From the *Department of Bioengineering, University of Louisville, Louisville, Kentucky; and †Department of Thoracic and Cardiovascular Surgery, Cardiovascular Innovation Institute, University of Louisville, Louisville, Kentucky.
Submitted for consideration January 2014; accepted for publication in revised form January 2014.
Disclosure: The authors have no conflicts of interest to report.
Reprint Requests: Steven C. Koenig, PhD, Departments of Bioengineering and Cardiothoracic Surgery, University of Louisville, Cardiovascular Innovation Institute, 302 East Muhammad Ali Boulevard, Room 408, Louisville, KY 40202. Email: firstname.lastname@example.org.
Continuous axial- and centrifugal-flow left ventricular assist devices (LVADs) have each grown in prominence and clinical use because of their small size, ease of implantation, reliability, and performance.1 However, differing opinions have emerged as to whether axial-flow versus centrifugal-flow pumps might have hemodynamic or clinical advantage(s). In the article by Senage et al.,2 the authors report that axial flow (HeartMate II; Thoratec, Pleasanton, CA) may offer significant clinical advantage(s) compared with centrifugal flow (VentrAssist; Ventracor, Chatswood, NSW, Australia) as evidenced by a more rapid reduction in ventricular pressure (unloading), but at a potentially higher risk of ventricular suction as observed in their mock circulation model. The authors propose that LVAD selection may one day be assigned according to patient-specific condition(s). For example, the authors suggest that axial flow may be of greater clinical benefit to patients with pulmonary hypertension, during exercise, or stress conditions. In this invited commentary, careful consideration of potential differences in technology, hemodynamic performance, and their clinical implications are presented.
In evaluating potential difference(s) between axial- and centrifugal-flow LVADs, it is important to recognize that differences in pump design (i.e., impeller and bearing) are also likely to influence hemodynamic performance or clinical outcomes. Similar reductions in left ventricular end-diastolic pressure and left ventricular end-diastolic volume can be achieved using axial- or centrifugal-flow LVADs simply by adjusting the systemic vascular resistance (blood pressure) or increasing pump speed. There may be value in measuring ventricular pressure-volume loops as a measure of pump performance and left ventricular “volume unloading” as well as distinguishing and quantifying left ventricular volume ejected through the aortic valve or the LVAD. In this mock flow loop model study, investigation of left ventricular “pressure unloading” with axial and centrifugal flow was limited to two devices (HeartMate II and VentrAssist). The authors’ findings raise several important engineering concepts worth consideration. First, the authors previously reported that centrifugal-flow LVADs offer the technological advantage of lower power consumption and are more sensitive to preload and afterload compared with axial-flow devices.2
Second, the hemodynamic performance of axial- and centrifugal-flow LVADs may be characterized by their respective pressure head and flow relationships (i.e., HQ curve), which can also be used to distinguish difference(s). An “ideal pump” will theoretically generate a constant pressure head across all pump flow rates (i.e., flat HQ line). However, a nonideal HQ “curve” is observed due to pressure head losses for a specific device. Although there is significant variability in the pressure head and flow relationships between pumps, centrifugal-flow LVADs generally have a flatter HQ curve compared with axial-flow LVADs (Figure 1A).3–5 Centrifugal pumps are typically more efficient devices because of lower losses (i.e., pressure head, friction), thereby resulting in lower power consumption compared with axial-flow LVADs.2 However, a flatter HQ curve will lead to a higher sensitivity to pressure head changes (i.e., preload and afterload).6 Higher pressure head sensitivity in centrifugal pumps results in a lower likelihood of ventricular suction events, but at the expense of lower flow rates at an elevated afterload.
Third, because of their high pressure head sensitivity, for a given pressure head variation, centrifugal pumps exhibit a larger change in LVAD flow compared with axial-flow devices.3,5 Thus, centrifugal-flow LVADs will exhibit greater flow “pulsatility” compared with an axial LVAD because of the cyclical pressure head variation across the pump resulting from native ventricular contraction (Figure 1). The higher flow “pulsatility” will result in a larger variation in left ventricular end-systolic and end-diastolic volumes and higher aortic pressure “pulsatility.”7 However, for any given mean LVAD flow rate at a constant preload and afterload, there will be no differences in mean aortic pressures or flows between axial- and centrifugal-flow LVADs (Figure 2). The mean aortic pressure difference observed by the authors between the two test devices may be due to the 5–10% difference in systemic vascular resistance between the axial and centrifugal data sets.2
Fourth, the steeper HQ curve produced by axial-flow LVAD will result in a nonlinear current-to-flow relationship, which may complicate LVAD flow estimates based upon intrinsic pump parameters. For example, in a clinical study of heart failure patients implanted with HeartMate II (axial flow) LVAD, the correlation between measured (Transonics flow probe) and estimated LVAD flow was inconsistent,8 which was suggested by Pennings et al.9 Blood viscosity should also be taken into account to fully characterize potential difference(s) between axial- and centrifugal-flow devices because of its impact on hydrodynamic performance.
The higher efficiency of centrifugal-flow LVAD may enable slightly smaller battery packs or extend the duration of support. However, the current incremental reduction in battery and controller size is unlikely to yield a significant technical advantage(s) or improvement in patients’ quality of life. Although centrifugal-flow LVADs have a higher preload and afterload sensitivity compared with axial-flow LVADs, the preload and afterload sensitivity of both types of LVADs are significantly lower than that of the human heart.10,11 Suction events are not isolated to axial-flow LVAD. Integration of suction prevention and physiologic control algorithm(s) for continuous-flow LVAD is in development.12,13 Centrifugal-flow LVADs produce a higher aortic pressure pulsatility compared with axial-flow LVADs at the same mean LVAD flow rate (Figure 2). However, pressure and flow waveform pulsatility are diminished for all continuous-flow devices.14,15 The slightly augmented pressure pulsatility with centrifugal LVAD is unlikely to mitigate potential adverse events associated with diminished pulsatility.16,17 To date, no differences in end-organ perfusion and function with axial or centrifugal LVADs have been discerned in a heart failure patients.18 Integration of pump speed (flow) modulation algorithm(s) may be used to augment aortic pressure pulsatility.19,20 Both axial- and centrifugal-flow LVADs have their unique advantages. In vitro, in vivo, and clinical studies are warranted to fully characterize and quantify their distinctive features. However, the differences between axial- and continuous-flow LVADs are unlikely to be as profound as the differences between pulsatile- and continuous-flow LVADs. Overall, the mock flow loop data presented by Senage et al.2 are a good start to help better understand potential differences in hemodynamic responses with axial- and centrifugal-flow pumps. However, their finding of “better unloading” with an axial-flow LVAD can be mitigated clinically by lowering systemic blood pressure or increasing pump speed when using a centrifugal-flow LVAD and only represents one aspect of the differing performance and subsequent clinical management of axial-flow versus centrifugal-flow LVADs.
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