Heart-Lung Machine

What is a Heart-Lung Machine? Cardiopulmonary bypass (CPB)? Pump-oxygenator?

It is a technique that temporarily takes over the function of the heart and lungs during surgery. It maintains the circulation of blood (blood flow) and the oxygen content of the body (respiration) for the patient while the heart is stopped.

When We Use Heart-Lung Machine? It is commonly used in operations requiring the opening of the chambers of the heart to support the circulation during the heart surgery period because of the difficulty of operating on the beating heart. Cardiac valve repair and/or replacement. Repair of heart defects. Transplantation (heart transplantation, lung transplantation, heart–lung transplantation).

Heart-Lung Machine History The first known operation involving open heart with temporary mechanical takeover of both heart and lung functions on April 5, 1951 at the University of Minnesota Hospital. The patient did not survive due to an unexpected complex heart defect. The first successful open heart procedure on a human utilizing the heart lung machine was performed on May 6, 1953 at Thomas Jefferson University Hospital in Philadelphia.

Basic look

Main Components The Heart-Lung machine consists of two main functional units, the pump and the oxygenator which remove oxygen-deprived blood from a patient's body and replace it with oxygen-rich blood through series of hoses.

Pumps: It typically includes up to five pumps assemblies. A pump can be used for circulation of the blood. The four remaining pumps provide fluid and gas for delivery or removal to the heart chambers and surgical field. An additional pump is available for emergency backup of the arterial pump in case of mechanical failure.

1- Roller Pump: The pump console usually comprises several rotating motor-driven pumps that "massage" tubing. This action gently propels the blood through the tubing. 2- Centrifugal Pump: By altering the speed of revolution (RPM) of the pump head, blood flow is produced by centrifugal force. Centrifugal Pumps are considered to be superior to roller pumps because it is thought to produce less blood damage and for the maintenance and control of blood flow during CPB

Both pumps can provide pulsed, as from a heartbeat, or non-pulsed blood flow to the systemic circulation.

Oxygenator (The artificial lung): The oxygenator is designed to transfer oxygen to infused blood and remove carbon dioxide from the venous blood. 1- Bubble Oxygenator: The venous blood is passed up a vertical column through which bubbles of oxygen are rising. Oxygen enters the blood and carbon dioxide is released.

2- Membrane Oxygenator: Uses a thin semi-permeable membrane to separate the blood and gas phases. Membrane oxygenators have supplanted bubble oxygenators since the 1980s. A- Heparin-coated Membrane Oxygenator: It is a new type which is believed to produce less inflammation and decrease the propensity for blood to clot in the CPB circuit.

Other Components Tubing: The components of the CPB circuit are interconnected by a series of tubes made of silicone rubber or PVC (Polyvinyl chloride is the third most widely produced plastic).

Cannulae: A cannula (plural cannulae) or canula is a tube that can be inserted into the body. Multiple cannulae are sewn into the patient's body in a variety of locations, depending on the type of surgery. A venous cannula removes oxygen deprived blood from a patient's body. An arterial cannula is sewn into a patient's body and is used to infuse oxygen-rich blood.

Cardioplegia: It is a separate circuit for infusing a solution into the heart itself to produce cardioplegia (i.e. to stop the heart from beating), and to provide myocardial protection (i.e. to prevent death of heart tissue).

Venous reservoir : Buffer (atrium) Air trap Defoaming Air filtering Suction blood Fluids Medication Emptying patient Reaction time!

Components The machine may consist of: 1. venous and arterial cannula (tubes), 2. polyvinyl chloride (PVC) or silicone tubing, 3. reservoir (to hold blood), 4. membrane oxygenator, 5. cardiotomy (filtered reservoir), 6. heat exchanger(s), tubes are placed in a water bath, 7. pump(s), typically includes up to five pump assemblies. 8. flow meter, 9. inline blood gas and electrolyte analyzer, 10. pressure-monitoring devices.

Operational Aspects Blood Loss Reduction: Any blood which escapes the circulation and spills into the operating field around the heart can be suctioned and returned to the pump. This feature is made possible because the blood has been rendered incapable of clotting by large doses of heparin. Returning blood into the heart-lung machine greatly preserves the patients own blood stores throughout the operation.

Control Body Temperature: The patient body temperature can be controlled by selectively cooling or heating the blood as it moves through the heart-lung machine. Thus the surgeon can use low body temperatures as a tool to preserve the function of the heart and other vital organs during the period of artificial circulation.

Medications during surgery: The bypass pump has connectors into which medications and anesthetic drugs can be given. In this way, medications arrive to the patient almost instantly by simply adding them to the blood within the heart-lung reservoir.

Specialized Technicians (The Perfusionist): The machine is attended to at all times by a specialized technician, called the perfusionist. Each heart operation requires a dedicated and trained perfusionist to manage all aspects of the heart-lung machine during the time period of artificial circulation. The perfusionist maintains the function of the pump and also monitors the delivery of blood back to the patient, the blood oxygen content, and other vital parameters.

During this time, the surgeon is then freed up to concentrate on the technical aspects of the heart operation itself. Operating a heart-lung machine is an excellent example of team work in action.