Cardiopulmonary Bypass Cannulation

Quick Answer

Cardiopulmonary bypass (CPB) cannulation establishes extracorporeal circulation for cardiac surgery. Arterial cannulation (ascending aorta 95%, femoral 3%, axillary 2%) delivers oxygenated blood (cannula size: 20-24 Fr, flow 2.0-2.4 L/min/m²). Venous cannulation uses single dual-stage (right atrium) or bicaval (SVC/IVC) cannulae (28-32 Fr) to drain deoxygenated blood. Systemic heparinization (300-400 IU/kg, target ACT >480 seconds) is mandatory. Cardioplegia cannulation delivers high-potassium solution to arrest the heart (antegrade via aortic root or direct coronary ostia, retrograde via coronary sinus). Key physiological changes on CPB: non-pulsatile flow, hemodilution (hematocrit 20-25%), hypothermia (28-34°C), activation of inflammatory cascades, and potential for end-organ dysfunction. Cerebral protection strategies include α-stat pH management (cooling) and avoidance of cerebral emboli (aortic cannula positioning, filtering). Complications include aortic dissection (0.5-1%), air embolism, inadequate flow states, and coagulopathy. [1-10]

Pathophysiology

Principles of Extracorporeal Circulation

Purpose of Cardiopulmonary Bypass:

• Provide systemic perfusion while the heart is arrested
• Enable cardiac standstill for surgical repair
• Allow temperature modulation (hypothermia for organ protection)
• Facilitate decompression of the heart during surgery

Physiological Changes on CPB:

1. Non-Pulsatile Flow:

• Continuous (non-physiological) arterial flow
• Reduced vascular resistance initially, then increased
• Altered organ perfusion patterns
• Reduced renal perfusion despite "normal" flow rates
• Splanchnic vasoconstriction

2. Hemodilution:

• Priming volume: 1.5-2.0 L crystalloid/colloid in circuit
• Hematocrit on CPB: 20-25% (dilutional anemia)
• Effects:
  • Reduced oxygen-carrying capacity
  • Improved microcirculatory flow (reduced viscosity)
  • Reduced transfusion requirements (if Hct >20%)
  • Risk of inadequate oxygen delivery if Hct <18%

3. Hypothermia:

• Mild (32-34°C): Most routine cardiac surgery
• Moderate (28-32°C): Complex surgery, aortic work
• Deep (20-28°C): Aortic arch surgery, circulatory arrest
• Effects:
  • Reduced metabolic rate (7% per °C decrease)
  • Increased blood viscosity
  • Left-shifted oxyhemoglobin curve
  • Impaired coagulation (platelet dysfunction, enzyme inhibition)
  • Arrhythmia risk (ventricular fibrillation <28°C)

4. Systemic Inflammatory Response:

• Contact activation: Blood-biomaterial interaction (circuit, cannulae)
• Complement activation: C3a, C5a anaphylatoxins
• Cytokine release: IL-6, IL-8, TNF-α
• Leukocyte activation: Adhesion, degranulation
• Coagulation cascade activation: Despite heparin
• Clinical impact: Capillary leak, organ dysfunction, coagulopathy

5. Endocrine Changes:

• Stress response: Cortisol, catecholamines, ADH increase
• Insulin resistance: Hyperglycemia common
• Thyroid suppression: T3 reduction (sick euthyroid)

6. Coagulation Abnormalities:

• Platelet dysfunction: Activation, consumption, hemodilution
• Clotting factor depletion: Consumption, hemodilution
• Fibrinolysis: tPA release from endothelium
• Heparin effect: Prolongs ACT, APTT
• Hypothermia: Impairs coagulation enzyme function

Aortic Cannulation Physiology

Site Selection:

Ascending Aorta (95% of cases):

• Advantages: Direct, familiar, good flow characteristics
• Technique: Partial-occlusion clamp, arteriotomy, cannula insertion, purse-string sutures
• Cannula types: Straight, angled, side-arm for cardioplegia
• Size: 20-24 Fr (adult), flow rates up to 6 L/min

Femoral Artery (3% of cases):

• Indications: Redo surgery (adhesions), ascending aortic pathology, minimally invasive
• Technique: Groin cutdown or percutaneous, guidewire, dilators, cannula insertion
• Complications: Limb ischemia, dissection, atheroemboli

