Continuous monitoring of glucose with a transcutaneous microdialysis probe

In vivo monitoring of lactate and glucose with microdialysis and enzyme reactors in intensive care medicine

Methods for the monitoring of glucose and lactate in intensive care units (ICU) based on microdialysis and continuous flow enzyme reactions plus some in vitro and in vivo characteristics of the probes used and the detection systems are described. Two microdialysis techniques were developed for clinical use: in sepsis patients a subcutaneous device for lactate monitoring was placed and in prematurely born infants a transcutaneous device was used for nearly non-invasive sampling of glucose from the skin. There was a relatively strong relationship between transcutaneously sampled and blood glucose in the neonates, on the other hand the relationship between subcutaneously sampled and blood lactate was highly significant but relatively weak. These results and our preliminary results obtained with transcutaneous ethanol monitoring (not presented here) show that in vivo possibilities of our techniques depend on the location of the sampling/detection devices and the chemical nature of the analyte, because these properties determine diffusion characteristics in vivo. The present approach may be an alternative to the use of the more integrated biosensor technology in vivo, since it avoids major problems related to biocompatibility.

Changes in metabolism and blood flow in peripheral tissue (skeletal muscle) during cardiac surgery with cardiopulmonary bypass: the biochemical microdialysis study

The aim of this study was to monitor the metabolism and blood flow in the interstitium of the skeletal muscle during cardiac surgery with cardiopulmonary bypass (CPB) and in the early postoperative period by means of microdialysis and to compare metabolic changes during CPB at normothermia (NT) and hypothermia (HT).

Surgical revascularization using CPB was performed in 50 patients, 25 patients (group HT) were operated using hypothermic CPB, 25 (group NT) using normothermic CPB. Interstitial microdialysis was performed by two CMA 60 probes (CMA Microdialysis AB, Solna, Sweden) inserted into the patient’s deltoid muscle. Constituents analysed in the obtained dialysates, collected at intervals, were glucose, urea, glycerol and lactate. Tissue blood flow was monitored by dynamic microdialysis with gentamicin as a marker.

In both groups, NT versus HT, similar dynamics of concentrations were found. Low initial concentrations were followed by gradual increases during CPB and in the following phase of the operation. Concentrations were higher in the NT group. Immediately after the operation, the decrease in values continued, with a gradual increase in the succeeding postoperative period in both groups. Similar dynamic changes in the lactate concentration were found in both groups. The gentamicin concentrations were lower in the NT group (versus the HT group).

The results showed dynamic changes in the interstitial concentrations of glucose, urea, glycerol and lactate, which depend on the phase of the surgery in the CPB and early postoperative phase in the both groups of patients. Higher tissue perfusion of the skeletal muscle was noted in those patients operated on in normothermia. The dynamics of the concentration changes of these substances in the interstitium of the skeletal muscle has been proven to be caused by both the metabolic activity of the tissue and by the blood flow through the interstitium of the muscle.

Overview of brain microdialysis

The technique of microdialysis enables sampling and collecting of small-molecular-weight substances from the interstitial space. It is a widely used method in neuroscience and is one of the few techniques available that permits quantification of neurotransmitters, peptides, and hormones in the behaving animal. More recently, it has been used in tissue preparations for quantification of neurotransmitter release. This unit provides a brief review of the history of microdialysis and its general application in the neurosciences. The authors review the theoretical principles underlying the microdialysis process, methods available for estimating extracellular concentration from dialysis samples (i.e., relative recovery), the various factors that affect the estimate of in vivo relative recovery, and the importance of determining in vivo relative recovery to data interpretation. Several areas of special note, including impact of tissue trauma on the interpretation of microdialysis results, are discussed. Step-by-step instructions for the planning and execution of conventional and quantitative microdialysis experiments are provided.

Measurements of Glucose on the Skin Surface, in Stratum Corneum and in Transcutaneous Extracts: Implications for Physiological Sampling

Obtaining representative physiological samples for glucose analysis remains a challenge especially when developing less invasive glucose monitoring systems for diabetic patients. In the present study the glucose content of the stratum corneum was compared with the amount of glucose obtained by short aqueous extractions from a site on the dorsal wrist, using high pressure liquid chromatography with pulsed amperometric detection. Ten successive aqueous 1-minute extractions of the site yielded a total of 60 ng cm(-2). The total glucose content of the stratum corneum of the site, determined from 30 successive tape-strippings of the site, was 360 ng cm(-2). After tape-stripping, the transcutaneous aqueous extraction rate was 86 +/- 13 ng cm(-2) min(-1), compared with rates of 80-600 ng cm(-2) min(-1) obtained with suction effusion or microdialysis after tape-stripping. Glucose on the surface of the skin and within the stratum corneum should be considered as sources of extraneous glucose contamination during testing of less invasive glucose monitoring devices.

Continuous glucose monitoring by means of the microdialysis technique: underlying fundamental aspects

The microdialysis technique allows extraction of substances (e.g., glucose) from fluids in the human body for quantitative measurements ex vivo. The microdialysis catheter can be inserted in many different tissues; for continuous glucose monitoring it is most often implanted in the subcutaneous fat tissue in the abdominal region. Perfusion of the thin catheter with an isotonic solution without glucose leads to a diffusion of glucose available in the interstitial fluid along the concentration gradient across the semipermeable membrane into the catheter. The glucose levels in the dialysate are measured quantitatively outside the body by means of specific sensors. A number of factors have a profound impact on the amount of glucose extracted (i.e., the glucose levels in the dialysate can be considerably lower than that in the interstitial fluid). However, as long as this proportion remains constant (independent of the prevailing glucose level), the sensor signal, which is related to the glucose level in the interstitial fluid, can be calibrated to the blood glucose level by means of a conventional blood glucose measurement. The microdialysis systems that are commercially available or in clinical development allow (after a run-in phase of some hours) continuous glucose monitoring with a good reliability over several days. Insertion of the microdialysis catheters cannot be performed by the patients themselves but requires professional help. From a technological point of view the microdialysis technique is demanding; consequently the costs of continuous glucose monitoring using this approach are considerable. However, further developments probably will allow development of cheaper patient self-care systems that can be used for longer periods of time.

