CARDIOPULMONARY BYPASS
As the name suggests, cardiopulmonary bypass is a technique that allows bypassing the patient’s cardiopulmonary system replacing it with an extracorporeal circuit that takes over the ventilation and perfusion function. In a nutshell, it is designed to take over the function of the heart and lungs, providing the surgeon with a bloodless and still surgical field.
However, the introduction of the extracorporeal system is a nonphysiological replacement and has a multitude of drawbacks, the prolonged contact of blood with artificial surface and the non-pulsatile characteristic of CPB flow being the most significant of all.
CPB CIRCUITRY
In its most basic form, the CPB circuit involves gravity-driven drainage of the blood from the heart and lung (venous cannulation), passing it through an artificial lung (oxygenator/ gas-exchange system) and artificial heart (roller pump) before returning the blood to the arterial side of the circulation (arterial cannulation).
CPB machines have six essential components
- Venous reservoir
- An oxygenator
- Heat exchanger
- Main pump
- An arterial filter
- Venous and arterial tubings conduct deoxygenated blood to the venous reservoir and deliver oxygenated blood to the patient, respectively.
The oxygenator, heat exchanger and reservoir are usually housed inside a single disposable unit, and most modern circuits allow connections of multiple accessory pumps or devices to allow advanced function. In addition, several safety alarms (pressure, oxygenation, temperature) and filters are usually incorporated within the CPB.
Modern CPB machines have systems for monitoring pressures, temperature, oxygen saturation, haemoglobin, blood gases, electrolytes, and safety features such as bubble detectors, oxygen sensors, and reservoir low-level detection alarms.
VENOUS CANNULATION:
The deoxygenated blood is collected from the body through one or two venous cannulas placed in the right atrium (or superior and inferior vena cava) and drained into the reservoir with the help of the venous tubings. Femoral vein cannulation, where a long cannula is inserted up to the right atrium (confirmed by TOE), can be utilised for minimally invasive or redo surgeries.
The cannulae and tubing are made of polyvinylchloride (PVC) and are wire reinforced to prevent obstruction due to kinking. PVC allows durability and an acceptable haemolysis rate.
The blood flow is gravity dependent.
FLOW INTO RESERVOIR α Height difference between the venous cannula and reservoir while inversely related to the resistance of the cannula and tubing.
Airlock in the cannula or tubing significantly decrease the flow and need careful exclusion. At times, it may require the addition of a centrifugal pump to get an adequate venous flow.
RESERVOIR:
The reservoir of the CPB machine receives blood from the patient. It can be an open or closed system.
Open reservoir:
- Commonly used
- Allow passive removal of entrained air
- Vacuum can be applied to assist drainage
- includes a cardiotomy suction and defoaming unit
A low blood level in the reservoir can entertain air and produce fatal air embolism. Maintaining the fluid level is critical, and the utmost care should be taken to avoid fatal air embolism.
Closed reservoir:
- Smaller unit with limited capacity
- Less inflammatory activation, better sterility, and low incidence of postoperative transfusion.
- Require a separate circuit for processing suctioned blood.
MAIN PUMP:
It is the propulsive force that allows continuous blood flow (replacing the function of the human heart).
Roller pump:
Two rollers positioned on a rotating arm compress a length of tubing to produce a continuous forward flow, irrespective of resistance encountered. Roller pumps are equipped with a hand-crank to allow manual pumping.
TO CONTINUE READING THE REST OF THE CHAPTER, DOWNLOAD THE CARDIOPULMONARY BYPASS MODULE BY TARGETEDAIC. THE CHAPTER COVERS THE BASICS OF CPB CIRCUIT, CPB CONDUCT, COMPLICATIONS, MANAGEMENT, AND ANAESTHETIC IMPLICATIONS OF A PATIENT UNDERGOING CARDIOPULMONARY BYPASS.
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