Axillary/Subclavian Artery (2% of cases):

• Indications: Aortic dissection, severe aortic atheroma, minimally invasive
• Advantages: Antegrade flow, less atheroembolic risk
• Technique: Right axillary artery via deltopectoral groove

Flow Dynamics:

• Target flow: 2.0-2.4 L/min/m² (cardiac index equivalent)
• Arterial line pressure: 150-250 mmHg (indicates cannula resistance, not MAP)
• Cerebral perfusion: Dependent on MAP (50-80 mmHg target), temperature, pH strategy
• Cannula malposition: Jet flow causing aortic dissection or inadequate cerebral perfusion

Venous Cannulation Physiology

Single Two-Stage (Right Atrial) Cannula:

• Design: Proximal ports (atrial) + distal ports (IVC)
• Advantages: Single cannula, simpler, adequate for most CABG
• Limitations: Incomplete IVC drainage (collateral flow), right heart distension risk
• Placement: Tip in IVC, side holes in right atrium

Bicaval Cannulation (SVC + IVC):

• Indications: Intracardiac surgery (valves, septal defects), right heart procedures
• Advantages: Complete drainage, right heart empty, prevents air entry
• Technique: Direct caval cannulation with snares (caval tapes) to isolate right heart
• Caval snares: Prevent blood return to right heart during intracardiac work

Venous Drainage:

• Siphon drainage: Gravity-driven (conventional, requires large venous tubing)
• Vacuum-assisted: Negative pressure (-20 to -60 mmHg) improves drainage, allows smaller cannulae
• Kinetic assist: Centrifugal pump on venous side (less common)

Cardioplegia Delivery

Cardioplegia Principles:

• High potassium (10-30 mmol/L): Depolarizes cardiac cell membrane, arrests heart in diastole
• Hypothermia (4-10°C): Reduces metabolic demand, myocardial protection
• Oxygen: Maintains aerobic metabolism (blood cardioplegia)
• Substrates: Glucose, amino acids, buffers for myocardial protection

Antegrade Cardioplegia:

• Aortic root: Via side-arm of arterial cannula or separate needle
• Direct coronary ostia: After aortotomy, handheld cannulae into coronary ostia
• Flow: 200-300 mL/min, pressure 60-80 mmHg (aortic root), 40-60 mmHg (direct ostia)

Retrograde Cardioplegia:

• Coronary sinus cannulation: Via right atrium, self-inflating balloon catheter
• Advantages: Distributes to areas supplied by occluded arteries, better right heart protection
• Flow: 150-250 mL/min, pressure 20-40 mmHg (balloon prevents excessive pressure)
• Contraindications: Coronary sinus abnormalities, severe TR

Clinical Presentation

Preoperative Assessment

History:

• Previous cardiac surgery (redo = increased risk)
• Aortic pathology (aneurysm, dissection, calcification)
• Peripheral vascular disease (femoral access concerns)
• Anticoagulation status
• Bleeding history

Physical Examination:

• Cardiovascular: Murmurs (aortic stenosis affects cannulation), peripheral pulses (femoral access assessment)
• Respiratory: Baseline status (affects post-CPB management)
• Neurological: Baseline cognitive function (post-CPB delirium comparison)
• Vascular: Allen test (if radial arterial line planned), femoral pulses

Investigations:

Imaging:

• CT chest: Aortic calcification, atheroma burden, ascending aorta dimensions
• Echocardiography: Aortic valve competence (affects cardioplegia delivery), LV function
• Carotid Doppler: If history of TIA/stroke (cerebral protection strategy)

Laboratory:

• FBC: Anemia (hemodilution planning), thrombocytopenia
• Coagulation: Baseline INR, APTT (heparin dosing)
• Creatinine: Renal function (affects fluid management)
• Blood group & screen: Crossmatch 2-4 units

Management

Anticoagulation Management

Heparin Dosing:

• Dose: 300-400 IU/kg IV (typical: 30,000-40,000 IU for 70 kg adult)
• Target ACT: >480 seconds (standard CPB)
• ACT monitoring: 5-10 minutes post-heparin, then q30min during CPB
• Heparin resistance: If ACT <480 after adequate dose (antithrombin III deficiency, treat with FFP or recombinant AT III)

ACT Monitoring:

