INTRODUCTION

A Ventricular Assist Device (VAD) is a mechanical circulatory device that is used to partially or completely replace the pumping action of a failing human heart. Unlike artificial heart, which completely takes over all the functions of the human heart, a Ventricular Assist Device, as the name suggests, aids the functioning of the ventricles of the heart. Any abnormality present in the ventricles can be effectively dealt by a Ventricular Assist Device.

Cardiac problems are a very common ailment in today’s world. People from all walks of life suffer from a plethora of cardiac complications. Ventricular malfunction is a common disease which leads to several problems viz. Lower Cardiac output, Angina Pectoris, poor circulation of oxygen and nutrients to the body, poor supply of deoxygenated blood to the lungs, etc. which leads to further physiological diseases. Ventricular Assist Device can effectively treat all such problems by treating the root cause of such ailments, the Ventricles. Proper contraction of Ventricular muscles will lead to a higher Cardiac output, which helps in delivering a higher volume of blood to the lungs, followed by proper delivery of oxygenated blood from the Left Ventricle. Thereby solving all the problems related to insufficient supply of oxygen and nutrients to the body.

All such aspects, classifications, working mechanisms, presently used Ventricular Assist Devices in the surgical arena, future of Ventricular Assist Devices and their shortcomings will be elaborated in the following pages. Although Ventricular Assist Devices have been in the world of cardiac surgery for more than a decade, this seminar work will stress upon Advanced Ventricular Assist Devices, which are technologically different from the conventional Ventricular Assist Devices. Technical details and methodology will be discussed as you browse through the pages to follow.

Happy reading and discovery!

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HISTORY AND PRESENT STATE OF VENTRICULAR ASSIST DEVICES

The very first Ventricular Assist Device was manufactured in the year 1994, the name coined was “HeartMate IP LVAS”. This device and its sister devices emulated the heart by using a pulsatile action where blood is alternately sucked into the pump from the left ventricle and then forced out of the aorta. FDA in the US had approved of such devices in October 1994. Such devices were termed as “First Generation VADs”.

Currently used Ventricular Assist Devices employ continuous flow pumps which can be classified into 2 types: Centrifugal pumps and Axial flow impeller driven pumps. These pumps have advantage over the former pulsatile action pumps because of their simplicity in working mechanism. Due to simplicity their sizes are also smaller. These devices are popularly known as “Second Generation VADs” or Advanced Ventricular Assist Devices. Working mechanism and classifications of Advanced Ventricular Assist Devices will be explained in details in the subsequent sections.

Ventricular Assist Devices are implanted to patients who have practically lost the ability of contraction of ventricles. The entire contraction mechanism of heart is taken over by this device, be it the right ventricle, the left ventricle or both. Since it takes over a major functionality of the normal heart, one might ask what is the difference between an artificial heart and a Ventricular Assist Device? The answer is, an artificial heart is a device that carries out all the functions of a healthy heart: intake of deoxygenated blood from the Vena Cavae, sending it to the lungs through pulmonary artery, receiving oxygenated blood from the pulmonary veins and finally releasing it through the aorta. The normal heart merely has a physical existence in this case. Ventricular Assist Device on the other hand performs with the impaired heart, it only assists the releasing/pumping of oxygenated blood from the impaired right ventricle to the aorta or pumping out the deoxygenated blood from right ventricle to the pulmonary artery. Basically, a Ventricular Assist Device is provided to support mere existence of the normal heart. Patients being implanted with Ventricular Assist Device must have their heart transplanted sooner or later because their heart has been severely damaged. Permanent Ventricular Assist Device do not have a huge utility because heart transplantation is the best solution for such an ailment rather than living on an artificial life support system.

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CLASSIFICATION OF VENTRICULAR ASSIST DEVICES

Advanced Ventricular Assist Devices can be classified under the following heads:

1) On the basis of period of use

Temporary Ventricular Assist Device: Ventricular Assist Devices which are meant for short term use, i.e when a patient is undergoing cardiac surgery or recovering from any cardiac ailment.

Permanent Ventricular Assist Device: Ventricular Assist Devices which are permanently transplanted into the human body for proper functioning of ventricles.

