Mechanical circulation support

Definition

Mechanical circulatory support is used to treat patients with advanced heart failure . A mechanical pump is surgically implanted to provide pulsatile or nonpulsatile flow of blood to supplement or replace the blood flow generated by the native heart . Types of circulatory support pumps include pneumatic and electro-magnetic pumps. Rotary pumps, which are also available, propel blood by axial or centrifugal force, or by the use of positive displacement roller pumps.

Purpose

Heart failure causes low cardiac output, which results in inadequate blood pressure and reduced blood flow to the brain , kidneys , heart, and/or lungs . Pharmaceutical and palliative surgical treatments are typically exhausted before mechanical circulatory support is initiated. The extent of failure exhibited by one or both ventricles of the heart determines if univentricular or biventricular support is required. In either case, blood flow is supplemented or replaced by a mechanical circulatory support device. The device works by removing blood from the inlet of the ventricle(s) and reinjecting it at the outlet of the ventricle(s) in order to increase blood pressure and blood flow to the brain, kidneys, heart, and lungs.

The Abiomed and Thoratec devices along with the intra-aortic balloon pump (IABP), centrifugal pump, and extracorporeal membrane oxygenation (ECMO) are systems that are meant to sustain the patient until the heart recovers. If recovery does not occur, or is not expected, then heart transplantation becomes the desired course of treatment. In this case intermediate-to long-term mechanical circulatory support devices are available. These longer-term devices include ECMO, Thoratec, Novacor, HeartMate, and Cardiowest products.

Description

Tertiary care facilities have the staff and equipment to provide treatment for heart failure patients, with the use of mechanical circulatory support devices. Short-, intermediate-, and long-term support requires bedside monitoring of the equipment and patient throughout treatment. The specialized nature of the equipment and intensive patient care requires dedicated staff who are able to provide continuous bedside treatment.

In most instances, patients receive pharmaceuticals that anticoagulate the blood by blocking the clotting factors from interacting with the foreign surfaces of the device and each other. Frequent laboratory testing determines the proper amount of medication required to prevent blood clots. To mimic the lining of blood vessels , some surfaces of the device attract native cells, which stick to the device surface, thereby eliminating the need for anticoagulation.

Blood flow generated by these devices is able to sustain blood pressure and flow to the heart, kidneys, liver , and brain. Temporary assist devices sustain vital organ tissues in situations where recovery of the heart function is anticipated. Long-term support devices sustain patients until a donor heart is available for transplantation.

Venoarterial ECMO circulatory support provides cardiopulmonary bypass. Both cardiac and pulmonary function can be supplemented with this device. The complexity of care and highly trained staff with specialized equipment limit the availability of ECMO to tertiary care facilities. Surgical cannulation is venoarterial, using the femoral or intrathoracic vessels. Postoperative care in the critical care unit requires dedicated bedside staffing.

Short-to intermediate-term devices

VENOARTERIAL ECMO. The positive displacement roller head pump provides pulsatile or non-pulsatile blood flow to the systemic circulation. Tubing connected to the venous cannula carries blood to the roller pump. The roller assembly rotates and engages the tubing, 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. Blood flow to the lungs is reduced as blood is drained from the venous circulation. Blood pumped by the left ventricle is also reduced as blood is returned directly to the systemic circulation. The heart is allowed to rest, pumping less blood than needed to maintain pressure and flow to the vital organs. An oxygenator is placed after the roller pump in the circuit. Gas exchange occurs prior to return of the blood to the arterial circulation.

As cardiac function improves, flow from ECMO support is reduced, allowing the heart to gradually resume normal function. The cannulae are surgically removed from the patient once the heart can maintain adequate cardiac output. Systemic anticoagulation is required throughout the length of support, and often leads to complications of stroke and coagulapathies. Long-term use of ECMO is limited since the patient is immobilized and sedated during treatment.

IABP. Ease of insertion for placement in the aorta makes the IABP the most often used univentricular assist device. Tertiary care centers provide this service in the cardiac catheterization laboratory, operating room, critical care unit, and emergency room. Secondary care level hospitals can also employ this technology. Well-trained staff are required to monitor equipment at regular intervals and troubleshoot problems.

