Heart-lung machines

Definition

The heart-lung machine is medical equipment that provides cardiopulmonary bypass or mechanical circulatory support of the heart and lungs . The machine may consist of venous and arterial cannula, polyvinyl chloride (PVC) or silicone tubing, reservoir (open or closed system), bubbler or membrane oxygenator, cardiotomy, heat exchanger(s), arterial line filter, pump(s) (usually centrifugal or roller-head), flow meter, inline blood gas and electrolyte analyzer, and pressure monitoring devices. Treatment provides removal of carbon dioxide from the blood, oxygen delivery to the blood, blood flow to the body, and/or temperature maintenance. Pediatric and adult patients both benefit from this technology.

Purpose

In the operating room the heart-lung machine is used primarily to provide blood flow and respiration for the patient while the heart is clinically arrested. Surgeons are able to perform coronary artery bypass grafting (CABG), open-heart surgery for valve repair or repair of cardiac anomalies, and aortic aneurysm repairs, along with treatment of other cardiac related diseases.

The heart-lung machine provides the benefit of a motionless heart in an almost bloodless surgical field. Cardioplegia solution is delivered to the heart by a dedicated pump resulting in cardiac arrest. The heart-lung machine is invaluable during this time since the patient is unable to maintain blood flow to the lungs or the body.

In critical care units and cardiac catheterization laboratory the heart-lung machine is used to support and maintain blood flow and respiration. The diseased heart or lungs are replaced by this technology providing time for the organ(s) to heal. Venoarterial extracorporeal membrane oxygenation (ECMO) is used primarily in the treatment of lung disease. Cardiopulmonary support is useful during percutaneous transluminal coronary angioplasty (PTCA) and stent procedures performed in the cardiac catheterization lab. Both treatments can be instituted in the critical care unit when severe heart or lung disease is no longer treatable by less invasive conventional treatments such as pharmaceuticals, intra-aortic balloon pump (IABP), and mechanical ventilation with a respirator.

Use of this treatment in the emergency room is not limited to patients suffering heart or lung failure. In severe cases of hypothermia, patient body temperature can be corrected by extracorporeal circulation with the heart-lung machine. Blood is warmed as it passes over the heat exchanger. The warmed blood returns to the body, gradually increasing the patients body temperature to normothermia.

Tertiary care facilities are able to support the staffing required to operate and maintain this technology. Level I trauma centers have access to this specialized treatment and equipment. Being that this technology serves both adult and pediatric patients specialized children¸s hospitals may provide treatment with the heart-lung machine for Venoarterial ECMO.

Description

The pump oxygenator developed by Dr. Gibbons saw its first success on May 6, 1953. A patient placed on the heart-lung machine survived an open-heart atrial septal defect repair. Continued research and design has allowed the heart-lung machine to become a standard of care in the treatment of heart and lung disease, while supporting other non-conventional treatments.

Foreign surfaces of the heart-lung machine activate blood coagulation proteins and platelets, which lead to clot formation. In the heart-lung machine, clot formation would block the flow of blood. As venous and arterial cannulas are inserted, pharmaceuticals are administered to provide anticoagulation of the blood. Clot formation is prevented and blood will flow through the heart-lung machine.

Large vessels, venous and arterial, are required for cannulation, to insert the tubes (cannulas) that will carry

the blood away from the patient to the heart-lung machine and return the blood from the heart-lung machine to the patient. Cannulation sites for venous access can include the inferior and superior vena cava, the right atrium, the femoral vein, or internal jugular vein. Oxygen rich blood will be returned to the aorta, femoral artery or carotid artery. By removing oxygen poor blood from the right side of the heart and returning oxygen rich blood to the left side, heart-lung bypass is achieved.

The standard heart-lung machine typically includes up to five pump assemblies. A centrifugal or roller head pump can be used in the arterial position for extracorporeal circulation of the blood. The four remaining pumps are roller pump in design to provide fluid, gas, and liquid for delivery or removal to the heart chambers and surgical field. Left ventricular blood return is accomplished by roller pump, drawing blood away from the heart. Surgical suction created by the roller pump removes accumulated fluid from the general surgical field. The cardioplegia delivery pump is used to deliver a high potassium solution to the coronary vessels. The potassium arrests the heart so that the surgical field is motionless during surgical procedures. An additional pump is available for emergency backup of the arterial pump in case of mechanical failure.