2) On the basis of impaired ventricle

Left Ventricular Assist Device ( LVAD): Ventricular Assist Device which helps in restoring the proper functioning of left ventricle.

Right Ventricular Assist Device (RVAD): Ventricular Assist Device which helps in restoring the proper functioning of right ventricle.

Bi- Ventricular Assist Device (BiVAD): Ventricular Assist Device which helps in restoring the proper functioning of both the left and right ventricles.

3) On the basis of Pumping Mechanism

Pulsatile Ventricular Assist Device: Ventricular Assist Devices which mimic the pumping action of the ventricles are termed as Pulsatile Ventricular Assist Devices.

Non-pulsatile Ventricular Assist Device: Centrifugal pumps or Axial flow pumps are employed to carry out the blood circulation through the ventricles.

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4) On the basis of suspension of rotors

Rotors are an important component of Ventricular Assist Device’s pump. This classification is of relevance to Advanced Ventricular Assist Devices only because the earlier versions did not propel the blood using rotors.

Bearing suspension: Advanced Ventricular Assist Devices use such rotor suspension technique where the rotor is suspended in the pumping chamber over a bearing chamber. As the rotor rolls over the bearings it creates a translatory motion in the chamber which pumps the blood out.

Electromagnetic or Hydrodynamic suspension: These technologies are still under research and has not been implemented in the device.

NEED OF VENTRICULAR ASSIST DEVICES

As mentioned in the introduction, cardiac ailments have become a major cause of mortality in the recent times, out of which ventricular problems find a great percentage. Invention of the Ventricular Assist Device has been able to solve this problem quite largely. Statistics of the US regarding cardiac ailments are listed below:

5 million people suffer from Congestive Heart Failure (CHF).

2,50,000 patients are in the advanced stage of CHF

5,00,000 new cases arrive each year

50,000 deaths are caused due to ventricular failure alone

Only-effective treatment for end stage ventricular failure is total Heart transplant.

But in the year 2008-2009:

7318 patients required a heart

2210 received one

623 perished waiting

ADVANCED VENTRICULAR ASSIST DEVICE

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Advanced Ventricular Assist Device is also termed as “Non-pulsatile Ventricular Assist Device”, because the main difference between the puslatile ones and the advanced is the pumping mechanism. Pulsatile Ventricular Assist Devices used positive displacement pumps to circulate blood which almost emulated the real contraction phenomenon of ventricles. But Advanced Ventricular Assist Devices uses continuous flow pumps which are of two types: Centrifugal and Axial flow pumps. The replacement of pulsatile technology with non-pulsatile technology was due to a few factors:

1) Irregular Haemodynamic flow of blood through the pulsatile pump.

Such a turbulent flow lead to haemolysis of blood corpuscles which lead to severe complications like clotting, emboli formation, thrombus formation. All such undesirable side effects lead to abandonment of the technology.

2) Large size

Large size of the pulsatile Ventricular Assist Device was undesirable as it had to be implanted within the body (most cases). Non pulsatile ones have smaller size.

3) Operating sound

Pulsatile Ventricular Assist Devices emitted sound during their operation which had an unaesthetic appeal. Non pulsatile have practically no sound during operation or at most may have a faint humming sound.

4) Large sized battery

Owing to its primitive design and working mechanism, the power consumption of pulsatile Ventricular Assist Device was high, thereby requiring a larger battery for its supply. Non pulsatile ones do not suffer from this problem.

Due to such positive desirable factors Non-pulsatile Ventricular Assist Devices became popular and are widely used now.

Key features of Advanced Ventricular Assist Device:

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Enabled with Continuous flow pumps

ECG independent

LVADs are the most popular Advanced Ventricular Assist Devices

Operation is practically soundless

Patient generally has a weak or non palpable pulse owing to the continuous pumping mechanism of the Ventricular Assist Device.

Patient may have a narrow pulse pressure and may not be measurable by normal blood pressure measuring instruments.

Mean arterial pressure is in the range of 65 to 90 mm Hg.

