Biomedical Engineering Reference
In-Depth Information
or arterial pressures. They were operated in open loop mode, which allowed manual
adjustments of the eject rate, duty cycle (systolic/diastolic ratio), and the pneumatic drive
pressure.
Pulsatile pumps that are actuated by a positive displacement mechanism include de-
vices such as the Thoratec Heartmate cam drive pump, Novacor spring decoupled solenoid,
and the Penn State roller screw pump. These are all controlled by adjusting the stroke vol-
ume or the pump rate. They have been shown to be effective for short-term implantation,
but they are bulky, have large power requirements, and use complex reciprocating pumping
motions.
Other types of ventricular assist devices are dynamic. These include centrifugal,
mixed-flow, and axial pumps. Their primary benefit is that they are smaller than the
pulsatile units and more energy efficient. Axial pumps, in particular, seem to be quite
promising for reasons of simplicity and efficiency. They can be implanted in the patient's
abdomen with catheters passing through the diaphragm connecting the pump to the left
ventricle, or in some cases they can be attached to the ventricle, inserted within it, or even
inserted within the aorta.
Rotary pumps are insensitive to the hydraulic head, which results in their being intol-
erant of mismatches between physiological demands and operating settings. In addition,
ensuring that the pump is operating at an appropriate speed is more critical than it is with
pulsatile devices. Rotary pumps in clinical use include the early Medtronic and Biomedicus
devices.
Pulsatile pumps still have a role because of their success in stimulating a highly
sensitive natural Starling law
1
response to venous pressure. Although nonpulsatile blood
flow is now known to be more acceptable than was once believed, it is not yet known with
certainty whether all patients will successfully adapt to the unnatural blood shear and flow
regimes of dynamic pumps.
In most of these devices, external batteries supply power via a cable through the
abdomen or via transcutaneous energy transfer devices using electromagnetic coupling.
The batteries are carried in underarm holsters or a waist pack.
To study the efficacy, safety, and cost-effectiveness of an LVAD for permanent use
compared with optimal medical management through medication therapy, a clinical trial
called the Randomized Evaluation of Mechanical Assistance for the Treatment of Conges-
tive Heart Failure (REMATCH) was undertaken. This trial was conducted on 129 patients
at 22 major hospitals across the United States and was financially supported by the Na-
tional Institutes of Health (NIH). The result of this study showed a 48% decrease in the
death rate from all causes with the LVAD over the first 2 years of use. Patients in the LVAD
group had a median survival period of 408 compared with 150 days in the medication ther-
apy group. Only 8% (1 of 12) survived 2 years in the optimal medical management group.
23% were alive at 2 years in the LVAD group. The quality of life was also improved in
the LVAD group, based on the questionnaire completed by patients from both groups at
1 year. The study was conducted on only the sickest patients with no other options (Jeffrey,
2001).
1
Starling law states that the increased volume of blood entering the heart stretches the ventricular wall,
causing cardiac muscle to contract more forcefully.