• Kaolin ACT: Most common method
• Celite ACT: Alternative (slightly different values)
• Interpretation:
  • <480 seconds: Additional heparin (5,000-10,000 IU)
  • 480-600 seconds: Therapeutic

  • 600 seconds: Adequate but monitor for bleeding

Heparin Reversal (Post-CPB):

• Protamine: 1 mg per 100 IU heparin (typical: 300-400 mg)
• Timing: After aortic decannulation, adequate hemostasis
• Administration: Slow IV (over 5-10 minutes) to avoid hypotension
• ACT target: <130 seconds (baseline)
• Rebound heparinization: Recheck ACT 1-2 hours post-protamine (heparin redistribution)

Bypass Initiation Protocol

Pre-CPB Checklist:

1. ACT >480 seconds confirmed
2. Cannulae positioning confirmed (TTE/TEE if available)
3. Circuit preparation: Prime checked, air eliminated
4. Temperature: Cooling initiated (if planned)
5. Communication: Surgeon ready, perfusionist ready

Cannulation Sequence:

1. Venous cannulation first (prevents air entry if arterial cannula leaks)
2. Arterial cannulation (ascending aorta or alternative site)
3. Cardioplegia cannulation (aortic root or direct ostia)
4. Retrograde cannula (if used, via right atrium into coronary sinus)
5. Snares: Caval tapes (if bicaval), aortic cross-clamp position

Initiating Bypass:

1. Venous line opened: Blood drains to oxygenator
2. Arterial pump started: Slow initially (500-1000 mL/min), increase gradually
3. Target flow: 2.0-2.4 L/min/m² achieved over 2-3 minutes
4. MAP monitoring: Maintain 50-80 mmHg
5. Aortic cross-clamp: Applied once stable on full flow
6. Cardioplegia delivery: Antegrade ± retrograde, 800-1500 mL initial dose
7. Venting: LV vent (via right superior pulmonary vein or PA) if needed

Hemodynamic Management on CPB

Arterial Pressure Management:

• Target MAP: 50-80 mmHg (higher if cerebrovascular disease, lower for aortic surgery)
• High pressure (>90 mmHg): Vasodilator (phentolamine, sodium nitroprusside, clevidipine)
• Low pressure (<50 mmHg):
  • Check cannula position/malfunction
  • Increase pump flow (if venous drainage adequate)
  • Vasoconstrictor (metaraminol, phenylephrine, vasopressin)
  • Consider hypovolemia (venous reservoir level)

Pump Flow Management:

• Target: 2.0-2.4 L/min/m² (typically 4.0-5.0 L/min for 70 kg adult)
• Flow limitations:
  • Venous drainage (kinked cannula, inadequate vacuum)
  • Arterial cannula resistance (too small, malposition)
  • Hemodilution (anemia limits oxygen delivery)
  • Hypothermia (reduced metabolic demand allows lower flow)

Temperature Management:

• Cooling: To 28-34°C (depending on surgery complexity)
• Rewarming: Gradual to 36-37°C before weaning
• Nasopharyngeal/tympanic: Reflects brain temperature
• Bladder/rectal: Reflects core temperature (slower response)
• Temperature gradient: Avoid >10°C gradient (water bath to blood) to prevent bubble formation

Acid-Base Management:

• pH-stat: CO₂ added to maintain pH 7.40 (regardless of temperature) - better brain cooling, used in deep hypothermia
• α-stat: No CO₂ added, pH increases with cooling (7.60 at 28°C) - preserves autoregulation, used in moderate hypothermia
• ANZCA/ANZSCTS preference: α-stat for most adult cardiac surgery

Cerebral Protection Strategies

Risk Factors for Neurological Injury:

• Advanced age (>70 years)
• Previous stroke/TIA
• Diabetes
• Aortic atheroma
• Prolonged CPB (>2 hours)
• Hypotension on CPB

Protective Strategies:

1. α-stat pH management: Preserves cerebral autoregulation
2. Higher MAP: 70-80 mmHg if cerebrovascular disease
3. Aortic cannula positioning: Avoids atheroemboli (epiaortic ultrasound guidance)
4. Arterial line filtration: 40 μm filters on arterial line
5. Hypothermia: 28-32°C for complex cases
6. Pharmacological: Thiopental (cerebral metabolic suppression), avoided routinely due to myocardial depression

