Traditionally, a sphygmomanometer is used for measuring blood pressure in the arteries. The word is derived from the Greek "sphygmus" (pulse), plus the scientific term manometer was introduced by Scipione Riva Rocci, an Italian Physician during 1896. Usually it consists of an inflatable cuff, a measuring unit and also a tube whereby, the inflation bulb is used along with stethoscope. The image of sphygmomanometer is given in Figure 2.1.
Figure 2.1 Sphygmomanometer
Due to technologies advancement, blood pressure testing devices now are using electronic instruments or digital readouts. In these cases, the blood pressure reading appears on a small screen or is signaled in beeps, and no stethoscope is used. Most of digital instruments have an automatic inflation mechanism, which replace the manual inflation bulb for simplicity and comfort. A digital system is widely known for its convenience and robustness even in noisy environment is preferable. Therefore, blood pressure meter now available is still adapting the same measuring techniques with added features. Some of available blood pressure meter are tabletop, wristband and also finger. Considerations need to be made when designing a digital blood pressure meter since electronic devices are very susceptible to operating temperature and also humidity.
2.2 Blood Pressure Measurement Method
There are few available techniques employed for blood pressure measurements in which have their own strengths and weaknesses. Two popular approaches can be classified into two major groups known as invasive and noninvasive methods. As the name implies, invasive method involve catheterization (cut) where the patient need to undergone a minor surgical process. On the other hand, the non invasive technique offers simplicity, convenience, and comfort procedure to the patient is more preferable.
The invasive method is undoubtedly yields the most accurate measurements, but it is rarely used since it is more risky and patient may suffer excessive blood loss. Even today, invasive catheterization procedures are seldom used due to the risk of infection. Although, non invasive sacrifice a degree of accuracy in the measurement, the procedures which are considering for patient safety are widely applied. Two major methods for non invasive measurement are known as Auscultatory and Oscillometric. In fact, there are various methods used for measuring blood pressure which will be discussed next.
2.2.1 Auscultatory technique
2.2.1.1 Auscultatory manual technique
This technique based on the ability of the human ear (expert practitioner) to detect and distinguished sounds. It was suggested by Korotkoff during 1905 has yet became the most common method of blood pressure measurement today. The clinician will use a stethoscope to listen for the Korotkoff sounds as the cuff deflates to determine the systolic, diastolic and estimate mean arterial pressure reading. The great advantage is clinician is allows to determine the quality of each measurement. However, the possible error may arise due to differences in hearing acuity from one clinician to another. Furthermore, the unqualified or inexperienced clinician may not be immune to outside noise and other interference, thus assessing inconsistent Korotkoff sounds during measurement. (The Auscultatory Method)
2.2.1.2 Automated Auscultatory technique
This particular technique was developed to replace to function of human ear by using microphone. A sound based algorithm was applied to estimate the systolic and diastolic readings. The drawback of this technique is lack of validation ability. In addition to noise artifact sensitivity, the algorithm may not adequately compensate or patient suffer low blood pressure (hypotension). Hence, the oscillometric technique was proposed to make the automated measurement more reliable. (The Automated Auscultatory Method)
2.2.2 Oscillometric technique
The name implies the procedure is done by measuring the oscillations caused by the arterial pressure pulse. These oscillations are the results of the coupling of the occlusive cuff to the artery. Oscillometric devices measure the mean but estimate both systolic and diastolic as proposed in Figure 2.2. The point of maximum amplitude is considered mean arterial pressure (MAP). Device using this technique do not use microphone, hence it is not affected by cuff placement and external noise. On the other hand, since is does not allow measurement validation, it is sensitive to patient movement. Error due to this technique may be generated from inaccurate determination of MAP. (K.G, Ng, & C.F, 1994)
Figure 2.2 Determination of blood pressure using oscillometric technique
2.2.3 Infrasound and ultrasound technique
Infrasound technique attempts to improve on the auscultatory method by detecting the low frequency Korotkoff sound vibrations below 50 Hz, in which including sub audible vibrations. On the other hand, ultrasound technique is not commonly used for measuring blood pressure. Usually, it is use in combination with other methods. Major feature of this method is, the values recorded by using ultrasound can be very operator dependent.
