Cardiopulmonary Bypass (CPB) Basics

• Cardiopulmonary bypass (CPB) is a technique that temporarily takes over the heart and lung functions during surgery, maintaining blood circulation and oxygen levels.
• CPB pumps are operated by Perfusionists.
• Perfusionists are specialized healthcare professionals managing a patient's physiological status with a heart-lung machine during cardiac surgery.

Introduction to CPB Use and Necessity

• CPB is commonly used in heart surgery due to the difficulty of operating on a beating heart.
• CPB supports circulation for operations requiring the opening of heart chambers.
• The CPB machine allows blood cells to continue cellular respiration during surgery.
• CPB induces total body hypothermia, which allows the body to be maintained for up to 45 minutes without blood flow.
• Permanent brain damage occurs if blood flow stops at normal body temperature for 3-4 minutes, potentially leading to death shortly after.
• CPB can rewarm individuals suffering from hypothermia.
• Surgical procedures using CPB include coronary artery bypass, cardiac valve repair or replacement, heart transplantation, and pulmonary thrombectomy.
• Blood is gravity drained from the heart into a reservoir via venous tubing and returned oxygenated to the aorta using a pump.
• Clear polyvinyl chloride (PVC) is used to manufacture the tubing.
• A cross-clamp is placed across the ascending aorta above the coronary ostia and proximal to the aortic cannula to create a dry, motionless operative area, preventing blood from entering the heart chambers.

Cardiotomy Reservoir Function

• Aspiration of blood from the cardiac chambers is necessary for adequate surgical field exposure during cardiac surgery.
• Recycling suctioned blood is achieved using a cardiotomy reservoir.
• Cardiotomy reservoirs include an air-fluid separation chamber, a temporary storage for priming solutions, filtration of particulate materials and the addition of fluids plus pharmacological agents.

Venous Reservoir

• The venous reservoir acts as a capacitance chamber to manage acute volume shifts during CPB due to hypothermia.
• Filtered venous blood is drawn from the reservoir and pumped towards the heat exchanger and oxygenator block.

Types of Arterial Pumps

• The two types of arterial pumps commonly used are partially occlusive peristaltic roller pump and a centrifugal pump.

Peristaltic Pump

• Fluid is contained within a flexible tube fitted inside a circular pump casing.
• A rotor with rollers compresses the flexible tube.
• As the rotor turns, the compressed part of the tube closes, forcing fluid to move through it.
• Fluid flow is induced as the tube opens to its natural state after the cam passes.

Centrifugal Pump

• Fluid is directed to the impeller eye and forced into a circular movement by the impeller vanes.
• The impeller vanes transfer mechanical work to the fluid.
• Centrifugal force presses fluid out of the impeller, increasing pressure and velocity.
• Centrifugal force (CF) is defined as mass of blood multiplied by the radius of the pump head multiplied by the speed of revolutions (RPM).

Cardiac Output Calculation

• The cardiac output supplied by the pump is calculated using the patient's surface area multiplied by a cardiac index ranging from 1.8 to 2.4 L/min/m², depending on patient temperature.
• Constant blood flow is delivered during CPB.
• Ideal blood pump characteristics should consider properties affecting blood flow.
• According to the Poiseuille equation: Blood flow rate = (Pressure gradient) multiplied by (Tube radius)^4 multiplied by π all divided by (Fluid viscosity) multiplied by (Tube length) multiplied by 8.
• Pumps should generate blood flow and pressure against resistance.

Heat Exchanger Function in CPB

• Heat Exchangers are integral components of all oxygenators used for CPB.
• Heat exchangers work with an external heater-cooler that pumps temperature-controlled water into the water phase, separated from the blood phase by a conductive material.
• A heat exchanger regulates the temperature of the blood perfusing the patient, raising or lowering body temperature depending on the surgery.
• CPB is performed under systemic hypothermia, typically between 25–32°C.
• More challenging operations need deep hypothermia (12–22°C) to allow low blood flow periods.
• At low temperatures the patient's metabolic rate is low, allowing the patient to be drained of all their circulating blood.
• At low temperatures circulatory arrest is possible for 45 minutes.

