Effects of a total artificial heart right stroke volume limiter on left-right hemodynamic balance

In a completely implantable total artificial heart (TAH), the left-right flow difference attributable to higher volumetric efficiency of the right pump and bronchial artery shunting has always been a significant problem. The automatic control of the Cleveland Clinic-Nimbus TAH accommodates for the left-right flow difference when the beat rate is below maximum (AUTO range). However, at its maximum beat rate (MAX), high left atrial pressure (LAP) (greater than 25 mmHg) with relatively low right atrial pressure (RAP) (less than 8 mmHg) were observed both in vitro and in vivo, suggesting the need for a stroke volume limiter (SVL) of the right pump. In vitro volume loading tests showed 10%, 15%, and 20% SVLs prevented a disproportionate increase of LAP. In vivo studies in five Holstein calves also showed a balanced LAP-RAP relationship with 10% and 15% SVLs at MAX. The left pump flow was not affected by this range of SVLs either in vitro or in vivo. Pulmonary function was maintained with either size SVL, with autopsies revealing absence of pulmonary congestion and minimal pleural effusions in two calves surviving for more than 1 month. Although additional studies are needed to determine the appropriate size of the SVL, both 10% and 15% SVLs were effective in maintaining left-right hemodynamic balance in this TAH.