2.2.4 Tonometry technique
This method uses a different approach where the arterial tonometry is realized by flattening the pressure non invasively to squeeze the artery against bone. The applied pressure required to maintain the flattened shape are recorded and accomplished by using array of pressure sensors. An algorithm must be used to calculate the blood pressure from the waveform obtained. Moreover, the waveform exhibits a similar pattern as catheter measurement (invasive). However, tonometry have several limitations which affecting its performance. Limitations like high sensitivity to sensor position and angle, measuring peripheral circulation, low interoperator reproducibility, and is also requires regular calibration. (Alternative Methods)
2.2.5 Ambulatory blood pressure monitoring technique (ABPM)
ABPM monitors patient blood pressure over a predefined length of time outside the clinic as the patients runs their daily life routines. Periodically, monitors will record the measurements and stores the results. When, monitoring period is over, clinician will have a set of data for analysis. The primary purpose of ABPM is to obtain a profile of patient's blood pressure under conditions outside clinical environment. It is believed that the blood pressure measured in clinic does not always representing the true value and may lead to identification of white coat hypertension and circadian rhythm of blood pressure. Clinical research for ABPM has led to the additional analysis techniques that allow clinician to obtain a clearer assessment of a patient's hypertensive condition. Some advantages offers by ABPM are reliable measurement, easier diagnosis and treatment development to help problematic patients. (Ambulatory Blood Pressure Monitoring)
2.2.6 Pulse dynamic technique
Pulse dynamic is a technology introduced by pulse metric proposed a variant of oscillometric method. The significant advantage of this method is, it combines the reliability of oscillometric technique while retaining the validation capability of manual auscultatory method.
2.2.7 Plethysmography technique
This method is also Volumetric changes cause changes in the electrical conductivity (impedance) of the measurement. If the impedance graph is plotted against time, the generated waveform looks similar to pressure generated oscillometric waveform. Therefore, blood pressure is estimated in a manner similar to oscillometric technique. (The Auscultatory Method)
2.2.8 Finger cuff technique
The technique was developed by Penaz and works on the principle of unloaded arterial wall. This method may give an accurate estimate of changes in systolic and diastolic pressure, although both may be underestimated when compared to brachial artery pressure. It is found that, this method is not suitable for obtaining absolute level of blood pressure due to its inaccuracy. Secondly, it is also costly compared to the other available methods.
2.3 Review of related literature
Blood pressure commonly measured by blood pressure meter or sphygmomanometer. It value is measure in millimeters of mercury (mmHg) although that the latest model of blood pressure meter didn't use mercury in the measurement process. For each heartbeat, blood pressure varies between systolic and diastolic pressures. Systolic pressure is peak pressure in the arteries, which occurs near the end of the cardiac cycle when the ventricles are contracting. Diastolic pressure is minimum pressure in the arteries, which occurs near the beginning of the cardiac cycle when the ventricles are filled with blood.
There are many methods in designing and examining blood pressure that has been use previously. However, the objective is still the same that is producing a reliable and efficient blood pressure meter with minimal errors. As we use digital blood pressure meter, the method that really suitable to use is oscillometric method. The researchers had emphasizing the usage of oscillometric method implemented to digital blood pressure meter.
Nowadays, there are many various type of digital blood pressure meter in the market. Generally, most of the automated clinical non invasive blood pressure meter use oscillometric method in process to measure the blood pressure on a person (K.G, Ng, & C.F, 1994).This is because the oscillometric method can gives better estimated measurements, more reliable result and reduce measurement error.
S. Mieke (S.Mieke, H.Grob, M.Ulbrich, & U.Frucht, 1993) proposed that the oscillometric method relies on analyzing the amplitude of relatively small pressure oscillations which may be contaminated by artefact caused by movement of the cuff. Even though this method can give more reliable result, but the result maybe degraded by the presence of artefact and patient's movement.
J N Amoore (J.N & W.B, 1996) had make the study about all the possible factors that affect non invasive blood pressure (NIBP) measurement and how well NIBP monitors cope with these conditions are examined. The result of the experiments state that most of monitors respond well to pulse strength but gives falsely high reading for systolic and diastolic. Therefore, some of monitors have leaning to record and measure slightly low systolic pressure.