Heat Transfer Forms

• Heat transfer occurs through conduction (through solids), convection (from solids to liquids, with motion of the liquid carrying fluid away from the solid-liquid interface), and radiation (electromagnetic mechanism).
• Within a CPB heat exchanger, convection is the major form of heat transfer where actively pumped water and blood pass the stainless steel interface.
• Water is pumped at a controlled temperature around 10 L/min, dictating patient temperature.
• Patients are often cooled (sometimes to 12°C for major aortic work) then rewarmed to normothermia (37°C).

Oxygenator Requirements and Function

• Arterial blood temperature should not exceed 37.5°C, and the gradient between the water and blood temperature should not exceed 10°C, this reduces the risk of protein breakdown.
• An oxygenator adds oxygen and removes carbon dioxide from the blood.
• Oxygenators use a membrane (membrane oxygenators) or bubble oxygenation to provide gas exchange in the blood.

Bubble Oxygenator

• Used when CPB is needed for less than 2 hours.
• Oxygen is infused into the oxygenator chamber through a diffusion plate.
• The diffusion plate produces thousands of small oxygen bubbles within the blood.
• Gas exchange occurs at the blood gas interface around each bubble.
• Oxygen diffuses into the blood, while carbon dioxide diffuses out.
• Bubbles are removed by filtration.

Membrane Oxygenator

• Membrane oxygenators are used when CPB is needed for more than 2-3 hours.
• Gas does not directly contact the blood.
• A membrane separates blood and gas flows in the CPB circuit.
• Oxygen diffuses from the gas side into the blood, and CO2 diffuses from the blood into the gas.

Principles of Diffusion

• The physical process of diffusion during gas exchange in the natural lung also applies to the artificial lung.
• Gas exchange is less efficient due to greater diffusional distances (approximately 200 µm compared to 10 µm).
• Surface area for gas exchange is 1.7–3.5 m² compared to 70–100 m² in the human lung.
• Gas exchange obeys Fick’s law of diffusion: V∝(ADδP)/T, where V is the amount of gas transferred, A is the surface area for gas exchange, δP is the partial pressure difference, D is the diffusion constant, and T is the membrane thickness.
• The diffusion constant D is proportional to the specific gas solubility (Sol) and inversely proportional to the square root of its molecular weight (MW): D = Sol/√MW

Hollow Fiber Membrane Oxygenator

• It can increase the surface of gas exchange with a 300 μm outer diameter and a 50 μm wall thickness.
• This type of hollow fiber membrane design increases surface of gas exchange, with a limited impact on blood trauma.

Oxygenator Challenges

• Red blood cells in pulmonary capillaries pass one at a time, making the distance for O₂ diffusion much shorter.
• Artificial lungs have less surface area for gas exchange, typically less than 10% of the natural lung's area.
• Current membrane lungs compensate by increasing the blood path length and the time available for blood exposure.
• Current membrane lungs compensate by increasing the number of hollow fibers and ventilation with 100% O₂ to maximize the driving pressure difference.

Cardioplegia

• Oxygenated flow is split into two divisions: a left division through a filter into the aorta and a right division through a cardioplegia pump.
• Cardioplegic techniques for myocardial protection involve delivering cardioplegic solutions to the myocardium.
• Administered via the aortic root or direct coronary access every 15–30 minutes.
• Potassium is the main agent to induce cardiac arrest at approximately 20 mmol Litre¯¹, causing a reduction in myocardial membrane potential.

Arterial Filter

• The arterial line filter is the last filtration point, designed to remove any microemboli before blood returns to the patient.
• It catches any microemboli (gaseous or particulate) generated after the cardiotomy reservoir filters.
• Filtration is important as vessel diameters decrease distally towards the heart.
• Large bubbles break into smaller bubbles with greater surface tension as they travel down this network.
• A critical point is reached when the bubble size is such and it is treated as a foreign body, instigating an inflammatory response eventually leading to ischemic cell death.

CPB Safety Features

• All monitored parameters have operator or mechanical safety devices, including audible alarms and arterial pump shut-off.
• Safety features include a bubble alarm, arterial pump flow, gas flows, arterial pressure, and arterial-venous temperature monitoring.
• Bubble alarm stops arterial pump.
• Arterial pump flow includes an audible alarm warns of high/low flows.
• Gas flows includes an audible alarm.
• Arterial pressure includes a pressure alarm that stops the arterial pump.
• Arterial and venous temperature includes an audible alarm.