Axial and centrifugal continuous-flow rotary pumps: A translation from pump mechanics to clinical practice
2012; Elsevier BV; Volume: 32; Issue: 1 Linguagem: Inglês
10.1016/j.healun.2012.10.001
ISSN1557-3117
AutoresNader Moazami, Kiyotaka Fukamachi, Mariko Kobayashi, Nicholas G. Smedira, Katherine J. Hoercher, Alex Massiello, Sangjin Lee, David J. Horvath, Randall C. Starling,
Tópico(s)Cardiac pacing and defibrillation studies
ResumoThe recent success of continuous-flow circulatory support devices has led to the growing acceptance of these devices as a viable therapeutic option for end-stage heart failure patients who are not responsive to current pharmacologic and electrophysiologic therapies. This article defines and clarifies the major classification of these pumps as axial or centrifugal continuous-flow devices by discussing the difference in their inherent mechanics and describing how these features translate clinically to pump selection and patient management issues. Axial vs centrifugal pump and bearing design, theory of operation, hydrodynamic performance, and current vs flow relationships are discussed. A review of axial vs centrifugal physiology, pre-load and after-load sensitivity, flow pulsatility, and issues related to automatic physiologic control and suction prevention algorithms is offered. Reliability and biocompatibility of the two types of pumps are reviewed from the perspectives of mechanical wear, implant life, hemolysis, and pump deposition. Finally, a glimpse into the future of continuous-flow technologies is presented. The recent success of continuous-flow circulatory support devices has led to the growing acceptance of these devices as a viable therapeutic option for end-stage heart failure patients who are not responsive to current pharmacologic and electrophysiologic therapies. This article defines and clarifies the major classification of these pumps as axial or centrifugal continuous-flow devices by discussing the difference in their inherent mechanics and describing how these features translate clinically to pump selection and patient management issues. Axial vs centrifugal pump and bearing design, theory of operation, hydrodynamic performance, and current vs flow relationships are discussed. A review of axial vs centrifugal physiology, pre-load and after-load sensitivity, flow pulsatility, and issues related to automatic physiologic control and suction prevention algorithms is offered. Reliability and biocompatibility of the two types of pumps are reviewed from the perspectives of mechanical wear, implant life, hemolysis, and pump deposition. Finally, a glimpse into the future of continuous-flow technologies is presented. The evolution of mechanical circulatory support from volume-displacement pulsatile pumps to continuous-flow (CF) rotary pumps has ushered in a new era for treatment of end-stage heart failure. Increasing use of these pumps in the clinical arena is related to multiple positive attributes of this class of pumps, including their smaller size, improved durability, and enhanced survival with less morbidity.1Miller L.W. Pagani F.D. Russell S.D. et al.Use of a continuous-flow device in patients awaiting heart transplantation.N Engl J Med. 2007; 357: 885-896Crossref PubMed Scopus (1396) Google Scholar, 2Slaughter M.S. Rogers J.G. Milano C.A. et al.Advanced heart failure treated with continuous-flow left ventricular assist device.N Engl J Med. 2009; 361: 2241-2251Crossref PubMed Scopus (2394) Google Scholar As acceptance of these classes of pumps continues to grow and newer-generation pumps are developed, a complete understanding of their mechanics and the inherent differences between centrifugal and axial-flow pumps becomes imperative for patient care and decisions regarding pump selection. In this review, we describe the basic engineering differences between the 2 types of CF pumps and provide a description of how these features translate clinically to patient management.Axial vs centrifugal CF pump design and theory of operationCF rotary pumps generally consist of blood inlet and outlet ports and a single rotating element that imparts energy to the blood to increase arterial blood flow and pressure. The rigid stationary housing(s) that surrounds and/or lies in the center of the rotating element incorporates some combination of motor windings, permanent magnets, electromagnets, or mechanical bearing surfaces that act to drive and support the rotating element.The primary difference between centrifugal-flow and axial-flow pumps lies in the design of their rotating elements (Table 1). When one considers the theory of operation of a centrifugal CF pump, its rotating element acts as a spinning disk with blades that can be viewed as a “thrower,” meaning that the fluid is captured and thrown tangentially out off the blade tips. In contrast, axial CF pump rotating elements operate like a propeller in a pipe and can be viewed as a “pusher.” This mechanism can also be