From the research by B P McGrath (McGrath, 2002), he states that ambulatory blood pressure (ABP) measurements give better prediction of clinical outcome than clinic or casual blood pressure measurements. It reports that this monitoring system should operate for 24-hours measurement and consideration of diary information and time of drug treatment. ABPM is indicated to exclude "white coat" hypertension and has a role in assessing apparent drug-resistant hypertension, symptomatic hypertension, in the elderly, in hypertension in pregnancy, and to assess adequacy of control in patients at high risk of cardiovascular disease. It is expected that ABPM will give accurate and reliable measurement.
In a study by J W Miao (J.W, 1992) he has designed a computer aided method for indirect measurement of arterial blood pressure by using hydraulic adaptive control based on vascular unloading technique. The error when we measured the blood pressure could be controlled to minimum state by using the adaptive control method. One important thing from the research is that how human finger is being use as the part of the body that we can test to get the blood pressure measurement. In the final of the research, he suggested that this method will not only reduce the error but also applicable to clinical application and safe to be use.
In order to suggested the applicability of human finger, Yamakoshi (K.Yamakoshi, A.Kamiya, H.Shimazu, & ., 1983)have developed a new method for non invasive measurement of beat-to-beat systolic and diastolic pressure based on mechanical volume servo-control system using vascular unloading technique. Not long after that, this method was then approved for its validity and high accuracy.
W B Geake (W.B, 1995) have proved that non invasive test instruments can generate range of conditions including variations in pulse rate, pulse strength, arterial pressure and also artefacts as it can improve the oscillator method. This study had proved that the oscillometric devices give better response in terms of functional evaluation.
There are many manufacturers that produced health care products range in the market. One of them is OMRON (Automatic and Manual Blood Pressure Monitors, 2003). The blood pressure monitor is available in the form of manual inflation, automatic inflation and also the wristband. Most of their products using a technology called "IntelliSenseTM" to ensure that home monitoring is easy and hassle free. "IntelliSenseTM" is a global brand name for bio-information sensing and high performance fuzzy-logic technology which embedded in their blood pressure meter. This technology enables blood pressure monitors make each measurement personalized, regardless of arm size, blood pressure level or the time of measurement.
C:\Documents and Settings\Owner\Desktop\PSM\blood pressure\OMRON\2848646346_418900b3b5.jpgC:\Documents and Settings\Owner\Desktop\PSM\blood pressure\OMRON\AB022_MX3.pngC:\Documents and Settings\Owner\Desktop\PSM\blood pressure\OMRON\AB031_R7.jpgC:\Documents and Settings\Owner\Desktop\PSM\blood pressure\OMRON\M1.jpg
Figure 2.3 : OMRON Blood pressure meter
This ensures the right level of cuff inflation is applied and an advanced control valve ensures quick deflation when measurement is complete. This self adjusting technology means maximum comfort and accuracy for each user. Moreover, with this excellent sensing technology, the monitor can achieve accurate measurement. Another technology by OMRON is known as "A.P.S TM (Advanced Positioning Sensor)" was designed to solve for error of wrist monitors. Ideally, wrist monitors requires it to be at the level of the heart when a reading is taken. User will be guided with arrows, instructing the user to either lower or elevate the monitor until it is in the proper position. The monitor's LCD will indicate to the user that the monitor is at the proper height and then the blood pressure monitor begins to inflate and take a reading.
In the same time, CITIZEN (Digital Blood Pressure Meter, 2001) under their main company Japan CBM Corporation which based in Tokyo has produced their own range of blood pressure meter. However, they still use oscillometric methods. The devices also give high accuracy with allowable pressure range is +/- 3mmHg or 2% of reading while pulse is +/- 5% of reading and this meets the standard given by AAMI.
C:\Documents and Settings\Owner\Desktop\PSM\blood pressure\CITIZEN\citizen.jpg C:\Documents and Settings\Owner\Desktop\PSM\blood pressure\CITIZEN\VHE05_1lg.jpg
C:\Documents and Settings\Owner\Desktop\PSM\blood pressure\CITIZEN\health002mar2008-18042008-125718.jpg
Figure 2.4 : CITIZEN Blood Pressure Meter
Although there are many digital blood pressure meter in the market, but most them use oscillometric method in order to measure the blood pressure. On the divergent, BIPITONE (Electronic Blood Pressure Meter) suggested the usage of auscultotary principle in their range of product. Some of the fact said that auscultotary method can give more accurate reading than oscillometric method. It is proven that the reading conforms to the reading obtained from mercury sphygmomanometer. No stethoscope is required and human error in determining blood pressure level is eliminated. The technology used an electronic adaptation of "Riva Rocci" principle, or applying a solid state integrated circuitry. By using the cuff embedded with multiple HIR Electrosteth transducers, it covers wide pick up area to determine blood pressure easily.