Left ventricular support with the IABP reduces the workload of the heart and increases blood flow to the vital organs. Once in position, the IABP times the inflation and deflation of the balloon catheter to the electrocardiogram or arterial blood pressures waveform. Helium or carbon dioxide gases are used to fill the balloon, although low molecular weight helium is preferred because it can be transported rapidly. Carbon dioxide has the advantage that it is highly soluble in the blood in case of balloon rupture. The balloon inflates during diastole to deliver increased oxygen saturated blood to the heart. Blood flow is also increased to the arteries distal to the balloon, since flow is not occluded in either direction. Deflation of the balloon occurs prior to systole. Less contractile force is required for the heart to eject blood against a decreased afterload.

With recovery of the heart, the device is timed to inflate with every second or third heart beat. The catheter is removed, non-surgically, when the heart can sustain blood pressure and systemic blood flow. Therapeutic anticoagulation is achieved with minimal pharmaceutical anticoagulant throughout the treatment. The device can be in place up to several weeks, but duration is limited because the patient must be immobilized during the treatment.

CENTRIFUGAL PUMPS. Centrifugal pumps are able to provide uni-ventricular or bi-ventricular support to the ventricles. Blood is removed from the left or right atrium and returned to the aorta or pulmonary artery, respectively, therefore surgery is required to place the device.

Tertiary care facilities have the staff and equipment to provide treatment to heart failure patients with the use of mechanical circulatory support devices. Post-operative care in critical care units requires continuous monitoring by dedicated staff.

The cannulae are passed through the chest wall to attach to a pump that is magnetically coupled to the control unit, which is kept at the patient's bedside during treatment. The centrifugal force draws blood into the device and propels it to the arterial cannula. Rotational speed determines the amount of blood flow, which is measured by a flowmeter. If rotational frequency is too low, blood may flow in the wrong direction since the system is non-occlusive in nature.

As the heart recovers, flow is decreased from the centrifugal pump until the device can be removed. The native heart is then able to maintain blood pressure and flow. Anticoagulant is delivered continuously during treatment with a centrifugal pump, and patient immobilization limits the length of support to several weeks.

Intermediate-to long-term devices

When short-term support devices, such as ECMO, IABP, and the centrifugal pump are ineffective to sustain the patient to recovery or organ transplantation, a medium or long-term device is required. An advantage of treatment with a medium to long-term device is that it allows the patient to be mobile. In some instances patients have been able to leave the hospital for continued treatment at home with the implanted device. Complete recovery of the heart has been demonstrated in 5–15% of patients being supported as a bridge to organ transplantation.

PNEUMATIC PULSATILE. Pneumatically driven pulsatile paracorporeal mechanical circulatory support devices provide pulsatile support for the left or right ventricle, or both. Devices implanted in at least 100 patients by January 2001 include Abiomed, Thoratec, HeartMate, and Cardiowest brands. Staff are trained to monitor and troubleshoot equipment, thus limiting use to tertiary care facilities.

Cannulation of the left or right atrium, along with the aorta or pulmonary artery, respectively, requires a surgical approach. The cables that connect to the control center are tunneled out of the chest wall and the housing is typically implanted in the chest cavity, except Abiomed, which remains extracorporeal. The rigid outer housing encloses two compartments separated by a flexible boundary. Valves located at the inlet and outlet of the device direct the path of blood flow from high to low pressure, preventing back flow after ejection. Inflation of the gas chamber creates pressure in the blood chamber,

KEY TERMS

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

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

Cardiac —Of or relating to the heart.

Cardiac output —The liter per minute blood flow generated by contraction of the heart.

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.

Console —A freestanding device that monitors, measures, and controls parameters associated with the mechanical circulatory support device it operates.

Extracorporeal —Circulation of blood outside of the body.

Intracorporeal —Circulation of blood inside the body.

Paracorporeal —Circulation of blood near or close to the body.

which opens the outlet valve. Blood is then ejected until the chamber empties and pressure in the chamber decreases, closing the outlet valve. The inlet valve opens when the pressure is low enough in the blood chamber. Blood fills from the atrium and the inlet valve closes once the blood volume has increased the pressure. The cycle repeats itself when the controller fills the gas chamber again. The ejection is not typically timed to that of the native heart. The heart is emptied of blood by the assist device so there is little ejection from the native heart.

