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"In the name of God"
Homework 2
Biomechatronic Systems
Instructor:
Dr. Delrobaei
Student:
Atiye Riasi(9800436)
Spring 2020
1.
Design an implantable-wearable system automatically control
hypertension. How to measure the blood pressure in a semi real-
time manner and send appropriate feedback to the neurostimulator.
Your device need to be unobtrusive and safe.
Keywords
What is Artrial blood pressure?
Diagram of a pulse blood pressure which shows Systolic, Diastolic
and Mean Artrial blood Pressure(MAP)
hypertension High blood pressure
hypotension Low blood pressure
What can cause an increase in blood pressure ?
Blood pressure is a force that blood exerts against the walls of their
vessels. So any reason which increase the volume of blood or
peripheral resistance can increase the blood pressure.
Hypertension can cause heart disease and stroke
It is very important to control the hypertension
How to control hypertension?
Following diagrams shows the role of kidney and nervous system to
control blood pressure in summary.
How baroreceptors decrease blood pressure?
Increasing the blood pressure Stretching the baroreceptors in
wall of carotid sinus Discharging baroreceptors
Sending messages to the vasomotor center in medulla via baro-
receptive afferent fibers including the carotid sinus nerve that fuses
with the glossopharyngeal nerve and the aortic depressor nerve that
runs in the vagal nerve
attenuating the efferent sympathetic nerve firing in response to the
baroreceptor discharge
reducing sympathetic tone/increasing renal function
decreasing peripheral resistance (by vasodilation)/decreasing blood
volume Decreasing blood pressure
What are possible treatments for hypertension?
1. Antihypertension drugs
It is a non-invasive and more acceptable methode
× For some patients blood pressure does not fall
× Blood pressure is reduced insufficiently
× May cause shifting the blood pressure set point (blood pressure
at rest) in a higher level.
2.Artificial stimulation of baroreceptor
is very useful for patients with drug resistance
× it is invasive
History of artificial stimulators
Early models :
1950s: Because of its invasive nature, it was never considered a
front-line and it was used just for patients who underwent neck
dissection for canser, they found out that increasing in the
intensity of electrical stimulation decreases MAP and HR.
Baropacers: the device was placed under the pectoral muscle
and the leads were connected to the carotid sinuses and the
device was turned on or off by placing a magnet over the
device.
Bilateral stimulators
Major limitation of these earlier devices:
Due to lack of ability to individualize for each patient, some patients
developed systemic symptoms such as orthostatic hypertension (a
sudden rise in systolic blood pressure of 20 mmHg or more when
standing), bradycardia and some developed local irritation from
stimulator implantation.
Matching bilateral stimulation frequency to heart rate as
tachycardia could mean an increased sympathetic tone, so the
stimulator did not abolish the body’s innate ability to increase
the sympathetic activity at the time of exercise.
Two major reasons cause this device fall out of favor:
1. Effectiveness and continues of oral anti-hypertensive agents in
1980s and 1990s.
2. Implantation of carotid sinus stimulator is an invasive
procedure.
Baro Activation Therapy(BAT) as an open loop neuro
modulation and long term blood pressure reduction
As the entry of new agents slowed and prevalence of resistance
hypertension increases, new devices such as Rheos
(𝐶𝑉𝑅𝑥,Minneapolis,MN) are developed, they consists of an
implantable pulse generator with leads attached to the carotid sinus
bilaterally.
In contrast to the older devices,
The program in this device could be adjusted after the
implantation.
Electronics have been much miniaturized.
× The most common approach is to set the voltage and the
frequency depending on the prevailing level of blood pressure
and heart rate and the patient’s response to stimulation. it is
necessary to use different settings during daytime and
nighttime. At any rate, it is a matter of trial and error to find
the optimal settings in a particular patient.
closed-loop baroreceptor stimulation lowers blood pressure without
inducing hypotension and adjusting the stimulation pattern in
response to the subject’s physiological state as well as the
environmental condition. Therefore, it would promise ideal blood
pressure control (tested in rats and rabbits)
bionic baroreflex system(BBS) basically consists of an artificial
pressure sensor, a neurostimulator for autonomic nerves and a
regulator that encodes blood pressure into neurostimulation.( New
proposed closed loop system to avoid hypotenstion after controlling
hypertension)
Remember: Bionic means that the device is designed to replicate the
dynamic characteristics of the native system to fully restore the
function
Implementation
It is important to point out that most neuromodulation devices
adopt an open-loop scheme which requires the user to manually
change the stimulation parameters, thus lacking safety, selectivity,
specificity and adaptability. Effective and safe neuromodulation
requires intelligent control to dynamically adjust the stimulation
pattern in response to the subject’s physiological state as well as the
environmental condition.
feasibility of the development of closed-loop blood pressure
control technology
Native baroreflex system
The arterial baroreflex system is a negative feedback system.
Neurostimulation at the afferent limb of the baroreflex inhibits the
efferent sympathetic nervous activity and lowers blood pressure.
Changes in SAP(systolic AP) induced by external disturbance in
pressure (Pd) is sensed by arterial baroreceptors. The change in
pressure initiates a reflex change in vasomotor sympathetic outflow.
Primary reflex center is located in brain stem.
