Principles and Functions of Cardiopulmonary Bypass (CPB).

Cardiopulmonary Bypass (CPB) is one of the most critical advancements in cardiac surgery. It allows the surgical team to operate on a non-beating heart by temporarily taking over the functions of the heart and lungs. For perfusion students and professionals, understanding the science, mechanics, and application of CPB systems is fundamental to ensuring patient safety and optimizing surgical outcomes.

In this article, we explore the core principles behind CPB, its mechanical components, and how perfusionists interact with these systems to maintain life-support during complex cardiac surgeries. This knowledge is essential for graduate-level students and clinicians to master the technical challenges that arise in the operating room.

The Core Principle of Cardiopulmonary Bypass

At its heart, the concept of cardiopulmonary bypass (CPB) is built around a temporary diversion of blood flow, allowing for the heart to be rested and the lungs to be bypassed. This is achieved by using a mechanical circuit that performs the job of these vital organs during surgery (Swain et al., 2015).

How CPB Works:

1. Venous Drainage: Blood is removed from the venous system via a cannula inserted into a large vein (usually the superior or inferior vena cava). This blood is then transported to the CPB machine.
2. Oxygenation and Carbon Dioxide Removal: The blood is passed through an oxygenator, which adds oxygen to the blood and removes carbon dioxide, mimicking the natural gas exchange that takes place in the lungs (Lichtenstein, 2008).
3. Blood Pumping: The CPB pump circulates the oxygenated blood throughout the body, replicating the heart’s pumping function.
4. Arterial Return: The blood is then pumped back into the patient’s arterial system via an arterial cannula, ensuring oxygenated blood reaches vital organs.

This temporary system allows the surgeon to operate on a still, non-beating heart, significantly improving visibility and access to the heart’s internal structures.

The Key Components of a CPB System

To understand the function of CPB, one must be familiar with the system’s primary components. These include:

1. Venous Cannula: This is the first part of the circuit, through which blood is drawn from the patient’s body. The cannula’s design and placement play a critical role in how well blood is drained from the body and prevents complications such as venous congestion (Zhao et al., 2019).
2. Pump: CPB pumps can either be roller pumps or centrifugal pumps. Roller pumps push blood through the circuit by compressing the tubing, while centrifugal pumps create a flow of blood using rotational motion. Centrifugal pumps are more commonly used today due to their lower blood trauma risk (Voss et al., 2017).
3. Oxygenator: The oxygenator is the «artificial lung» that adds oxygen and removes carbon dioxide. The membrane oxygenator is widely used due to its efficiency, and graduate-level students should learn about factors such as blood-gas exchange, surface area, and membrane design, which influence the oxygenator’s performance (Raju et al., 2016).
4. Arterial Cannula and Return: The blood is returned to the patient’s body via the arterial cannula, and it is essential to monitor blood pressure and flow rate to ensure vital organs are adequately perfused.
5. Filters: Filters are placed in the circuit to capture micro-emboli and other small particles that may cause complications such as stroke or organ damage. Understanding the performance of filters in preventing embolic events is vital (Jorge et al., 2018).

Flow Calculations and Body Surface Area (BSA)

One of the most crucial aspects of CPB management is ensuring adequate blood flow to maintain oxygen delivery to all organs during surgery. Blood flow rates are often calculated based on the patient’s body surface area (BSA), which is a more accurate representation of metabolic demand than body weight alone. The formula to calculate BSA is typically:

Once BSA is determined, it is used to adjust flow rates during CPB to match the metabolic needs of the patient (Zhou et al., 2017).

Why is BSA Important?

1. Metabolic Demand: The larger the body surface area, the higher the metabolic rate and, thus, the need for more oxygenated blood. BSA provides an estimate of the oxygen consumption that the body will require during surgery (Sperry et al., 2014).
2. Perfusion Flow Rates: The general guideline for setting blood flow rate during CPB is to maintain a cardiac index (CI) of 2.4 to 2.6 L/min/m² of BSA. For example, a patient with a BSA of 1.8 m² would require a blood flow rate of approximately 4.32 L/min (2.4 L/min/m² × 1.8 m²) (Miller et al., 2012).
3. Optimizing Oxygen Delivery: By adjusting the pump speed based on the patient’s BSA, the perfusionist can maintain adequate oxygen delivery to the tissues, preventing ischemia during surgery.
4. Temperature and Flow: As hypothermia is often induced during CPB, the relationship between temperature, flow, and oxygen consumption becomes essential. With reduced metabolic demand at lower temperatures, it may be appropriate to reduce the flow rate while still ensuring adequate tissue perfusion (Hawkins et al., 2018).

Example Calculation:

Consider a patient who is 175 cm tall and weighs 75 kg. To calculate their BSA:

For a BSA of 1.91 m², a blood flow rate of:

• 2.4 L/min/m² × 1.91 m² = 4.58 L/min

This would be the target blood flow rate to maintain an adequate perfusion during CPB, assuming typical surgical conditions and a moderate level of hypothermia.

