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In Vivo Blood Pressure Sensor
A. Pillai and D. P. Nair
Introduction to Biosensors Final Report
ECE Dept., UMass Lowell,
Abstract
Blood pressure measurement is an important diagnostic tool for ailments like dizziness, cardiac arrest, strokes etc. Conventional methods, also called non-invasive methods are not accurate when it comes to real time measurement of blood pressure. Hence, there is an urgent need for in-vivo blood pressure sensors. The advantage of having an in-vivo sensor is that it is an attractive proposition for continuous blood pressure monitoring and hence avoiding long term biological effects. In this study, we began by reviewing existing in-vivo blood pressure sensors and then found out what were the drawbacks for these pressure sensors. We are also proposing two solutions to overcome the two problems that were affecting the performance of these sensors.
Keywords: Blood pressure, in-vivo, noise improvement, power dissipation
Introduction
Blood pushing against the walls of the arteries causes a force, when the blood is pumped by the heart. This is called Blood Pressure. Damage to the body can be caused in many ways if this pressure rises and stays overtime. Coronary heart disease, Heart failure, Kidney Failure and stroke are few of the symptoms of High blood Pressure (HBP) along with many other serious health conditions.
It has been found that 1 in 3 adults in United States has a medical condition caused by High Blood Pressure .Because HBP by itself shows no symptoms there is a good chance that a person can have the disease for a long time without knowing about it. And during this time of unawareness, the HBP can cause damage to the blood vessels, the Heart, kidneys and other vital organs of the body. Checking and maintaining blood pressure numbers is important, even when the person is feeling fine regardless of age and state of health. High blood pressure on the other hand needs treatment and may prevent further damage of the vital organs. Low blood pressure needs as much attention as high blood pressure, as it poses as many health risks as HBP.
One of the tricky things about BP is that it is never the same value. Activities like sleeping and relaxing bring down the Blood pressure levels. Similarly blood pressure is expected to rise when the person gets up or even gets excited, stressed out. or nervous. It is also high with levels of activeness. If the numbers of BP remain above normal level even during moderate levels of activity that is when there is a risk for health problems. Risk factors increase as the number increases above 120/80 mm Hg or drop below 120/800 mm Hg.
A condition called "Prehypertension" basically states that there is a good chance a person will end up having HBP unless steps are taken to prevent it . If a person is currently under treatment for HBP and show consistent readings in the normal range, the blood pressure is considered to be under control. However, you still have the condition. You should see your doctor and follow your treatment plan to keep your blood pressure under control. Your systolic and diastolic numbers may not be in the same blood pressure category. In this case, the more severe category is the one you're in. For example, Stage 2 HBP is when a person has a systolic number of 160 and a diastolic number of 80. If a person has systolic number is 120 and a diastolic number is 95, the condtion is called Stage 1 HBP. If the person has additional risk of having diabetes or chronic kidney disease, HBP is defined as 130/80 mmHg or higher. Children and teenagers exhibit different HBP numbers. Age is one main factor that influences HBP. Following a healthy lifestyle is usually the best solution to delay or prevent HBP
In some cases, besides age and lifestyle, other diseases maybe responsible for raising blood pressure. Problems such as chronic kidney disease, sleep apnea, thyroid disease, may cause blood pressure to rise.. Medicines used to control certain diseases may also may raise a person’s blood pressure. Asthma medicines and cold-relief products are few of the examples.
Damages caused to the body when the blood pressure numbers stay high over a long time include the following:
· An abnormal larger or weaker heart, leading eventually to heart failure. This is a condition where in the heart cannot pump enough blood to meet the body's needs.
· An abnormal bulge in the wall of an artery is a medical condition called aneurysm .The main artery carries blood from the heart to the body; and these are the main spots for aneurisms to occur. The brain, legs, and intestines all have arteries and this might cause them to shut down. .
· Kidney failure may arise due to narrowing of the kidney vessels.
· Affecting the arteries by narrowing them which are responsible, this limits blood flow (especially to the heart, brain, kidneys, and legs). Causing medical conditions like heart attack, stroke, kidney failure, or amputation of part of the leg.