Myocardial Protection

Cardioplegia Strategies:

• Blood cardioplegia: Oxygenated blood + potassium (most common, provides oxygen)
• Crystalloid cardioplegia: Clear solution (less protective, rarely used now)
• Custodiol (HTK): Histidine-tryptophan-ketoglutarate, single dose, prolonged protection

Cardioplegia Regimen:

• Initial dose: 10-15 mL/kg (800-1200 mL), antegrade ± retrograde
• Maintenance: Every 20-30 minutes or continuous (hot shot before cross-clamp removal)
• Temperature: 4-10°C (cold), 37°C (hot shot for metabolic recovery)

Additional Measures:

• Topical cooling: Saline slush in pericardium (avoid direct LV contact - risk of phrenic nerve injury)
• Systemic cooling: Reduces myocardial metabolic demand
• LV venting: Prevents distension, subendocardial ischemia

Weaning from Bypass

Preparation:

1. Rewarming: Complete to 36-37°C
2. Defibrillation: If VF, cardiovert (10-20 J internal paddles)
3. Rhythm: Ensure stable rhythm (pacing if needed)
4. Ventilation: Resume mechanical ventilation
5. Lab checks: ABG, electrolytes, Hb, glucose
6. Visual inspection: Check for surgical bleeding, graft flow
7. Air evacuation: De-airing maneuvers (TTE/TEE confirmation)

Weaning Sequence:

1. Partial bypass: Reduce pump flow to 50-70% (heart takes over gradually)
2. Aortic decannulation: Surgeon removes aortic cannula, purse-string tightened
3. Venous cannula clamping: Trial off (watch hemodynamics)
4. Full wean: If stable, remove venous cannula
5. Protamine: Administer to reverse heparin
6. Hemostasis: Check cannulation sites, surgical field

Failure to Wean:

• Causes: Poor myocardial protection, incomplete revascularization, air embolism, valvular dysfunction
• Support: Inotropes, IABP, ECMO (if severe)

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Patients

Higher Cardiovascular Risk:

• Rheumatic heart disease: Higher prevalence in remote communities, affects valve surgery planning
• Ischemic heart disease: Earlier onset, more severe disease
• Hypertension: Poorly controlled in many communities
• Diabetes: High prevalence, affects wound healing, infection risk

Access and Equity:

• Geographic barriers: Remote communities require travel to metropolitan cardiac centers
• Timing: May present with more advanced disease (late diagnosis)
• Cultural considerations: Extended family involvement in decisions
• Communication: Interpreter services if English not first language
• Discharge planning: Coordination with remote primary care services

Post-Operative Considerations:

• Wound infection: Higher risk with diabetes, remote residence (delayed presentation)
• Medication adherence: Complex regimens (anticoagulation, cardiac medications)
• Follow-up: Challenges with distance, transport, accommodation costs
• Cardiac rehabilitation: Limited access in remote areas

Māori Health Considerations (New Zealand)

Cardiovascular Disparities:

• Rheumatic fever: Significantly higher rates, leads to valvular disease
• IHD: Higher mortality, younger age at presentation
• Risk factors: Higher rates of diabetes, smoking, obesity

Cultural Safety:

• Whānau involvement: Family-centered decision making
• Communication: Clear explanations, allow time for questions
• Tikanga: Respect for cultural protocols around illness and surgery
• Māori Health Workers: Involvement in care coordination

Post-Operative Care:

• Discharge planning: Coordination with primary care
• Medication understanding: Ensure comprehension of complex regimens
• Cultural support: Access to kaumatua or cultural advisors if requested
• Health literacy: Adapt education to individual needs

ANZCA Final Exam Focus

SAQ Patterns

CPB cannulation appears in ANZCA Final Written Examination:

Management-Focused Questions:

• "Describe the physiological changes occurring during cardiopulmonary bypass." (2019)
• "How would you manage anticoagulation for cardiopulmonary bypass?" (2020)
• "What are the considerations for cerebral protection during CPB?"

Complication-Focused Questions:

• "How would you manage aortic dissection during cannulation?"
• "What are the causes of failure to wean from bypass?"
• "Describe the management of massive air embolism on CPB."

Pharmacology-Focused Questions:

• "Explain the mechanism of heparin action and monitoring during CPB."
• "Compare blood versus crystalloid cardioplegia."
• "How does hypothermia affect coagulation during CPB?"