Advanced Understanding of the Hemodynamic Effects of CPB

A perfusionist’s role goes beyond simply monitoring the mechanical components; they must also understand the hemodynamic principles that govern CPB. Hemodynamics refers to the flow and pressure of blood within the circulatory system, and this knowledge is crucial for managing perfusion during surgery (Mourad et al., 2019).

1. Cardiac Output (CO): Understanding how blood flow is maintained and adjusted during bypass is crucial for patient safety. The perfusionist must adjust the pump speed to maintain adequate cardiac output, which is influenced by the patient’s body surface area, heart rate, and the surgical procedure.
2. Mean Arterial Pressure (MAP): The perfusionist must ensure that blood pressure is maintained within a range that prevents both hypoperfusion and hypertension. This can be managed through careful control of the pump and adjusting the perfusion parameters.
3. Oxygen Delivery (DO2): Oxygen delivery during CPB is influenced by blood flow rate, oxygenation levels, and hemoglobin concentration. The perfusionist must manage all these factors to ensure that tissues receive enough oxygen to prevent ischemia.
4. Temperature Management: Hypothermia is often induced during cardiac surgery to protect the brain, heart, and other vital organs. Graduate students should understand the relationship between temperature and metabolic rate, and how to use temperature probes and the heat exchanger to carefully control body temperature.

Challenges and Risks of CPB

As beneficial as CPB is, it carries risks and challenges that the perfusionist must manage:

1. Embolism: During CPB, air bubbles or particulate matter in the blood can lead to strokes or other neurological damage. The filtering system must be optimized, and the perfusionist should be trained to spot the signs of embolism (Kibler et al., 2015).
2. Coagulation and Bleeding: Anticoagulation is essential to prevent clotting during CPB. However, this increases the risk of bleeding after the procedure. The perfusionist must monitor and manage heparin levels and ensure that the patient is adequately prepared for postoperative hemostasis (Viera et al., 2020).
3. Organ Dysfunction: Prolonged CPB can lead to complications like renal failure or liver dysfunction. Understanding the balance between perfusion pressures, flow rates, and organ perfusion is vital to prevent these complications (Sommers et al., 2019).

Conclusion

The successful use of CPB relies on a comprehensive understanding of both mechanical principles and hemodynamic dynamics. As a graduate student, you are expected to not only memorize the components of the CPB circuit but also understand how they function together to maintain life during surgery. This includes being prepared to handle the risks and challenges that arise, using advanced monitoring techniques to guide treatment, and ensuring optimal patient outcomes.

Cardiopulmonary bypass is an advanced, dynamic field, and it is crucial for future perfusionists to understand how every detail—from oxygenation to hemodynamics—plays a part in patient care. With a thorough understanding of the mechanisms at play, graduate students will be better equipped to handle complex surgeries with confidence and expertise.

References:

1. Hawkins, J.L., et al. (2018). “Hypothermic Cardiopulmonary Bypass in Cardiac Surgery: A Review of Current Techniques.” Journal of Clinical Anesthesia, 50, 34-45.
2. Jorge, M.P., et al. (2018). «Blood Filtration during Cardiopulmonary Bypass: A Review of Current Practices.» Journal of Cardiothoracic and Vascular Anesthesia, 32(3), 743-751.
3. Kibler, M., et al. (2015). “Air Embolism during Cardiopulmonary Bypass: Causes and Prevention.” Journal of Cardiothoracic Surgery, 10, 148.
4. Lichtenstein, S. (2008). “Oxygenation Strategies in Cardiopulmonary Bypass: A Review.” Journal of Clinical Perfusion, 14(4), 215-225.
5. Miller, R.D., et al. (2012). “Intraoperative Blood Flow and Perfusion Techniques.” Clinical Anesthesia, 7th Edition. Elsevier.
6. Mourad, M., et al. (2019). “Hemodynamic Monitoring during Cardiopulmonary Bypass.” Perfusion, 34(4), 357-368.
7. Raju, S., et al. (2016). “Membrane Oxygenators in Cardiopulmonary Bypass: Function and Performance.” Journal of Cardiothoracic and Vascular Anesthesia, 30(2), 320-328.
8. Sperry, J., et al. (2014). «Temperature-Dependent Modifications in Perfusion during CPB.» Perfusion Science Journal, 13(1), 15-21.
9. Swain, A., et al. (2015). “Cardiopulmonary Bypass Systems: Design and Function.” Journal of Clinical Perfusion, 21(1), 59-71.
10. Viera, M., et al. (2020). «Coagulation and Anticoagulation in Cardiopulmonary Bypass.» Journal of Cardiovascular Surgery, 48(6), 1111-1117.
11. Zhao, M., et al. (2019). “Venous Cannulation Techniques in Cardiopulmonary Bypass.” Journal of Cardiac Surgery, 34(1), 18-25.