What Is Cardiopulmonary Bypass?
To repair most cardiac defects, the cardiothoracic surgeon requires a bloodless, motionless field in which to work. To achieve this, the motion of the heart and lungs must be stopped.
For this to occur, there needs to be a way for blood to circulate throughout the body, delivering the nutrients and oxygen necessary for life, while the heart and lungs are not functioning. This is made possible through a process known as cardiopulmonary bypass (CPB).
Tubing made of clear polyvinyl chloride (PVC) contains the patients' blood as it is diverted from the body. Large bore catheters (called cannula) are placed in the right side of the heart, allowing the desaturated blood from the body to enter the cardiopulmonary bypass circuit.
The PVC tubing runs through a mechanical pump that can be regulated to the proper cardiac output for a given patient.
For example, a 5-year-old child has a much smaller cardiac output than an adult does; thus the pump must run at a higher rate for the adult patient than the child. More PVC tubing delivers blood from the mechanical pump to a gas exchange device called an oxygenator, or artificial lung.
The oxygenator performs the same job as the lungs: oxygenation of the blood as well as removal of carbon dioxide.
This re-oxygenated blood is then returned to the body via another cannula placed in the aorta.
In this way, cardiopulmonary bypass permits the patients' blood to bypass the heart and lungs, achieving the desired bloodless, motionless operative field and still supplying all the other organs of the body with a constant supply of oxygen and nutrient-rich blood.
The sum total of the mechanical pump, oxygenator, cannula and PVC tubing is often referred to as the heart-lung machine, or simply "the pump."
When a patient is being supported by a heart-lung machine, the patient is said to be "on bypass," or "on the pump."
Conversely, when a patient is taken off of this support, it is termed "off bypass," or "off the pump."