Residency · Residency · Cardiothoracic Surgery

Principles of Cardiopulmonary Bypass

Introduction

Cardiopulmonary bypass (CPB) is the fundamental enabling technology for open heart surgery, providing temporary support of circulation and gas exchange while the heart is arrested and opened. A thorough understanding of CPB physiology, circuit components, and management is essential for every cardiothoracic surgeon, as decisions made regarding bypass strategy directly impact patient outcomes.

Circuit Components

Venous Drainage

Venous cannulae provide drainage from the systemic venous circulation. A two-stage single cannula placed in the right atrium is the most common configuration, while bicaval cannulation with separate SVC and IVC cannulae is used for procedures requiring right heart access. Drainage occurs by gravity siphon into the venous reservoir, and vacuum-assisted venous drainage (VAVD) allows use of smaller cannulae with improved drainage.

Venous Reservoir

The venous reservoir may be a hard-shell open type or a soft-shell closed type. It serves as a compliance chamber and volume buffer. Open reservoirs facilitate air removal, while closed reservoirs reduce the blood-air interface.

Pump

Two pump types are in use. The roller pump is an occlusive device that generates pulsatile or non-pulsatile flow and is simple and reliable, but may cause hemolysis with excessive occlusion. The centrifugal pump is afterload-sensitive and non-occlusive, with reduced hemolysis and air embolism risk, and is increasingly preferred for longer bypass runs.

Oxygenator

The membrane oxygenator uses hollow-fiber polymethylpentene membranes to separate blood and gas phases. Gas exchange occurs across the membrane, with sweep gas FiO2 and flow rate controlling oxygenation and CO2 removal respectively. An integrated heat exchanger provides temperature management.

Arterial Filter and Line

A 40-micron arterial line filter removes microemboli, particulate matter, and air. Arterial return is achieved via ascending aortic cannulation (standard), femoral artery, or axillary artery.

Additional Components

The circuit includes cardiotomy suction to return shed blood from the surgical field to the reservoir, a cell saver to wash and concentrate red blood cells for reinfusion, and a hemoconcentrator for ultrafiltration to remove excess fluid.

Physiology of Cardiopulmonary Bypass

Hemodilution

The circuit prime of 1000-1500 mL of crystalloid dilutes the patient's blood volume, causing the hematocrit to drop to 20-28% on bypass. The acceptable nadir hematocrit remains debated but is generally above 21-24%. The benefits of hemodilution include reduced viscosity and improved microcirculatory flow, while the risks include organ injury and increased transfusion need with excessive hemodilution.

Non-Pulsatile Flow

Standard CPB provides continuous non-pulsatile flow. Target flow rates are 2.2-2.4 L/min/m2 at normothermia. Adequacy of perfusion is monitored by mixed venous oxygen saturation (SvO2 above 65%), lactate, and urine output. Pulsatile flow can be generated with roller pumps or intra-aortic balloon pumps, though evidence for clinical benefit is mixed.

Temperature Management

Normothermic bypass (36-37 degrees C) is increasingly used for shorter procedures and avoids rewarming injury. Mild hypothermia (32-35 degrees C) is the most common strategy, reducing metabolic demand by 7% per degree Celsius. Moderate hypothermia (26-31 degrees C) is used for complex aortic surgery. Deep hypothermic circulatory arrest (DHCA) at 18-20 degrees C allows periods of total circulatory arrest for aortic arch surgery, safe for 20-30 minutes. Rewarming should be slow and controlled at less than 1 degree C/min, and hyperthermia above 37 degrees C must be avoided.

Anticoagulation

Unfractionated heparin at 300-400 units/kg IV is administered before cannulation, targeting an activated clotting time (ACT) above 400-480 seconds. Additional heparin is given during bypass as needed. Protamine reversal at 1-1.3 mg per 100 units of total heparin is administered at termination of bypass, with monitoring for protamine reactions including hypotension, pulmonary vasoconstriction, and anaphylaxis.

Cannulation Strategies

Standard Cannulation

Aortic cannulation is performed in the ascending aorta with purse-string sutures, directing the cannula toward the aortic arch and avoiding atheromatous areas using epiaortic ultrasound guidance. Venous cannulation uses a two-stage right atrial cannula for most procedures.

Alternative Sites

Femoral artery and vein cannulation is used for redo surgery, aortic dissection, or minimally invasive approaches. The axillary artery provides antegrade cerebral perfusion during aortic arch surgery. The innominate artery serves as an alternative to the axillary for antegrade cerebral perfusion.

Management During Bypass

Monitoring Parameters

Key monitoring parameters include arterial line pressure (target MAP 50-80 mmHg, adjusted for patient age and cerebrovascular disease), venous line pressure (negative pressures with gravity drainage), continuous inline SvO2 monitoring (above 65% indicating adequate oxygen delivery), blood gas analysis every 15-30 minutes using alpha-stat or pH-stat management, ACT monitored every 30 minutes and maintained above 400 seconds, and urine output targeting above 1 mL/kg/hr.

pH Management Strategies

Alpha-stat management uses blood gas values uncorrected for temperature, maintaining constant alpha-imidazole ionization, and is preferred for most adult cardiac surgery. pH-stat management corrects blood gas values to patient temperature and adds CO2 to maintain pH 7.40 at hypothermic temperature, increasing cerebral blood flow, and is preferred in pediatric surgery and DHCA.

Troubleshooting

High arterial line pressure suggests kinking, aortic dissection, or a malpositioned cannula. Low venous drainage indicates hypovolemia, cannula obstruction, or air lock. Falling SvO2 reflects inadequate flow, anemia, or increased metabolic demand. Massive air embolism requires immediate Trendelenburg positioning, pump cessation, retrograde cerebral perfusion, and de-airing maneuvers.

Separation from Bypass

Separation from bypass requires confirmation of adequate rewarming (core above 36 degrees C, peripheral above 35 degrees C), adequate cardiac function by TEE, optimized heart rate and rhythm (with pacing if needed), and initiation of inotropes or vasopressors as required. Venous drainage and pump flows are gradually reduced while the heart is volume-loaded from the bypass circuit. Aortic and venous cannulae are removed after heparin reversal with protamine.

Key Clinical Pearls

ACT must exceed 400 seconds before initiating bypass; inadequate anticoagulation causes consumptive coagulopathy and circuit thrombosis. Epiaortic ultrasound should be used to identify aortic atheroma and guide safe cannulation site selection. Mixed venous oxygen saturation is the single best real-time indicator of adequacy of perfusion on bypass. Alpha-stat pH management is standard for adult cardiac surgery, while pH-stat is preferred for deep hypothermia and pediatric cases. Slow, controlled rewarming and avoidance of hyperthermia are critical to preventing neurological injury.

References

1. Hessel EA, Edmunds LH. Extracorporeal circulation: perfusion systems. In: Cohn LH, ed. Cardiac Surgery in the Adult. 5th ed. McGraw-Hill; 2018:295-370.
2. Engelman R, Baker RA, Likosky DS, et al. The Society of Thoracic Surgeons, The Society of Cardiovascular Anesthesiologists, and The American Society of Extracorporeal Technology: clinical practice guidelines for cardiopulmonary bypass. Ann Thorac Surg. 2011;91(1):163-173.
3. Hogue CW, Palin CA, Arrowsmith JE. Cardiopulmonary bypass management and neurologic outcomes. Anesth Analg. 2006;103(1):21-37.
4. Murphy GS, Hessel EA, Groom RC. Optimal perfusion during cardiopulmonary bypass: an evidence-based approach. Anesth Analg. 2009;108(5):1394-1417.

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