Uncovering the Beauty Behind Cardiopulmonary Bypass
- priyakchahal1
- Aug 5, 2025
- 8 min read
Have you ever wondered how the machinery commonly used to perform heart surgeries works? Or what it means to be placed “on bypass”? In this article, we will identify the critical components of the cardiopulmonary bypass machine, outline the necessary steps for operating this device during surgery, and, most importantly, gain an appreciation for this groundbreaking technology.
What is Cardiopulmonary Bypass?
Cardiopulmonary bypass (CPB) is a machine-based technology that temporarily functions as both the heart and lungs during cardiac surgeries. The first successful operation involving a CPB machine was performed by American surgeon Dr. John Gibbon on May 6th, 1953, to correct a congenital heart defect – marking a revolutionary achievement in the field of cardiac surgery.
The CPB or heart-lung machine creates a bloodless surgical field and is capable of immobilizing the heart, thereby enabling cardiovascular surgeons to perform complex procedures such as cardiac tumor resections, valve replacements, and heart transplants. Notably, the heart-lung machine is controlled by a certified cardiovascular perfusionist, who works closely with surgeons, anesthesiologists, and other healthcare professionals to ensure operations proceed smoothly.
Basic Overview of the Circulatory System
Before proceeding to describe the CPB machine, it is important for you to have a basic understanding of the circulatory system. The circulatory system is composed of organs and blood vessels that pump blood throughout the body. The major organ of the circulatory system is the heart, which is marginally situated on the left side of the body, between the two lungs. Specifically, the heart is separated into four muscular chambers and consists of four valves. These valves – tricuspid valve, pulmonary valve, bicuspid (or mitral) valve, and aortic valve – ensure blood flows in the correct direction and prevents backflow.
Chamber of the Heart | Location & Description |
Right Atrium | -The upper right chamber of the heart -Receives deoxygenated (oxygen-poor) blood from the body through two large veins: the superior and inferior vena cava |
Right Ventricle | -The lower right chamber of the heart -Pumps deoxygenated blood to the lungs via the pulmonary arteries |
Left Atrium | -The upper left chamber of the heart -Receives oxygenated (oxygen-rich) blood from the lungs via the pulmonary veins |
Left Ventricle | -The lower left chamber of the heart -Pumps oxygenated blood to the remainder of the body via the aorta |
Summary of Blood Flow Through the Heart & Lungs
Deoxygenated blood enters the right atrium via two large veins: the superior and inferior vena cava. The superior vena cava receives blood from the upper body, whereas the inferior vena cava receives blood from the lower body.
Deoxygenated blood from the right atrium passes through the tricuspid valve to enter the right ventricle.
The blood subsequently exits the right ventricle, passes through the pulmonary valve, and enters the pulmonary artery. The pulmonary artery divides into the left and right pulmonary arteries, transporting the deoxygenated blood to each respective lung.
Within the lungs, gas exchange occurs where the blood collects oxygen and relinquishes carbon dioxide.
Oxygenated blood enters the heart via the pulmonary veins and drains into the left atrium.
The blood is then pumped into the left ventricle through the bicuspid valve.
Finally, the oxygenated blood exits the left ventricle via the aortic valve, entering the aorta to be circulated throughout the body.
When CPB is used, the body’s natural circulatory system is temporarily halted and replaced with an artificial circuit, ensuring that vital organs receive oxygenated blood needed to survive during surgery.
Key Components of the Cardiopulmonary Bypass Machine
The CPB machine is considered a form of extracorporeal circulation, where extracorporeal refers to “outside the body.” In essence, blood is rerouted from the patient’s heart and lungs to the CPB machine, where it undergoes oxygenation and further processing before being returned to the body. The major components of the heart-lung machine include the cannulae, reservoir, pumps, heat-exchanger, oxygenator, and the arterial line filter. Extra tubing is necessary to connect these parts together to form a complete circuit.
CPB Components | Function |
Cannula (Plural: Cannulae) | Tubes connecting the patient to the CPB machine, allowing for the passage of blood -Venous Cannula: Diverts deoxygenated blood from the patient to the CPB circuit -Arterial Cannula: Delivers oxygenated blood to the patient from the CPB circuit |
Reservoir | A container that holds deoxygenated blood collected from the patient |
Main Pump | Mimics the heart by driving blood forward through the circuit, eventually returning it to the patient |
Heat-Exchanger | Controls the temperature of the blood |
Oxygenator | Mimics the lungs by oxygenating the blood |
Arterial Line Filter | Forms part of the oxygenator, removing non-biological particles and air bubbles (air emboli) from the blood |
How Does the Cardiopulmonary Bypass Machine Work?
The venous and arterial cannulas are inserted into the heart through a process referred to as cannulation. Venous cannulas are commonly placed in either the right atrium, inferior vena cava, or superior vena cava. Conversely, arterial cannulas are often inserted into the ascending portion of the aorta, but may instead be placed in peripheral arteries such as the femoral artery, located in the thigh.
During CPB, deoxygenated blood from the right atrium flows through the venous cannula and drains into the reservoir via gravity. Importantly, this is achieved by positioning the reservoir beneath the heart. The main pump propels the blood from the reservoir through the oxygenator, where gas exchange occurs. From this point, the oxygenated blood is segregated into two distinct cascades:
Cascade 1: The oxygenated blood is passed through the arterial cannula, where it enters the aorta to be circulated throughout the body.