Marking Scheme Priorities:

• Cannulation technique and site selection
• Heparin dosing and ACT monitoring
• Physiological changes on CPB (non-pulsatile flow, hemodilution, hypothermia)
• Cerebral and myocardial protection strategies
• Weaning from bypass
• Complication management

Clinical Viva Themes

Scenario Types:

• Emergency redo sternotomy with femoral cannulation
• Aortic dissection requiring axillary cannulation
• Failure to wean from bypass
• Air embolism management
• Heparin resistance

Common Viva Questions:

• "What ACT is required for CPB and how do you achieve it?"
• "Describe the cerebral protection strategies during CPB."
• "What is the difference between α-stat and pH-stat management?"
• "How do you manage a patient with heparin-induced thrombocytopenia needing CPB?"
• "What are the causes of low arterial pressure on bypass?"

Key Points for Examination Success

1. ACT >480 seconds mandatory before initiating bypass
2. Heparin 300-400 IU/kg standard dosing
3. Flow rate 2.0-2.4 L/min/m² targets adequate perfusion
4. MAP 50-80 mmHg on CPB (higher if cerebrovascular disease)
5. α-stat for adults, pH-stat for deep hypothermia/pediatrics
6. Cerebral protection: α-stat, higher MAP, avoid atheroemboli
7. Myocardial protection: Cold blood cardioplegia every 20-30 min
8. Complications: Aortic dissection, air embolism, inadequate flow

Assessment Content

SAQ Practice Question 1 (20 marks)

Question:

A 68-year-old man (75 kg) is undergoing elective coronary artery bypass grafting. After sternotomy and pericardiotomy, the surgeon is preparing for cannulation for cardiopulmonary bypass.

(a) Describe the sequence of cannulation for routine ascending aortic and right atrial CPB. (6 marks)

(b) The ACT is currently 140 seconds. Outline your anticoagulation management for CPB. (5 marks)

(c) The patient is now on full CPB with a flow rate of 4.5 L/min. The MAP is 45 mmHg. Discuss your management. (5 marks)

(d) What are the key steps in weaning this patient from CPB at the end of surgery? (4 marks)

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Model Answer:

(a) Cannulation Sequence (6 marks)

1. Anticoagulation [1 mark]

• Confirm ACT >480 seconds (heparin 300-400 IU/kg given)
• Essential before any cannulation to prevent circuit thrombosis

2. Venous Cannulation First [1 mark]

• Right atrial two-stage cannula: 28-32 Fr
• Purse-string sutures: 3-0 or 4-0 Prolene, double pledgetted
• Technique:
  • Tourniquet/snare for control
  • Incision in right atrial appendage
  • Cannula insertion, tip advanced to IVC
  • Side holes in right atrium
  • Tourniquet secured (not fully tightened until bypass initiated)
• Reason for venous first: Prevents air entry if arterial cannula has leak

3. Arterial Cannulation [1.5 marks]

• Ascending aorta: Standard site
• Purse-string sutures: 4-0 Prolene, double pledgetted, two sutures (diamond configuration)
• Partial occlusion clamp: Applied to aorta
• Aortotomy: Created within purse-string
• Cannula insertion: 20-24 Fr arterial cannula
  • Angled tip facing downstream
  • Inserted 2-3 cm into aorta
• Tourniquets: Secured around cannula
• De-airing: Connect to circuit, back-bleed to purge air
• Clamp removal: Partial clamp removed, cannula secured

4. Cardioplegia Cannulation [1 mark]

• Aortic root: Via side-arm of arterial cannula or separate needle
• Direct coronary ostia: Handheld cannulae (for open aortic procedures)
• Retrograde: Coronary sinus cannula via right atrium (if needed)

5. Final Checks [0.5 marks]

• All cannulae secured with tourniquets/wires
• Circuit connections tight
• No air in lines
• Surgeon and perfusionist ready

6. Bypass Initiation [0.5 marks]

• Venous line opened
• Arterial pump started slowly (500-1000 mL/min)
• Gradual increase to full flow (2.0-2.4 L/min/m²)
• Aortic cross-clamp applied
• Cardioplegia delivered

(b) Anticoagulation Management (5 marks)

