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 non-pulsatile flow of blood to supplement or replace the blood flow generated by the native heart. Types of circulatory support pumps include pneumatic and electromagnetic pumps. Rotary pumps are also available.


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 surgical treatments (other than transplantation) are all 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.

Some 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 next desired course of treatment. In this case, intermediate- to long-term mechanical circulatory support devices are required.


Description

Short-, intermediate-, and long-term support requires bedside monitoring of the equipment and patient throughout treatment. The specialized nature of the equipment and the intensive patient care require dedicated staff who are able to provide continuous bedside treatment.

In most instances, patients receive anticoagulants, drugs that prevent clots in the blood. 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 the body's cells, which stick to the device surface and eliminate 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.


Short- to intermediate-term support devices

ECMO circulatory support provides cardiopulmonary bypass. Both cardiac and pulmonary (lung) function can be supplemented with this device. The complexity of care and the need for highly trained staff with specialized equipment limit the availability of ECMO to specialty care facilities. Surgical cannulation (placement of tubes) is required. Postoperative care in the critical care unit requires dedicated bedside staffing.

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. 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.

Ease of insertion for placement in the aorta makes the intra-aorta balloon pump (IABP) the most often used ventricular assist device . Specialty 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 (the lower left chamber of the heart) support with the IABP reduces the workload of the heart and increases blood flow to the vital organs. The balloon inflates during diastole (the filling phase of the heart) to deliver increased oxygen-saturated blood to the heart; blood flow is also increased to the arteries. Deflation of the balloon occurs prior to systole (the emptying phase of the heart).

With recovery of the heart, the IABP 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. Anticoagulation is achieved with minimal drugs 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 are able to provide support to one or both ventricles. Blood is removed from the left or right atrium (upper chamber) and returned to the aorta or pulmonary artery, respectively; therefore, surgery is required to place the device. Specialty care facilities have the staff and equipment to provide treatment to heart failure patients with the use of mechanical circulatory support devices. Postoperative 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 draws blood into the device and propels it to the arterial cannula. As the heart recovers, blood flow is decreased from the centrifugal pump until the device can be removed. An anticoagulant drug is delivered continuously during treatment with a centrifugal pump, and patient immobilization limits the length of support to several weeks.


Intermediate- to long-term support 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.

Pulsatile paracorporeal mechanical circulatory support devices provide pulsatile support for the left or right ventricle, or both. Cannulation of the left or right atrium, along with the aorta or pulmonary artery, respectively, requires a surgical approach. The heart is emptied of blood by the assist device, so there is little ejection from the body's heart.

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


Destination therapies

Destination therapies intended to supplement or permanently replace the body's heart are provided by chronic implantation of the mechanical circulatory support system. For example, total artificial hearts (TAH) replace the body's 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. Destination therapies are currently in clinical trials, offering those patients not eligible for organ transplantation a promising future.


Preparation

General anesthetic is given to the patient if a chest incision will be used to expose the heart or if blood vessels need to be exposed. Sedation with local anesthetic is sufficient if the vessels can be accessed with a needle stick. Cardiac monitoring will be performed, including electrocardiograph and cardiovascular pressures. Blood tests prior to surgery are used to measure blood elements and electrolytes. Once all sterile connections are complete, the physician will request that mechanical circulatory support be initiated. Adjustments may be frequent initially, but decrease as the patient stabilizes.


Normal results

Once stable following device implant, the patient is cared for in the intensive care unit (ICU). Any change in patient status is reported to the physician. Around-the-clock bedside care is provided by trained nursing staff.

These patients are very ill when they require device implant, often suffering from multi-system organ failure as a result of poor blood flow. The long-term survival is superior at one year when compared to medical treatment alone. Patients that continue to improve on intermediate-, long-term, and TAH increase in activity level and begin a regular exercise program. Eventually, with proper training about device maintenance, they are able to leave the hospital to live at home, returning to a normal lifestyle, until further medical treatment is required.


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, and Joe R. Utley. Cardiopulmonary Bypass: Principles and Practice, Second Edition. Philadelphia: Lippincott Williams & Wilkins, 2000.


periodicals

Stevenson, Lynne W., 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 St., Denver, CO 80218. (303) 320-7701. http://www.caahep.org .

Extracorporeal Life Support Organization (ELSO). 1327 Jones Dr., Ste. 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, IL 60181. (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 Joan Spiwak, BS, CCP



User Contributions:

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Athena
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Mar 23, 2012 @ 9:09 am
You've really captured all the esseintals in this subject area, haven't you?

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