viewed as an “auger” trying to screw itself into the inlet fluid, against the “resistance force” at the outlet, to overcome the difference between pre-load and after-load.Table 1Comparison of Axial and Centrifugal Rotary Pump Response to Physiologic Conditions↓, decreased; ↑, increased; CBC, circulating blood volume; Pump dP, pressure difference across pump inlet and outlet; RHF, right heart failure; SVR, systemic vascular resistance. aAxial pumps in clinical use do not show a linear power vs flow relationships over the entire ventricular assist device operating flow range, making characterization of flow vs power relationships difficult. Open table in a new tab Bearing designA further distinction made between CF pumps is the method used to support the rotating element (rotor). Listed below and illustrated in Figure 1 are several different kinds of bearing elements that are used to accomplish this:•Mechanical/pivot: Rotor is suspended with mechanical bearings on spherical surfaces rotating in sockets (eg, HeartMate II, Thoratec Corporation, Pleasanton, CA).•Hydrodynamic: Rotor derives lifting from fluid forces in thin, fluid blood films separating rotor and pump housing based on the relative motion of surfaces (eg, HeartWare HVAD, HeartWare International Inc, Framingham, MA).•Electromagnet/position sensor: Suspends rotor using electronic position control and electromagnets (eg, DuraHeart, Terumo Heart Inc, Ann Arbor, MI; and HeartMate III, Thoratec Corp).•Permanent magnet: Repelling magnets in rotor and pump housing suspend the rotor. Permanent magnets alone cannot suspend a rotating element because magnetic forces change continuously with the position of the rotor. Therefore, permanent magnets are generally used in combination with one of the other bearing types. In a magnetically suspended pump, stabilization in one or more directions of motion is achieved by using hydrodynamic or electromagnetic bearing elements.As a practical matter, the primary difference in blood pump bearings lies in their complexity and reliability. Mechanical bearings have tiny and precise ceramic components whose advantage is that they position the rotating assembly in all 6 directions of motion and are stable at all speeds and operating conditions. Although their small size limits the surface area exposed to mechanical contact and friction, these surfaces are potential sites for thrombus/fibrin deposition, which is not present in other bearing designs that completely suspend the rotor. The concentration of hydraulic loads on the rotor at these small pivot bearings also makes them theoretically life-limiting for wear and fragile with respect to impact.Electromagnetic rotor positioning generates real-time electromagnetic forces on the rotor to actively counteract the surrounding fluid and magnetic forces that could cause unstable rotor operation. Among the advantages of this approach are that there is no contact between the bearing surfaces, so there is no life-limiting wear, and that it provides the lowest shear stress to the blood because of relatively large clearances between the rotating element and the housing. However, these systems are complex, requiring position sensors, electromagnets and extra conductors, connector pins, and electronics to execute the dedicated position control algorithm. If, for example, there is an electrical contact failure in a connector pin, or momentary instability encountered in the control algorithm, pump failure can occur. To make the system fail-safe, electromagnetic bearing elements are frequently backed up with hydrodynamic bearings, which require no control.Hydrodynamic bearings similarly produce no life-limiting contact between the bearing surfaces, but in contrast, are simple and reliable. Once the rotor starts spinning, it essentially “water skis” on a film of blood, deriving lift and separation from its own motion. However, the load-bearing blood fluid film is prone to higher shear stress, and theoretically, more hemolysis can occur. Another design issue is that the rotor and housing surfaces are in contact at startup and shutdown, when there is no relative motion. The bearing surface material properties need to accommodate this friction to avoid damage at these 2 times. Surface coatings, if used, require extended endurance testing to verify multiyear reliability.Hydrodynamic performanceThe pressure across the inlet and outlet of any hydrodynamic pump is termed by engineers as the “pump delta P” or “head pressure.” Figure 2A shows the typical pressure difference between the left ventricle (LV; pump inlet) and aorta (pump outlet) of a failing heart supported by a rotary pump. Figure 2B and C compare the typical hydrodynamic performance curves (pump head curves) for centrifugal vs axial CF pumps, respectively. A pump head curve compares the relationship between pump flow and pressure difference across the pump ports (delta P) at one operating pump speed. Figure 3 shows a full set of