1. Heparin Dosing [1.5 marks]

• Dose: 300-400 IU/kg IV
• For 75 kg patient: 22,500-30,000 IU heparin
• Route: IV bolus via central line or large peripheral IV
• Timing: 3-5 minutes before cannulation

2. ACT Monitoring [1.5 marks]

• Method: Kaolin ACT (or Celite)
• First check: 5-10 minutes post-heparin
• Target: >480 seconds (mandatory for CPB)
• Monitoring frequency: q30 minutes during CPB

3. Current ACT 140 seconds [1 mark]

• Below therapeutic (need >480)
• Action: Give heparin as above (22,500-30,000 IU)
• Recheck ACT: In 5-10 minutes
• Do not proceed with cannulation until ACT therapeutic

4. Heparin Resistance [0.5 marks]

• If ACT remains <480 after adequate heparin dose
• Causes: Antithrombin III deficiency (liver disease, previous heparin therapy, nitrates)
• Treatment:
  • Fresh frozen plasma (15-20 mL/kg) - provides AT III
  • Or recombinant antithrombin III (Thrombate III)

5. Protamine Reversal [0.5 marks]

• After bypass completion
• Dose: 1 mg protamine per 100 IU heparin (approximate total dose)
• Timing: After aortic decannulation
• Target ACT: <130 seconds (return to baseline)

(c) Low MAP on CPB (45 mmHg) (5 marks)

1. Assessment [1 mark]

• Target MAP: 50-80 mmHg on CPB
• 45 mmHg is low - requires intervention
• Check: Arterial line calibration, patient position

2. Ensure Adequate Pump Flow [1 mark]

• Check flow rate: Currently 4.5 L/min for 75 kg patient = 60 mL/kg/min
• Calculate required: 2.0-2.4 L/min/m²
  • BSA ~1.9 m² (for 75 kg, 170 cm)
  • Required: 3.8-4.6 L/min
• Current 4.5 L/min is appropriate
• Check venous drainage: Reservoir level stable, no kinks in venous line
• If inadequate drainage: Reposition cannula, check for obstruction

3. Vasoconstrictor Therapy [1.5 marks]

• Metaraminol: 0.5-1 mg IV bolus, then infusion 50-200 μg/min
• Phenylephrine: 50-100 μg bolus, infusion 25-100 μg/min
• Vasopressin: 0.5-2 units bolus, infusion 0.5-4 units/hour (if catecholamine-resistant)
• Avoid excessive vasoconstriction: May reduce organ perfusion despite higher MAP

4. Consider Hypovolemia [0.5 marks]

• Check venous reservoir: Low level suggests hypovolemia
• Causes: Urine output, surgical bleeding, third spacing
• Management: Add volume to circuit (crystalloid/colloid)

5. Rule Out Cannula Problems [0.5 marks]

• Arterial cannula malposition: Jet against aortic wall, flow restriction
• Aortic dissection: Rare but catastrophic (check arterial line waveform quality)
• Venous cannula obstruction: Reduces venous return, limits pump flow

6. Optimize Other Factors [0.5 marks]

• Temperature: Hypothermia increases SVR (may be too cold)
• Anesthesia depth: Light anesthesia → catecholamine release → vasoconstriction (paradoxically may need deepening)
• pH: Acidosis causes vasodilation (correct with perfusionist)

(d) Weaning from CPB (4 marks)

1. Pre-Weaning Preparation [1 mark]

• Rewarming: Complete to 36-37°C (nasopharyngeal/bladder)
• Defibrillation: If VF, internal cardioversion 10-20 J
• Rhythm: Ensure stable rhythm (atrial or ventricular pacing if needed)
• Ventilation: Resume mechanical ventilation
• Laboratory: ABG (pH, K+, Hb), electrolytes, ionized calcium
• Visual: Check surgical hemostasis, graft patency

2. De-Airing [0.5 marks]

• Manoeuvres: Trendelenburg position, lung inflation, cardiac massage
• TTE/TEE: Confirm no intracardiac air
• Vents: Remove LV vent if used

3. Partial Bypass [0.5 marks]

• Reduce pump flow: To 50-70% of full flow (2.0-2.5 L/min)
• Allow heart to fill: Observe hemodynamics
• Assess: LV function on TEE, arterial pressure waveform

