Conference Lectures

Mechanical Circulatory Assistance
Prof. VK Arya MD, FRCPC.
Visiting Professor University of Manitoba,
Medical Staff Anesthetist WRHA Cardiac
Anesthesia Programme, St. Boniface Hospital,
Winnipeg, Canada. R2H 2A6

During the past decade, mechanical cardiac assist devices have gained widespread
acceptance as therapeutic instruments for the treatment of cardiac insufficiency. The first
indication for use of mechanical circulatory assist was proposed for the treatment of
postcardiotomy cardiogenic shock. Initially some circulatory devices were designed for this
purpose, “pending recovery of the natural heart.” At present time, orthotopic heart
transplantation is acknowledged as the best therapy for end stage congestive cardiac failure.
Many of the circulatory assist devices developed subsequently are currently being used for a
second indication: temporary cardiac support until a donor heart becomes available for
transplantation or “bridge to transplantation.” Current indications for left ventricular assist device
(LVAD) insertion include reversible ventricular dysfunction occurring after cardiac surgery,
temporary support for patients awaiting heart transplantation, and destination therapy for patients
who are not candidates.
   In 1958 Harken described for the first time a method to treat left ventricular failure by
using counter-pulsation or diastolic augmentation. He suggested removing a certain blood
volume from the femoral artery during systole and replacing this volume rapidly during diastole.
By increasing coronary perfusion pressure, this concept would therefore augment cardiac output
and unload the dysfunctioning heart simultaneously. Then in the early 1960s Moulopoulus et al.
from the Cleveland Clinic developed an experimental prototype of the intra-aortic balloon (IAB)
whose inflation and deflation were timed to the cardiac cycle. In 1968 the initial use in clinical
practice of the IABP and its further improvement was realized. In 1966, DeBakey and colleagues
implanted a blood pump in a patient who had undergone double valve replacement, making this
the first successful use of an LVAD for postcardiotomy heart failure.
Types of Left ventricular assist devices:
I. Counterpulsation: Intra Aortic Balloon Pump (IABP)
II. Blood pumps:
1. Displacement pumps ( Pulsatile VAD):
a. Extra carporial : Thoratec, Abiomed b.
Implantable : Heartmate I, Novacor, CardioWest,
2. Rotary pumps (Non-pulsatile continuous flow VAD)
a. Axial flow pumps: Impella, Hemopump, Nimbus, Heartmate II, DeBakey
b. Radial (Centrifugal) pumps: Rotaflow, Biopump, Heartmate III
c. Diagonal pumps: Ventrassist, Heartquest, HIA microdiagonal
3. Total artificial heart (TAH): CardioWest C-70 TAH, Nimbus TAH
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I. Counterpulsation:
IABP is the simplest and most widely used device throughout the world for supporting
patients after cardiac surgery, angioplasty, and myocardial infarction or with various low-output
syndromes. It increases perfusion to the coronary arteries with diastolic inflation and decreases
afterload and, thus, left ventricular work and oxygen consumption with systolic deflation, but it
cannot provide complete mechanical support.
Physiologic Effects of IABP Therapy
After correct placement of the IAB in the descending aorta with its tip at the distal aortic arch
(below the origin of the left subclavian artery) the balloon is connected to a drive console. The
console itself consists of a pressurized gas reservoir, a monitor for ECG and pressure wave
recording, adjustments for inflation/deflation timing, triggering selection switches and battery
back-up power sources. The gases used for inflation are either helium or carbon dioxide. The
advantage of helium is its lower density and therefore a better rapid movement and diffusion
coefficient. Whereas carbon dioxide is highly soluble in blood and thereby reduces the potential
consequences of gas embolization following a balloon rupture. Inflation and deflation are
synchronized to the patients’ cardiac cycle. Inflation at the onset of diastole results in proximal
and distal displacement of blood volume in the aorta. Deflation occurs just prior to the onset of
systole. The primary goals of IABP treatment are to increase myocardial oxygen supply and
decrease myocardial oxygen demand. Secondary, improvement of cardiac output (CO), ejection
fraction (EF), an increase of coronary perfusion pressure, systemic perfusion and a decrease of
heart rate, pulmonary capillary wedge pressure and systemic vascular resistance occur.