· Blood vessels in the eyes to burst or bleed. This may lead to vision changes or blindness.
When blood pressure is low, that is when another medical condition called Hypotension strikes. It happens mostly because the body cannot bring the pressure back to its normal level at all, or even fast enough. Low blood pressure sometimes occurs in some people all the time. This usually means there are no signs or symptoms that cause them any discomfort, and a low blood pressure s normal to them. In other people, certain conditions or factors cause abnormally low blood pressure. Less blood and oxygen flow to the body organs is the result of this. For the most part, hypotension is a medical concern only if it causes signs or symptoms or is linked to a serious condition, such as heart disease. Signs and symptoms of hypotension may include dizziness, fainting, cold and sweaty skin, fatigue (tiredness), blurred vision, or nausea (feeling sick to your stomach).
The signs and symptoms of orthostatic hypotension and neurallu mediated hypotension (NMH) are similar. They include:
· Dizziness or light-headedness
· Blurry vision
· Confusion
· Weakness
· Fatigue
· Nausea
When low blood volume (from major blood loss, for example) or poor pumping action in the heart (from conditions like heart failure, for example) causes shock:
· The skin becomes cold and sweaty. It often looks blue or pale. If pressed, the color returns to normal more slowly than usual. A bluish network of lines appears under the skin.
· The pulse becomes weak and rapid.
· The person begins to breathe very quickly.
These two medical conditions are reason enough why measuring blood pressure is important.
Conventional Methods of Blood Pressure Measurement
Noninvasive
Unlike invasive techniques non invasive techniques are less expensive and virtually have no complication at all. They are simpler and quicker, require less expertise and are least unpleasant and less painful for patients. Their biggest disadvantage however lies in the fact that these methods usually provide less accurate results with small differences in numerical values and also cannot be used for long term continuous monitoring.. Routine examinations and monitoring usually uses Non Invasive method of BP measurement.
Palpation
A minimum systolic value can be roughly estimated bypalpation, most often used inemergency situations.Historically, students have been taught that palpation of a radial pulse indicates a minimum BP of 80mmHg, a femoral pulse indicates at least 70mmHg, and a carotid pulse indicates a minimum of 60mmHg. However, at least one study indicated that this method often overestimates patients' systolic BP.
Auscultatory
The auscultatory method (from the Latin word for "listening") uses astethoscopeand asphygmomanometer. This comprises an inflatable (Riva-Rocii)cuffplaced around the upperarmat roughly the same vertical height as the heart, attached to a mercury oraneroidmanometer. The mercury manometer, considered the gold standard, measures the height of a column of mercury, giving an absolute result without need for calibration and, consequently, not subject to the errors and drift of calibration which affect other methods. The use of mercury manometers is often required inclinical trialsand for the clinical measurement ofhypertension in high-risk patients, such as pregnant women.
Fig. 1: Auscultatory Method
Oscillometric
The oscillometric method was first demonstrated in 1876 and involves the observation of oscillations in the sphygmomanometer cuff pressure. which are caused by the oscillations ofblood flow , i.e., thepulse. The electronic version of this method is sometimes used in long-term measurements and general practice. It uses a sphygmomanometer cuff, like the auscultatory method, but with an electronicpressure sensor(transducer) to observe cuff pressure oscillations, electronics to automatically interpret them, and automatic inflation and deflation of the cuff. The pressure sensor should be calibrated periodically to maintain accuracy.
Oscillometric measurement requires less skill than the auscultatory technique and may be suitable for use by untrained staff and for automated patient home monitoring.
Fig. 2: Mercury Manometer
Invasive
The most common techniques for monitoring blood pressure in small laboratory animals rely on using an invasive catheter-tip transducer inserted into an artery. Tonometry is a minimally invasive technique for a continuous measurement of pressure in blood vessels. The principle is that if a vessel is pressed against a flat surface of a pressure sensor diaphragm until vessel flattening occurs, according to Laplace’s law the pressure measured by the sensor will be equal to the pressure inside the vessel.
