Low bearing wear in explanted HeartMate II left ventricular assist devices after chronic clinical support.

The primary objective of this study was to evaluate bearing wear during clinical use of the HeartMate II (HMII) left ventricular assist device. Bearings obtained from HMII pumps explanted after clinical use in the Destination Therapy and Bridge to Transplantation clinical trials were analyzed for wear using surface profilometry. Geometric profile variations measured on the inlet bearing ball were used to calculate the wear. Bearing wear was normalized to the total pump support duration to obtain an annualized bearing wear rate. Bearing life was estimated assuming a linear wear rate, as the time to reach a wear limit of 25 µm, which includes a 3× safety factor, to ensure that there is no contact between the rotor blades and the blood bore housing. One hundred and eighty-three bearings from left ventricular assist devices implanted in 181 patients were analyzed. Average age of the patients was 56.3 ± 14.6 years, 76% were male, 46% had an ischemic etiology of heart failure. Mean support duration for the pumps was 363 ± 349 days (median: 238, range: 1-1,621 days). Sixty pumps (33%) were explanted at heart transplantation, 20 (11%) after device replacement, 6 (3%) for ventricular recovery, 94 (51%) after patient death, and 3 (2%) were explanted for other reasons. Mean bearing wear was 0.59 ± 0.37 µm (median: 0.46 µm [5-95% interval: 0.25-1.48]). The median bearing wear rate for patients supported for at least 1 year was 0.30 [5-95% interval: 0.09-0.94] µm/yr. The 5-95% limits of the bearing wear rate corresponded to an estimated bearing life between 27 and 269 years. The pump having the highest bearing wear rate (1.46 µm/yr) had an estimated bearing life of at least 17 years. HMII bearing wear is extremely small, with an estimated bearing life well in excess of 17 years; it is not a limiting factor for long-term support with the HMII left ventricular assistive system.

Controller for an Axial Flow Blood Pump.

A rotary blood pump inherently provides only one noninvasive "observable'" parameter (motor current) and allows for only one "controllable" parameter (pump speed). To maintain the systemic circulation properly, the pump speed must be controlled to sustain appropriate outlet Hows and perfusion pressure while preventing pulmonary damage caused by extremes in preload. Steady-state data were collected at repeated intervals during chronic trials of the Nimbus AxiPump (Nimbus, Inc., Rancho Cordova, California, U.S.A.) in sheep (n = 7) and calves (n = 12). For each data set, the pump speed was increased at increments of 500 rpm until left ventricular and left atrial emptying was observed by left atrial pressure diminishing to zero. The effect of decreasing preload was evaluated perioperatively by inferior vena cava occlusion at a constant pump speed. Fourier analysis established a relationship between changes in the pump preload and the power spectra of the pump current waveform. Based on these results, a control method was devised to avoid ventricular collapse and maintain the preload within a physiologic range. The objective of this controller is the minimization of the second and third harmonic of the periodic current waveform. This method is intended to provide a noninvasive regulation of the pump by eliminating the need for extraneous transducers.