Cardiopulmonary Bypass Machine

Cardiopulmonary Bypass Machine

A technique known as cardiopulmonary bypass involves temporarily replacing the heart and lungs with a machine during surgery to keeps the patient’s blood flowing and oxygen levels high.

Perfusionists use cardiopulmonary bypass equipment to bypass the heart and lungs during open heart surgery. Before being pumped back into the arterial circulation, blood to the heart is first diverted through a heart-lung machine (a pump oxygenator). The device performs the functions of the heart (pump blood) and lungs (supply oxygen to red blood cells).

Robert Hooke invented the idea for the oxygenator in the 17th century, and it was French and German experimental physiologists who turned it into a useful extracorporeal oxygenator in the 19th century. Direct contact oxygenators are bubble oxygenators that do not have a barrier between the blood and oxygen. The first known open cardiotomy with a brief mechanical takeover of the heart and lungs was carried out on April 5, 1951, at the University of Minnesota Hospital by a group under the direction of Dr. Clarence Dennis. Due to an unexpectedly complex congenital heart abnormality, the patient did not survive. John Gibbon performed the first successful open-heart surgery using the heart-lung machine on a patient on May 6, 1953, at Philadelphia’s Thomas Jefferson University Hospital. An 18-year-old woman had an atrial septal defect that he fixed. Cardiopulmonary bypass development has allowed for heart surgery (CPB) advancements. 

Heart-lung bypass machine:

In situations like open heart surgery

ry, a heart-lung machine performs the functions of the heart and lungs by pumping blood and oxygenating it, respectively (Galletti and Colton, 1995). The machine’s main job is to oxygenate the body’s venous blood supply and pump that blood back into the arterial system. Before being redirected back to the arterial circulation, blood going to the heart first passes through the device. Pumps, oxygenators, temperature regulators, and filters are a few of the most crucial elements of these devices. Additionally, the heart-lung machine regulates temperature and offers intracardiac suction.

CPB circuit or unit:

A filter for the arterial line is included in the CPB circuit, along with pumps, cannulae, tubing, a reservoir, and an oxygenator. The monitoring systems used in modern CPB machines include those for pressure, temperature, oxygen saturation, hemoglobin, blood gases, and electrolytes. They also have safety measures, including oxygen sensors, oxygen bubble detectors, and reservoir low-level detection alarms.

  • Pump:

A roller pump has two rollers mounted on an arm that rotates, compressing a length of tubing to provide forward flow. Hemolysis and tube debris may result from this process, and their frequency may rise over time. As a result, using roller pumps for prolonged treatments is not recommended. Impellers or stacking cones are the main components of centrifugal pumps. The rapid rotation creates negative pressure at one inlet and positive pressure at the other, which forces the blood forward. Since they are afterload dependent, the cardiac output produced will decrease if the patient’s systemic vascular resistance (SVR) rises until the flow through the pump is raised. In more prolonged situations, centrifugal pumps may enhance renal function, neurological outcomes, and platelet preservation.

  • Cannulae:

Patients and CPB machines are linked together through cannulas. These are wire-reinforced polyvinyl chloride (PVC) pipes to avoid kinking-related blockage. 

  • Venous cannulae: Single-stage cannulae, consisting of two cannulae put into the superior and inferior vena cava and connected by a Y-piece, are utilized for most open-heart procedures. A single cannula is put into the right atrium during closed-heart surgeries, which use dual-stage cannulae. Through gravity, drainage takes place. Smaller cannulae and tubing can be used, which reduces the circuit volume when the vacuum is applied to the reservoir.

In minimally invasive or redo procedures, a long cannula may be placed up to the right atrium through the femoral vein as an alternate location for cannulation. TOE, or trans-esophageal echocardiography, aids in determining where it should be placed. A vent is necessary for blood to exit the left side of the heart via the bronchial veins. 

The ascending aorta is often punctured using an arterial catheter. In cases like emergencies, second surgeries, minimally invasive surgeries, or to achieve regional perfusion in procedures involving the ascending aorta and arch, alternative locations include the femoral, innominate, or axillary arteries.

  • Oxygenator:

In the era of membrane oxygenators, bubble oxygenators are primarily of historical importance.

Microporous hollow polypropylene fibers with an internal diameter of 100 to 200 m make up membrane oxygenators. Gases and blood are separated by the thread, with the former flowing outside the latter. They provide better blood gas control accuracy and have a lower tendency for air embolism. More recent designs incorporate an emboli management filter, eliminating the need for extra arterial filters.

To prevent gaseous emboli from being released due to changes in the temperature of saturated blood, a heat exchanger is integrated with the oxygenator and positioned next to it.

  • Tubing:

Due to PVCs’ longevity and modest hemolysis rate, they are typically built of this material. Flexible plasticizers like di(2-Ethylhexyl) phthalate have been observed to seep from the tubing and may be harmful. Dioctyl adipate, a more recent plasticizer with less leaching, is being researched.

  • Reservoir:

They gather the heart’s exhausted blood. Open reservoirs are used more frequently. They offer the option of providing suction to aid drainage and allow passive evacuation of entrained venous air. To handle suctioned blood, they incorporate a separate cardiotomy and defoaming circuit. When in use, the reservoir is kept at a safe blood level to prevent air from entering the artery.

Closed reservoirs give a lower area of blood interaction with artificial surfaces but have a limited volume capacity. Less inflammation is activated, sterility is improved, and post-operative transfusion is decreased. However, they need a different circuit just for suctioned blood processing.

Cardioplegia system:

Cross-clamping the aorta is required for intracardiac repair, which makes the heart ischemic. To elicit electromechanical arrest, which lowers myocardial oxygen consumption, the heart is perfused with a solution in a procedure known as cardioplegia. The aortic cannula is placed distal to the clamp, and the cardioplegia cannula is inserted proximally. Cardioplegia is administered using a different pump, either anterogradely into the aortic root, retrogradely into the coronary sinus, or both. The coronary sinus cannula with a balloon tip can be inserted using TOE as a guide. When used alone, retrograde cardioplegia provides insufficient proper ventricle protection. When aortic regurgitation is severe, ostial cardioplegia is administered.

Cardioplegia can be administered sporadically and can be crystalloid- or blood-based (warm or cold). Solutions based on potassium are frequently employed. The ratio of oxygenated blood to crystalloid in blood cardioplegia ranges from 1:1 to 8:1.

The gas line and blender, which deliver new gas to the oxygenator in a controlled mixture, are other circuit components. On the bypass, the total flow determines PaCO2, while the set FiO2 decides PaO2. The arterial line filter distorts the pump and eliminates particles larger than 20 to 40 micrometers.

The surface of the circuit has been coated with various materials to increase biocompatibility, reduce inflammation, and prevent thrombus formation. The advantages of one coating type over another in terms of clinical outcomes are still debatable.

 

References:

  • https://www.ebme.co.uk/articles/clinical-engineering/cardiopulmonary-bypass-machine-cpb
  • https://en.wikipedia.org/wiki/Cardiopulmonary_bypass
  • https://www.sciencedirect.com/topics/nursing-and-health-professions/heart-lung-machine
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