A Review of Blood Gas Sensors and a ProposedPortable Solution

N. Joglekar, J. Chandran, R. Veljanovski, A. Stojcevski*, A. Zayegh*, M. Dorairaj**School of Electrical Engineering*, Faculty of Health, Engineering and Science, Victoria University

Melbourne, Australia* Director,Marian Cardiac Centre and Research Foundation

Pune, Maharashtra, India

Abstract- Blood gasses are one of the vital signs monitored by Oxygen in the lungs is carried to tissues through themedical practitioners for critically ill patients and as preventive bloodstream, but only a small amount of this oxygen canhealth care measures for potential heart disease patients. actually dissolve in arterial blood. How much dissolvesMeasurement of blood gasses in a quick, effective way, without depends on the partial pressure of the oxygen (the pressure thatcompromising on accuracy is one of the fundamental topics for the gas exerts on the walls of the arteries). Therefore, testingresearch today and is in tune with the demands of the medical the partial pressure of oxygen is actually measuring how muchfraternity. oxygen the lungs are delivering to the blood. The remainder ofThe objective of this paper is to review the theory of blood gas oxygen that is not dissolved in blood combines withmeasurement and the current technological solutions used to hemoglobin, a protein-iron compound found in red blood cells.measure blood gasses non-invasively and continuously. This paper The oxygen content measurement in an ABG analysis indicatesfinally presents proposed research that will foresee portable, non- how much oxygen is combined with the hemoglobin. A relatedinvasive blood gas monitoring solutions. value is the oxygen saturation, which compares the amount of

I. INTRODUCTION oxygen actually combined with hemoglobin to the total amountof oxygen that the hemoglobin is capable of combining with.

Non invasive measurement of blood gasses and other blood Carbon dioxide dissolves more readily in the blood thananalytes is widely acknowledged to be one of the most oxygen, primarily forming bicarbonate and smaller amounts oftechnological and path breaking advances m monitoring carbonic acid. When present in normal amounts, the ratio ofclinical patients[l][Traditionally blood gasses were measured carbonic acid to bicarbonate creates an acid-base balance in theby taking samples from patients and sensing the samples to a blood, helping to keep the pH at a level where the body'slaboratory for further analysis. This has proven to be a tedious cellular functions are most efficient. The lungs and kidneysand time consuming process and the availability of the results both participate in maintaining the carbonic acid-bicarbonatebecomes very important and a hindrance for the doctor to take balance. The lungs control the carbonic acid level and thefurther steps when treating a critically ill patient. The delay kidneys regulate the bicarbonate. If either organ is notbetween the event itself and blood sampling, plus the delay in functioning properly, an acid-base imbalance can result.obtaining the results, means that this sort of analysis may be Determination of bicarbonate and pH levels, aids in diagnosingmisleading. [2] the cause of abnormal blood gas values [5].The paper is divided into 6 sections. Part II deals with thetheory of blood gasses, part III deals with the different Sr. Quantities Measured in Blood Gas Normal Values for ameasurement techniques, part IV deals with current solutions No Analysis Healthy Patientand their inherent drawbacks, part V deals with a proposed 1. Partial pressure of oxygen (PaO2) 75-100 mm Hgsolution and part VI summarizes the discussion. 2. Partial pressure of carbon dioxide 75-100 mm HgI(PaCO2)

3. Oxygen content (02CT) 35-45 mm Hg4. Oxygen saturation (SaO2) 94-100%

Blood gas analysis, also known as arterial blood gas (ABG) 5. Bicarbonate (HCO3) 22-26 mEq/litreanalysis, is a test which measures the amounts of oxygen and 6. Acidity (pH) 7.35-7.45carbon dioxide in the blood, as well as the acidity (pH) levelsof blood. An ABG analysis evaluates how effectively the lungs TABLE 1: Normal values for blood analysis in a healthy patient [5]are delivering oxygen to the blood and how efficiently they areeliminating carbon dioxide from it. The test also indicates howwell the lungs and kidneys are interacting to maintain normal III. MEASUREMENT AND TECHNIQUESblood pH (acid-base balance). In addition, the component of A. Oxygen Tension:the test provides information on kidney function [5]. The partial pressure of oxygen is always measured in the

244

1-4244-0797-4/07/$20.OO ( 2007 IEEE

arteries. The oxygen from the arterial blood diffused into the Io / I = 1 + K(PO2) (3)surrounding tissue and this action is dependant on the partialpressure of oxygen at that point of time. If the partial pressure Where I(PO2) is the fluorescence intensity in presence ofof oxygen is higher than the surrounding tissue the oxygen will oxygen, 1o is fluorescence intensity in absence of oxygen and Kbe diffused into the tissue till equilibrium is achieved. is the overall quenching constant.However, if the reverse is true then no oxygen is diffused andthis may cause tissue damage. So to check the oxygenationstatus of blood is alone not enough and the capacity of the B. Oxygen Saturation:blood to deliver the oxygen is also important. [31 Oxygen saturation in blood is one more important factor that isThe concentration of oxygen, C, is given by: measured by medical practitioners. There is an important

