A heart-lung machine, also called a cardiopulmonary bypass machine, is a medical device that temporarily does the work of both the heart and lungs during some open-heart surgeries. It lets surgeons stop or slow the heart while still keeping oxygen-rich blood flowing to the brain, kidneys, and other organs. This technology is essential for procedures such as valve replacement, coronary artery bypass grafting, and repair of certain heart defects.
It matters because body tissues can be damaged quickly if blood flow or oxygen delivery stops.
Key Facts
- Cardiopulmonary bypass temporarily replaces the pumping function of the heart and the gas exchange function of the lungs.
- Blood leaves the body through a venous cannula, passes through a pump and oxygenator, then returns through an arterial cannula.
- Flow rate is often estimated by body surface area: cardiac index = flow rate / body surface area.
- Typical adult bypass flow may be about 2.0 to 2.5 L/min/m^2 of body surface area.
- Oxygen delivery depends on blood flow and oxygen content: DO2 = blood flow x arterial oxygen content.
- Heparin is used to reduce clotting in the tubing, pump, and oxygenator during bypass.
Vocabulary
- Cardiopulmonary bypass
- A procedure in which a machine temporarily pumps and oxygenates blood while the heart and lungs are partly or fully bypassed.
- Oxygenator
- The part of the heart-lung machine that adds oxygen to the blood and removes carbon dioxide.
- Cannula
- A tube inserted into a blood vessel or heart chamber to drain or return blood during surgery.
- Perfusion
- The delivery of blood through body tissues so cells receive oxygen and nutrients.
- Heparin
- An anticoagulant medicine used to reduce blood clot formation during cardiopulmonary bypass.
Common Mistakes to Avoid
- Thinking the machine only pumps blood is wrong because it also oxygenates blood and removes carbon dioxide, replacing key lung functions.
- Reversing the blood flow direction is wrong because venous blood usually leaves the body first, passes through the machine, and oxygenated blood returns to the arterial circulation.
- Ignoring clot prevention is wrong because blood touching artificial tubing can clot unless anticoagulation such as heparin is used.
- Assuming bypass is risk-free is wrong because changes in blood pressure, clotting, temperature, and inflammation must be carefully controlled by the surgical team.
Practice Questions
- 1 A patient has a body surface area of 1.8 m^2. If the target cardiac index during bypass is 2.4 L/min/m^2, what pump flow rate is needed?
- 2 During bypass, blood flows through the oxygenator at 4.5 L/min for 20 minutes. What total volume of blood passes through the oxygenator in that time?
- 3 Explain why a heart-lung machine needs both a pump and an oxygenator rather than only a pump.