MECHANICAL CIRCULATION SUPPORT
Category: Cardiovascular Surgery
Abstract :
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
pu
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-theclock 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.
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