A pump is required to produce blood flow. Pump technology has advanced from the Sigmamotor pump, a fingerlike contraption that uses peristalsis to propel blood, through tubing, from the oxygenator to the body. This technology was abandoned for more reliable roller pump technology. Currently roller and centrifugal pump designs are the standard of care. Both modern designs can provide pulsatile or non-pulsatile blood flow to the systemic circulation.

The roller assembly rotates and engages the tubing, PVC or silicone, which is then compressed against the pump's housing, propelling blood ahead of the roller head. Rotational frequency and tubing inner diameter determine blood flow. Because of its occlusive nature the pump can be used to remove blood from the surgical field by creating negative pressure on the inflow side of the pump head.

The centrifugal pump also has a negative inlet pressure. As a safety feature this pump becomes inefficient when gaseous emboli are introduced. The centrifugal force draws blood into the center of the device. Blood is propelled and released to the outflow tract tangential to the pump housing. Rotational speed determines the amount of blood flow, which is measured by a flowmeter placed adjacent to the pump housing. If rotational frequency is too low, blood may flow in the wrong direction since the system is non-occlusive in nature. Magnetic coupling links the centrifugal pump to the control unit.

A reservoir collects blood drained from the venous circulation. Tubing connects the venous cannulae to the reservoir. Reservoir designs include open or closed systems. The open system displays graduated demarcations corresponding to blood volume in the container. The design is open to atmosphere allowing blood to interface with atmospheric gasses. The pliable bag of the closed system eliminates the air blood interface, while still being exposed to atmospheric pressure. Volume is measured by weight or by change in radius of the container. The closed reservoir collapses when emptied as an additional safety feature.

Bubble oxygenators use the reservoir for ventilation. When the reservoir is examined from the exterior the blood is already oxygen rich and appears bright red. As blood enters the reservoir gaseous emboli are mixed directly with the blood. Oxygen and carbon dioxide are exchanged across the boundary layer of the blood and gas bubbles. The blood will then pass through a filter that is coated with an antifoam solution, which helps to remove fine bubbles. As blood pools in the reservoir it has already exchanged carbon dioxide and oxygen. From here tubing carries the blood to the rest of the heart-lung machine.

In opposition to this technique is the membrane oxygenator. Tubing carries the oxygen poor blood from the reservoir through the pump to the membrane oxygenator. Oxygen and carbon dioxide cross a membrane that separates the blood from the ventilation gasses. As blood leaves the oxygenator it is oxygen rich and bright red in color.

As blood is ready to be returned from the heart-lung machine to the patient the arterial line filter will be encountered. This device is used to filter microemboli that may have entered, or been generated by the heart-lung machine. After this filter tubing completes the blood path as it returns the blood to the arterial cannula to enter the body.

Fluid being returned from the left ventricle and surgical suction require filtration before the blood is reintroduced to the heart-lung machine. Blood enters a filtered reservoir, called a cardiotomy, which is connected with tubing to the venous reservoir. Other fluids, such as blood products and pharmaceuticals are also added in the cardiotomy for filtration, of particulate.

Heat exchangers allow body and organ temperature to be adjusted. The simplest heat exchange design is a bucket of water. As the blood passes through the tubing placed in the bath the blood temperature will change. A more sophisticated system separates the blood and water interface with a metallic barrier. As the water temperature is changed so is the blood temperature, which enters the body or organ circulation changing the tissue temperature. Once the tissue temperature reaches the desired value the water temperature is maintained. Being able to cool the blood helps to preserve the organ and body by metabolizing fewer energy stores.

Because respiration is being controlled, and a machine is meeting metabolic demand, it is necessary to monitor the patient's blood chemical makeup. Chemical sensors placed in the blood path are able to detect the amount of oxygen bound to hemoglobin. Other, more elaborate sensors can constantly trend the blood pH, partial pressure of oxygen and carbon dioxide, and electrolytes. This constant trending can quickly analyze the metabolic demands of the body.

Sensors that communicate system pressures are also a necessity. These transducers are placed in areas where pressure is high, after the pump. Readings outside of normal ranges often alert the operator to obstructions in the blood flow path. The alert of high pressure must be corrected quickly as the heart-lung machine equipment may disengage under the stress of abnormally elevated pressures. Low-pressure readings can be just as serious alerting the user to faulty connections or equipment. Constant monitoring and proper alarms help to protect the integrity of the system.