Vasomotor center(controller)
Artificial Baroreflex system
The bionic baroreflex system basically consists of an artificial
pressure sensor, a neurostimulator for autonomic nerves and a
regulator that encodes blood pressure into neurostimulation
𝐻𝑛𝑎𝑡𝑖𝑣𝑒 + +
Pd
SAP
artificial vasomotor center(controller):
In anesthetized rats, bilateral carotid sinus baroreceptive areas
were vascularly isolated with preservation of carotid sinus
nerves
Then a servo-controlled piston pump that can generate real-
time AP in the intracarotid sinus was connected to bilateral
common carotid arteries. Bilateral aortic depressor nerves
were sectioned to eliminate interaction
𝐻𝑆𝐴𝑃−𝑆𝑇𝑀 𝐻𝑆𝑇𝑀_𝐴𝑃
+ +
Pd
SAP
Neurostimulation electrodes were attached to the proximal
end of the sectioned aortic depressor nerves. An AP sensor
was placed in the aortic arc
With this procedure, the ‘‘bionicbaroreflex’’ works through the aortic
depressor nerve, meanwhile the‘‘nativebaroreflex’’works through
the carotid sinus baroreceptor
transfer function from intracarotid sinus pressure to
AP(𝐻𝐶𝑆𝑃−𝐴𝑃) using white noise analysis
transfer function from aortic depressor nerve stimulation to
AP(𝐻𝑎𝑑𝑛𝑆𝑇𝑀−𝐴𝑃)
opening rule for artificial vasomotor center:
𝐻𝐶𝑆𝑃−𝑆𝑇𝑀=𝐻𝐶𝑆𝑃−𝐴𝑃
𝐻𝑎𝑑𝑛𝑆𝑇𝑀−𝐴𝑃
Since the identified HCSP-adnSTM approximated the delta
function, a simple gain as a simplified operating rule is used
set the ‘‘bias’’ stimulation frequency that could maintain AP at
the same level of native closed-loop AP. In other words, the
‘‘bias’’ functions as the modulator of mean AP, whereas the
‘‘gain’’ works as the pressure buffer in reducing pressure
variability.
The bionic baroreceptor therefore allows not only AP lowering
by adjusting the ‘bias’ frequency like BAT therapy but also AP
stabilization by increasing/decreasing stimulation frequency in
response to AP. Thus, the bionic baroreceptor might benefit
hypertensive patients with comorbid orthostatic hypotension.
Practical issues for clinical application
× physical and logical neural interfaces and development of
miniaturized implantable sensor for detecting feedback
signals
× nerve damage due to electrode physical contact and
unpleasant reactions caused by current leakage to
surrounding tissue and nonselective recruitment of the fiber.
× invasive surgical approach to the splanchnic sympathetic
nerve is not practical in humans
× Spinal cord stimulation for blood pressure control therapy
seems more feasible, but the application would be limited to
anesthetized patients because of unpleasant sensation and
muscle twitching. Regarding baroreceptor afferent limb
activation, the baroreflex activation therapy (BAT) system
developed by CVRx Inc. was approved for clinical trial
× we have to introduce a clinically approved, implantable,
durable and safe AP sensors such intravascular pressure
sensor that is used for implanting in the pulmonary artery
FDA approved in 2014
× safety against clot
× it is unclear whether the dynamic transduction from AP to
neurostimulation remains unchanged over time or under the
awake condition
A Wireless Miniaturized Neural Implant system on chip (SoC)
the above mentioned system is not implantable and relies on wires
connecting the stimulator and the pressure sensor with the external
controller, impeding its clinical usage.
For an implantable device we need
to support adaptive power regulation to stabilize the potential
power fluctuation at the implant side due to environment
changes such as coil displacement.
Using inductive powering because of high power transmission
efficiency
Class-E power amplifier with optimized coil pair is used for wireless
power transfer. The loading and/or coupling coefficients change is
dynamically compensated by sensing the implant’s power
information and transmitting it back to the primary power
transmitter in order to adjust the level of delivered power
The unexpected power loss would decrease the efficiency of
the power link and even lead to the malfunction of the implant.
Using a wireless rechargeable battery
a neural implant must be capable of monitoring the electrode-
tissue interface to ensure an effective and safe stimulation
protocol.
An effective approach is time domain analysis on the electrode-
tissue impedance: a biphasic stimulus as a by-product
supported by most stimulators is injected into the electrode-
tissue interface and the electrode potential are measured at
three specific time points to estimate the equivalent circuit
model of the bio-impedance.
Biphasic electrical stimulation is widely adopted to ensure a
zero Faradic charge residual at the stimulation site
the sensed electrode voltage is sent to a proportional-integral-
derivative (PID) controller to precisely control the width of
either the anodic or the cathodic current pulses.
A critical and paramount feature for the closed-loop implant is
the capability of adapting the stimulation parameters in real-
time
The capability of performing selective and focal stimulation to
specific nerves would greatly ameliorate the treatment efficacy.
a multi-channel stimulator should be able to independently
configure the stimulation parameters in each driver and
support various waveforms. This would allow the shaping and
steering of the electric field at the stimulating electrode to
improve the stimulation spatial resolution and accordingly
perform selective nerve stimulation.
Miniaturizing the device to to reduce its surgical invasiveness
would greatly enhance patient acceptability.
There are two approaches widely adopted to execute the control
algorithm for the closed-loop implant:
1) preloading the re-configurable firmware in the implant
2) configuring the algorithm in the external controller that
communicates with the implant