In Vivo Blood Pressure Measurement
In vivo(Latinfor "within the living") is experimentation using a whole, livingorganismas opposed to apartialor dead organism, or anin vitro ("within the glass", i.e., in a test tube or petri dish) controlled environment.Animal testing andclinical trialsare two forms ofin vivoresearch.In vivotesting is often employed overin vitrobecause it is better suited for observing the overall effects of an experiment on a living subject.
Two types of In Vivo Blood pressure Measurement
1. Long-Term Implantable Blood Pressure Monitoring System
2. Wireless Battery less In VIVO Blood Pressure
Sensing Micro system
Long-Term Implantable Blood Pressure Monitoring System
The system employs an instrumented elastic cuff, wound around a blood vessel, operating in a linear “diameter v.s. pressure” region of the vessel for real time blood pressure monitoring. . The elastic cuff is made of silicone or latex rubber, filled with low viscosity bio-compatible insulating fluid with an immersed highly sensitive MEMS pressure sensor. The MEMS sensor enclosed in the cuff measures the pressure waveform, which represents a scaled version of the blood pressure in the vessel, independent of the cuff bias pressure exerting on the vessel. This method avoids vessel insertion, bleeding, and potential blood clotting. Furthermore, since the cuff is made of soft elastic material such as latex or silicone rubber, and the stiffness of the cuff can be much smaller than that of a blood vessel, the restrictive effect on the blood vessel is thus substantially minimized while the soft cuff is in close contact with the vessel. This can reduce the sliding-motion-induced signal drift, thus attractive for tolerating long-term implant variations and minimizing adverse biological effects.
Wireless Battery less In VIVO Blood Pressure Sensing Micro system
A proposed wireless less-invasive implantable blood pressure sensing microsystem is depicted in figure. The system employs an instrumented elastic circular cuff, wrapped around a blood vessel, to sense real-time blood pressure waveforms. The elastic circular cuff is made of bio-compatible elastomer and is filled with low viscosity bio-compatible insulating fluid, for example silicone oil, with an immersed MEMS pressure sensor and integrated electronic system. The MEMS sensor measures the pressure waveform in the cuff coupled from the expansion and contraction of the vessel.
The measured waveform represents a down-scaled version of the vessel blood pressure waveform and can be processed by a nearby integrated electronic system, consisting of a sensor interface circuitry, an analog-to-digital converter (ADC), and a system configuration and control unit for signal conditioning and coding, followed by a wireless data transmitter to an external transceiver. The overall electronic system architecture is shown in the figure below. An adaptive RF-DC power converter is incorporated in the system design to provide a sufficient and stable energy to the microsystem implanted in an un-tethered animal.
RF powering is used to eliminate the need of an implanted battery, thus substantially reducing the overall implant size and weight. A miniature RF coil, can be employed to receive an incoming RF energy to power the entire microsystem due to a low system power dissipation.
Fig. 3 Wireless in-vivo sensor
The adaptive RF powering capability was enabled to provide a reliable power supply for the microsystem implanted in the freely moving laboratory mouse or rat. The measured digital blood pressure information was transmitted to a nearby external receiver by the on-chip FSK oscillator based transmitter.
The most common techniques for monitoring blood pressure in small laboratory animals rely on using an invasive catheter-tip transducer inserted into an artery. Tail cuff devices require animal restraint, thus resulting in a stress-induced signal distortion. Furthermore, tail cuffs can only obtain systolic and diastolic blood pressure levels instead of a continuous blood pressure waveform with detailed signatures, which are desirable for advanced biomedical research. Both technologies, therefore, are inadequate for real-time long-term monitoring. This is where wireless, batteryless long term implantable blood pressure monitor is desirable.
Microsystem Architecture
Fig. 4 System architecture
The overall electronic system architecture is presented in Figure 4. An adaptive RF-DC power converter is incorporated in the system design to provide a sufficient and stable energy to the microsystem implanted in an un-tethered animal. RF powering is used to eliminate the need of an implanted battery, thus substantially reducing the overall implant size and weight. A miniature RF coil, can be employed to receive an incoming RF energy to power the entire microsystem due to a low system power dissipation.