C = axP02 (1) relationship between oxygen saturation and partial pressure ofoxygen. Partial pressure of oxygen falls at a linear rate with a

Where oc is the solubility coefficient [4] fall in oxygen saturation and is a good indicator of ability ofoxygen to diffuse into the body tissue. Partial pressure of

Similarly the rate of diffusion of oxygen will increase with a 13.3KPa (100mm Hg) translates into 100% saturation possible.difference in partial pressures of oxygen in blood and the If P2 is more than 13.3 KPa it indicates presence of oxygensurrounding tissue. The oxygen delivery rate, Do is defined by which is not bound by hemoglobin and does not diffuse into

D0(CaO2-Cv02)xR (2) the tissue. In the above discussion P02 relates to oxygensaturation in arterial blood (SaO2) and not venous bloodWhere Do is the oxygen delivery rate, CaO2 is the concentration oxygen saturation (SaO2). [3]of arterial oxygen and CvO2 is the concentration of venous Oxygen Saturation in blood can be measured in three basicoxygen [4]. ways:

The partial pressure of oxygen can be measured in three basic 1. Transmission Oximetry: The basic concept in transmissionways oximetry is to transmit light through a blood sample where the

1. Electrochemical Measurements: It has been observed that blood absorbs a determined amount of light according to Beersoxygen has an affinity to platinum and the reaction generates a law.[3] The assumption made is the hemoglobin present incurrent proportional to the oxygen content. For the blood is basically a combination of two substances, oxygenatedmeasurement a reference value is set (preferably to zero) hemoglobin or oxyhemoglobin (02Hb) and deoxygenatedChange in partial pressure of oxygen will change the current hemoglobin (RHb). Both of the substances can be measured byflow accordingly. [3] using two separate wavelengths of light. The wavelengths used

are the red (660nm) and the infrared (940nm)[8]. Red has the2. Transcutaneous partial pressure of oygen: Transcutaneous largest difference between the two extinction curves of O2Hboxygen tension measurement (TCOM) is a measurement of and RHb while the infrared has maximal difference after theoxygen perfusion of the skin. Transcutaneous monitoring is isobestic point where the two extinction curves cross.noninvasive and relatively simple to use. In neonates and small The Oxygen saturation equation isinfants, this monitoring technique may provide very useful SaO2=Co(Co+Cr)x100 (4)clinical information. Transcutaneous gas monitoring, usingconventional electrochemical techniques, provides a means oftrending the values of PaO2 and PaCO2 in most patients with Where Cr is the concentration of deoxygenated hemoglobinrelatively normal cardiovascular function. Typically the skin of and Co is the concentration of oxygenated hemoglobin. [3]the patient is heated to 44 'C. [4]

2. Pulse Oximetry: It is a simple non-invasive method of3. Optical based measurement: The principle used in optical monitoring the percentage of hemoglobin (Hb) which issensors is the fluorescent quenching properties of oxygen. The saturated with oxygen. The pulse oximeter consists of a probedevice includes an optical waveguide and an oxygen sensing attached to the patient's finger or ear lobe which is linked to amedium disposed on the waveguide. The sensing medium computerized unit. The unit displays the percentage of Hbfluoresces in response to light from a light source such that the saturated with oxygen together with an audible signal for eachintensity of fluorescence is dependent on the partial pressure of pulse beat, a calculated heart rate and in some models, aoxygen in the environment. The sensing medium includes an graphical display of the blood flow past the probe. The basis ofoxygen sensitive fluorescent dye in a matrix consisting of a pulse oximetry relies on the difference in optical absorbance ofplasticized polymer. A zero state can be measured by filling the deoxyhemoglobin (Hb) and oxyhemoglobin (HbO2). HbO2chamber with oxygen concentration mixture. The measured absorbs less light in the red region of the spectrum (660nm)intensity is compared with the incident light source and is used than does Hb. Therefore oxygenated blood is distinctively redto calculate the concentration of the sample. The oxygen and venous blood has a bluish tinge. For a infrared wavelengthtension is calculated using the Stern-Volmer quenching of light at 940nm, the absorbance of HbO2 is slightly greaterformula and the PG2 value is calculated empirically [3]. than that of Hb.

Oxygen saturation is given by the equation

2007 IEEE International Symposium on Integrated Circuits (ISIC-2007) 245

patient's movement is overlapped with that of the patient'sAAi pulse wave which is measured by the received light sensor.

