Continuous measurement of subcutaneous lactate concentration during exercise by combining open-flow microperfusion and thin-film lactate sensors

Monitoring of Lactate in Interstitial Fluid, Saliva and Sweat by Electrochemical Biosensor: The Uncertainties of Biological Interpretation

Lactate electrochemical biosensors were fabricated using Pediococcus sp lactate oxidase (E.C. 1.1.3.2), an external polyurethane membrane laminate diffusion barrier and an internal ionomeric polymer barrier (sulphonated polyether ether sulphone polyether sulphone, SPEES PES). In a needle embodiment, a Pt wire working electrode was retained within stainless steel tubing serving as pseudoreference. The construct gave linearity to at least 25 mM lactate with 0.17 nA/mM lactate sensitivity. A low permeability inner membrane was also unexpectedly able to increase linearity. Responses were oxygen dependent at pO2 < 70 mmHg, irrespective of the inclusion of an external diffusion barrier membrane. Subcutaneous tissue was monitored in Sprague Dawley rats, and saliva and sweat during exercise in human subjects. The tissue sensors registered no response to intravenous Na lactate, indicating a blood-tissue lactate barrier. Salivary lactate allowed tracking of blood lactate during exercise, but lactate levels were substantially lower than those in blood (0–3.5 mM vs. 1.6–12.1 mM), with variable degrees of lactate partitioning from blood, evident both between subjects and at different exercise time points. Sweat lactate during exercise measured up to 23 mM but showed highly inconsistent change as exercise progressed. We conclude that neither tissue interstitial fluid nor sweat are usable as surrogates for blood lactate, and that major reappraisal of lactate sensor use is indicated for any extravascular monitoring strategy for lactate.

Lactate and glucose measurement in subepidermal tissue using minimally invasive microperfusion needle

Knowing the concentrations of biological substances can help ascertain physiological and pathological states. In the present study, a minimally invasive microperfusion needle was developed for measuring the concentrations of biological substances in subepidermal tissue. The microperfusion needle has a flow channel with a perforated membrane through which biological substances from subepidermal tissue are extracted. Since this device uses a thin steel acupuncture needle as the base substrate, it has sufficient rigidity for insertion through the skin. The efficacy of the needle was examined by measuring lactate and glucose concentrations in mice. Lactate was injected intraperitoneally, and changes in lactate concentrations in subepidermal tissue over time were measured using the device. Lactate concentrations of blood were also measured as a reference. Lactate was successfully collected using the microperfusion needle, and the lactate concentration of perfused saline was significantly correlated with blood lactate concentration. Glucose solution was administered orally, and the glucose concentration of perfused saline was also correlated with blood glucose concentration. The newly developed microperfusion needle can be used for minimally invasive monitoring of the concentrations of biological substances.

Estimation of human leptin concentration in the subcutaneous adipose and skeletal muscle tissues

BACKGROUND

Interstitial leptin concentrations in subcutaneous adipose and skeletal muscle tissues were determined by open-flow microperfusion.

METHOD

In 12 lean male subjects (age: 25.6 ± 1.1 years), a zero flow rate experiment using different flow rates was applied. Recovery was determined by urea as an internal reference. In the no-net-flux experiments, catheters were perfused with five solutions containing different concentrations of leptin. Concentrations of interstitial leptin were calculated by applying linear regression analysis to perfusate as opposed to sampled leptin concentrations.

RESULTS

The zero flow rate protocol showed significantly higher concentrations of leptin in the interstitial fluid of subcutaneous adipose compared to skeletal muscle tissue [36.8 ± 10.32 vs. 7.1 ± 2.5% of the corresponding plasma level (P = 0.018)]. The recovery of urea in the samples was comparable for all catheters [79.4 ± 6.8 vs. 83.0 ± 5.8 of the corresponding plasma level, flow rate of 0.3 μL/min; (P = ns)] and was higher when compared to leptin. In the no-net-flux protocol, the concentration of leptin in subcutaneous adipose tissue was almost identical to plasma [90. 5 ± 7.0%] and the skeletal muscle tissue concentration of leptin was 23.7 ± 2.5% of the corresponding plasma level.

