopegia
  • Confirm coronary sinus cannula is correctly sited
    Via TOE. Used for retrograde cardioplegia.
  • Monitor for LV distension with TOE
    May occur with anterograde cardioplegia if there is aortic regurgitation.

Surgeon:

  • Places pericardial stay sutures
    May impair atrial filling and cause hypotension.
  • Places aortic purse-string sutures
  • Places atrial purse-string sutures
    May cause atrial arrhythmia. If unstable:
    • Temporise with vasopressor and volume, then go onto bypass
    • Synchronised DCR
      Clutters surgical field.
  • Places aortic return cannula
  • Places atrial access cannula
    • At this point, bypass may commence although the heart is still beating
    • Access cannula may be
      • Bi-caval
        Two separate cannulas, one to the IVC and one to the SVC. Used if the RA has to be empty for surgical conditions (e.g. atrial myxoma, tricuspid valve surgery).
      • Single, larger dual-stage cannula
  • Places retrograde cardioplegia cannula
  • Places anterograde cardioplegia catheter
    • An LV vent (typically through the right upper pulmonary vein) may be placed if the anterograde cardioplegia causes LV distension
  • Monitors heart as CPB initiated
  • Cross-clamps aorta once on bypass

Perfusionist:

  • Gradually releases clamp on venous line, ↑ bypass flow and reducing right heart CO
  • Left heart empties due to reduced right sided CO
    Significant left sided CO at this stage implies a problem:
    • Obstruction of venous bypass line
    • Significant AR
    • PDA
  • Deliver cardioplegia once aorta is cross clamped
    Typically contain high potassium and magnesium concentrations.
    • Produce asystole when rapidly infused
    • Cold cardioplegia is cardioprotective for ischaemia
    • Cardioplegia may be delivered:
      • Retrograde via the coronary sinus
        Typically retrograde will preserve LV function better than anterograde, especially when there is AR or severe CAD. The same protection does not extend to the RV, as the RV is drained via the anterior cardiac veins.
        • Cardioplegia delivery will cause rise in coronary sinus pressure
      • Anterograde via the coronary arteries

During CPB

Anaesthetist:

  • Cease ventilation

Perfusionist:

  • Deliver nonpulsatile flow of ~2.4L/min/m2 to approximate a normal CI
  • Aim MAP 50-70mmHg
  • Continue anaesthesia
    • Via TIVA
    • Via volatile agent
      Can be vapourised into bypass circuit.
  • Cool to 28-34°C
    Cooler temperatures provide better cerebral protection but require longer times for rewarming.
  • Monitor blood gases and ACT
    Typically every 30 minutes, aiming:
    • Normal gas exchange
    • Hb > 60
    • BE < 2.5
    • ACT >400
  • Continues to deliver cardioplegia as required
    • Typically every 10-15 minutes or at surgeon request
    • May deliver a ‘hot shot’ Warmed cardioplegic blood given as the last bolus prior to rewarming.
      • ↓ chance of immediate post-operative arrhythmia
  • Rewarms patient prior to weaning
    Ensure that central-peripheral temperature gradient is <2°C as this ↑ the risk of neurological complications.
    • Slow down rate of rewarming if this occurs

Prior to Weaning

Complete a pre-wean checklist and confirm:

  • A:
    • ETT is in situ
  • B:
    • Hand ventilate both lungs and visually check for inflation
      Re-establish FRC with a high FiO2 to reduce immediate shunt.
    • Appropriate blood gases on bypass circuit
  • C:
    • Pacing box present
    • De-airing
      Assess for residual air in cardiac chambers if open cardiac surgery performed.
    • Estimate what the patients cardiac state will likely be during wean and aim to optimise each facet:
      • Rate and Rhythm
        Pacing wires in situ and pacing is set. A suggested approach:
        • AOO/AAI or DOO/DDD
        • High amplitudes (e.g. 16mA)
        • High sensing threshold if using a synchronous mode
          ↓ chance of diathermy interference.
      • Preload
      • Afterload
        Start vasopressors or dilators that may be required.
      • Contractility
        Start inotropes that may be required.
    • Reperfusion
      Coronary flow prior to weaning off CPB. 10 minutes per hour of cross-clamp time.
  • D:
    • Confirm BSL 6-10 mmol/L
    • Method for delivery anaesthesia is present
  • E:
    • Temperature >36.5°C
    • Ensure central-peripheral temperature gradient is < 2°C for rewarming
      Slow down rate of separation if this temperature gradient exceeds this.
  • F:
    • pH is 7.35-7.45
    • Potassium 4.5-5mmol/L
  • H:
    • Hb >70
    • Anticipate possible coagulopathy
      Product should only be administered once separated from CPB.
      • Platelets
        May be required if:
        • CKD
        • Long pump run
        • Antiplatelets pre-operatively
        • Reoperation
      • Factors/cryoprecipitate
      • DDAVP
        • CKD
        • von Willebrand disease
        • Platelet function abnormalities

