Avances en DiabetologÃa - Sociedad EspaÃ±ola de Diabetes

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16Vol. 21 Núm. 1 - enero-marzo 2005L. HeinemannINTRODUCTIONA number of different innovative technologies for insulindelivery and glucose monitoring are currently under development.In the year 1987 Prof. Dr. E. Pfeiffer published areview article about “Automated blood glucose regulationin diabetes”, in which he described his opinions about thedark past, the grey present and the rosy future of glucosemonitoring 1 . In this article, and many others published inthe years thereafter, the statement was always, that glucosemonitoring systems would be available for daily treatmentof patients with diabetes within three to five years. Morethan 17 years later we have the first continuous glucose monitoringsystems on the market for some time now, however,they are not available for every day use in patients withdiabetes. The aim of this mini-review is to provide a criticalassessment of the current status of glucose sensor developmentand to look a bit into the (hopefully rosy) future.GLUCOSE MONITORINGWe have come a long way from urine glucose measurementsto the first continuous glucose monitoring systems(Table I). Over time all glucose-monitoring techniqueshave been improved considerably. For example, from thebulky early blood glucose meters to the most recent versions,massive changes in size, weight and performance havetaken place. The recent versions are tiny, precise and somealso allow control of external insulin pumps. Most probablyalso the glucose monitoring systems, which are currently farfrom optimal (see below) will see a rapid improvement intheir performance in the next years.CURRENTLY AVAILABLE CONTINUOUS GLUCOSEMONITORING SYSTEMS (GMS)At present there are only two glucose monitoring systemscommercially available in Europe, even if three have aCE-mark:1. CGMS (continuous glucose monitoring system, MedtronicMiniMed).2. GlucoDay (A. Menarini Diagnostics, Italy; available inEurope only).3. PENDRA (Pendragon Medical, Switzerland; this systemhas a CE-mark but is not on the market).The CGMS was the first system that comes to the marketand it was used in a number of clinical trials and also byphysicians under many different circumstances. This systemis also a good example how improvements in the implementedsoftware and in the manufacturing process of theTABLE I. Glucose monitoring systems• Urine glucose test strips• Blood glucose test strips• Blood glucose meters• Integration of meters and insulin pens• Sensors/Monitoring systems (sc, iv, non-invasive)glucose electrodes can improve the performance of a systemconsiderably from one generation to the next one. TheGlucoDay system, which is based on the microdialysis technique,showed also a reliable performance in clinical-experimentalstudies and in small, short-term clinical studies 2 . Probablythe next generation of this system will come to the marketvery soon.Although it was announced that the PENDRA systemcomes to the market at the occasion of the EASD in Munichin 2004, its market introduction was postponed for an unspecifiedperiod of time due to the fact that use of the systemcaries a potential risk for patients with diabetes. It appearsthat issues with the calibration procedure are the main reasonfor this decision. This situation remembered the recentlyappearance of GlucoWatch, another interesting glucosemonitoring system, which will vanish from the market. Thissystem was on the market in the US for some years and fora short period of time in Great Britain as well. Even if it isdeclared that it is still possible to buy this novel system, theremainder of the company Cygnus, which has developed theGlucoWatch, was sold to the company Animas recently.LIMITATIONS OF THE CURRENTLY AVAILABLE GMSIn general, the introduction of the GMS to the marketrepresents a major breakthrough for glucose monitoring ingeneral, however, there is a considerable list of disadvantagesof the currently available systems:• Application of the CGMS and the GlucoDay is still aninvasive procedure, which requires help of healthcareprofessionals.• Limited functional period, between 2-4 days.• No on-line display (in case of the CGMS).• Possible side effects as cutaneous infections, etc.• Several re-calibrations (= finger pricking) per day arenecessary to achieve a good measurement quality (incase of the CGMS).Having these disadvantages in mind, what are thedemands for the ideal GMS? This should provide:
