Reliability model from the in vitro durability tests of a left ventricular assist system

Currently available ventricular-assist devices: capabilities, limitations and future perspectives

The continuous progress in ventricular-assist device (VAD) technology and the management of patients with VADs has broadened the treatment options for end-stage heart failure patients. The available line-up of clinical devices provides the current optimal therapies to meet the specific needs of each patient. The extended durability, safety, efficacy and improved quality of life of the patients provides sufficient proof for the VAD to be a likely alternative therapy to heart transplantation. The sequential progress from the first-, to the second- and to the third-generation VAD technology is expected to bring increasing benefits to clinical outcomes. This article reviews the current status, capabilities, limitations and future perspectives of currently available VADs by generally classifying them via support duration, alignment of pump devices and via pulsatile or nonpulsatile mode of perfusion. Furthermore, the future direction of research and development for next-generation VADs is presented based on the lessons learned from currently available VADs.

Mechanical circulatory support: a clinical reality

Mechanical circulatory support is becoming an alternative therapeutic option for patients in cardiogenic shock or advanced cardiac failure who cannot be improved by maximal medical therapy. More than 30 years of engineering development and clinical research have led to a level of efficacy and reliability of ventricular assist devices, which allows promotion of this approach for the most difficult patients. Uses include a gaining-time strategy as a bridge to cardiac transplantation or recovery of native cardiac function, as well as permanent support with the device. The large variety of devices permits every cardiac surgical unit, even those not used to cardiac transplantation, to propose this option to the patient. Recent experience with small silent implantable pumps suggests that the pioneering period of mechanical circulatory support is probably over, and the time has come for precise prospective trials to optimize both patient selection and the timing for utilization. In countries where cardiac transplantation has not developed, there is now an easily accessible technique for management of patients with cardiac failure.

The Status of Failure and Reliability Testing of Artificial Blood Pumps

Artificial blood pumps are today's most promising bridge-to-transplant, bridge-to-recovery, and destination therapy solutions for patients with congestive heart failure. There is a critical need for increased reliability and safety as the next generation of artificial blood pumps approach final development for long-term destination therapy. To date, extensive failure and reliability studies of these devices are considered intellectual property and thus remain unpublished. Presently, the Novacor N100PC, Thoratec VAD, and HeartMate LVAS (IP and XVE) comprise the only four artificial blood pumps commercially available for the treatment of congestive heart failure in the United States. The CardioWest TAH recently received premarket approval from the US Food and Drug Administration. With investigational device exemptions, the AB-180, AbioCor, LionHeart, DeBakey, and Flowmaker are approved for clinical testing. Other blood pumps, such as the American BioMed-Baylor TAH, CorAide, Cleveland Clinic-Nimbus TAH, HeartMate III, Hemadyne, and MagScrew TAH are currently in various stages of mock loop and animal testing, as indicated in published literature. This article extensively reviews in vitro testing, in vivo testing, and the early clinical testing of artificial blood pumps in the United States, as it relates to failure and reliability. This detailed literature review has not been published before and provides a thorough documentation of available data and testing procedures regarding failure and reliability of these various pumps.

Methods of Failure and Reliability Assessment for Mechanical Heart Pumps

Artificial blood pumps are today's most promising bridge-to-recovery (BTR), bridge-to-transplant (BTT), and destination therapy solutions for patients suffering from intractable congestive heart failure (CHF). Due to an increased need for effective, reliable, and safe long-term artificial blood pumps, each new design must undergo failure and reliability testing, an important step prior to approval from the United States Food and Drug Administration (FDA), for clinical testing and commercial use. The FDA has established no specific standards or protocols for these testing procedures and there are only limited recommendations provided by the scientific community when testing an overall blood pump system and individual system components. Product development of any medical device must follow a systematic and logical approach. As the most critical aspects of the design phase, failure and reliability assessments aid in the successful evaluation and preparation of medical devices prior to clinical application. The extent of testing, associated costs, and lengthy time durations to execute these experiments justify the need for an early evaluation of failure and reliability. During the design stages of blood pump development, a failure modes and effects analysis (FMEA) should be completed to provide a concise evaluation of the occurrence and frequency of failures and their effects on the overall support system. Following this analysis, testing of any pump typically involves four sequential processes: performance and reliability testing in simple hydraulic or mock circulatory loops, acute and chronic animal experiments, human error analysis, and ultimately, clinical testing. This article presents recommendations for failure and reliability testing based on the National Institutes of Health (NIH), Society for Thoracic Surgeons (STS) and American Society for Artificial Internal Organs (ASAIO), American National Standards Institute (ANSI), the Association for Advancement of Medical Instrumentation (AAMI), and the Bethesda Conference. It further discusses studies that evaluate the failure, reliability, and safety of artificial blood pumps including in vitro and in vivo testing. A descriptive summary of mechanical and human error studies and methods of artificial blood pumps is detailed.

Clinical results with an ePTFE inflow conduit for mechanical circulatory support

BACKGROUND

Neurologic complication is an adverse event associated with mechanical circulatory support. To decrease the incidence of embolic cerebrovascular accidents (CVA) during support with the Novacor left ventricular assist system (LVAS), an expanded polytetrafluoroethylene (ePTFE) inflow conduit has been developed and introduced clinically.

METHODS

Using clinical data from Europe and Canada, we conducted a retrospective analysis of the incidence of embolic CVA with the ePTFE inflow conduit (n=88) in comparison with the previously used polyester inflow conduits (n=310, including Vascutek n=155 and Cooley n=155). We calculated freedom from embolic CVA, risk reduction for embolic CVA, and linearized rates of embolic CVA.

