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Ann Thorac Surg 2006;81:2338-2339
© 2006 The Society of Thoracic Surgeons

Correspondence

MagScrew Total Artificial Heart

Soren Schenk, MD, Stephan Weber, Dipl Ing, William A. Smith, DEng, PE, Kiyotaka Fukamachi, MD, PhD

Department of Biomedical Engineering/ND20, The Cleveland Clinic Foundation, 9500 Euclid Ave, Cleveland, OH 44195

(Email: fukamak{at}ccf.org).

To the Editor:

We appreciated seeing our article in The Annals of Thoracic Surgery [1], and we read Dr Holman's [2] invited commentary with interest. In response, we would like to provide some additional information addressing the points he raised, which were not issues addressed by our original article.

The maximum pressure in the original version of the HeartMate® (Thoratec Corp, Pleasanton, CA) left ventricular assist system has been reported to be 400 mm Hg [3]. Thoratec has since made improvements in the HeartMate's ejection dynamics [4, 5]. Measurements on the MagScrew total artificial heart (TAH) show maximum pressure values in the range of 120 to 140 mm Hg. These values are within the acceptable range defined for us by Edwards LifeSciences (Irvine, CA), the valve manufacturer. We have also studied some design refinements that would further lower the maximum pressure and the related rate of pressure change. It should also be noted that the valve endurance tests at Edwards are run at rates of 1,000 to 1,200 beats/min, far exceeding the beat rates of the pump. Actual in vivo durability of the valves has not yet been shown and cannot be effectively tested in calf implants because of higher juvenile calcium metabolism and high infection rates of calf models.

With respect to the effect of beat rate on durability in the MagScrew TAH, the resulting internal velocities and loads are accounted for in the design calculations and tests. Analysis indicates that the main ball bearings would be the life-limiting mechanical component, but they should also have a ≥90% survival at 15+ years. The MagScrew TAH beat rates do not pose major issues with respect to durability. Flow visualization studies show good wash flow patterns at all beat rates. Perhaps the flow patterns are more a function of the percent stroke than of the speed of stroke.

Similarly the MagScrew TAH does not have efficiency problems related to its beat rate. The magnetic screw concept, with its lack of friction, may contribute to this. We do not have data on the AbioCor® battery (ABIOMED Inc, Danvers, MA). However, it seems to be about the same length and width but twice the thickness of the equivalent MagScrew TAH system component. With the MagScrew implanted battery, a 45-minute to 90-minute duration on a charge is possible, depending on the loading condition. The MagScrew external battery weighs 1.6 pounds and provides 3 to 4 hours of use on a charge.

With respect to left-to-right shunt compensation, bovine implants are a very challenging situation, because the animals quickly outgrow the pump's capacity and drive it above the intended operating flow. That the system works acceptably under these conditions provides us with confidence that it will work well in the human clinical situation. Our design philosophy explicitly resists a dynamic compensation approach. The MagScrew TAH controls beat rate on the basis of left pump filling. The left pump also has 20% more pumping capacity than the right pump. This ensures that the left pump can stay ahead of the right pump under almost all conditions. With a set of normal valves, the left atrial pressure does not exceed 15 mm Hg at 10 L/min flow; even in calves with 90% left atrial valve stenosis and 10 L/min flow, the left atrial pressure did not exceed 30 mm Hg. The fundamental configuration results in right atrial pressure rising more than left atrial pressure under pathologic conditions. Under normal conditions, at 6 L/min flow, the left atrial pressure is about 6 mm Hg, and the right atrial pressure about 2 mm Hg.

We hope this information clarifies the points raised in Holman's [2] commentary, and we look forward to publishing more information on our program as it proceeds.


   References
Top
 References
 

  1. Schenk S, Weber S, Luangphakdy V, et al. MagScrew total artificial heart in vivo performance above 200 beats per minute Ann Thorac Surg 2005;79:1378-1383.[Abstract/Free Full Text]
  2. Holman WL. Invited Commentary Ann Thorac Surg 2005;79:1383.[Free Full Text]
  3. Krause M, Manka M, Martin J, et al. Klappeninsuffizienz bei assist-device-systemen undderen moegliche pathogenese Kardiotechnik 2003;3101–6.
  4. Farrar DJ, Bataille O, Cotter C, Kwan K, Aulenbach C, Poirier VL. Design improvements of the HeartMate® VE LVAS and their effectiveness in reducing adverse events J Heart Lung Transplant 2003;22(Suppl 1):S83.
  5. Dowling RD, Park SJ, Pagani FD, et al. HeartMate VE LVAS design enhancements and its impact on device reliability Eur J Cardio-Thorac Surg 2004;25:958-963.[Abstract/Free Full Text]




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