Although the systolic pressure is quite below the normal range of a healthy human being, one must not compare the blood pressure with that of a healthy human because the patient who is being put on a Ventricular Assist Device has practically lost his heart contraction ability. A mean blood pressure of 65 to 90 mm Hg is sufficient to circulate the blood through the entire body because the pumping action is continuous rather than pulse variant.

Components of an Advanced Ventricular Assist Device:

Outflow tube: attached to aorta

Inflow tube: attached to the apex (bottom) of left ventricle

Power source: driving the continuous flow pump, placed outside the body.

Driveline: connects the pump to the controller, placed outside the body.

Controller: controls the pump’s functioning, displays the status of the system and has alarm system.

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Fig1: Arrangement of Advanced VAD (Axial Pump) and its external components.

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Fig 2: Arrangement of Advanced VAD (Centrifugal Pump) and its external components.

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The Pump:

This is the heart of the Advanced Ventricular Assist Device. These devices use Continuous flow pumps, which are of two types:

1) Centrifugal flow pumps:

A central rotor is present which is suspended on bearings inside the pump chamber. Controlled electric currents running through coils contained in the pump housing apply forces to the electromagnets which causes the rotors to spin.

Fig 3.: Centrifugal flow pump.

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2) Axial flow pump:

This type of pumps are mostly preferred in manufacturing Advanced Ventricular assist devices. Similar to the centrifugal ones, the rotors are suspended over a bearing set but here the blades are helical in shape and are parallely aligned with the rotor’s axis.

Fig. 4: Axial flow pump.

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Key features of the continuous flow pumps:

1) Flow:

Measured in liters per minute (lpm): General range: (3-10)lpm

Flow is directly proportional to pump speed: Higher the pump speed greater is the flow rate.

2) Speed:

Measured in revolutions per minute (rpm): General range: (8000-15000) rpm

Flow speed is determined and fixed by the surgeons and cannot be manipulated. If, however this speed changes, it indicates thrombus formation inside the pump.

3) Power:

The amount of power a VAD consumes to run continuously at the set speed.

4) Pulsatility Index (PI):

It is the measure of the pressure difference inside the Ventricular Assist Device pump and the heart’s ventricle(s) if it is functioning partially. Patients suffering from severe cardiac problem have a PI of Zero.

Such important parameters are displayed on the Ventricular Assist Device’s console or controller.

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LIST OF VENTRICULAR ASSIST DEVICES USED:

Device Manufacturer TypeApproval Status as of

July 2009

Novacor World Heart Pulsatile.

Was approved for use in North America, European Union and Japan. Now defunct and no longer supported by the manufacturer.

HeartMate XVE Thoratec Pulsatile.

FDA approval for BTT in 2001 and DT in 2003. CE Mark Authorized. Rarely used anymore due to reliability concerns.

HeartMate II ThoratecRotor driven continuous axial flow, ball and cup bearings.

Approved for use in North America and EU. CE Mark Authorized. FDA approval for BTT in April 2008. Recently approved by FDA in the US for Destination Therapy (as at January 2010).

HeartMate III Thoratec

Continuous flow driven by a magnetically suspended axial flow rotor.

Clinical trials yet to start, uncertain future.

Incor Berlin Heart

Continuous flow driven by a magnetically suspended axial flow rotor.

Approved for use in European Union. Used on humanitarian approvals on case by case basis in the US. Entered clinical trials in the US in 2009.

Jarvik 2000 Jarvik Heart Continuous flow, axial rotor supported by

Currently used in the United States as a bridge to

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ceramic bearings.

heart transplant under an FDA-approved clinical investigation. In Europe, the Jarvik 2000 has earned CE Mark certification for both bridge-to-transplant and lifetime use. Child version currently being developed.

MicroMed DeBakey VAD

MicroMedContinuous flow driven by axial rotor supported by ceramic bearings.

Approved for use in the European Union. The child version is approved by the FDA for use in children in USA. Undergoing clinical trials in USA for FDA approval.

VentrAssist Ventracor

Continuous flow driven by a hydrodynamically suspended centrifugal rotor.