Glucose monitoring in clinical diabetes: reality and promises Vol. 21 Núm. 1 - enero-marzo 2005 17• Continuous glucose monitoring for prolonged periodsof time.• Stable measurements without drift.• A precise and accurate measurement.• On-line display of the measured data• Trend information• No need for frequent re-calibrations• No side effect (intact skin/ no skin reactions), and itshould be• Small, with nearly no weight and with “no” additionalcosts related to one-used accessories like sensors,etc.WHO IS DEVELOPING NEW GMS?A considerable number of companies are currently workingon the development of minimal-invasive GMS; most ofthem are based in the US (Abbott, Animas, Bayer (Germany/US),Dexcom, iSense, Medtronic MiniMed, MenariniDiagnostics (Italy), Roche Diagnostics (Germany), SpectRx,Therasense). Also a number of companies are workingon the development of non-invasive systems (Animas, CMETelemetrix (Canada), Glucon Medical Ltd (Israel), IntegrityApplications (Israel), InLight Solutions, LifeTrac Systems,LighTouch, MedOptix, MicroSense, OrSense (Israel), Pendragon(Switzerland), Sensys). Most probably this list is notexhaustive. Due to the fact that this area of research is ahighly competitive one, the information politics of some ofthe companies appear to be driven by the idea to make alot of smoke out of a little fire. This is understandable inview of the fund raising problems many companies have,but it blurs the view.PHYSIOLOGYThe technological achievements that have been madewith the currently available GMS in terms of measurementprecision and reliability allow rising of physiology relatedquestions, e.g., what are the relationships between variousmeasurement signals and changes of glucose levels in blood,extra-vascular compartments and within different tissues.The data observed in the first studies looking in this makeclear that this is a complex story. A number of aspects haveto be taken into account:• Which glucose levels are measured in which compartment(blood/interstitial fluid/mixed signal) and at whichbody site?• If different sensors measure glucose in different compartment,are the results obtained comparable?• What is the impact of insulin in varying levels on therelationship between glucose levels in blood and theinterstitial fluid?• Which impact has the calibration procedure on the displayedresults?In order to learn more about these relationships, moredetailed studies are required especially experiments withrapid changes in blood glucose levels. Such studies can alsoclarify if there are differences in the relationship betweenglucose levels in blood and interstitial fluid at states ofincreased or decreased glucose clearance. Nevertheless,under such conditions it can be difficult to differentiatebetween physiology and technology related factors that influencethe observed results. Other factors (e.g. local woundreactions with invasive systems) can also have an impact onthe results obtained.CALIBRATIONUsually the glucose sensors do not provide an absoluteglucose measurement, but the measured signal (most oftenan electric current) must be calibrated to the currently prevailingblood glucose level by means of a conventional capillaryblood glucose measurement. Ideally the glucose levelsin blood and interstitial fluid should be identical whilethis procedure takes place. In case of profound differencesin glucose levels between both compartments no calibrationshould be performed. If this precaution is ignored, all subsequentlyshown results can be severely flawed, which canresult in potentially life threatening errors 3 . This is an issuethat is difficult to overcome. However, also attempts aremade to develop GMS that need no re-calibration at all orat least in very long intervals. In view of all the factors thathave an impact on a “successful” calibration, one has to statethat this is an elusive goal.“AST-LIKE” PHENOMENONThe limited reliability of the glucose monitoring withGMS so far has not allowed to study the relevance of the socalled Alternate Site Test (AST)-phenomenon. This termdescribes the differences in both the time and the concentrationdomain that were observed during rapid changes incapillary blood glucose when the sample was not collectedfrom the fingertip but from the forearm or the abdomen.Capillary blood samples are usually collected from the upperdermal layers (< 2 mm), whereas GMS (i.e., CGMS and GlucoDay)measure glucose changes in the interstitial fluid inthe subcutaneous tissue, i.e., in deeper skin layers.
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