Another manufacturer known as HEALTH-O-METER (Blood Pressure Monitors) also provides wide range of blood pressure meter using oscillometric method. The sensor used is semiconductor type which is known to be very susceptible to noise and very sensitive. It is 100% latex free to signify the unique design. The meter is equipped with large amount of memory capacity, and it will shut off automatically, 1 minute after last button was pressed. One significant of this device is, it gives rather satisfactorily short operation time estimated around 2 minutes. This is proven to be 50% faster compared to average fuzzy logic blood pressure monitor. The technology called "ComfortReadTM" and "MicroStepTM" are two major technologies implemented in their product range.
C:\Documents and Settings\Owner\Desktop\PSM\blood pressure\HEALTH-O-METER\B000B66RQE.01._SCMZZZZZZZ_.jpg
Figure 2.5 : HEALTH-O-METER Blood Pressure Meter
In order to fulfill the need of the users that want a product that have high quality and comfortable to them. Then, LUMISCOPE (Digital Blood Pressure Meters, 2001) has adapted fuzzy logic sensor in their blood pressure meter range. This blood pressure meters are able to decide on suitable cuff size, to start or stop inflation automatically. LUMISCOPE has basically two major type that is semi automated and fully automated which operate on oscillometric principle. One specialty of this product is that this package is included with 2 sides of cuff to suits people with small and large arms so it can give the user comfortable. As addition, the LUMISCOPE also has design the blood pressure meter that can measure the blood pressure at human finger.
C:\Documents and Settings\Owner\Desktop\PSM\blood pressure\LUMISCOPE\lum1145.jpgC:\Documents and Settings\Owner\Desktop\PSM\blood pressure\LUMISCOPE\lm-1092_FULL.jpg
C:\Documents and Settings\Owner\Desktop\PSM\blood pressure\LUMISCOPE\thap pressure.JPG
Figure 2.6 : LUMISCOPE Blood Pressure Meter
In addition to various manufacturers mentioned, FORECARE (Blood Pressure Monitors, 2001) has its own blood pressure meter available in the market. By using "BrightSensorTM" technology, operating under oscillometric principle, it determines the optimum inflation depending on person's arm/ wrist circumference. Hence, this will not only give a reliable result, but it also high in accuracy and precise. A micro pump is used to control the inflation via automatic pressurization. Through this mechanism, automatic pressurization will be supplied when cuff pressure is sufficient. Moreover, for deflation purpose and for rapid air release a solenoid valve which produces constant air release is used.
C:\Documents and Settings\Owner\Desktop\PSM\blood pressure\FORECARE\Forecare_1315_250x212.jpgC:\Documents and Settings\Owner\Desktop\PSM\blood pressure\FORECARE\Forecare_6400_300x179.jpg
Figure 2.7 : FORECARE Blood Pressure Meter
2.4 Different characteristics of manufactured blood pressure
By comparing all the information in the entire manufactured blood pressure meter, the comparison between all the manufactured blood pressure meters had been made. The comparison was made by comparing its characteristics.
Table 1.0 : Different characteristics of manufactured blood pressure
Characteristics;
Manufacterer
Method chosen
Clinical Accuracy
Fully automatic Operation
Minimum error
Portable (lightweight and compact)
Memory
Removable memory
(memory card)
Home monitoring
Cuff type
OMRON
Oscillometric
√
√
√
X
X
X
√
Arm
CITIZEN
Oscillometric
√
√
√
√
X
X
√
Arm
BIPITONE
Auscullotory
√
X
√
X
X
X
X
Arm
HEALTH-O-METER
Oscillometric
√
√
√
√
√
X
X
Arm
LUMISCOPE
Oscillometric
√
√
√
X
X
X
√
Arm
FORECARE
Oscillometric
√
√
√
√
X
X
√
Arm and finger
Portable Blood Pressure Meter with Daily Monitoring Application
Oscillometric
√
√
√
√
√
√
√
Arm
(K.G, Ng, & C.F, 1994)
In view of all related literatures, the blood pressure monitor using FPGA was developed by using oscillometric principles. The principle was chosen because it is easier to be implemented in an electronic device and it can reduce overall measurement error. The advantages of oscillometric which is a non invasive method over patients made it more suitable to be used.
