Cardiopulmonary bypass, also known as "heart-lung machine", is a technique used in certain heart surgeries and complex procedures to support the cardiopulmonary functions of the patient during the surgery. The cardiopulmonary bypass equipment establishes an extracorporeal circulation of the patient's blood to allow temporary control of the patient's blood flow and to permit operations to be performed on a bloodless and still heart.
Components of the Cardiopulmonary Bypass Circuit
The main components that make up the cardiopulmonary bypass circuit are:
Oxygenator: This component of the circuit temporarily takes over the gas exchange function of the lungs by oxygenating deoxygenated blood from the veins and removing carbon dioxide from it. Modern oxygenators use a membrane to allow gas exchange without mixing blood and gas.
Blood Pump: The blood pump component of the circuit is used to regulate and maintain blood flow through the extracorporeal circuit at the desired rates. Roller pumps and centrifugal pumps are commonly used as blood pumps during cardiopulmonary bypass.
Arterial Filter: An arterial filter is placed in the arterial line of the circuit before the blood re-enters the patient. The filter removes any air, microparticles, clots or debris from the blood before it is returned to the patient.
Cardiotomy Reservoir: During the surgery, any blood suctioned from the surgical site will be collected in the cardiotomy reservoir. This removes air and allows sedimentation of solid debris before the blood is returned to the circuit.
Heat Exchanger: A heat exchanger is used within the circuit to maintain the patient's normal body temperature of 37°C during cardiopulmonary bypass. This helps prevent complications from hypothermia.
Tubing and Connections: Standard tubing and mechanical connectors link together the individual components of the cardiopulmonary bypass circuit and allow blood to flow through it in a regulated manner. The tubing must be biocompatible.
Monitoring Equipment
Advanced monitoring equipment attached to the Cardiopulmonary Bypass Equipment circuit helps the perfusionist closely watch multiple parameters during surgery:
- Blood Flow Meters: Ultrasonic or electromagnetic blood flow meters attached to the arterial and venous lines precisely measure blood flow rates through the circuit in liters per minute.
- Temperature Probes: Temperature probes placed before and after the heat exchanger, oxygenator and other locations ensure proper temperature control is maintained.
- Gas Analyzers: In-line gas analyzers sample blood gases from the arterial and venous lines to monitor pH, pCO2, pO2 and other parameters important for oxygen delivery and carbon dioxide removal.
- Pressure Transducers: Pressure transducers placed at key areas record blood pressure readings to optimize pump function and ensure safe perfusion pressures are maintained.
- Additional Sensors: Other sensors may include hematocrit, lactate and glucose monitors incorporated into the circuit.
Precise Monitoring and Control
The integration of advanced monitoring equipment enables the perfusionist to precisely control all aspects of extracorporeal circulation during cardiopulmonary bypass. Through adjustments to pump flows, heat exchanger settings and other parameters, safe perfusion is maintained that optimally supports the patient's physiologic needs during open heart surgery.
Venous Cannulation
The vascular access for initiating cardiopulmonary bypass requires venous cannulation - the insertion of cannulae into major veins to drain deoxygenated blood from the body into the bypass circuit. Common sites for venous cannulation include:
- Superior and Inferior Vena Cava: Large bore double-stage cannulae are directly inserted through sternotomy incisions into the right atrium or great cardiac veins that drain into the venae cavae.
- Femoral Vein: Percutaneously inserted femoral vein cannulae provide alternative venous drainage, especially for more complex procedures.
- Right Atrial Appendage: A separate cannula may be inserted into the right atrial appendage to enhance venous drainage.
Arterial Return and Aortic Cross-Clamping
For delivery of oxygenated blood from the bypass circuit back into systemic circulation, an arterial cannula is inserted into the ascending aorta. With a cross-clamp applied above the cannula site, the aorta can then be safely opened during procedures such as coronary artery bypass grafting without back-bleeding. Removal of the cross-clamp at the end of bypass re-establishes normal blood flow pathways.
Cardiopulmonary bypass equipment establishes an extracorporeal circulation allowing temporary control of the patient's blood during complex cardiac surgery. Through integration of advanced monitoring and precise adjustments by the perfusionist, the cardiopulmonary bypass circuit enables operations to be safely performed on an empty, motionless heart while optimal perfusion is maintained. This surgical bypass technique has enabled countless lifesaving cardiac procedures over several decades.
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