pump head curves over the full range of operating pump speeds for the axial HeartMate II and the centrifugal HeartWare HVAD and Terumo DuraHeart left ventricular assist device (LVAD).3Larose J.A. Tamez D. Ashenuga M. Reyes C. Design concepts and principle of operation of the HeartWare ventricular assist system.ASAIO J. 2010; 56: 285-289PubMed Google ScholarFigure 2(A) Representation of the systemic arterial pressure (AoP) and left ventricular pressure (LVP) relationship is shown during rotary LVAD support in a failing ventricle. Comparison of (B) centrifugal-flow and (C) axial rotary pump performance characteristics are shown as pressure differential across pump inlet and outlet in mm Hg and pump flow in liters/min. (D) Representative centrifugal and axial rotary pump flow waveforms during rotary LVAD support in a failing ventricle are compared. Reprinted with permission from Pagani FD. Continuous-flow rotary left ventricular assist devices with “3rd generation” design. Semin Thorac Cardiovasc Surg 2008;20:255–263. © 2008 Elsevier, Inc.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 3Comparison of head curves for axial vs centrifugal continuous flow (CF) pumps is shown for the (A) HeartMate II axial CF, the (B) HeartWare HVAD* centrifugal CF,3Larose J.A. Tamez D. Ashenuga M. Reyes C. Design concepts and principle of operation of the HeartWare ventricular assist system.ASAIO J. 2010; 56: 285-289PubMed Google Scholar and the (C) Terumo DuraHeart centrifugal CF. Reprinted with permission from Frazier OH et al. Optimization of axial-pump pressure sensitivity for a continuous flow total artificial heart. J Heart Lung Transplant 2010;29:687–91. © 2010 Elsevier Inc. and Alex Medvedev, Terumo Heart, Inc.*The HeartWare HVAD is an investigational device limited by Federal Law to investigational use.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Most important to note is that most centrifugal pumps have what is called a flat head curve, where they operate over a very wide range of flows for a very small change in delta P across the pump. The example in Figure 3 shows that for 1 cardiac cycle in which pump delta P swings from 40 to 80 mm Hg, centrifugal pumps have a very large swing in flow (0 to 10 liters/min), acting almost like a pulsatile pump with high peak systolic flows and low, even negative, diastolic flows. One can think of a centrifugal pump with a flat head curve as an on/off CF pump cycling between high-flow and low-flow as its output surges with the beating of the ventricle. This creates inherent high pump flow pulsatility in response to changing LV pressures (Figure 2D). In contrast, a typical axial-flow pump has a steep head curve where there is a linearly related increase and decrease in flow with decreasing and increasing pump delta P.4Pagani F.D. Continuous-flow rotary left ventricular assist devices with “3rd generation” design.Semin Thorac Cardiovasc Surg. 2008; 20: 255-263Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar In this example, the 40- to 80-mm Hg swing across the pump conduits produces less flow pulsatility, ranging from 3 to 7 liters/min during a cardiac cycle. As the text that follows explains, this difference in pump-flow pulsatility affects the diagnostic and control feedback available to the respective pump controllers. Figure 3B and C demonstrate that the actual degree of flatness of centrifugal pump head curves can vary with design, with the HeartWare HVAD being less flat than the Terumo DuraHeart design.Inlet cannula suction and control of pump operating speedThe likelihood of high inlet cannula suction in axial vs centrifugal CF pumps is related to the flow vs delta P relationship described above. A CF pump is blind to the absolute value of the pressures at the inlet and outlet ports and responds only to the total differential pressure across the pump. At any given pump speed, a CF pump would have the same flow at inlet and outlet pressure values of 0/100, 100/200, and –50/50 mm Hg, because it sees only the delta P of 100 mm Hg. This has clinical significance during a low LV volume state in which pump flow decreases, such as one might expect during conditions related to hypovolemia associated with right ventricular (RV) failure or bleeding. As can be seen from the head curves in Figure 2B, a centrifugal LVAD with a flat head curve has a fixed differential pressure as flows decrease from 5 to 0 liters/min, so no significant increase in inlet suction occurs at low flows or even total occlusion. However, as a result of the steep head curve characteristic of axial pumps, the increasing pressure differential generated across them as flow decreases translates into much higher suction developed at the inlet during conditions of low LV volume or inlet cannula obstruction.4Pagani F.D. Continuous-flow rotary left ventricular assist devices with “3rd generation” design.Semin Thorac Cardiovasc Surg. 2008; 20: 255-263Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar This means axial pumps pull the hardest at the lowest flow, creating the potential for a self-latching condition wherein the pump continues to increase suction as flows fall, resulting in the ventricular wall being sucked in around the inlet. Therefore, centrifugal pumps have an advantage over axial pumps in the avoidance of ventricular suck down at low flow.A complication of intermittent LV suck down is ventricular arrhythmias. Morshuis et al5Morshuis M. Schoenbrodt M. Nojiri C. et al.DuraHeart magnetically levitated centrifugal left ventricular assist system for advanced heart failure patients.Expert Rev Med Devices. 