Centrifugal Flow or Axial Flow Left Ventricular Assist Devices: An Overview

This article was collaboratively written with Zil-e-Huma Sabir, a fourth-year PharmD candidate at Chicago State University College of Pharmacy.

Advanced heart failure results in significant limitations impacting survival, functional status, and quality of life despite optimal medical treatment with evidence-based therapies.^1,2 The gold standard treatment for patients with advanced heart failure is heart transplantation, but due to a paucity of organs available, left ventricular assist device (LVAD) utilization has increased in recent years. Left ventricular assist devices augment the function of the failing heart by providing mechanical circulatory support to restore blood flow and circulation. These devices can be employed while patients are waiting for a heart transplant or it can serve as a permanent alterative to transplantation, known as destination therapy. Functionally, LVADs decrease preload, decrease cardiac workload, increase systemic circulation and tissue perfusion, and decreases the neurohormonal response seen in heart failure.

Currently, there are 2 types of FDA-approved LVADs, pulsatile and non-pulsatile, also known as continuous flow LVADs. Pulsatile LVADs mimic the natural rhythmic action of the heart, while continuous flow LVADs use a motor at fixed speeds leading to constant ejection of blood to the systemic circulation. This is the reason patients with continuous flow LVADs often lack a pulse upon palpation. Continuous flow LVADs can be further divided into 2 categories, axial flow [HeartMate II™ Left Ventricular Assist Device (Thoratec Corporation, San Diego, CA)] and centrifugal flow [HeartWare^® Ventricular Assist System (HeartWare International, Framingham, MA) or HeartMate 3™ Left Ventricular Assist System (Abbott Laboratories, Lake Forest, IL)]. These designations refer to the way the blades rotate within the pump. Although LVADs improve mortality and other outcomes, there are many complications associated with these devices such as pump hemolysis/thrombosis, gastrointestinal (GI) bleeding, right ventricle (RV) failure, and stroke. Historically, a debate has existed regarding whether axial flow or centrifugal flow LVADs have a better benefit to risk ratio for patients although few studies have directly compared the therapies.

Rogers and colleagues in a multicenter noninferiority trial evaluated outcomes of 446 destination therapy patients randomized in a 2:1 fashion for implantation of either the Heartware or HeartMate II device.² The primary end point was survival at 2 years free from disabling stroke or device removal for malfunction or failure. The results of this study indicated that patients who received the axial flow HeartMate II device were more likely to require device replacement, explantation, or urgent transplantation. However, the Heartware cohort was significantly more likely to present with ischemic and hemorrhagic strokes, right sided heart failure, and sepsis. Furthermore, treatment with either device was associated with sustained improvements in functional and quality-of-life measures. Therefore, the authors of the study concluded that the centrifugal flow device was non-inferior to the axial flow device in terms of survival free from disabling stroke or device removal due to malfunction or failure.

Mehra and colleagues conducted a randomized noninferiority and superiority trial comparing the HeartMate 3 with the HeartMate II device. The trial randomly assigned 366 advanced heart failure patients irrespective of the intended goal of support (bridge to transplantation or destination therapy) and evaluated 2-year outcomes.³ The primary end point of this study was survival at 2 years free of disabling stroke or survival free of reoperation to replace or remove a malfunctioning device.

The centrifugal flow pump was found to significantly increase 2-year survival free of disabling stroke or survival free of reoperation compared to the axial flow device (HR, 0.46; 95% confidence interval [CI], 0.31 to 0.69; 2-tailed P<0.001). This result was primarily driven by higher rates of reoperation in the axial flow LVAD cohort. Ischemic and hemorrhagic stroke rates were significantly higher in the axial flow LVAD cohort as were rates of pump thrombosis, but disabling stroke rates were not different. Bleeding rates were not deemed to be significantly different between cohorts, although the confidence interval did not cross the pre-specified upper limit, the p value was above the specified cutoff of 0.05 (HR, 0.71; 95% CI, 0.53-0.96; P=.07). Similar to the previous study, treatment with either device was associated with similar improvements in quality-of-life measures. The authors concluded that a fully magnetically levitated centrifugal flow pump was superior to a mechanical-bearing axial flow pump with regard to survival free of disabling stroke or reoperation to replace or remove a malfunctioning device.

As mentioned earlier, few studies have directly compared the efficacy and safety of centrifugal flow and axial flow LVADs. In addition, several endpoints such as stroke rates differed between the axial flow and centrifugal flow pump cohorts depending on the study. Based on the trial results, the implantation of centrifugal flow devices in patients with advanced heart failure have either better or non-inferior outcomes versus the axial flow devices, primarily because of the lower rates of reoperation. Neither study, however, showed improved mortality with one device versus the other. It seems like centrifugal flow devices have several benefits over those with axial flow based on the data, but at this time it cannot be said that one device is superior to another. A study comparing the 2 commonly used centrifugal flow devices has not been completed to date, but would be a welcome addition to this subset of literature for clinicians and patients alike.

References:

1. Go AS, Mozaffarian D, Roger VL, et al. Executive summary: heart disease and stroke statistics-2014 update: a report from the American Heart Association. Circulation. 2014;129(3):399-410.

2. Rogers JG, Pagani FD, Tatooles AJ, et al. Intrapericardial Left Ventricular Assist Device for Advanced Heart Failure. N Engl J Med. 2017;376:451-460.

3. Mehra MR, Goldstein DJ, Uriel N et al. Two-Year Outcomes with a Magnetically Levitated Cardiac Pump in Heart Failure. N Engl J Med. 2018;378(15):1386-1395.