RESULTS

A significant decrease in the incidence of embolic CVA was demonstrated with the ePTFE conduit (ePTFE 10% vs polyester 23%, p=0.002). Kaplan-Meier analysis of freedom from embolic CVA at 180 days after implantation was 86% for the ePTFE group vs 72% for the polyester group (log-rank test, 0.0185). We also found an associated risk reduction of 55% in CVA occurrence in the ePTFE group when compared with the Polyester group (hazard ratio, 0.445; 95% confidence limit, 0.222-0.890; p=0.0221). Linearized CVA rates also were decreased at all time intervals after implantation in the ePTFE group.

CONCLUSIONS

Preliminary clinical results with the newly introduced ePTFE inflow conduit provide compelling evidence that the ePTFE conduit material significantly decreases thromboembolic complications during mechanical circulatory support with the Novacor LVAS.

Novacor Left Ventricular Assist System Long-Term Performance: Comparison of Clinical Experience with Demonstrated In Vitro Reliability

Since the first implant of the Novacor wearable left ventricular assist system (LVAS) in 1993, median implant duration worldwide has increased from 93 days (max 2.2 years) to 202 days (max 4.1 years) in May 2001. In vitro reliability/durability testing of the Novacor LVAS has previously demonstrated a mean time to failure of 4.2 (3.04-5.59) years. These tests revealed a single failure mode--main bearing wear--with measurable symptoms gradually appearing before degradation of pump function. An ongoing clinical study of 37 recipients implanted for more than 1 year has shown that a simple noninvasive method of pump surveillance, derived from the in vitro experience, is well tolerated in the clinical setting. The overall clinical experience is consistent with in vitro reliability/durability tests that demonstrated 99.9%, 98.5%, and 87.4% freedom from wear at 1, 2, and 3 years (80% confidence). So far, the clinical study has shown 99.4%, 91.5%, and 91.5% freedom from wear at the same 1, 2, and 3 year intervals (95% confidence). Furthermore, the clinical findings have corroborated the in vitro experience that the wear mechanism is generally measurable and gradual, enabling elective clinical LVAS replacement or transplantation. Clinical valve performance was also monitored, using an exercise protocol and collecting comparative data on peak flows across the valves. It was determined that developing valve dysfunction could be diagnosed early and, in the failures that did occur (n = 2), these were related to the patient's disease state. In conclusion, although clinical conditions exposed the LVAS to a wide range of different environmental and hydraulic stresses, the surveillance program described appears practical and reliable, and its findings broadly parallel those of the earlier in vitro study. Additional data needed to complete formal validation continue to be collected.

The Novacor left ventricular assist system: clinical experience from the Novacor registry

The electrically powered Novacor left ventricular assist (LVAS) system was first used clinically as a bridge to transplant in 1984. The configuration has evolved to the current wearable model used clinically for the first time in 1993. In 1998, the inflow conduit was modified, reducing embolic events by 50%. Over 1100 implants have been performed worldwide with cumulative support greater than 300 patient years, and only 0.7% requiring replacement. The Novacor is a safe and effective device for bridge to transplant, bridge to recovery, or potentially permanent implant with reliable long-term support for periods as long as 4 years.

The totally implantable novacor left ventricular assist system

The Novacor Left Ventricular Assist System (LVAS) (Novacor Corp, Oakland, CA) was initially console-based and has been available since 1993 in a wearable configuration. It has been successfully used for the past 16 years as a bridge to cardiac transplantation in patients with end-stage congestive heart failure. The Stanford experience represents 53 patients (48 male, 5 female) with a mean age of 44 +/- 13 years (16 to 62) and a mean support time of 56 +/- 76 days (1 to 374). Complications with LVAS use consisted predominantly of bleeding (43%), infection, (30%), and embolic cerebrovascular events (24.5%). Sixty-six percent of the supported patients were successfully bridged to cardiac transplantation. In animal studies, 4 sheep had the totally implantable configuration in place for a cumulative duration of 1 year with 1 animal supported for 260 days. The next generation Novacor LVAS will be small, quiet, and fully implantable without the need for volume compensation. It will also provide physiologic pulsatile flow and will be fail-safe.

Mechanical circulatory support for advanced heart failure: effect of patient selection on outcome

BACKGROUND

Use of wearable left ventricular assist systems (LVAS) in the treatment of advanced heart failure has steadily increased since 1993, when these devices became generally available in Europe. The aim of this study was to identify in an unselected cohort of LVAS recipients those aspects of patient selection that have an impact on postimplant survival.

METHODS AND RESULTS

Data were obtained from the Novacor European Registry. Between 1993 and 1999, 464 patients were implanted with the Novacor LVAS. The majority had idiopathic (60%) or ischemic (27%) cardiomyopathy; the median age at implant was 49 (16 to 75) years. The median support time was 100 days (4.1 years maximum). Forty-nine percent of the recipients were discharged from the hospital on LVAS; they spent 75% of their time out of the hospital. For a subset of 366 recipients, for whom a complete set of data was available, multivariate analysis revealed that the following preimplant conditions were independent risk factors for survival after LVAS implantation: respiratory failure associated with septicemia (odds ratio 11.2), right heart failure (odds ratio 3.2), age >65 years (odds ratio 3.01), acute postcardiotomy (odds ratio 1.8), and acute infarction (odds ratio 1.7). For patients without any of these factors, the 1-year survival after LVAS implantation including the posttransplantation period was 60%; for the combined group with at least 1 risk factor, it was 24%.

CONCLUSIONS

Careful selection, specifically implantation before patients become moribund, and improvement of management may result in improved outcomes of LVAS treatment for advanced heart failure.