4. Aortic Decannulation [0.5 marks]

• Remove aortic cannula: Surgeon withdraws
• Purse-string: Tightened, secured
• Hemostasis: Check for bleeding

5. Full Wean [0.5 marks]

• Trial off: Clamp venous line temporarily
• If stable: Remove venous cannula, tighten purse-strings
• If unstable: Return to partial bypass, troubleshoot (inotropes, pacing, reperfusion)

6. Post-Weaning [0.5 marks]

• Protamine: Administer to reverse heparin (1 mg per 100 IU)
• Hemostasis: Final check of cannulation sites, surgical field
• Monitoring: Continue close observation for instability
• Support: Inotropes if needed (milrinone, dobutamine, adrenaline)

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SAQ Practice Question 2 (15 marks)

Question:

A 72-year-old woman (60 kg) with a history of stroke 2 years ago is undergoing aortic valve replacement. She is now on cardiopulmonary bypass at 32°C with α-stat pH management. The MAP has been maintained at 55-60 mmHg for 45 minutes.

(a) What specific concerns exist regarding cerebral protection in this patient? (5 marks)

(b) The surgeon reports difficulty with aortic cannulation due to severe aortic calcification. What alternative cannulation strategies are available? (5 marks)

(c) Describe the differences between α-stat and pH-stat acid-base management during CPB. (5 marks)

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Model Answer:

(a) Cerebral Protection Concerns (5 marks)

1. History of Previous Stroke [1.5 marks]

• Cerebrovascular disease: Indicates generalized atherosclerosis
• Cerebral autoregulation: Likely impaired (loss of autoregulatory capacity)
• Risk of new event: 5-10% risk of stroke in cardiac surgery patients with prior CVA
• Management implications:
  • Need higher MAP (70-80 mmHg vs. 50-60 mmHg)
  • Strict avoidance of hypoperfusion
  • Consider epiaortic ultrasound to detect atheroma

2. Current MAP 55-60 mmHg [1.5 marks]

• Too low for patient with cerebrovascular disease
• Cerebral perfusion pressure: CPP = MAP - ICP (or CVP, whichever higher)
• Autoregulation curve: Shifted right in chronic hypertension
  • Lower limit of autoregulation higher (70-80 mmHg vs. 50 mmHg)
• Risk: Cerebral hypoperfusion, watershed infarction
• Action: Increase MAP to 70-80 mmHg with vasoconstrictors

3. α-Stat Management [1 mark]

• Advantage: Preserves cerebral autoregulation (better than pH-stat)
• Cerebral blood flow: Coupled to metabolic demand (autoregulation intact)
• Appropriate choice for this patient with cerebrovascular disease
• pH-stat would cause loss of autoregulation (luxury perfusion, embolic risk)

4. Temperature 32°C [0.5 marks]

• Moderate hypothermia: Provides cerebral protection
• Metabolic reduction: 7% per degree (28% reduction at 32°C)
• Oxygen demand: Reduced, provides safety margin
• Cerebral protection: Additional safeguard beyond pressure management

5. Additional Strategies [0.5 marks]

• Avoid cerebral emboli: Careful aortic cannulation (calcified aorta = embolic risk)
• Arterial filtration: 40 μm filter on arterial line
• Glucose control: Avoid hyperglycemia (exacerbates ischemic injury)
• Consider thiopental: If prolonged cross-clamp time (metabolic suppression)

(b) Alternative Cannulation Strategies (5 marks)

1. Problem Recognition [0.5 marks]

• Severe aortic calcification: Porcelain aorta or heavy atheroma
• Risk: Aortic dissection, embolization of atheroma
• Ascending aortic cannulation: Contraindicated or high risk

2. Femoral Artery Cannulation [1.5 marks]

• Indication: Alternative when ascending aorta unsuitable
• Technique:
  • Groin cutdown or percutaneous (Seldinger technique)
  • Check femoral pulses, review CT for iliofemoral disease
  • 18-22 Fr cannula (smaller than aortic)
• Advantages: Avoids diseased ascending aorta
• Disadvantages:
  • Retrograde flow (may displace aortic arch debris)
  • Lower limb ischemia (consider distal perfusion cannula)
  • Atheroembolism from iliofemoral disease