Constant scanning of all components and monitoring devices is required. Normal values can quickly change due to device failure or sudden mechanical constrictions. The diagnosis of a problem and quick troubleshooting techniques will prevent additional complications.

Operation

The perfusionist or ECMO specialist monitors hemodynamic parameters associated with the patient and the device. Therefore, they are able to use the relationship of pressure, flow and resistance to make the appropriate changes to maintain adequate blood flow to vital organ beds. The standing orders from the physician, along with specific requests during the procedure are met by controlling the flow, temperature, gas exchange , pharmaceutical delivery and blood gas and electrolyte values. All requests and changes from protocol require documentation on the patient record.

Patient blood is drawn at 30-minute intervals to document arterial or venous blood gasses along with electrolytes and anticoagulation status. Decisions are made about device operation and pharmaceutical delivery based on these parameters. Abnormal values may indicate mechanical device failure.

Maintenance

Biannual preventive maintenance ensures proper device operation. The preventive maintenance as determined by the manufacturer should be followed. Electrical leakage tests are performed biannually on electrical components. The Joint Commission on Accreditation of Healthcare Organizations (JCAHO) requires documentation of these activities. A pre-bypass checklist is also completed before each procedure for review of the following categories: patient, sterility, pump, electrical, gas supply, lines/pump tubing, cardioplegia, safety mechanisms, monitoring, temperature control, supplies, anticoagulation and backup. Documentation of a completed checklist is kept with the patient record, and must be signed with date and time, by the device operator.

Health care team roles

At the sterile field the physician must react with the perfusionist or ECMO specialist in correcting problems that may interfere with the proper operation of the heart-lung machine. At the termination of device support the perfusionist or ECMO specialist must communicate clearly to the physician all changes in support status. This allows the entire team to assess changes in patient parameters that are consistent with the patient becoming less dependent on the device, while the native heart and lungs meet the metabolic demands of the body.

It is the responsibility of the perfusionist or ECMO specialist to be at the device controls at all times. Continuous scanning of all patient monitors is necessary for proper treatment and troubleshooting. Documentation of patient status is acquired every 15 to 30 minutes. This information allows the physician and nursing staff to follow trends that will help better manage the patient once treatment is discontinued.

KEY TERMS

Anticoagulant —Pharmaceuticals to prevent clotting proteins and platelets in the blood to be activated to form a blood clot.

Cannula —Tubes that provide access to the blood once inserted into the heart or blood vessels

Cardiopulmonary bypass —Diversion of blood flow away from the right atrium and return of blood beyond the left ventricle, to bypass the heart and lungs.

Extracorporeal —Circulation of blood outside of the body.

Training

A perfusionist earns a certificate of completion from a program accredited by the Commission on Accreditation of Allied Health Education Programs (CAAHEP). A bachelor's degree is required before entering the certificate program, or is received at the time of completion of the certificate-granting program. Professional certification can be earned from the American Board of Cardiovascular Perfusion or the International Board of Circulation Technologists. Licensure is required in some states.

Perfusionists, nurses, and respiratory therapists can receive additional training, as ECMO specialists, in the operation of ECMO equipment and institution specific protocols to operate venoarterial ECMO. The institution, in accordance with the guidelines established by the Extracorporeal Life Support Organization (ELSO), performs the training.

Resources

BOOKS

Gravelee, Glenn P., Richard F. Davis, Mark Kurusz, and Joe R. Utley. Cardiopulmonary Bypass: Principles and Practice. 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2000.

ORGANIZATIONS

American Society of Extra-corporeal Technology. 503 Carlisle Dr., Suite 125, Herndon, VA 20170. (703)435-8556. <http://www.amsect.org>.

Commission on Accreditation of Allied Health Education Programs. 1740 Gilpin Street, Denver, CO 80218. (303) 320-7701. <http://www.caahep.org>.

Joint Commission on Accreditation of Health Organizations. One Renaissance Boulevard, Oakbrook Terrace, IL60181. (630) 792-5000. <http://www.jcaho.org/>.

Extracorporeal Life Support Organization (ELSO). 1327 Jones Drive Suite 101, Ann Arbor, MI 48105. (734) 998-6600. <http://www.elso.med.umich.edu/>.

Allison Joan Spiwak, BS, CCP