Overview of microdialysis

The technique of microdialysis enables the monitoring of neurotransmitters and other molecules in the extracellular environment. This method has undergone several modifications and is now widely used for sampling and quantitating neurotransmitters, neuropeptides, and hormones in the brain and periphery. This unit describes the principles of conventional and quantitative microdialysis as well as strategies in designing a dialysis experiment. It establishes the groundwork for the basic techniques of preparation, conduct, and analysis of dialysis experiments in rodents and subhuman primates. Although the methods described are those used for monitoring CNS function, they can be easily applied with minor modification to other organ systems.

New technologies for amperometric biosensors

Amperomeric-based detectors have successfully been used as personal monitors for blood glucose levels. However, there is a desire to increase the number of compounds measured in a small blood sample, the speed of detection and enhance the reliability of the measurement. Furthermore, with the increasing use of microdialysis as a clinical sampling method in metabolic medicine, paediatric medicine and neurointensive care, there is a need for rapid on-line detection of analytes such as lactate, glucose and glutamate in low microlitre volume samples. Two approaches to these problems are described. The first uses enzymes immobilized in a packed bed with electrochemical detection of a ferrocene mediator as a flow-injection assay for use with microdialysis. Results from microdialysis of the brain of freely moving rats are described. In the second approach, thin-film techniques are used to fabricate arrays of microdisk and micro line electrodes. The properties of these arrays in free solution and in a flow cell are presented together with an example using multiple arrays to identify an analyte by oxidation potential. Finally, different enzymes are entrapped onto the surface of two arrays by electrochemical polymerization of o-phenylenediamine. The resulting device detects glucose and lactate in real-time.

Clinical microdialysis: the role of on-line measurement and quantitative microdialysis

The use of microdialysis in the clinic is examined in the light of lessons learnt from microdialysis in freely moving rats. Changes in concentrations of metabolites are an important index of the state of health of tissues. For effective therapeutic intervention rapid assays are essential Enzyme-based on-line assays for glucose and lactate are described. By combining two of these assays simultaneous measurements of glucose and lactate, sampled at 2 min intervals can be obtained. The relation between dialysate concentrations and the true extracellular concentration of an analyte is dependent on conditions in the tissue sampled and cannot be calculated from in vitro probe recoveries. Furthermore, with acute implantation of the probe and possibly rapidly changing tissue conditions, there will be changes in probe recovery in vivo. Quantitative microdialysis allows the measurement of the true extracellular concentration and the probe recovery in vivo. The clinical applicability of a number of quantitative microdialysis methods is discussed, and three approaches highlighted. By increasing membrane length and reducing flow rate recovery in vivo can be increased to 100%. In this case dialysate concentrations equal extracellular ones. By perfusing an inert exogenous compound an index of changes to extracellular volume and hence tissue oedema can be obtained. In the zero net flux method the infusion of a few concentrations of the analyte under study allows the direct determination of both the ECF concentration and the in vivo recovery. The latter can provide valuable information about changes in the physical as well as chemical state of the tissue. This can guide rapid effective therapeutic intervention.

Bi-enzyme reactor for electrochemical detection of low concentrations of uric acid and glucose

An enzyme-based flow-injection amperometric analysis system (FIA) for monitoring of uric acid and glucose is described. The oxidase and peroxidase enzymes are physically coimmobilised in a sandwich-type reactor and ferrocene serves as a mediator. The assays are based on the measurement of a reduction current resulting from the enzymatic reactions, at a glassy carbon electrode held at 0.00 mV (vs. Ag/AgCl). The high selectivity (ascorbic acid did not interfere) is coupled to high sensitivity (a detection limit of 30 and 60 nmol/l for uric acid and glucose, respectively; signal/noise = 3) and good stability (the enzymes remained active for more than 6 weeks at 30 degrees C). The usefulness of the assay in clinical chemistry is illustrated by the measurement of human serum uric acid and glucose concentration. The results obtained were in fairly good agreement with those obtained using conventional hospital laboratory methods.

Reverse iontophoresis: development of a noninvasive approach for glucose monitoring

Solvent flow generated during iontophoresis can be used to convect neutral molecules through the skin, thereby greatly enhancing their flux. This concept was exploited to realize noninvasive glucose measurement by its iontophoretic extraction from the subcutaneous tissue. The hypothesis was tested in vitro using hairless mouse skin. The dermal surface was bathed with a glucose solution; chambers on the epidermal surface housed the current delivery electrodes. Iontophoresis (at 0.36 mA/cm2) was performed for 2 hr, at the end of which the solutions in contact with the electrodes were analyzed. The amount extracted was proportional to the glucose solution concentration bathing the dermis. Higher radioactivity levels were found at the anode than at the cathode, possibly because of glucose metabolism during its outward transport across the skin. Glucose biotransformation results in negatively charged metabolites which migrate to the anode. Two sensitive glucose sensors were developed; one was selective for glucose, the other for glucose and related compounds. Both sensors indicated the presence of glucose at the cathode but an abnormally high value was also recorded at the anode. This signal, however, was not due to glucose but rather to electroactive ascorbate withdrawn from the skin. Finally, a system has been developed with which glucose can be extracted noninvasively from the subcutaneous tissue and unambiguously measured. Whether iontophoretic glucose sampling in vivo will be equally successful remains to be answered.