Biomedical Engineering Reference
In-Depth Information
FIGURE 8-57
Waveform changes
for different rotation
speeds of a
Thoratec Heartmate.
(a) 9,000 rpm.
(b) 10,000 rpm.
(c) 11,000 rpm.
(Adapted from
(Antaki, Boston
et al., 2003).]
Flow rate and pressure difference (or head) are key variables needed in the control of
implantable rotary blood pumps. However, use of flow or pressure probes can decrease
reliability and increase system power consumption and expense. For a given fluid viscosity,
the flow state is determined by any two of the four pump variables: flow, pressure difference,
speed, and motor input power. Thus, if viscosity is known or if its influence is sufficiently
small, flow rate and pressure difference can be estimated from the motor speed and motor
input power (Tanaka, Yoshizawa et al., 2001).
As discussed in Paden, Ghosh et al., (2000) and Antaki, Boston et al., (2003), any
cardiac augmentation system must function within the following constraints:
• Cardiac output should be above a minimum value. This is nominally 5 L/min but
will vary between 3 and 6 L/min depending on the size of the patient.
• Left atrial pressure should be maintained below 10 to 15 mmHg to avoid pulmonary
edema and above 0 mmHg to avoid suction.
• Systolic arterial pressure should be maintained within specific limits to ensure an
adequate oxygen supply while avoiding risks associated with hypertension.
• System should be maximally efficient in terms of blood flow and pump power.
In general, it is not possible to minimize all of these simultaneously, so control sys-
tems are designed that optimize performance based on cost functions associated with
deviation from the constraint. These cost functions are normally asymmetrical because of
hard minima below which the patient cannot function. An intelligent controller based on
multiobjective optimization of these parameters as well as information about the patient's
activity level is used to control the pump motor speed, as shown in Figure 8-58.
Other control architectures have been developed. For example, Vollkron discusses a
closed-loop controller for a DeBakey VAD that uses venous return based on flow pulsatility
(peak-to-peak flow variation) as well as the available return derived from the patient's
own heart rate (desired flow) along with power use and minimal flow as inputs. These
are analyzed on a beat-to-beat basis within a 10-second moving window before the motor
speed is adjusted (Vollkron, Schima et al., 2006).