CONCLUSION

Open-flow microperfusion enables the estimation of leptin concentrations in subcutaneous adipose and skeletal muscle tissues in humans in vivo. This is the first documentation on the use of open-flow microperfusion to demonstrate that relevant amounts of leptin are also found in skeletal muscle tissue.

Measurement of subcutaneous biological substances using thin metal needle with micro flow channel

Concentrations of biological substances are useful as indicators of physiological and pathological states. In order to monitor biological substances in daily life, we developed a minimally invasive needle type device with which biological substances are extracted through a microperfusion system inserted under the skin. The perfusion needle has a flow channel with perforated membrane through which biological substances from subepidermal tissue are extracted. The efficacy of the device was examined by measuring lactate concentration of exercising mice. Lactate was successfully collected from the back skin of the mice running on a treadmill using a fabricated microperfusion needle. Lactate concentration of perfused solution correlated with blood lactate concentration.

Interstitium: the next diagnostic and therapeutic platform in critical illness

BACKGROUND

For decades we have been testing blood either ex vivo or else placing monitors directly in the bloodstream to "see" what might be going on in tissues. In the last 20 yrs, conceptual and practical advances in interstitial monitoring have begun to challenge traditional approaches. In this review we explore how interstitial monitoring might be used as a platform for future diagnostics and therapy in critical illness.

RESULTS

From a diagnostic perspective, interstitial analysis has been instructive about the pathophysiology of critical illness. Valuable insights have been gained into the pathophysiology of critical illness. To this end, examples from the areas of interstitial oxygenation and acid base, endocrine pathophysiology, and head injury monitoring have been used. From a therapeutic perspective, the main focus has been on antibiotic therapy and an improved understanding of pharmacokinetics and pharmacodynamics in critical illness.

CONCLUSIONS

Monitoring of the interstitium is feasible and can be achieved through minimally invasive techniques. It has improved the understanding of the pathophysiology of critical illness, holds potential in the diagnosis and management of sepsis, may allow early prediction of organ deterioration, and finally offers the possibility of reduction of blood testing and minimizing blood loss. While all of these hold promise, randomized trials will need to be conducted based on interstitial end points rather than plasma end points. This will pave the way for a more rational approach to the therapy of critically ill patients.

Micromachined sensor for lactate monitoring in saliva

A miniaturised sensor for continuous lactate measurement in saliva was developed and tested. The sensor was built using silicon microfabrication technologies. The size of the chip is 5.5 mmx6.4 mmx0.7 mm and features a working, a counter and an Iridium reference electrode. The chip has a cavity whose floor is perforated by fine pores. The cavity contains the enzyme lactate oxidase (LOD), which is immobilised in an agarose gel. Prior to the amperometric detection of the reaction product hydrogen peroxide at the working electrode, the analyte lactate has to pass the pores to reach the cavity with the lactate oxidase by diffusion. To test the silicon sensor, capillary blood and saliva samples were obtained during standardised ergometer tests. Salivary lactate concentrations were determined with the sensor and compared to photometrically derived data from a lab-automate. In addition the saliva data were compared to standard capillary blood lactate concentrations measured with a pocket photometer. Lactate concentration versus load graphs were plotted and compared visually showing very similar progressions. The novel approach enables a location independent, permanent real-time measurement of the lactate concentration during exercise.