SaO2 = A -B A2 (5) Accordingly, it is difficult to filter the motion artifact of theA22 pulse wave by using a filter which has a cut-off frequency thatis fixed. In this study, the motion artifact is removed using a

Where A and B are functions of the specific absoptivities ofUb filter bank and a matched filter. Compared with traditionaland HbO2 respectively and Tis wavelength of light [6] adaptive filter methods, the ratio variation is 50% lower than3. Reflectance Oximetry: This method is similar to pulse that of the moving average filter, allowing more stableoximetry with difference being in the placement of the light measurement of oxygen saturation despite the patient'semitters and the photo diode receptors. In reflectance oximetry movement. But presence of venous blood in the pulstatilethe two are placed side by side and are separated by a optical moment. issence a prous[9].barrier. The capillary bed under the skin is used to reflect the component reading is still a problem [9].incident rays back to the photo diode [3]. A more practical problem would be that of power consumption

by the whole device. The device is to work as a continuous andmobile device on the body of the patient. So the whole design

IV. PROBLEMS FACED WITH NON INVASIVE SENSORS AND has to be very energy conscious and the development of aRECENT DEVELOPMENTS energy saving algorithm specific to the device in question is a

must. The choice of the right design for the transmitter is alsoessential as the RF part of the device will consume the mostfuture aim of the medical fraternity. The current bottleneck to

achieve this aim is directly related to the current technologies. power m its duty cycle.It is a major challenge to implement a portable marketable The above mentioned solution is a oxygen saturationsolution as compared to today's the bulky blood gas analyzers measuring device. But the relationship between SpO2 and P02currently in the market. One of the key factors which prevent is nonlinear and sigmoidal. The relationship is also dependentnon invasive sensors over the traditional solution is accuracy on pH and temperature, thus under normal conditions thewhich is very important considering that the very reason for the relation stays true but deviates in altered conditions e.g. lowanalysis is healthcare. The problem is due to the nature of noninvasive~ ~ ~~.sesrwhc.rhbtdrc otc ihtebodt perfusion. Thus 10000 oxygenation saturation does not translatebnvasive sensors whcchonntae on to effective or medically recommended tissue perfusion.

be analyzed. Instead,the noninvasiv. sensoTherefore detecting the P02 of oxygen is critical rather thandetecting and measuring secondary factors which are in turndependant on changes in the blood gas values.A good example is a pulse oximeter, because the measurement The current solutions for measuring P02 are all either invasiverelies on detecting a change in the absorbance of light due tothe pulsating arterial bed (AC component). If the pulse is not or bulky Fig 1 is an example of the current analyzers used instrong enough, the variations in the Ac component would be htoo small to process further. So this puts a physical limitationon where the sensor should be placed on the body and at thesame time renders the readings inaccurate for a patient whosuffers from problems like hypotension, vasoconstriction orhypothermia [8]. A second problem is with motion artifacts,because the sensor is on the surface of the skin any transientmotion relative to the skin and the sensor (shivering, seizureactivity) on it will introduce errors when the changes in thepulsatile readings are taken. The variations in the ACcomponent are very small compared to the whole signal andany errors in this variation will translate into a huge loss inaccuracy. Some manufacturers use the R wave (analogous to Fig 1: AVL Compact 2 Blood Gas Analyzer (Current Solution)[10]the pulsating AC component in pulse oximetry) of the patient'selectrocardiogram to synchronize the optical measurements andthere by improve the detection of noisy pulsatile signals by Choice of sensor is itil Elthil sensors areenhancing the signal to noise ratio through the use of multiple mostly electrodes which require taking samples. There aretime averages signals [8]. some electrodes which can sense blood gas through the skinThe Department of Electronic Engineering, Bioelectronic but in all cases are not suitable for continuous monitoring ofLaboratory, Gyeongsang National University and Department patient. In addition to the electrochemical reaction on theof Computer and Information Technology, YONAM Collegeof Engineering has come with a solution to counter motion transucer and c nd deto on thetsdueri surfaceartifacts in pulse oximetry sensors. Generally speaking, the are further issues and need to be addressed periodically tofrequency band of the motion artifact signal generated from the

246 2007 IEEE International Symposium on Integrated Circuits (ISIC-2007)

analysis and also allow the practitioner to provide a quickV. PROPOSED SOLUTION response to the change in the conditions. The portability of the

