28-05-2013, 12:51 PM
Novel Long-Term Implantable Blood Pressure Monitoring System with Reduced Baseline Drift
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Abstract
A novel long-term less-invasive blood pressure
monitoring system with fluid-filled cuff is proposed for
advanced biological research. The system employs an
instrumented elastic cuff attached with a rigid isolation ring on
the outside wall of the cuff. The cuff is wrapped around a blood
vessel for real-time blood pressure monitoring. The elastic cuff
is made of bio-compatible soft silicone material and is filled with
bio-compatible insulating silicone oil with an immersed MEMS
pressure sensor. This technique avoids vessel penetration and
substantially minimizes vessel restriction due to the soft cuff
elasticity, thus attractive for long-term monitoring. A rigid
isolation ring is used to isolate the cuff from environmental
variations to suppress baseline drift in the measured waveform
inside the monitoring cuff. The prototype monitoring cuff is
wrapped around the right carotid artery of a laboratory rat to
measure real-time blood pressure waveform. The measured in
vivo blood waveform is compared with a reference waveform
recorded simultaneously by using a commercial catheter-tip
transducer inserted into the left carotid artery, showing
matched waveforms with a scaling factor about 0.03 and a
baseline drift of 0.6 mm Hg. The measured baseline drift is
three times smaller compared to using a cuff without a rigid
isolation ring.
INTRODUCTION
NA sequencing of small laboratory animals together
with in vivo real-time biological information, such as
blood pressure, temperature, activity and bio-potential
signals, is ultimately crucial for various biomedical and
genetic research to identify genetic variation susceptibility to
diseases, for example hypertension, obesity, epilepsy and
cancers [1], and to potentially develop new treatments for
diseases. A small-size, light-weight, long-term, reliable biosensing
implantable system with two-way wireless telemetry
capability is highly desirable to capture the real-time
biological information from a “free” roaming animal housed
in its home cage as shown in Figure 1. The implantable
microsystem employs a micro-fabricated sensor array for
multi-channel vital signal monitoring and integrated
electronics for sensor interfacing, RF powering and two-way
data telemetry. This paper focuses on the development of
blood pressure monitoring system, which is important for
biological research [2].
BASELINE DRIFT ANALYSIS AND NEW CUFF DESIGN
The baseline drift is likely caused by the soft outside wall
of the cuff. Because of the soft nature of the cuff outside
wall, the pressure inside the cuff is quite susceptible to
environmental variations, such as animal muscle and tissue
movement. The cuff outside wall can be made more rigid to
decrease the effects, however, at the same time the restraint
to the vessel will increase, thus leading to a trade off between
measurement baseline drift and the vessel constraint.
However, it is difficult to address the trade off due to the
limited research currently available on the long-term
influence on animals with different amount of vessel
constraint. Therefore, a rigid isolation ring is employed to
isolate the cuff from the outside environment in the new
design.
Figure 3 shows the top view and cross-sectional view of
the proposed cuff design with a rigid isolation ring. The rigid
isolation ring is attached to the outside wall of the original
cuff to isolate the cuff from the environmental variation. At
the same time, the isolation ring is designed so that an air
cavity between the isolation ring and the cuff outside wall
will be formed upon completion of the fabrication process,
as shown in the figure. As a result, the cuff outside wall can
move freely and would not influence the stiffness of the cuff
in the center.
IN VIVO RESULTS
A laboratory rat from Charles River Co. with a weight of
630 g is used for implant evaluation due to their relatively
large artery size around 1 mm. The blood pressure
monitoring cuff is wrapped around the right carotid artery
and secured by suture threads; a commercial catheter-tip
transducer from Micro-Med Inc. is inserted into the left
carotid artery as a reference for comparison. Figure 6 shows
the relative position of the monitoring cuff and catheter-tip
transducer during the implant measurement. Blood pressure
waveform is measured by the monitoring cuff and the
catheter-tip transducer and recorded simultaneously by a
two-channel data acquisition system sampled at 1 KHz.
CONCLUSION
A novel less-invasive blood pressure monitoring system
with fluid-filled cuff is proposed for advanced biological
research. The system employs an instrumented elastic cuff
attached with a rigid isolation ring on the outside wall of the
cuff. The cuff is wrapped around a blood vessel for real-time
blood pressure monitoring. The proposed method avoids
vessel occlusion, bleeding, and potential blood clotting, thus
is suitable for long-term implant applications. In vivo blood
pressure measurement in a rat shows the new cuff design
with a rigid isolation ring can suppress the baseline drift of
the measured waveform. Thus, blood pressure waveform
with higher fidelity and accuracy can be obtained. The
proposed cuff-based sensing architecture can be used to
realize a complete implantable wireless small animal
monitoring system for advanced biological research and can
be potentially useful for human implant monitoring to
improve health care quality in the future.