DDAVP (desmopressin) is:

  • A synthetic vasopressin analogue
  • Dosed at 0.3μg/kg
    • Response is variable between patients, but usually consistent for the one patient
    • Tachyphylaxis occurs after 3-5 days
  • Used for a variety of pro-coagulant effects:
    • Release of stored vWF
      Effective for mild quantitative and some qualitative von Willebrand disease.
    • Release of factor VIII
      Effective for mild haemophilia A.
    • ↑ Platelet aggregation
      • ↑ Surface GP receptors
      • ↑ Platelet-dependent thrombin generation

Weaning from CPB

Anaesthetist:

  • Monitor cardiac function as heart is slowly refilled
    Adjust infusions if necessary. Ensure good flow rates of carrier fluid to enable rapid titration and reduce the effect of dead space.
  • Optimise:
    • Preload
      Evaluate with:
      • TOE
      • PADP
      • CVP
    • Contractility
      • Evaluate with:
        • Ventricular performance on TOE Global EF and wall function
        • Rate of rise of the arterial line waveform
        • Visual appearance of heart in chest
      • ↑ with:
        • Inotropes
        • IABP
        • ECMO
    • Afterload Note perfusing pressures (aim SBP <100mmHg during aortic decannulation), and optimise with:
      • Vasodilators
      • Vasopressors
    • Rate
      Usually 80-100, though different physiology may be optimised at different rates.
      • Correct HR >120 prior to weaning
        Usually ↑ preload during wean will reduce HR via atrial stretch.
      • Check epicardial pacing lead function
      • Consider pacing
        Native conduction typically superior.
        • Generally pace 88-90 to improve CO during weaning period.
          Atrial pacing preferred if AV node is functional.
        • DDD 88-90 at 15mA is a safe starting mode
        • Use higher sensitivities (e.g. ~7mV) for diathermy
    • Rhythm
      Defibrillate if required; arrhythmias are common.
      • 10-20J, announce when charged
      • Defibrillate when surgeon instructs
      • May need multiple shocks if persistent VT/VF
      • Pharmacotherapy may be required:
        • IV amiodarone
        • Magnesium
        • Esmolol
  • Reverse heparinisation when fully off bypass
    Give 1mg protamine per 100 units of UFH.
    • Don’t draw up protamine (or hide it somewhere) until reversal is required
    • Monitor for systemic hypotension/pulmonary hypertension, high airway pressures, circulatory arrest
      May need to crash back onto bypass.
    • Give test dose of 1-2ml initially
    • Given slowly, typically 2-3ml/min, titrating to patient blood pressure
    • Alert surgeons when half to 1/3rd of protamine given
      Avoid complete reversal whilst cardiotomy suction is still being used.
  • Check ACT 3 minutes after protamine has finished

Protamine may cause haemodynamic compromise by:

  • True anaphylaxis
  • Direct histamine release from mast cells
    Dose and rate related.
  • Thromboxane release
    Heparin-protamine combination may lead to profound ↑ in PVR and cardiovascular collapse, which may be mistaken for anaphylaxis.

Perfusionist:

  • Progressive clamping of the venous line
    Allows the heart to fill and eject.
    • Partial bypass can be sustained for some time if required

Surgeon:

  • De-air the circulation if required
    i.e. if aorta or cardiac chambers opened.
  • Ensure adequate haemostasis
  • Remove venous access cannula
  • Remove cardioplegia lines
  • Remove aortic cannula

Difficulty Weaning from Bypass

Assess cause; may be:

  • Rate/rhythm
    • Conduction failure
    • Failure of pacing
  • Preload
    • Inadequate volume
    • Tamponade
    • Bleeding
  • Contractility
    • Ischaemia
      Look for new RWMA.
      • Graft occlusion/dysfunction
      • Valve dislodgement
      • Air embolus
    • Myocardial stunning
      • Long bypass time
    • Metabolic
      • Electrolyte abnormalities
        • Residual cardioplegia
      • Acidosis
      • Hypothermia
    • Pulmonary HTN and RV failure
      • Hypoxia
      • Hypercarbia
      • Protamine
  • Afterload
    • Vasoplegia
    • Residual cardioplegia