An ideal blood gas analyzer would be one, which is solution also allows diagnosing of patients who don't havenoninvasive, portable and if possible continuous. All these access to the facilities and also patients who are being treated ataspects are difficult to integrate into a single solution with the home. The solution is minimally invasive hence will be morecurrently available technologies. The reason for that is related accurate than the current noninvasive solutions.to the very nature of noninvasive measurements, wherein VI. CONCLUSIONsecondary parameters are measured from which informationabout blood gases is derived. The problem lies in the fact that This paper reviews the existing methodologies available forthe parameters sensed do not translate into true values of blood measurement of blood gases. The paper details out the variousgases unless the measurement is performed under known techniques and aims to propose an alternative and viableconditions, hence there is a trade off between accuracy and method for blood gas analysis. The current noninvasivenoninvasive nature of instruments. solutions are not fully accurate and there is no way to test if the

sensor is working. On the other hand the blood gas analyzersThe proposed research is aimed at an optical based blood gas currently used in hospitals are highly accurate but have thesensor solution for critically ill patients which will be both disadvantage of being bulky and not portable. The future inaccurate and portable. A block diagram of the proposed blood gas analysis lies in combining the best feature of thesolution is presented in Fig. 2. The sensor will feature three invasive and noninvasive measurement techniques usingsets of red and infrared LEDs sources and photo detectors existing technology to provide the medical fraternity with awhich will detect the gases. The sensor will detect blood gas device that not only guarantees high accuracy but also isvalues (PCO2, PG2 and pH) from a drawn blood sample. The considerable smaller in size and portable. The proposedproposed technologies to realize such a solution are Micro solution is a step in bridging these gaps in existingElectro Mechanical Systems (MEMS) and advanced VLSI methodologies.techniques. These techniques will aid in reducing powerconsumption. The use of MEMS technologies will aid in REFERENCESdeveloping a sensor, which is considerably smaller andportable than the current bulky analysers. The sensor will also [1] Boo-Ho Yang, Sokwoo Rhee and Haruhiko Asada " A 24 Houtfeature the necessary computing power required and telemetry Tele-Nursing System Using a Ring Sensor" Proceedings of theto transmit the data reducing the time to get the results 1998 IEEE confon robotics and automationconsiderably. [2] E.Roupie Critical Care. 1997; 1(1): 11-14. Published online 1997

August 13. doi: 10.1186/cc2. Copyright ©D 1997 Current ScienceS ens or V Ltd)

[3] John G Webster "Measurement, Instrumentation and Sensors" CRCr-D_riP | Press, 1999 pp 78.7 - 78.19,

I [4] M. Franklin "Transcutaneous measurement of partial pressure ofoxygen and carbon dioxide" 1195 Sept L: PMID: 9390854fPubMed - indexedforMEDLINE

[5] C.Turkington "Gale Encyclopedia ofMedicine", The Gale GroupDecember 2002

v [6] J.Bussink, J. Kaanders, A. Strik, B. Vojnovic, A.Van Der Kogel."Optical sensor-based oxygen tension measurements correspondwith hypoxia marker binding in three human tumor xenograft lines"

Fig 2: Ba Diagramfofthe prpoped soi Department ofRadiation Oncology, UMC St Radboud, Joint Centrefor Radiation Oncology Arnhem-Ni/megen, P.O. Box 9101, 6500The Netherlands.

The signal from the three photo detectors will be digitized [7] Lopez-Silva et.al. "Transmittance photoplethysmography andpulseusing a low power biomedical ADC. The ADC featured in the oximetry with near infrared laser diodes Instrumentation andMeasurement Technology" Conference, 2004. IMTC 04.solution will be low speed (maximum 100 kilocycles) and high Proceedings of the 21"t IEEE Volume 1, 18-20 May 2004resolution (10 to 12 bits). The signal is then passed on to the Page (s):718 - 722 Vol.]digital signal processing block where the signal will be [8] L. Lovinsky "Urgent problems of metrological Assurance ofanalyzed and the blood gas values will be computed based on Optical Pulse Oximetry" Instrumentation and Measurement, IEEE

analysis algorithms. The ..,,, Transactions Publication Date: June 2006 Volume: 5, Issue: 3 Onratiometric Thalys1s algor1tnms. lne DSP block wi ll also page(s): 869- 875feature a controller which will control the multiplexer which [9] L. Juwon, et.al."Design of filter to reject motion artifact of pulsecontrols property to be measured and will switch on the oximetry"Computer Standards & Interfaces Volume 26, Issue 3relevant circuitry to measure it. The DSP will execute the May 2004, Pages 241-249

algorithm accordingly to computethevaluesrequired.These [10] AVL Compact 2 Blood Gas Analyser, Global Medicalalgorithm accordingly to compute the values required. These Instrumentation, Inc. 6511 Bunker Lake Blvd, Ramsey, Minnesotameasures will aid in reducing the power consumption. The 55303 USAcomputed values will be transmitted via wireless telemetry. Www)mi nccom roducts A V70o20bloodgas%20compac%0o2.hJiim

The proposed solution being portable will reduce the time for

2007 IEEE International Symposium on Integrated Circuits (ISIC-2007) 247