l-lactate measures in brain tissue with ceramic-based multisite microelectrodes

A newly developed multisite array microelectrode for in vivo measurements of L-lactate is presented. The resulting microelectrode is composed of three functional layers. First, Nafion is used to repel interfering electroactive anions, such as ascorbate. Second, L-lactate oxidase immobilized onto the recording sites is used to convert L-lactate to hydrogen peroxide. The H2O2 produced is proportional to L-lactate concentrations and is quantified at the platinum recording sites. Third, a layer of polyurethane is coated over the L-lactate oxidase to adjust the linear range of the microelectrode to one that is compatible with in vivo measurements. This layer reduces the amount of L-lactate that diffuses to the enzyme while not significantly limiting oxygen diffusion. The resulting L-lactate microelectrodes were linear to 20 mM (R2 = 0.997 +/- 0.001) and beyond in some cases with detection limits of 0.078 +/- 0.013 mM (n = 12). The selectivity and response time of these electrodes make them suitable for in vivo measurements in brain tissue. Self-referencing recordings may be utilized to further improve the selectivity of the recordings. However this is not necessary for most applications in the brain, because the resting and stimulated levels of dopamine (DA), norepinephrine (NE), and other potentially interfering cations are two to three orders of magnitude lower than that of in vivo L-lactate, which is in the millimolar range. Preliminary in vivo measures of L-lactate in the brain of anesthetized rats support that the microelectrodes are capable of measuring rapid endogenous changes in vivo.

Simultaneous assay of glucose, lactate, L-glutamate and hypoxanthine levels in a rat striatum using enzyme electrodes based on neutral red-doped silica nanoparticles

An electrochemical method suitable for the simultaneous measurement of cerebral glucose, lactate, L-glutamate and hypoxanthine concentrations from in vivo microdialysis sampling has been successfully performed for the first time using a neutral red-doped silica (NRDS) nanoparticle-derived enzyme sensor system. These uniform NRDS nanoparticles (about 50 +/- 3 nm) were prepared by a water-in-oil (W/O) microemulsion method, and characterized by a TEM technique. The neutral red-doped interior maintained its high electron-activity, while the exterior nano-silica surface prevented the mediator from leaching out into the aqueous solution, and showed high biocompability. These nanoparticles were then mixing with the glucose oxidase (GOD), lactate oxidase (LOD), L-glutamate oxidase (L-GLOD) or xanthine oxidase (XOD), and immobilized on four glassy carbon electrodes, respectively. A thin Nafion film was coated on the enzyme layer to prevent interference from molecules such as ascorbic acid and uric acid in the dialysate. The high sensitivity of the NRDS modified enzyme electrode system enables the simultaneous monitoring of trace levels of glucose, L-glutamate, lactate and hypoxanthine in diluted dialysate samples from a rat striatum.

A wire-based dual-analyte sensor for glucose and lactate: in vitro and in vivo evaluation

Continuous measurement of lactate is potentially useful for detecting physical exhaustion and for monitoring critical care conditions characterized by hypoperfusion, such as heart failure. In some conditions, it may be desirable to monitor more than one metabolic parameter concurrently. For this reason, we designed and fabricated twisted wire-based microelectrodes that can measure both lactate and glucose. These dual-analyte sensors were characterized in vitro by measuring their response to the analyte of interest and to assess whether they were susceptible to interference from the other analyte. When measured in stirred aqueous buffer, lactate sensors detected a very small amount of crosstalk from glucose in vitro, although this signal was less than 3% of the response to lactate. Glucose sensors did not detect crosstalk from lactate. Sensors were implanted subcutaneously in rats and tested during infusions of lactate and glucose. Each sensing electrode responded rapidly to changes in its analyte concentration, and there was no evidence of in vivo crosstalk. This study constitutes proof of the concept that oxidase-based, amperometric wire microsensors can detect changes in glucose and lactate during subcutaneous implantation in rats.