Removal of the device occurs at the time of cardiac transplant, unless the native heart has healed during support. Anticoagulation is achieved by low doses of pharmaceuticals. Some patients regain mobility while assisted by these devices.

ELECTRICAL PULSATILE. Novacor and HeartMate make devices that run electrically. Pusher plates activate the compression of the blood chamber for pulsatile blood flow. Cannulation and cable positioning are the same as for pneumatic devices. Valves are required for direction of blood flow and operation is the same as for pneumatic mechanical circulatory support. Electronic connections that use magnetic induction to cross the skin barrier, rather than cables tunneled through the chest wall decrease the risk of infection .

DESTINATION THERAPIES. Destination therapies intended to supplement or permanently replace the native heart are provided by chronic implantation of the mechanical circulatory support system. The Nimbus/TCI IVAS, the Jarvik 2000 IVAS, and DeBakey Micromed IVAS axial flow pumps are expected to achieve "first generation" chronic device trials in the United States. Low volume centrifugal pump technology includes the AB-180 Circulatory Support System, the HeartMate III LVAD, and the CorAide centrifugal blood pump. Pulsatile assist devices include the Thoratec Intracorporeal Ventricular Assist Device (IVAD), the Novacor II, the Worldheart HeartSaver VAD, and the Arrow Lionheart VAD. Total artificial hearts (TAH), made by Abiomed (AbioCor) and Penn State/3M, will replace the native heart. Upon removal of the native heart the TAH will be attached to the major blood vessels, thereby supplying blood pressure and flow to both the pulmonary and systemic circulation. No blood contact will be required with the Abiomed Heart Booster. The next five years, beyond 2001, expect to find these products in clinical trials, offering patients not eligible for organ transplantation a promising future.

Operation

The operator powers up the control console as equipment in the surgical field is inserted into the patient. Any tubing that will be connected to the patient is filled with crystalloid solution, which displaces any air that would be harmful to the patient if it entered the bloodstream. Once all sterile connections are complete, the physician will request that mechanical circulatory support be initiated. Rotational frequency is then increased or pneumatic pumping commences. Initial adjustments may be frequent, but decrease as the patient stabilizes.

Maintenance

Electrical maintenance is performed biannually to check consoles for leakage currents exceeding 100 mAmps. The Joint Commission on Accreditation of Healthcare Organizations (JCAHO) requires documentation of all electrical and mechanical maintenance activities. Specific manufacturer maintenance protocols must be followed to prevent mechanical failure. Physician orders provide the nursing and allied health staff with specific treatment instructions for maintenance of the patient on the support device.

Health care team roles

The physician, nursing, and allied health staff work as a team when patients are treated with mechanical circulatory support. Support initiation requires clear communication by the device operator of changes in device status that will alert the team to the changing condition of the patient. Once stable, the patient is transported to the intensive care unit (ICU). Any change in patient status is reported to the physician. Around-the-clock bedside care is provided by nursing staff trained to operate the mechanical circulatory support, or by nursing staff and an allied health professional trained in the operation of the particular mechanical circulatory support device in use.

Training

A circulation technologist 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 certification program, or is achieved by the time of completion of the certificate-granting program. Registered nurses usually receive in-service training from a circulation technologist, an experienced nurse, or a manufacturer representative. A respiratory therapist can pursue additional training as an ECMO specialist. Those who receive on-the-job training may also provide support services. Electrical and mechanical maintenance of the control unit is provided by biomedical engineers, who hold an associates or undergraduate degree in engineering.

Resources

BOOKS

DeBakey, Michael, and Antonio M. Gotto. The New Living Heart. Holbrook: Adams Media Corporation, 1997.

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

PERIODICALS

Stevenson, Lynne W. and Robert L. Kormos, et al. "Mechanical Cardiac Support 2000: Current Applications and Future Trial Design." The Journal of Heart and Lung Transplantation (January 2001): 1-38.

ORGANIZATIONS

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

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

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

OTHER

"Spare Hearts: A Houston Chronicle Four-Part Series." The Houston Chronicle, October 1997. <http://www.chron.com/content/chronicle/metropolitan/heart/index.html>.

Allison Spiwak