Cardiopulmonary Bypass

What Is Cardiopulmonary Bypass?

To repair most cardiac defects, the cardiothoracic surgeon requires a bloodless, motionless surgical field. To achieve this, the motion of the heart and lungs must be stopped.

Cardiopulmonary bypass (CPB) is the process that allows blood, oxygen, and nutrients to circulate throughout the body while the heart and lungs remain motionless.

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.

Large bore catheters called cannulas are placed in the right side of the heart, diverting blood from the body through tubing made of clear polyvinyl chloride (PVC). Next, this desaturated blood from the body enters the cardiopulmonary bypass circuit. This circuit features a mechanical pump. that can be regulated to the proper cardiac output, or blood flow for a given patient.

For example, a 5-year-old child has a much smaller cardiac output than an adult does; therefore, the pump must run at a higher rate for the adult patient than the child. After running through the mechanical pump, the blood continues its PVC journey to the oxygenator.

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."

Cardiopulmonary Bypass Risks

Although the origins of cardiopulmonary bypass can be traced back to the 19th century, the field has developed rapidly in the last 50 years.

The first attempt to use a heart-lung machine for total CPB occurred at the University of Minnesota in 1951. Since that time, cardiopulmonary bypass has become a standard, widely-used, low-risk procedure.

Despite this widespread acceptance, there are risks to using CP. All organ systems are affected by cardiopulmonary bypass, mainly the heart, lungs, brain and kidneys.

These effects can range from mild to severe based on how sick the patient is before surgery, the length of time that a patient is supported by CPB, and the complexity of the operation being performed.

The function of the heart may be compromised to a degree after bypass surgery. Some patients have subtle neurologic changes after bypass surgery. The occurrence of stroke or seizures during or after bypass remains low, but is a possibility.

The kidney may experience damage ranging from decreased urine output to complete renal failure. Areas of the lung may fail to fully expand after bypass. This condition is known as atelectasis.

During CPB, the patients' blood is in contact with the foreign surfaces (PVC tubing) that make up the heart-lung machine. This leads to a reaction by the patient's body that is best characterized as a generalized inflammatory response characterized by swelling of different body parts, fever, and abnormal blood tests. Research has shown this response could be potentially damaging to certain tissues in the body. We try to minimize this inflammatory response with preoperative and intraoperative steroids.

Due to the extra fluid volume needed to fill the cardiopulmonary bypass circuit, the patients' blood volume is diluted. This may require transfusion of blood products to the patient while on CPB and blood clotting abnormalities during the post-operative period.

The risk of serious complications related to cardiopulmonary support depends on the age of the patient, how ill they are at the time of the operation, and the complexity of the surgery.

In most cases this risk is below one percent, but in higher, more-complex situations, the risk may increase to 10 percent to 20 percent.

Who Operates the Heart-Lung Machine?

The conduct of CPB is an elaborate process. The cardiovascular perfusionist is the member of the open-heart surgical team responsible for setting up and running the heart-lung machine.

The cardiovascular perfusionist will assemble the proper circuit for a given patient, based on body size and the specific operation performed.

While a patient is supported by the heart-lung machine, the perfusionist is responsible for monitoring various pressures, ECG, coagulation, blood gases and electrolytes. The goal is to keep the patient in a normal physiologic state.

Currently there are 18 accredited programs across the United States specializing in perfusion training. Individuals who complete a perfusion training program earn a post-baccalaureate degree. Areas of study include cardiac anatomy, pharmacology, physiology, biochemistry and fluid dynamics.

Cardiovascular perfusionists on staff at Cincinnati Children's Hospital Medical Center are board-certified by the American Board of Cardiovascular Perfusion.