2010; 7: 173-183Crossref PubMed Scopus (33) Google Scholar reported a 65% lower prevalence of ventricular arrhythmia than reported for axial flow LVADs in their clinical experience with the DuraHeart centrifugal pump. Another pathologic condition that higher LV inlet suction can create is leftward shift of the interventricular septum. This not only negatively alters the mechanics of the septal contribution to RV output but can also increase tricuspid valve regurgitation secondary to the anatomic connection of the septal leaflet of this valve, causing it to be “pulled” away.6Santamore W.P. Gray Jr, L.A. Left ventricular contributions to right ventricular systolic function during LVAD support.Ann Thorac Surg. 1996; 61: 350-356Abstract Full Text PDF PubMed Scopus (99) Google Scholar, 7John R. Lee S. Eckman P. Liao K. Right ventricular failure—a continuing problem in patients with left ventricular assist device support.J Cardiovasc Transl Res. 2010; 3: 604-611Crossref PubMed Scopus (48) Google ScholarRecent trends in patient management have been to operate CF LVADs at lower speeds and at less than the maximum level of cardiac support to ensure continuous or intermittent opening of the aortic valve during operation. This is to prevent reported complications of aortic valve fusion and reduce the potential for aortic regurgitation, but it also reduces the likelihood of suction events.8John R. Mantz K. Eckman P. Rose A. May-Newman K. Aortic valve pathophysiology during left ventricular assist device support.J Heart Lung Transplant. 2010; 29: 1321-1329Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar, 9Adamson R.M. Dembitsky W.P. Baradarian S. et al.Aortic valve closure associated with HeartMate left ventricular device support: technical considerations and long-term results.J Heart Lung Transplant. 2011; 30: 576-582Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, 10Cowger J. Pagani F.D. Haft J.W. Romano M.A. Aaronson K.D. Kolias T.J. The development of aortic insufficiency in left ventricular assist device-supported patients.Circ Heart Fail. 2010; 3: 668-674Crossref PubMed Scopus (259) Google ScholarFlow estimation accuracy based on flow vs power relationshipsCentrifugal pumps have a linear current-to-flow relationship across the full range of operating pump flows. In addition, the characteristic current and flow pulsatility in response to the pressures generated across the pump makes the centrifugal pump motor current an accurate sensorless index of pump flow and a sensitive virtual index of the LV pressures during the cardiac cycle. This information allows centrifugal pump controllers to accurately monitor pump flow and the degree of LV unloading by simply monitoring the motor current or power.4Pagani F.D. Continuous-flow rotary left ventricular assist devices with “3rd generation” design.Semin Thorac Cardiovasc Surg. 2008; 20: 255-263Abstract Full Text Full Text PDF PubMed Scopus (87) Google ScholarIn contrast, the correlation between flow and current in axial pumps is not nearly linear over the full operating flow range, is not as well defined as in centrifugal pumps, and offers less accuracy for flow estimation. For example, the HeartMate II axial flow pump does not display flows below 3.0 liters/min. In an intraoperative study of estimated flow accuracy in 20 HeartMate II patients, Slaughter et al11Slaughter M.S. Bartoli C.R. Sobieski M.A. et al.Intraoperative evaluation of the HeartMate II flow estimator.J Heart Lung Transplant. 2009; 28: 39-43Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar concluded that the device’s estimated flow values can be used only “to provide directional information for trend purposes rather than absolute values of pump flow.” The MicroMed DeBakey (MicroMed, Houston, TX) axial pump relies on an ultrasonic flow probe incorporated into its outlet conduit for flow determination. The centrifugal HeartWare HVAD and Terumo DuraHeart LVAD flow estimations derived from motor power and speed inputs have been generally considered accurate and reliable enough to use in clinical assessment.Accurate flow estimation for axial and centrifugal rotary pumps also depends on the viscosity of blood, which can be estimated from the patient’s hematocrit. Currently, the HeartWare HVAD has a programmable hematocrit setting, whereas the HeartMate II does not. Implementation of automatic control algorithms is expected to be more difficult in axial pumps without direct flow measurements than in centrifugal pumps.Axial vs centrifugal physiology and controlPre-load SensitivityCardiac output in the natural heart is determined by the interaction of after-load, myocardial contractility, heart rate, pre-load sensitivity, and LV compliance. Pre-load sensitivity, as it relates to mechanical circulatory assist devices, mimics the relationship between LV filling pressures and ventricular stroke volume defined by the Frank-Starling curves.12Patterson S.W. Starling E.H. On the mechanical factors which determine the output of the ventricles.J Physiol. 