Ventricular Assist Devices: Which Device for Which Patient?

The use of mechanical support of the circulation with ventricular assist devices for refractory heart failure is increasing. This is due to expanding indications, including for acute heart failure and shock, and the development of a number of new types and designs, both for temporary and more long-term support. Pulsatile volume displacement pumps have evolved to continuous flow pumps, a whole new type of design for the long-term pumps(Slide 1). These new pumps are a major step forward in the care of patients due to significantly reduced size and weight, and are also associated with a decrease in adverse events as well as improved survival, quality of life, and functional capacity (Slide 2). Despite that nearly half of patients in the recent Heartmate II clinical trial were inotrope-dependent, 84% of these patients were alive and had either undergone heart transplant or were still being successfully supported on the VAD. These results are far superior to those reported with pulsatile design pumps.

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Slide 1.

Pulsatile vs Continuous Flow VADs. The large volume displacement type of LVAD is the prototype of the first generation pumps, while an example of the newer second generation of continuous flow pump is the Heartmate II pump on the right.The HM II pump allows instantaneous adjustment of pump speed and LV unloading.

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Slide 2.

Survival with use of the HM II LVAD as a Bridge to Transplant in recent trial, including both the original cohort reported and the additional patients enrolled in a continuous access protocol. The outcomes improved with more experience of the centers later in the trial experience and now equal nearly 84% survival at 6 months of support.

Citation:
As of Sep 14, 2007 There was a trend towards improved survival in the CAP cohort compared to the Primary cohort. Survival at 12 months was 71% in the Primary cohort and 75% in the CAP cohort.

The devices now available for acute cardiogenic shock can be either placed percutaneously in the cath lab by interventional cardiologists or surgically implanted via a sternotomy. These temporary devices are designed to provide only a few days of support, but can allow 1) an adequate assessment of recovery of native cardiac function followed by discontinuation, or 2) stabilization and recovery from shock to allow the placement of devices suited for long-term support, typically for support to a heart transplant. The advantages of the percutaneous devices include speed of implant and no requirement for surgery, while the surgical devices provide more blood flow and likely more rapid recovery of organ function from shock. The comparative maximum flow ranges from 2.5 to 3.5 liters with the percutaneous devices and 5 to 6 liters with the surgically implanted temporary devices.

The major application of VADs has been for long-term support (typically for months and even years) as a so-called Bridge to Transplant (BTT) until a donor heart becomes available. Until recently, the most commonly used devices were those that used a pulsatile, volume displacement design. They were very effective for short duration of support, but they had suboptimal durability due to failure of the multiple moving parts, including prosthetic valves or bearings. Their use for longer periods in patients who were not transplant candidates demonstrated the durability problem, with over 50% of pumps requiring replacement by 18 months of support.

Matching the individual patient to the optimal VAD can be challenging at times, but the new second generation of continuous flow pumps have proven to be quite versatile and can effectively support nearly any type of patient for any indication (Slide 5). One advantage that these new pumps provide is marked reduction in size, which has allowed their use in a much higher percentage of women (especially small to average size women, who were previously limited due to the large pump size). The smaller women supported in recent trials have had outcomes as good as those of larger men and women, including survival both while on support and post heart transplant. The current lower limits of body surface area for VAD candidates has been reduced from 1.5 to 1.3 meters² with the continuous flow pumps.

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Slide 5.

List of specific indications and relative advantages of different types of pumps to help decide Which Pump for Which Patient.

Secondary endpoints of the clinical study included:

The newest rotary pump is the Heartware HVAD (Slide 3). This device is unique in that it is smaller than other continuous flow pumps, but more importantly, the pump is placed totally within the pericardium and therefore does not require a surgically created pocket. This feature should reduce the long-term complication of late pump pocket infection. The pericardium also serves to keep the canula within the LV cavity in a stable and fixed location. This pump is now in clinical trials in the US.

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Slide 3.

The new third generation rotary pump is the Heartware HVAD. This pump is smaller but more importantly sits entirely within the pericardium which eliminates the need for a pump pocket which is a long term risk of late infection. Trials of this new pump are now under way in the US and Europe.

Other advantages of the new second generation of continuous flow design LVADs is that they operate silently, unlike the bellows sound of the pulsatile pumps. The also have only one moving part, the rotor, which provides a tremendous improvement in projected durability, now estimated at 5 to 10 years without replacement. Poor durability is a major cause of morbidity and mortality with VADs, and this improvement is one of the important contributions to the longer-term outcomes now being reported. The results of the Heartmate II DT trial will be available at the AHA meeting in November, and will confirm the durability at the study end point of two years of support.

The was a question as to whether the smaller pumps would be adequate to support patients with a large BSA. The range of patients supported in a recent trial with the Heartmate II pump was 1.4 to 2.7 m², with equally good outcome in the larger patients and flows adequate for functional capacity.

Another concern was whether end organ dysfunction or other comorbidities could be reduced or significantly improved with a period of VAD support. Of patients implanted with the Heartmate XVE device from 2002 to 2005 as destination therapy (not as bridge to transplant), 13% had sufficiently recovered renal function or reduced pulmonary hypertension to sucessfully undergo heart transplant, with survival of 88% at one year. Often patients supported with an LVAD are the very best patients undergoing heart transplantation due to ability to exercise and recover functional capacity; such patients have equal, if not superior, outcomes compared to candidates not supported with an LVAD.

There are several new devices, such as the Circulite pump (Slide 4), that employ unique design, including one that is very small in size and placed by local surgical incision much like a pacemaker pocket. It is designed to only provide 2 to 2.5 liters of flow and is for use in patients before their disease advances and they require higher levels of flow and support.

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Slide 4.

New LVAD designed for much lower levels of flow (2.0-2.5 L/min) for patients with earlier, less severe heart failure.

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