Cardiopulmonary Bypass (CPB)

Definition and Purpose

The CPB Circuit

1. Venous Reservoir

• Receives deoxygenated blood from the patient via cannulae placed in the right atrium, SVC, IVC, or femoral vein
• Blood returns by gravity drainage (siphon effect); flow depends on the height difference between patient and reservoir and cannula resistance
• Acts as a buffer to accommodate fluctuations in venous return

2. Oxygenator

• Adds O2 and removes CO2 from the blood
• Two types:
  • Bubble oxygenator: oxygen froths directly with blood; settling chambers allow froth to coalesce back to blood
  • Membrane oxygenator (now standard): blood contacts oxygen across a thin membrane - no foaming, less trauma to blood elements
• Pye's Surgical Handicraft, 22nd ed.

3. Heat Exchanger

• Blood cools below 37°C during CPB
• The heat exchanger (essentially a water bath) restores or adjusts temperature
• Used to induce deliberate hypothermia and to rewarm at the end of bypass

4. Main Pump (Roller Pump)

• The roller pump is the standard type; output is variable according to motor speed
• Centrifugal pumps are an alternative (less hemolysis, less risk of air embolism)
• Flow is characteristically non-pulsatile, which has physiological consequences (see Complications)

5. Arterial Line Filter

• A very fine mesh filter (pore size 20-30 μm) removes debris including air, fatty particles, and aggregated blood cells before blood is returned to the patient
• Critical for preventing microvascular embolism

6. Arterial Cannula

• Oxygenated blood is returned to the patient, usually via the ascending aorta (most common) or a femoral artery
• When inserted into the ascending aorta, blood flows retrograde, stopped at the closed aortic valve, and perfuses the coronary arteries and all systemic arteries
• Pye's Surgical Handicraft, 22nd ed.

Circuit Prime

• Colloid (albumin or starch)
• Mannitol (promotes diuresis)
• Heparin (500-5000 units)
• Bicarbonate

Anticoagulation

• Heparin is given before cannulation (typically 300-400 units/kg) to prevent clotting in the circuit
• Adequacy is confirmed by the activated clotting time (ACT) - a target of >480 seconds is usual
• After bypass, heparin is reversed with protamine sulfate (1 mg per 100 units of heparin given)
• Matching heparin and protamine doses can be difficult; over- or under-dosing contributes to postoperative coagulopathy

Systemic Hypothermia

• Metabolic O2 requirements roughly halve with each 10°C drop in temperature
• Allows safer periods of reduced perfusion and protects organs
• "Tepid bypass" (30-35°C drift) is increasingly used for shorter cases
• Deep hypothermic circulatory arrest (DHCA): cooling to 15-18°C allows total cessation of CPB flow for up to 60 minutes - used for complex aortic arch repairs and pediatric congenital surgery
• Adverse effects: platelet dysfunction, coagulopathy, depression of myocardial contractility
• Rewarming via the heat exchanger restores normothermia at procedure end - Morgan and Mikhail's, 7e

Myocardial Protection

Cardioplegia

• Potassium cardioplegia is the standard: a K+-rich solution (10-40 mEq/L) is infused into the coronary circulation, raising extracellular K+, reducing transmembrane potential, and arresting the heart in diastole
• Can be delivered:
  • Antegrade: through a catheter in the proximal aorta (between the cross-clamp and aortic valve)
  • Retrograde: through a catheter in the coronary sinus
• Repeated every ~30 minutes due to gradual washout and rewarming
• Solution also contains reduced Na+ (<140 mEq/L), calcium (0.7-1.2 mmol/L), magnesium (1.5-15 mmol/L), and a buffer (bicarbonate or histidine)
• Blood cardioplegia (4:1 blood:crystalloid) is increasingly preferred for its oxygen-carrying capacity and buffering

Topical Hypothermia

• Ice slush bathing the heart maintains myocardial temperature at 10-15°C
• Reduces basal metabolic O2 consumption

Ischemia and Reperfusion Injury

• Aortic cross-clamp times >120 minutes significantly increase risk
• Reperfusion after ischemia generates reactive oxygen species, intracellular Ca2+ overload, and endothelial-leukocyte interactions
• Myocardial stunning (reversible systolic and diastolic dysfunction) is common; responds to inotropes
• Myocardial necrosis is irreversible - Morgan and Mikhail's, 7e

Physiological Changes During CPB

Neurological Monitoring During CPB

• EEG / Processed EEG: hypothermia, hemodilution, and drug changes all alter EEG, making ischemia detection difficult; approximate entropy analysis can help quantify temperature-related changes
• Transcranial Doppler (TCD): monitors cerebral blood flow, detects emboli, and can identify misplaced cannulae; limited by probe instability and signal absence in some patients
• NIRS (Near-Infrared Spectroscopy): non-invasive cerebral oximetry; combined with TCD identifies patients with impaired autoregulation who are at higher risk for postoperative cognitive dysfunction and stroke. These patients may benefit from higher MAP during CPB
• NIRS + TCD combined is the most useful approach; neither modality alone has convincing outcome data for improving neurological results
• Miller's Anesthesia, 10e