Cascade 2: The oxygenated blood is infused with cardioplegia, a specialized solution used to temporarily arrest the heart.
This division is crucial as it makes certain that the heart is the sole recipient of cardioplegia whilst the body collects oxygenated blood.
For internal heart repairs, cross-clamping of the aorta is a compulsory step, despite it causing ischemia – a condition where the heart receives insufficient amounts of blood. Hence, the cardioplegic solution is delivered once clamping is completed. Why, you may ask? Aside from stopping the heart from beating, cardioplegia neutralizes the effects of clamp-induced ischemia by lessening the oxygen demand of the heart, thereby protecting this organ during intricate surgical procedures.
Cardioplegia is delivered to the heart by a separate pump either antegrade via the aortic root (portion of the aorta connected to the heart), retrograde via the coronary sinus (located at the back of the heart; the largest vein of the heart that drains deoxygenated blood collected from the heart itself into the right atrium), or both.
As the heart-lung machine continues circulating blood, modulatory elements, alongside both the vents and suckers, aid in refining and recovering blood, respectively.
Heat exchanger: A temperature modulator that generally tends to precede the oxygenator, as the reverse arrangement would cause air bubbles to form in the blood.
Vents: Withdraws blood from the heart to avert swelling.
Suckers: Captures blood from the surgical field to mitigate its loss and ensure clear visualization. The gathered blood is subsequently drained into the reservoir to be pumped through the heart-lung machine.
Numerous safety adjuncts, such as the arterial line filter, are incorporated throughout the CPB circuit to avoid serious complications (e.g., air embolism).
Steps For Operating Cardiopulmonary Bypass During Surgery
To ensure successful operation of the heart-lung machine, surgical teams must follow a series of steps prior to initializing the circuit.
1) Setup CPB Circuit | The equipment necessary to establish the CPB circuit is retrieved and organized. |
2) Priming & Anticoagulation | Priming is necessary for deairing the CPB circuit and hemodilution (that is, increasing the fluidity and decreasing the thickness of the blood). These priming solutions are often composed of colloids (large molecules) and crystalloids (small molecules). If the circuit is not adequately deaired, air bubbles can emerge in the blood pumped into the patient’s body, resulting in an air embolism. Furthermore, patients requiring CPB may be subjected to hypothermic conditions due to its ability to protect vital organs. This, in turn, increases the thickness of blood, which is evaded through the use of priming solutions. Afterwards, the patient will receive the anticoagulant heparin (that is, a blood-thinner medication that reduces the likelihood of blood clots). In our body, blood flows through vessels lined with endothelial cells that prevent clotting. However, tubes of the CPB machine lack these endothelial cells and therefore render the patient susceptible to blood clot formation, underscoring the importance of administering heparin before initializing the heart-lung machine. Of note, some individuals who are at an increased risk of developing thrombocytopenia (that is, low platelet production) from heparin are given bivalirudin as an alternative. |
3) Going “on bypass” | Once the necessary criteria and parameters for commencing CPB are met, patients are connected to the heart-lung machine and both the heart and lungs are ceased. |
Throughout surgery, the patient’s status and temperature are monitored. Following the completion of the operation, the patient is weaned off the heart-lung machine.
1) Rewarming | Since the patient may have been subjected to hypothermic conditions, their body must be rewarmed. This is gradually achieved using both the heat exchanger and heated blankets. |
2) Deairing | Any new air that was introduced into the circuit during surgery is removed. |
3) Pacing | Heart rate and rhythm are evaluated to ensure they are within normal range. |
4) Dismantling the Circuit | Once the necessary criteria and parameters are satisfied for dismantling the circuit, the patient is removed from the CPB machine to allow the heart and lungs to resume their normal function. The patient also receives a dose of protamine, which counteracts the anticoagulant effect of heparin. |
Final Takeaways
The CPB machine is an innovative piece of technology that has helped save countless lives. Following the success of this machine, extracorporeal membrane oxygenation (ECMO) was developed to temporarily replace heart and lung function in patients over longer durations, allowing these organs to effectively heal.
If you are interested in learning more about CPB from the perspectives of cardiovascular perfusionists and surgeons, feel free to check out the links below:
An Inside Look into Preparing & Operating the CPB Machine During Heart Surgery – Dr. Patrick Parrino, a cardiothoracic surgeon based at the Ochsner Medical Center in New Orleans, details the operating room setup for cardiac surgery, the heart-lung machine equipment, and the steps necessary to initialize the CPB machine during an actual open heart procedure.
An Overview of the Steps Necessary to Go on Bypass – Anesthesiologist Dr. Max Feinstein and cardiovascular perfusionist Caleb Varner provide a demonstrative rundown of the steps necessary to start the CPB machine.
References
Cleveland Clinic. (2022, August 31). Cardiopulmonary Bypass. https://my.clevelandclinic.org/health/treatments/24106-cardiopulmonary-bypass
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Cleveland Clinic. (2024, January 26). Heart.
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U.S. Food and Drug Administration. (2018, July 17). Cardiopulmonary Bypass Arterial Line Blood Filter 510(k) Submissions – Final Guidance for Industry and FDA. https://www.fda.gov/medical-devices/guidance-documents-medical-devices-and-radiation-emitting-products/cardiopulmonary-bypass-arterial-line-blood-filter-510k-submissions-final-guidance-industry-and-fda
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