Inside Story Issue 3

The technology of heart transplants has been honed for almost 40 years now. Survival rates are constantly improving and great progress has been made on the problem of organ rejection. In fact, a challenge arguably tougher than surgery today is the ever-widening gap between the number of cardiac transplant candidates and the number of donor organs available to help them.
Issue 3
4th Quarter
2006
A Mitutoyo America Publication

The good news is that a technological “bridge” solution to the donor shortage has emerged in recent years. It’s called a Ventricular Assist Device or VAD. A VAD is a surgically implanted pump designed to help a weakened heart circulate blood. VADs are now being used as an effective and safe therapy to help patients in the transplant waiting line.  In the last few years, VADs are also being used to provide long term support to many non-transplant eligible patients.

The VAD (or LVAD, when the pump is specifically implanted to assist the left ventricle—the heart’s main pumping chamber) comes in two varieties. The first is a device whose pulsatile pumping action fairly closely emulates that of the heart itself. Developed somewhat later, the rotary (centrifugal) pump provides a continuous flow by utilizing a rotor, sweeping along blood, as its only moving part.

In traditional rotary models, the rotor turns on a mechanical bearing.  In others, a thin layer of blood lubricates the bearing surface. However, the HeartQuest VAD from WorldHeart Corporation of Oakland, California, (www.worldheart.com), now in a European clinical trial, eliminates the need for any bearing—blood-lubed or mechanical. The HeartQuest rotor is both levitated and spun by magnetic fields generated within the third-generation device—a principal benefit being that the clearance between the bottom of the titanium-alloy rotor and titanium-alloy housing is large enough so that fragile blood components never experience destructive shearing forces.

The HeartQuest’s proprietary MagLevTM technology uses a combination of passive magnetic fields (from permanent magnets embedded in the rotor and housing) and a single-axis of active magnetic control (from pulsed fields set up by stator windings in the housing) to both suspend and spin the rotor. A sensor internal to the housing works with a feedback controller to keep the rotor centered in the housing.  And with no bearing of any kind to wear, the useful working life of the implant should be greatly extended.

The innovators
WorldHeart Corporation, source of the innovative technology in the HeartQuest VAD, specializes in developing, manufacturing and marketing implantable heart assist devices for late-stage and end-stage congestive heart failure.  Established in 1986, the company expanded in August, 2005 by acquiring MedQuest Products, Inc., headquartered in Salt Lake City, Utah. This strategic move added MedQuest’s advanced rotary pump technology to WorldHeart’s existing pulsatile device portfolio (their Novacor™ LVAS). As a result, WorldHeart is currently the only firm offering both types of pump to serve the needs of heart failure patients.

A closer look at HeartQuest’s rotary VAD
The HeartQuest’s housing is a small, well-radiused, almost round titanium-alloy structure, about 3” in diameter and about 1.5” thick.  It consists of three, match-machined parts.  A large portion of the interior of the housing is a blood flow pathway that contains the rotor.  Also in the blood path, on the downstream side of the rotor, is a critically-shaped cavity called a “volute,” machined into the housing walls. Its shape helps control blood flow.  The housing also includes securely potted electronic circuitry, a sensor that can measure position of the rotor to within less than 1 mil, and magnet (stator) wiring.  Inlet and outlet cannulas are attached to the blood pathway.  After assembly, the housing is sealed by laser-welding and then helium leak-tested.

During implantation, the HeartQuest’s blood inlet cannula is sutured into the apex of the left ventricle. The outlet cannula is sewn to the ascending aortic arch, which sends oxygenated blood to the entire body.  A lead wire extends from the pump out through the patient’s skin to permit communication with an external controller and receipt of power from a rechargeable battery, both of which the patient can wear on a belt for mobility.

A blood pump’s internal surfaces are critical
In the HeartQuest pump, the 3-dimensional contoured volute on the downstream side of the rotor enhances hydraulic performance. It converts kinetic energy (arising from the blood’s velocity) to potential energy (pressure or head), and straightens and equalizes flow coming out of the rotor. However, the contour must not encourage blood stagnation and the possibility of thrombosis or hemolysis. In general, a critical concern in an implantable pump is avoidance of trauma to cellular components of blood by assuring smooth surface transitions. Friction heating can also damage blood. Areas presenting angular changes in internal pump geometry must be avoided since they not only heat the blood but force it to take abrupt directional changes, mechanically working it. Another way to reduce friction and discourage blood stagnation is to create a surface “texture” which promotes/stimulates natural deposition of blood constituents to create a stable intermediary surface, which keeps blood flowing smoothly over it.

How WorldHeart tests form and surface on its rotary pump
To check the quality of surfaces in the interior of the titanium housing of the HeartQuest, WorldHeart uses a Mitutoyo SJ-400 compact profilometer.  Says Neal Maughan, a Quality Engineer at the company, “We supply roughness specs via CAD files to the outside supplier who does our machining—so we’re simply checking their compliance.”

Controlling this parameter allows the company to assure that its pump’s internal surfaces are such that blood will experience no abrupt transitions.  Maughan explained, “Essentially, we want a surface that won’t interrupt flow or cause any turbulence that could damage red blood cells.”

The Mitutoyo SJ-400 is capable of making as many as 36 kinds of roughness measurement to satisfy the various requirements of the latest standards issued by ISO, DIN, ANSI and JIS. The instrument provides a measuring resolution of 0.000125µ-m/0.005µ-in. and a traverse range of 800µ-m/0.032µ-in. Surface roughness is analyzed and evaluated using SURFPAK-SJ, the SJ-400’s roughness analysis software program.

To verify the conformance by the machining supplier to their specifications regarding the form (geometry) of the volute in their pump, Maughan says WorldHeart relies on high-accuracy contour measurements from a Mitutoyo Contracer CBH-400 equipped with Mitutoyo FORMPAK software. Measured profile geometry is overlaid on a CAD model to check for deviations from the design. According to Maughan, “We’re especially pleased with the ability of our Contracer to amplify measurements as it creates a profile trace for us. That lets us make measurements on the volute and rotor more accurately.”

FORMPAK makes it possible to get precise contour readouts on all curved surfaces at any magnification up to 200X. The instrument has a measuring range of 4 in. for the X axis and 1.57 in. for the Z axis.

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