In particular systolic wall tension uses approximately 30% of myocardial oxygen
demand. Wall tension itself is affected by intra-ventricular pressure, afterload, end-diastolic
volume and myocardial wall thickness. Regarding to the studies of Sarnoff et al. the area under
the left ventricular pressure curve, (TTI = tension-time index), is an important determinant of
myocardial oxygen consumption. On the other hand, the integrated pressure difference between
the aorta and left ventricle during diastole (DPTI = diastolic pressure time index) represents the
myocardial oxygen supply (i.e. hemodynamic correlate of coronary blood flow). Balloon
inflation during diastole augments diastolic pressure and increases coronary perfusion pressure
as well as improving the relationship between myocardial oxygen supply and demand (DPTI to
TTI ratio).
To achieve optimal effect of counter-pulsation, inflation and deflation need to be
correctly timed to the patient’s cardiac cycle. This is accomplished by either using the patient’s
ECG signal or the patient’s arterial waveform. An intrinsic pump rate is used on bypass or when
ECG and pressure waveform can not be used. The most common method of triggering the IAB is
from the R wave of the patient’s ECG signal. Mainly balloon inflation is set automatically to
start in the middle of the T wave and to deflate prior to the ending QRS complex.
Tachyarrhythmias, cardiac pacemaker function and poor ECG signals may cause difficulties in
obtaining synchronization when the ECG mode is used. In such cases the arterial waveform for
triggering may be used. It is important that the inflation of the IAB occurs at the beginning of
diastole, noted on the dicrotic notch on the arterial waveform. Deflation of the balloon should
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occur immediately prior to the arterial upstroke. Balloon synchronization starts usually at a beat
ratio of 1:2. This ratio facilitates comparison between the patient’s own ventricular beats and
augmented beats to determine ideal IABP timing. Errors in timing of the IABP may result in
different waveform characteristics and a various number of physiologic effects.
If the patient’s cardiac performance improves, weaning from the IABP may begin by
gradually decreasing the balloon augmentation ratio (from 1:1 to 1:2 to 1:4 to 1:8) under control
of hemodynamic stability. After appropriate observation at 1:4 or less counter-pulsation the
balloon pump is removed.
Indications and Contraindications
Early purposed indications for intraaortic balloon pumping have included cardiogenic
shock or left ventricular failure, unstable angina, failure to separate a patient from
cardiopulmonary bypass and prophylactic applications, including stabilization of preoperative
cardiac patients as well as stabilization of preoperative non-cardiac surgical patients. Today more
extending indications are: Cardiac patients requiring procedural support during coronary
angiography and PTCA, or as a bridge to heart transplantation. Further on in pediatric cardiac
patients and as well as in patients with stunned myocardium, myocardial contusion, septic shock
and drug induced cardiovascular failure the IABP can be life-saving.
IABP therapy should only be considered only for use in patients who have the potential
for left ventricular recovery, or to support patients who are awaiting cardiac transplantation.
Absolute contraindications of IABP are relatively few. These includes: severe aortic valve
insufficiency, aortic dissection, irreversible brain damage and in patients with acute trauma to the
descending aorta.
Complications: Incidence of complications has decreased significantly as experience with
the device has increased; however, IABP therapy in today’s patients` population does still hold a
risk for complications. In elderly (68 - 80 years), and female patients who very often may suffer
from severe peripheral vascular disease and hypertension or diabetes, most common vascular
complication is limb ischemia. It may occur in 14-45% of patients receiving IABP therapy.
Therefore the patient must be consistently observed for any symptoms of ischemia during IABP
counter-pulsation. If signs of ischemia appear the balloon should be removed. Other
complications include arterial injury, femoral artery thrombosis, peripheral embolization,
infection and hemorrhage. In general, vascular injuries should be dealt with directly by surgical
interventions and repair. Balloon related problems like perforation/ rupture of balloon, incorrect
positioning and gas embolization require removal and / or replacement of the IAB.
II. Blood pumps:
Generally, pumps can be classified into two main categories: Displacement pumps and
rotary pumps. The energy transfer in displacement pumps is characterized by periodic changes of
a working space. In rotary pumps, the energy transfer to the fluid is established by velocity
changes within the impeller vanes. It can be generally stated that, for large volume flows and low
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pressures, the advantages of rotary pumps predominate, while for low-volume flows and high
pressures displacement pumps are generally more suitable.
1. Displacement pump: These (e.g. roller pumps) have been established over decades for
cardiopulmonary bypass in heart–lung machines or in dialysis machines. Roller pumps are still
predominant and are commonly used for up to several hours. Their main advantages are
simplicity of operation, low cost of disposable tubing and reliability, whereas disadvantages
include blood damage and particulate spallation. The Novacor VAD and Thermedics VAD
represent prototypes of implantable displacement pump devices for cardiac assist. Both devices
work in a similar fashion and provide pulsatile assistance to the failing left ventricle. These
devices are indicated for profound cardiogenic shock that is unresponsive to maximal inotropic
support and IABP after cardiac surgery; but mainly used as a bridge to heart transplantation.