Microdevice with integrated dialysis probe and biosensor array for continuous multi-analyte monitoring

The simultaneous on-line determination of glucose and lactate using a microdevice that consisted of a dialysis sampling system incorporated to the flow-through cell of a microfabricated biosensor array is presented. The fluidic connections between the different device's components were realized by subsequent processing of stacked dry resist layers on a plastic support that provided also the means for electric connections. The performance of the device was evaluated in vitro. The cross-talk effect on the downstream sensor was investigated and found to be negligible. Recoveries of over 95% for both analytes were achieved when flow rates of the perfusion fluid </=0.5 microl/min were used. At this flow rate, the response time of the device was 2.4 min, which is acceptable for on-line analysis. The linear response concentration range extended up to 30 mM for glucose and 15 mM for lactate. Interference from electroactive species such as ascorbic acid, 2-acetamidophenol and uric acid, was minimal (less than 5% increase in biosensors signal for all substances tested). In addition, the device presented long-term run stability both in buffer and serum samples.

BioMEMS device with integrated microdialysis probe and biosensor array

The fabrication of a microdevice for continuous sampling and on-line monitoring of glucose is described. The device comprised a microdialysis sampling system integrated on the flow through channel of a microfabricated enzyme sensor. The sensor was produced by thin film technology and was assembled to a printed circuit board (PCB) that provided the means for both electrical and fluidic connections. A polyacrilonitrile fibre, with a cut-off of 50 kDa, was used in the fabrication of the sampling probe. The performance of the device was evaluated in-vitro. High sampling efficiency of the microdialysis probe was achieved by appropriate selection of the perfusion fluid flow rate. Response times varying from 1.5 to 3.0 min were determined for flow rates ranging between 1 and 0.2 micro l/min. The linear response range was up to 30 mM glucose and interference from other electroactive substances was almost negligible. The device showed excellent stability under continuous operation for at least 5 days and sensitivity variation less than 3% over a period of 15 days.

Role of the sympathoadrenergic system in adipose tissue metabolism during exercise in humans

1. The relative roles of sympathetic nerve activity and circulating catecholamines for adipose tissue lipolysis during exercise are not known. 2. Seven paraplegic spinal cord injured (SCI, injury level T3-T5) and seven healthy control subjects were studied by microdialysis and (133)xenon washout in clavicular (Cl) and in umbilical (Um) (sympathetically decentralized in SCI) subcutaneous adipose tissue during 1 h of arm cycling exercise at approximately 60 % of the peak rate of oxygen uptake. 3. During exercise, adipose tissue blood flow (ATBF) and interstitial glycerol, lactate and noradrenaline concentrations increased significantly in both groups. Plasma catecholamine levels increased significantly less with exercise in SCI than in healthy subjects. The exercise-induced increase in interstitial glycerol concentration in subcutaneous adipose tissue was significantly lower in SCI compared with healthy subjects (SCI: 25 +/- 12 % (Cl), 36 +/- 20 % (Um); healthy: 60 +/- 17 % (Cl), 147 +/- 45 % (Um)) and the increase in ATBF was significantly lower (Cl) or similar (Um) in SCI compared with healthy subjects (SCI: 1.2 +/- 0.3 ml (100 g)(-1) min(-1) (Cl), 1.0 +/- 0.3 ml (100 g)(-1) min(-1) (Um); healthy: 2.8 +/- 0.7 ml (100 g)(-1) min(-1) (Cl), 0.6 +/- 0.3 ml (100 g)(-1) min(-1) (Um)). Accordingly, in both adipose tissues lipolysis increased less in SCI compared with healthy subjects, indicating that circulating catecholamines are important for the exercise-induced increase in subcutaneous adipose tissue lipolysis. In SCI subjects, the exercise-induced increase in subcutaneous adipose tissue lipolysis was not lower in decentralized than in sympathetically innervated adipose tissue. During exercise the interstitial noradrenaline and adrenaline concentrations were lower in SCI compared with healthy subjects (P < 0.05) and always lower than arterial plasma catecholamine concentrations (P < 0.05). 4. It is concluded that circulating catecholamines are important for the exercise-induced increase in subcutaneous adipose tissue lipolysis while sympathetic nerve activity is not.