2.5 FPGA
Field-programmable gate arrays (FPGAs) are integrated circuits (ICs) that contain an array of identical logic blocks with programmable interconnections. The user can program the function realized by each logic block and the connections between the blocks. FPGAs have revolutionized the way prototyping and designing are done. (H.Roth Jr & John, 2007) The flexibility offered by reprogrammable FPGAs has enhanced the design process. There are a variety of FPGA products available in market now. Xilinx, Altera, Lattice Semiconductor, Actel, Cypress, QuickLogic and Atmel are examples of companies that design and sell FPGAs.
One of the major defining characteristics of the FPGA is that it can be programmed. Programming an FPGA is very different from programming a microprocessor. The FPGA's program (also called a personality) is interwoven into the logic structure of the FPGA. An FPGA does not fetch instructions- the FPGA's programming directly implements logic functions and interconnections. (Field-Programmable Gate Array, 2010). To define the behavior of the FPGA, the user provides a hardware description language (HDL) or a schematic design. The HDL form is more suited to work with large structures because it's possible to just specify them numerically rather than having to draw every piece by hand. However, schematic entry can allow for easier visualisation of a design. The most common HDLs are VHDL and Verilog, although in an attempt to reduce the complexity of designing in HDLs, which have been compared to the equivalent of assembly languages, there are moves to raise the abstraction level through the introduction of alternative languages.
2.6 Verilog
A Hardware Description language (HDL) language defines a behavioral language for analog systems. Verilog HDL is a textual format for describing electronic circuits and systems. Applied to electronic design, verilog is intended to be used through simulation, for timing analysis, for test analysis and for logic synthesis.
2.6.1 Design Flow using Verilog
The diagram below summarises the high level design flow for an ASIC (ie. gate array, standard cell) or FPGA. In a practical design situation, each step described in the following sections may be split into several smaller steps.
http://www.doulos.com/knowhow/verilog_designers_guide/design_flow_using_verilog/implicn2.gif
(verilog, 2010)
2.6.2 Levels of Abstraction
Verilog descriptions can span multiple levels of abstraction and can be used for different purposes at various stages in the design process.
http://www.doulos.com/knowhow/verilog_designers_guide/levels_of_abstraction/levels3.gif
(verilog, 2010)
Verilog supports Register Transfer Level (RTL) descriptions, which are used for the detailed design of digital circuits. Synthesis tools transform Register Transfer Level (RTL) descriptions to gate level. Verilog supports gate and switch level descriptions, used for the verification of digital designs, including gate and switch level logic simulation, static and dynamic timing analysis, testability analysis and fault grading. Verilog can also be used to describe simulation environments; test vectors, expected results, results comparison and analysis.
2.7 VHDL
VHDL is the VHSIC Hardware Description Language. VHSIC is an abbreviation for Very High Speed Integrated Circuit. It can describe the behaviour and structure of electronic systems, but is particularly suited as a language to describe the structure and behaviour of digital electronic hardware designs, such as ASICs and FPGAs as well as conventional digital circuits. VHDL can be used to describe hardware at the gate level or in a more abstract way. Successful high level design requires a language, a tool set and a suitable methodology.
2.7.1 Levels of Abstraction
(Levels of abstraction of VHDL, 2005)
In the diagram above, the Register Transfer Level (RTL) level of abstraction is highlighted. The gate level is too low a level for describing hardware. Conversely, the algorithmic level is too high a level, most commercially available synthesis tools cannot produce hardware from a description at this level. Until then, VHDL coding at Register Transfer Level (RTL) for input to a synthesis tool will give the best results.
2.7.2 Design Flow using VHDL
The diagram below summarizes the high level design flow for an ASIC or FPGA. In a practical design situation, each step described in the following sections may be split into several smaller steps.
(Design Flow using VHDL, 2005)
2.7.3 Benefits of VHDL
Executable specification
Validate spec in system context (Subcontract)
Functionality separated from implementation
Simulate early and fast (Manage complexity)
Explore design alternatives
Get feedback (Produce better designs)
Automatic synthesis and test generation (ATPG for ASICs)
Increase productivity (Shorten time-to-market)
Technology and tool independence (though FPGA features may be unexploited)
Portable design data (Protect investment)
(Benefits of using VHDL, 2005)