Novacor is electromechanical device. It is implanted in the left upper quadrant of the abdomen,
anterior to fascia of the rectus abdominis muscle. Its weight is 3.3 Kg and occupies volume of
400 ml. Inflow to device is from a hole in the apex of ventricle and out flow is to ascending aorta
or abdominal aorta. It can produce flows up to 8 litres per minute. Novacor VAD can be
triggered in three different settings: 1) at a fixed rate 2) following the native QRS complex of
ECG and 3) following change in the rate of pump filling. Mostly it is used in third setting that
allows the ventricular systole to fill the device and trigger device ejection. Device has been used
in VF also where filling of device occurs by pressure gradient; 10 mm Hg is the minimum
required. The Thermedics VAD is totally implantable, pneumatically driven dual chamber pump
in rigid titanium housing. It is implanted intra-abdominally in the left upper quadrant. It has
porcine valves at inlet and outlet for unidirectional flow. Relative exclusion criteria for Novacor
pumps are: 1) Weight < 50Kg, 2) BSA <1.5 m 2 , 3) age > 65 yrs and 4) severely depressed RV
with EF <10%, in addition to other general contraindications for VAD devices.
2. Rotary pumps:   In these pumps a series of rotating vanes within the device’s body move
blood forward, producing non-pulsatile flows. Depending on impeller geometry, rotary blood
pumps can be classified into three main categories: axial, radial (centrifugal) and diagonal
(mixed flow) pumps. Rotary pumps are best suited for high flows up to 20 l/min at differential
pressures lower than 500 mmHg. The device is after load dependent, with increasing peripheral
resistance lowering output. The radial design is the one most capable of producing high pressures
and low flows, whereas axial pumps generate high flows at low pressure differences and
diagonal pumps, often referred to as mixed flow systems, tend to have the capability of high
generated pressures and high flows. This simple classification of potential pump designs is
normalized with respect to pump size, taking into consideration that a 60 mm diameter
centrifugal pump can naturally pump more fluid at significantly higher pressures than a 6 mm
diameter axial pump. Due to several theoretical and practical advantages of rotary pumps in
terms of lower blood damage, smaller size, lower filling volume, better transportability and
absence of spallation, a number of rotary pump types have been introduced for medical
application in recent years. The majority of them are of the centrifugal type. Rotary blood pumps
have a number of potential physiological and technological advantages that are characterized by
low blood trauma, lower anticoagulation levels and, thus, less hemorrhage. Depending on pump
design, priming volume may be low and a surface modification such as heparin coating is
possible.
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a) Axial Flow Pump: Hemopump (Medtronic, Inc, Minneapolis, MN) was conceived by Dr
Richard Wampler in 1975 and it was designed around 1982 while Dr Wampler worked with
Nimbus Corporation in Rancho Cordova, California. As a temporary cardiac assist system, it is
intended to assume up to 80% (nonpulsatile flow up to 3.5L/min) of the workload of the resting
heart for at least 7 days, thus giving the heart in cardiogenic shock the opportunity to rest and
recover. It shares some of the simplicity of the IABP in the sense that this device is actually a
miniature axial flow pump at the end of a catheter. It is inserted through a 12 mm woven graft
that is sutured to a femoral or iliac artery. The cannula is advanced into aorta, across the aortic
valve and the tip is positioned at the apex of the left ventricle under fluoroscopic guidance. The
axial pump actually sits in the descending aorta in a 7×16 mm cylindrical housing at the end of a
20 cm long flexible inflow cannula. It aspirates blood from the LV and pumps it directly into the
descending aorta. This pump generates 3L/min when rotating at 24,500 rpm, discharging into an
output load of 100-mmHg pressures. Hemopump is a disposable, onetime use device.
Disadvantage of this device is that it cannot provide RV support, useful for short duration, (13-
120 hrs), requires an artery with 7-10 mm lumen, injury to aortic valve, hemolysis and ischemic
injury to extremity used for placement.
b) Centrifugal pumps: This family of pumps utilizes centrifugal force to generate a non-
pulsatile flow that depends on the rotational speed of the pump. Blood enters the pump at the
apex and is accelerated outwards. The pump is electromagnetically coupled to a motor that
makes pump console completely sealed from blood contacting surface of pump head. These
pumps can be used to assist RV or LV or both. The inflow cannula to pump is joined to right or
left atrium and out flow goes to pulmonary artery or aorta. They can produce high flows with
essentially all cardiac output moving through the pump or can operate at reduced speeds to serve
as a ‘‘booster’’ to cardiac output, with some blood flow through the pump and some through the
patient’s own aortic valve.   These pumps are extracorporeal and require ACT above 1.5 times
normal with continuous heparin infusions.
