Cardiopulmonary Bypass: Components, Mechanisms of Gas Exchange, Priming Solutions, and Additives

Introduction

• The introduction of CPB, also known as the “heart-lung machine,” in the 1950s and its subsequent evolution revolutionized the field of cardiac surgery, enabling precise heart and great vessel repair while supporting cardiopulmonary functions.¹
• CPB offers extracorporeal replacement of pulmonary function (oxygenation and ventilation), as well as cardiac function and tissue oxygen delivery (replacing native cardiac output).¹
• There are several components to the CPB machine; understanding these components, their roles, and potential disadvantages allows for delicate coordination and tight regulation to ensure patient stability and minimize complications.²

Components of the CPB Circuit

The CPB system comprises many interconnected components that together support the cardiovascular and pulmonary systems during cardiac surgery (Table 1).^1-3 These include:

• Cannulas: Initially, an arterial cannula is placed in the ascending aorta, or less frequently in the axillary or femoral artery. Its role is to return oxygenated blood from the CPB circuit into the systemic circulation. Venous cannulas are then inserted into the right atrium, or the superior and inferior vena cava, either centrally or peripherally, to deliver deoxygenated blood into the CPB circuit. These cannulas come in various shapes and sizes to facilitate appropriate drainage.^1-3
• Pumps: Flow within the circuit is regulated by one of two types of pump systems: roller pumps or centrifugal pumps. Roller pumps work by compressing flexible tubing, generating a force that propels the blood forward. Centrifugal pumps use stacked cones that rotate rapidly, like an impeller, to generate a pressure gradient that produces flow. Centrifugal pumps are gentler on blood cells and cause less trauma.³
• Oxygenator: Oxygenation occurs as blood passes through the oxygenator. Historically, blood was oxygenated through its contact with oxygen bubbles, using bubble oxygenators. These are not widely used anymore due to their risk of air embolism and blood trauma. More commonly, membrane oxygenators, in which blood flows on one side and gases flow on the other side of a semipermeable membrane of microporous polypropylene fibers, facilitate gas exchange by diffusion. This technique carries a lower risk of complications, including air emboli, and offers better accuracy in blood gas control.^2,3
• Reservoir: Blood returning from the patient is collected in a reservoir. Open reservoirs are more widely used, allowing easy access to deliver medications, fluids, or products. However, these produce a blood-air interface that may activate blood components, as well as pose a higher risk of air embolism and contamination. Conversely, closed reservoirs have better sterility but lower capacity and require a separate suction circuit.^2,3
• Heat Exchanger: Thermoregulation occurs via a heat exchanger, where a temperature-controlled fluid (e.g. water) and the patient’s blood flow in separate compartments, allowing heat transfer and enabling precise adjustment of blood temperature.^2,3
• Tubing: The CPB circuit is connected by standardized polyvinyl chloride tubing.
• Others: additional equipment, such as cardiotomy suckers, vents, hemofilters, arterial line filters, and safety monitors, all facilitate optimal and secure bypass operation (Table 1).^2,3

Mechanism of Gas Exchange

• In a CPB circuit, gas exchange occurs within the membrane oxygenator. The membrane oxygenator enables the passage of blood and sweep gas via hollow microporous polypropylene fibers, promoting gas exchange.³
• The sweep gas mixture resides within the lumen of the fibers, whilst blood flows along the external surface of the fibers, and the partial pressure gradient facilitates gas exchange.⁵
• As the partial pressure of the sweep oxygen exceeds that of the venous blood, oxygen diffuses across the semipermeable membrane into the patient’s blood as it traverses the oxygenator.5 Similarly, the partial pressure of CO₂ in venous blood exceeds that of the sweep gas mixture, facilitating its diffusion across the membrane and subsequent entry into the gas flow exiting the oxygenator.⁵
• The sweep gas flow rate is inversely proportional to the rate of CO₂ elimination: as the sweep gas flow rate increases, the sweep gas CO₂ decreases, creating a larger partial pressure gradient and increasing CO₂ elimination, and vice versa.^5,6 Similarly, oxygen depends on blood flow rate and oxygen-carrying capacity for its exchange.⁶
• The function of an oxygenator is dependent on both the gas compartment (sweep flow, FiO₂) and the blood compartment (flow rate, hemoglobin, distribution). Any issue with either side may result in serious complications in gas exchange during CPB.⁶

Priming Solutions

• Priming solutions must occupy the CPB circuit before extracorporeal circulation initiation to assist in regulating fluid balance, hemodilution, and perioperative outcomes⁷ (Table 2).
• Crystalloid solutions serve as an important priming solution for CPB due to their accessibility, cost-effectiveness, and compatibility with the CPB circuit.⁸ Common crystalloids used for priming include balanced salt solutions and saline-based formulations, both of which expand intravascular volume and have a known electrolyte composition.⁸
• Nonetheless, they are imperfect, as they may result in different levels of hemodilution, and their diminished oncotic pressure facilitates the movement of fluids into the interstitial space.⁸
• In contrast, colloid solutions contain larger molecules that exert greater oncotic pressure, thereby restricting hemodilution during CPB. Colloids, such as albumin and synthetic colloids, help preserve intravascular volume while minimizing fluid transfer into the interstitial space.⁸
• The selection of priming solutions is a crucial aspect of preoperative planning.⁷

Hemodilution

• Hemodilution occurs at the beginning and throughout CPB when the priming solution and cardioplegia mix with the patient’s circulating blood, resulting in decreased hematocrit levels. The extent of hemodilution is based on the ratio of the patient’s blood to the priming volume in the CPB circuit.⁹
• A reduced hematocrit during CPB results in a decreased oxygen-carrying capacity, thus reducing oxygen supply to the vital organs despite sufficient pump flow. Increased hemodilution results in compromised intraoperative oxygen delivery, exhibited by postoperative hyperlactatemia, indicating insufficient tissue oxygenation during bypass.⁹
• Hemodilution represents a trade-off between the beneficial effects of reduced blood viscosity and the detrimental consequences of inadequate oxygen supply. To optimize hemodilution, it is essential to consider the circuit prime volume and select appropriate intraoperative perfusion objectives to maintain an adequate hematocrit level.⁹
• Strategies to mitigate the degree of hemodilution during CPB include minimizing circuit prime volume, preoperative acute normovolemic hemodilution, blood-containing prime solutions, retrograde autologous priming, and ultrafiltration during bypass.⁹

Additives

• Additives are mixed into the priming solution to change its properties and affect how the body responds during bypass (Table 3).⁸
• Mannitol is a commonly used additive. Mannitol increases plasma osmolality, leading to increased urine production during bypass. However, it inadvertently leads to a measurable change in electrolytes. Its use has not consistently been shown to improve postoperative renal outcomes. It is also thought to have free radical scavenging properties.¹⁰
• Sodium bicarbonate is another common additive that is added to the priming solution. Sodium bicarbonate is used to correct acid-base imbalances that may occur when CPB causes hemodilution or subsequently due to lactic acidosis.⁸
• Heparin may also be included in the priming fluid to sustain anticoagulation within the circuit, in addition to systemic heparinization.⁸
• These additives demonstrate that CPB priming fluids are dynamic fluids modified not only to occupy the circuit volume but also to regulate osmotic, acid-base, and anticoagulation characteristics during extracorporeal circulation.