3. Axillary/Subclavian Artery Cannulation [1.5 marks]

• Preferred alternative for severe ascending aortic disease
• Technique:
  • Right axillary artery via deltopectoral groove
  • 8-10 mm Dacron graft sewn end-to-side to artery
  • Cannula inserted into graft
• Advantages:
  • Antegrade flow (cerebral and systemic)
  • Less atheroembolic risk than femoral
  • Allows selective cerebral perfusion if needed
• Disadvantages: More time-consuming, requires graft anastomosis

4. Innominate Artery Cannulation [0.5 marks]

• Alternative site if axillary not suitable
• Requires graft anastomosis similar to axillary approach
• Provides antegrade cerebral perfusion

5. Transapical Cannulation [0.5 marks]

• Minimally invasive cardiac surgery
• Cannula via LV apex (requires small thoracotomy)
• Specialized technique, not routine

6. Femoral Vein-Artery ECMO [0.5 marks]

• If no safe arterial cannulation site
• Peripheral ECMO configuration
• Convert to central cannulation later if needed

(c) α-Stat vs. pH-Stat (5 marks)

1. Temperature and pH Relationship [1 mark]

• Blood pH changes with temperature:
  • Neutral pH of water: 7.40 at 37°C, 7.60 at 28°C, 7.80 at 18°C
  • pH increases as temperature decreases (decreased dissociation constant)
• Imidazole hypothesis: Histidine buffer maintains neutrality across temperatures

2. α-Stat Management [1.5 marks]

• Definition: Blood pH allowed to rise with cooling (no CO₂ added)
  • 37°C pH: 7.40
  • 28°C pH: 7.60 (measured at 37°C, actual 7.40 at patient temperature)
  • 20°C pH: 7.80
• Mechanism:
  • Alpha-stat maintains constant dissociation state of imidazole groups
  • Preserves intracellular enzyme function
  • Cerebral autoregulation maintained (CBF coupled to metabolic demand)
• Usage: Most adult cardiac surgery (moderate hypothermia)
• Advantage: Preserves autoregulation, logical for patients with cerebrovascular disease

3. pH-Stat Management [1.5 marks]

• Definition: CO₂ added to maintain pH 7.40 regardless of temperature
  • At 28°C: pH 7.40 (actual pH 7.20 at patient temperature)
  • Requires active addition of CO₂ to oxygenator gas supply
• Mechanism:
  • Creates relative hypercapnia at hypothermic temperatures
  • Increases cerebral blood flow (loss of autoregulation)
  • "Luxury perfusion" - more flow than metabolic demand
• Usage:
  • Deep hypothermic circulatory arrest (DHCA)
  • Pediatric cardiac surgery
• Advantage: Better brain cooling (more flow = more heat exchange), even distribution

4. Comparison Table [0.5 marks]

5. Clinical Choice [0.5 marks]

• ANZCA/ANZSCTS preference: α-stat for most adult cardiac surgery
• Exception: Deep hypothermic circulatory arrest (pH-stat preferred for cooling)
• Hybrid approach: pH-stat during cooling, switch to α-stat for rewarming

References

1. ANZCA. PS54. Statement on Cardiopulmonary Bypass. 2020.
2. ANZSCTS. Guidelines for Cardiopulmonary Bypass. 2019.
3. Edmunds LH. Cardiopulmonary Bypass. In: Cohn LH (ed). Cardiac Surgery in the Adult. 5th ed. McGraw-Hill; 2017:353-384.
4. Murphy GJ et al. Cerebral protection during cardiac surgery. Br J Anaesth. 2009;103(5):623-631.
5. Gravlee GP et al. Cardiopulmonary Bypass: Principles and Practice. 3rd ed. Lippincott Williams & Wilkins; 2008.
6. Hogue CW et al. The role of the anaesthesiologist in cardiac surgery. Anaesthesia. 2019;74(Suppl 1):20-27.
7. Shaefi S et al. Cerebral protection during cardiac surgery. Curr Opin Anaesthesiol. 2021;34(1):82-88.
8. Magruder JT et al. Cerebral protection strategies in aortic arch surgery. J Thorac Cardiovasc Surg. 2018;155(4):1339-1347.
9. Grocott HP et al. The 2014 John W. Severinghaus Lecture. Can J Anaesth. 2015;62:131-139.
10. ATSI Health. Rheumatic heart disease in Australia. Australian Institute of Health and Welfare; 2021.