c) Diagonal pumps: These pumps, often referred to as mixed flow systems, tend to have the
capability of high generated pressures and high flows. When high pressures in the order of up to
600 mmHg are mandatory, as in heart–lung machines, radial pumps with a significantly larger
diameter are the first choice. Therefore, it is likely that both centrifugal and diagonal pumps may
be used in extracorporeal circulation, whereas axial pumps will operate at physiological
pressures in heart assist or local organ perfusion applications.
3. Total artificial heart:
When a biventricular failure occurs, a TAH can be implanted as bridge to transplantation or
destination therapy. The most common TAHs are the AbioCor (Abiomed Inc., Danvers,MA,
USA) and the CardioWest (CardioWest Technologies Inc., Tucson, AZ, USA). AbioCor is a
construct of titanium and a polymer with artificial heart valves. It is the first TAH system that
does not require percutaneous lines. The internal components include the AbioCor thoracic unit,
the battery, the controller, and the TET system. The external components are a TET coil with a
module, batteries, and a bedside console. The thoracic unit is placed in the orthotopic position in
the chest and consists of an energy converter and two pumping chambers that are compressed by
a miniaturized centrifugal pump driven by a brushless DC motor. There is a chamber with a low
viscosity hydraulic fluid and a 2-position valve to alternate the flow between the right and left
pumping chambers. The blood-contacting surfaces of the thoracic unit
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are made of polyurethane (Angioflex; Abiomed Inc.). By the means of a radiofrequency
telemetry, important device performance data can be transmitted from the internal controller to
the bedside console. The TAH AbioCor has a large dimension, making an implantation only
possible in a few male candidates with appropriate chest dimensions, but not in female
candidates and children due to the risk of pulmonary vein compression. Furthermore, pulmonary
problems arise if there is a mismatch between the right and left heart blood flow. The first
AbioCor model is not available anymore, but a smaller AbioCor II is under development. The
CardioWest is also implanted in the orthotopic position and is pneumatically driven. The rigid
pump housing contains dual spherical polyurethane chambers. The inflow and outflow conduits
are made of DacronTM (CardioWest Technologies Inc.) and contain Medtronic Hall valves
(Medtronic Inc). This device lacks a portable control consult. Another electromechanical TAH is
a double-chamber diaphragm pump that replaces the explanted ventricles functionally and
anatomically. The main components are the two diaphragm pump chambers with inlet and outlet
valves and an electromechanical energy converter. Whereas the inlets are connected to the
natural atrias, the outlets are connected to the aorta and the pulmonary artery. The energy for the
motor is delivered via the TET coils, a buffer battery, and a power supply that is worn in a
shoulder belt.
Indications and contraindications for implantation:
Today, there are three indications for the implantation of long-term mechanical cardiac
support devices:
(1) Bridge to cardiac transplantation,
(2) Bridge to recovery, and
(3) Destination therapy.
Usually, it is indicated to implant a cardiac support device if the patient suffers from a
severe heart failure therapy refractory to the conservative treatment options. Following criteria
should be met for placement of these devices: 1) Approved transplant /destination candidate, 2)
SBP< 90 mmHg or MAP < 60 mm Hg with CVP/PAOP ≥ 20 mm Hg and CI < 2L/min/m 2 ,
despite maximal inotropic and IABP support and 3) pre-renal failure (urine output <20 mL/hour).
Contraindication for VAD devices are: 1) CRF requiring hemodialysis with in 1 month
prior to surgery, 2) unresolved pulmonary infarction secondary to PTE, 3) fixed PAH with PVR
> 6 wood units after trial of PGE 1 or 100% O 2 therapy, 4) severe hepatic failure with total
bilirubin >10 mg/dL or biopsy proven cirrhosis, 5) unresolved/ metastatic malignancy, 6) severe
blood dyscrasia, 7) irreversible neurological deficit or severe cerebrovascular / peripheral
vascular disease, 8) history of stroke or TIA, 9) acute systemic infection/ sepsis and 10) positive
HIV status.
Complications and side effect: With all these devices infection remains the main problem.
With non-pulsatile flow generators development of gastrointestinal vascular malformations and
GI bleed remains main problem. With pulsatile pumps hemolysis is main problem due to RBC
trauma by mechanical pump. New small pumps have been developed whose battery can be
charged by electromagnetic field and do not have wires to come out from body.