Approved for use in European Union and Australia. Company declared bankrupt while clinical trials for FDA approval were underway in 2009. Company now dissolved and intellectual property sold to Thoratec.

MTIHeartLVADMiTiHeart Corporation

Continuous flow driven by a magnetically suspended centrifugal rotor.

Yet to start clinical trials.

C-Pulse Sunshine HeartPulsatile, driven by an inflatable cuff around the aorta.

Currently in clinical trials in the US and Australia.

HVAD HeartWare Miniature "third generation" device with centrifugal blood path and hydromagnetically suspended rotor that may

Obtained CE Mark for distribution in Europe, January 2009. Initiated US BTT trial in October 2008 (completed February 2010)

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be placed in the pericardial space.

and US DT trial in August 2010.

DuraHeart TerumoMagnetically levitated centrifugal pump.

CE approved, US FDA trials underway as at January 2010.

Thoratec PVAD (Paracorporeal Ventricular Assist Device)

Thoratec

Pulsatile system includes three major components: Blood pump, cannulae and pneumatic driver (dual drive console or portable VAD driver).

CE Mark Authorized. Received FDA approval for BTT in 1995 and for post-cardiotomy recovery (open heart surgery) in 1998.

IVAD - Implantable Ventricular Assist Device

Thoratec

Pulsatile system includes three major components: Blood pump, cannulae and pneumatic driver (dual drive console or portable VAD driver).

CE Mark Authorized. Received FDA approval for BTT in 2004. Authorized only for internal implant, not for paracorporeal implant due to reliability issues.

COMPLICATIONS AND SIDE EFFECTS

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Like every technology, this technology too has its share of shortcomings. Some of the complications are:

1) Coagulation and Thrombosis:

This is one of the most predominant side effects of the Advanced Ventricular Assist Device. This happens mainly due to the flow of blood over non-biological surfaces. This problem has been eliminated by constructing the pump components with anticoagulant coated materials viz. Heparin or a fibrin based compound and polyurethane compounds.

2) Haemolysis:

It is a phenomenon where red blood corpuscles are destroyed due to non-laminar flow of blood. The non-laminar flow occurs across various sections of the Ventricular Assist Device viz. the pump, the conduits carrying the blood from the ventricles or to the aorta. Such a problem is eliminated by using elastic conduits which more or less resemble the elastic nature of normal blood vessels and by using blood straightener devices before and after the pump which creates a laminar flow.

3) Infection:

Ventricular Assist Devices infections can be caused by a large number of different organisms viz.:

Gram positive bacteria (Staphylococci, etc)

Gram negative bacteria (Pseudomonas aeruginosa, etc)

Fungi (Candida, etc)

Treatment of Ventricular Assist Device related infection is very difficult despite optimal treatment. But every effort is made to contain proliferation of such organisms. Practicing sterile procedures during every step of the operation and also during maintenance is necessary.

4) Portability:

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This remains one of the major disadvantages of this technology. The majority of the Advanced Ventricular Assist Devices present today are somewhat bulky, therby rendering normal movement of human a bit difficult and uneasy. The smallest device, the HeartMate II weighs about 0.45 kg and measures 7.6 cm. This has been proven the best device for women and children for whom other devices would be bulky. As technology progresses we hope to see even more compact and reliable devices being discovered and used.

CONCLUSION

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Advanced Ventricular Assist Devices have been a boon to cardiac patients suffering from severe ventricular failure. It has been a long time since this device was first designed, the device has evolved and through extensive research and development it has reached almost a supreme state. But as other technologies and methodologies would develop this present technology would gradually become outdated. For instance, the latest form of Ventricular Assist Devices, still under development, employs Hydrodynamic suspension of the rotor and its blades within the pump. This technology would drastically decrease the power consumption of the device, probably would eliminate the usage of blood flow straighteners, thereby making the devices more compact. FDA and other medical bodies would be there to ascertain the credibility of such a technology and its implications. Till then, this present technology would continue to serve innumerable patients suffering from ventricular ailments and gift them better lives.

REFERENCES

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1) www.thoratec.com

2) www.jarvikheart.com

3) www.google.com

4) Google Scholar

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