1914; 48: 357-379PubMed Google Scholar It is calculated from the ratio of pump output to pump filling pressures at the pump inlet in liters/min/mm Hg. Salmonsen et al13Salamonsen R.F. Mason D.G. Ayre P.J. Response of rotary blood pumps to changes in preload and afterload at a fixed speed setting are unphysiological when compared with the natural heart.Artif Organs. 2011; 35: E47-53Crossref PubMed Scopus (114) Google Scholar reported an average pre-load sensitivity for the DuraHeart, HeartWare HVAD, HeartMate II, and INCOR (Berlin Heart AG, Berlin Germany) CF rotary pumps of 0.105±0.096 liters/min/mm Hg, which is almost 3 times lower than the 0.275 liters/min/mm Hg reported for the human heart.14Levine B.D. Lane L.D. Buckey J.C. Friedman D.B. Blomqvist C.G. Left ventricular pressure-volume and Frank-Starling relations in endurance athletes. Implications for orthostatic tolerance and exercise performance.Circulation. 1991; 84: 1016-1023Crossref PubMed Scopus (262) Google Scholar The limited pre-load sensitivity of CF devices theoretically limits their rate of increased output in response to increasing LV venous return. There is no evidence in the literature to suggest that axial or centrifugal pumps would have any significant clinical difference in pre-load sensitivity.Reports of greater LV volume unloading by pulsatile vs CF LVAD pumps15Klotz S. Deng M.C. Stypmann J. et al.Left ventricular pressure and volume unloading during pulsatile versus nonpulsatile left ventricular assist device support.Ann Thorac Surg. 2004; 77 (discussion 149–50): 143-149Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar, 16Haft J. Armstrong W. Dyke D.B. et al.Hemodynamic and exercise performance with pulsatile and continuous-flow left ventricular assist devices.Circulation. 2007; 116: I8-15Crossref PubMed Scopus (145) Google Scholar may be a consequence of the low pre-load sensitivity of the CF LVADs. Interestingly, a review of all bridge-to-recovery cases at the German Heart Institute from 1992 to 2009 showed that patients with a pulsatile-flow LVAD had an almost 3-fold chance for myocardial recovery compared with those who received CF devices. This may be due to greater volume unloading of the LV by pulsatile pumps.17Krabatsch T. Schweiger M. Dandel M. et al.Is bridge to recovery more likely with pulsatile left ventricular assist devices than with nonpulsatile-flow systems?.Ann Thorac Surg. 2011; 91: 1335-1340Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar To date, no published reports have compared the exercise tolerance of patients with axial vs centrifugal CF LVADs.After-load sensitivityData from Salamonsen et al13Salamonsen R.F. Mason D.G. Ayre P.J. Response of rotary blood pumps to changes in preload and afterload at a fixed speed setting are unphysiological when compared with the natural heart.Artif Organs. 2011; 35: E47-53Crossref PubMed Scopus (114) Google Scholar on after-load sensitivity of CF rotary pumps support the general understanding that these pumps have higher after-load sensitivity (0.09±0.034 liters/min/mm Hg) than the human heart (0.03±0.01 liters/min/mm Hg). This high after-load sensitivity creates the need to control systemic vascular resistance (SVR) in these patients to guarantee sustained outputs.Typically, the targeted mean systemic arterial pressures for CF pump patients is 70 to 90 mm Hg, with pressures exceeding 90 mm Hg to be avoided.18Slaughter M.S. Pagani F.D. Rogers J.G. et al.Clinical management of continuous-flow left ventricular assist devices in advanced heart failure.J Heart Lung Transplant. 