Check and optimise:

  • Preload
    • Check EDV on TOE and filling pressures CVP, PADP)
    • Give volume
  • Contractility
    • Check systolic function on TOE, CI on PAC, and visual assessment over drapes
    • Give inotropes, consider IABP
  • Afterload
    • Give vasopressors
  • Rate
    Pace if <88.
  • Rhythm
    Restore sinus rhythm.
  • Anaesthetic factors
    • Electrolytes
    • Temperature
    • Gas exchange
  • Surgical factors
    • Absence of major surgical problem
    • Absence of dynamic obstruction
    • Major bleeding controlled
    • Chambers de-aired

After Weaning from CPB

Anaesthetist:

  • Aim SBP 80-140mmHg
  • Maintain high-normal potassium
  • Check ABG, ACT, and TEG post reversal
  • Check pacing
    • VOO or DOO commonly used for weaning
      Change to a safer mode when option for internal defibrillation is no longer available (i.e., during sternal closure).
    • Capture thresholds should be checked
      • <5mA acceptable
      • 5-10mA
        Discuss with the surgeon about:
        • Repositioning
        • Reversing polarity
      • />10mA
        Unacceptably high, should be replaced due to high risk of post-operative failure.

Complications

Use of bypass is associated with several complications including:

  • A
    • Air emboli
      • CVA
      • CAGE
  • C
    • Myocardial stunning
      EF is reduced immediately post-bypass.
    • Dysrhythmias
    • Impaired organ perfusion, leading to post-bypass:
      • AKI
      • Post-pump vasoplegia
  • D
    • ↑ risk of anaesthetic awareness
  • H
    • Coagulopathy

Key Studies

  • TRICS-III (2017)
    • 5092 non-pregnant, non-lactating, adult standard cardiac surgical patients (not heart transplants or VAD) with EUROSCORE I >6
    • Randomised, allocation concealed, assessor blinded, multicentre (73), international RCT
    • 90% power for 3% non-inferiority margin for composite of death, MI, inpatient RRT
    • Restricted vs. liberal transfusion
      • Restrictive
        Transfused if Hb <75g/dL.
      • Liveral
        Transfused if Hb <95g/dL intraoperatively or in ICU, or <85g/dL once discharged from ICU.
    • No difference in primary outcome (11.4% vs. 12.5%), or any secondary outcomes
      • Subgroup showed restrictive transfusion ↓ primary outcome in >75 year olds
    • ~50% of the liberal group were transfused an average of 2 units, compared to 7~3% of the liberal group (3 units)
  • ATACAS (2017)
    • ~4600 at-risk CABG patients
    • Multicentre (22) block randomised RCT, stratified by site and on-pump vs. off-pump surgery
    • 90% power for 3% ARR from a 10% composite of death and thrombotic events (MI, CVA< PE, AKI, bowel infarction)
    • 2-by-2 factorial design
      • Aspirin vs. placebo
        • Aspirin 100mg 1-2 hours pre-surgery
        • Aspirin armed discontinued eraly due to higher-than-expected even rate
      • TXA vs. placebo
        • TXA initially 100mg/kg 30 minutes post-induction, ↓ to 50mg/kg partway due to seizure concern
    • No change in primary outcome for TXA (16.7% vs. 18.1%) or aspirin (19.3% vs. 20.4%)
    • Significantly ↓ return to theatre (1.4% vs 2.8%) and blood product use with TXA
    • No significant change in return to theatre or blood product use with aspirin
    • ↑ Seizures in TXA group (0.75% vs. 0.1%)

References

  1. Melanson, P. Management of post-op cardiac surgery patients. 2001. McGill.
  2. Koster A, Faraoni D, Levy JH. Antifibrinolytic Therapy for Cardiac Surgery: An Update. Anesthesiology. 2015 Jul;123(1):214-21.
  3. Eichhorn EJ, Diehl JT, Konstam MA, Payne DD, Salem DN, and Cleveland RJ. Protective effects of retrograde compared with antegrade cardioplegia on right ventricular systolic and diastolic function during coronary bypass surgery. Circulation. 1989;79:1271-1281, originally published June 1, 1989
  4. Mazer CD, Whitlock RP, Fergusson DA, et al. Restrictive or Liberal Red-Cell Transfusion for Cardiac Surgery. New England Journal of Medicine. 2017;377(22):2133-2144. doi:10.1056/NEJMoa1711818