Complications of CPB

1. Hemorrhage

• The clotting cascade is disrupted at multiple points: heparin/protamine dosing mismatches, protein degradation from O2 exposure, reduced platelet numbers and function
• A haemorrhagic tendency post-surgery is common
• Thresholds for re-exploration: >500 mL loss in 1 hour, or >300 mL/hour for 3 consecutive hours
• Treatment: protamine, platelet transfusion, fresh frozen plasma, desmopressin, or fresh blood (<8 hours old)
• Pye's Surgical Handicraft; PMID: 40473511 - recent meta-analysis on desmopressin efficacy (2025)

2. Organ Dysfunction (Post-Pump Syndrome)

• Non-pulsatile flow + small micro-emboli (20-40 μm; air, fat, aggregated cells) impair precapillary arteriolar flow
• Results in mild tissue hypoxia affecting virtually every organ system:
  • Brain: confusion, delayed awakening, nightmares, or (rarely) stroke
  • Kidneys/Liver: transient or (rarely) permanent dysfunction
  • Lungs: impaired oxygenation ("post-pump lung")
  • Heart: reduced myocardial contractility
  • GI tract: ileus is common for 1-2 days post-op
• Organ damage increases with bypass duration; damage mild at 2 hours may be lethal at 4 hours
• Prevention: arterial line filter, hemodilution with crystalloid prime
• Pye's Surgical Handicraft

3. Cerebral Air Embolism

• When the heart is opened (e.g., valve surgery), air fills the cardiac chambers
• Even with meticulous de-airing, residual air enters the arterial circulation at heart restart
• Air emboli preferentially enter the right coronary ostium (superior position in supine patient)
• Consequences range from delayed awakening to irreversible widespread cerebral damage and death
• Considered the major hazard of open cardiac surgery
• Management: supportive care, steroids for cerebral oedema; no specific reversal

4. Vasoplegic Syndrome

• Post-CPB distributive shock from systemic inflammatory response to CPB
• Refractory to vasopressors; treated with methylene blue or hydroxocobalamin
• A 2024 systematic review (PMID: 39438181) found both agents effective for vasoplegic shock after CPB

5. Myocardial Damage

• Inadequate preservation, reperfusion injury, coronary embolism, graft vasospasm
• Manifests as reduced cardiac output, arrhythmias, or TEE-detected ventricular dysfunction at bypass termination

6. Inflammatory Response

• Blood-surface contact activates complement and inflammatory cascades
• Particularly pronounced in children (large surface area relative to blood volume)
• Managed with corticosteroids; in pediatric patients, modified ultrafiltration post-CPB removes cytokines and vasoactive substances while correcting hemodilution

Weaning from CPB

• Core temperature restored to normothermia via heat exchanger
• Cardiac rhythm confirmed (defibrillation if needed)
• Ventilation recommenced
• Venous inflow to bypass reservoir is gradually reduced while the heart takes over ejection
• TEE is used to assess ventricular function and identify residual surgical defects
• Inotropic support (dopamine, epinephrine, milrinone) if weaning is difficult
• Protamine is administered to reverse heparin anticoagulation

Special Considerations

Pediatric CPB

• Circuit prime volume is often 200-300% of the infant's blood volume → blood must be added to prime
• Higher flow rates required (up to 200 mL/kg/min)
• pH-stat management (maintaining pH by adding CO2 under hypothermia) may yield better neurological outcomes in children undergoing circulatory arrest
• DHCA at 15°C allows up to 60 minutes of complete circulatory arrest for complex repairs
• Calcium homeostasis is more often impaired; ionized calcium monitoring is essential

Off-Pump Cardiac Surgery (OPCAB)

• Coronary bypass can be performed on the beating heart without CPB, avoiding its systemic effects
• Requires specialized stabilizer devices; technically demanding
• Reduces risk of CPB-associated complications but not universally applicable

Key Points Summary

• Morgan and Mikhail's Clinical Anesthesiology, 7e, Chapter 22
• Pye's Surgical Handicraft, 22nd Edition, Chapter 17
• Miller's Anesthesia, 10e, Chapter 14 (neurological monitoring)
• Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e
• Recent evidence: vasoplegic shock after CPB - hydroxocobalamin vs. methylene blue (PMID: 39438181, J Cardiothorac Vasc Anesth, 2024); desmopressin for bleeding reduction (PMID: 40473511, Heart Lung Circ, 2025); nitric oxide for organ protection (PMID: 40610908, BMC Anesthesiol, 2025)