2010; 29: S1-39Abstract Full Text Full Text PDF PubMed Scopus (715) Google Scholar Although axial and centrifugal CF pumps share this generalized categorization, analysis of their head curves suggests some relative differences that can have significant clinical implications. Because centrifugal pumps operate on a flatter head curve than axial pumps, they demonstrate larger changes in flow for any given change in pressure across the pump. If SVR were to increase (increase in delta P), the pump outlet pressure increase would produce an instantaneous drop in pump flow to maintain a constant pump outlet pressure, which means that centrifugal pumps cannot pump against a high blood pressure, thereby causing a lower flow condition.In contrast, the steeper head curve of axial pumps responds to an increase in SVR by increasing the pressure generated across the pump ports, limiting the decrease in flow by increasing outlet pressure. At low-flow conditions, this retains the capability to enforce high blood pressures with adequate LV volume. However, it also results in high inlet suction that in a low LV volume state can potentiate arrhythmias, create suction events, or lead to hemolysis.Pump flow pulsatility and automatic physiologic pump controlThe degree of pulsatility in the pump flow waveform during a cardiac cycle for CF rotary pumps is termed the flow pulsatility and is quantified in various ways for different pump systems as a pulsatility index (PI). It is typically displayed as some form of the ratio of the peak flow during systole (flowmax) or the total swing in flow during a cardiac cycle (flowmax – flowmin) over the average pump flow (flowavg).3Larose J.A. Tamez D. Ashenuga M. Reyes C. Design concepts and principle of operation of the HeartWare ventricular assist system.ASAIO J. 2010; 56: 285-289PubMed Google Scholar, 18Slaughter M.S. Pagani F.D. Rogers J.G. et al.Clinical management of continuous-flow left ventricular assist devices in advanced heart failure.J Heart Lung Transplant. 2010; 29: S1-39Abstract Full Text Full Text PDF PubMed Scopus (715) Google Scholar, 19Doi K. Golding L.A. Massiello A.L. et al.Preclinical readiness testing of the Arrow International CorAide left ventricular assist system.Ann Thorac Surg. 2004; 77: 2103-2110Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar, This is normally derived by averaging the recorded instantaneous peak flows or flow swings over each cardiac cycle for approximately 10 to 15 seconds and then calculating the average total pump flow over that same period. The formula for calculating a PI for the HeartMate II (PIHM II )20Griffith B.P. Kormos R.L. Borovetz H.S. et al.HeartMate II left ventricular assist system: from concept to first clinical use.Ann Thorac Surg. 2001; 71 (discussion S124–6): S116-120Abstract Full Text Full Text PDF PubMed Scopus (217) Google Scholar is {PIHM II = [(flowmax – flowmin)/flowavg]× 10.For example, if rotary pump flow values for flowmax, flowmin,, and flowavg were 8, 4, and 6 liters/min, respectively, the PIHM II would be 6.7. Multiply the PIHM II by 10, and it can be interpreted as the percentage of the average flow that the pump flow swings through during a cardiac cycle, which for this example would be a 4 liters/min swing or 67% of the average flow of 6 liters/min. The average PIHM II for the axial HeartMate II bridge-to-transplantation (BTT) trial was 5.0±0.9, and the average PI calculated as PIHM II for the centrifugal CorAide European clinical trial was 12.1±2.6, demonstrating more than twice the flow pulsatility.3Larose J.A. Tamez D. Ashenuga M. Reyes C. Design concepts and principle of operation of the HeartWare ventricular assist system.ASAIO J. 2010; 56: 285-289PubMed Google Scholar, 18Slaughter M.S. Pagani F.D. Rogers J.G. et al.Clinical management of continuous-flow left ventricular assist devices in advanced heart failure.J Heart Lung Transplant. 2010; 29: S1-39Abstract Full Text Full Text PDF PubMed Scopus (715) Google Scholar, 19Doi K. Golding L.A. Massiello A.L. et al.Preclinical readiness testing of the Arrow International CorAide left ventricular assist system.Ann Thorac Surg. 2004; 77: 2103-2110Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar Although flow pulsatility data for the HeartWare HVAD clinical trial are not available, it is recommended to maintain a minimum of 2 to 4 liters/min for pump flow operating ranges of 3.5 to 7.0 liters/min, which equates to a PIHM II value of 5.7.The significance of flow pulsatility for rotary blood pumps lies in the fact that the degree of flow pulsatility is inversely related to the degree of LV unloading by the pump and directly proportional to the strength of LV contraction and as such can be used as a measure of the LV function under VAD support. Any significant decrease in pump flow pulsatility without a change in pump speed should be investigated clinically for causes of decreasing LV pressures during LVAD support. This typically includes decreasing LV contractility or low LV volume states caused by right heart failure or dehydration. Reasons for the flow PI to increase, without a change in pump speed, include an increase in LV contractility via inotropes, myocardial recovery, and exercise or increased pre-load Starling effects.It is not surprising that automatic physiologic pump control algorithms have not been used to any significant extent in the clinical axial-flow pumps given their previously stated poor flow estimation accuracy, lower flow pulsatility, and the increased danger of LV suction events. They are typically operated only in fixed-speed mode with a suction-detection control algorithm
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