Application of transcutaneous microdialysis and continuous flow analysis for on-line glucose monitoring in newborn infants.

The combination of a transcutaneous microdialysis probe and continuous flow analysis was tested for continuous glucose monitoring in eight newborn infants who were fed intravenously. The probe was placed on the skin, which was first stripped with cellophane tape to increase the skin permeability. The skin was stripped until the transepidermal water loss reached values greater than 80 gm/m2/hr. Dialysate concentrations were monitored for 165 minutes while the blood glucose concentrations were manipulated by changing the infusion rate of glucose. Blood glucose concentration was linearly related to the dialysate concentration. Because the dialysate/blood glucose ratio varied among the infants, the dialysate concentration was calibrated to estimated blood values with a single-point and a multiple-point calibration method. The latter method yielded more accurate estimates of the blood values. We conclude that transcutaneous microdialysis may be used for glucose monitoring in newborn infants.

Lactate monitoring with subcutaneous microdialysis in patients with shock: a pilot study.

We describe the use of subcutaneous microdialysis for continuous sampling of lactate to monitor the plasma lactate concentration in eight patients with shock. The dialysate lactate concentrations were significantly correlated with the plasma lactate concentrations (r = 0.8229), but the linear regression lines varied between patients. Therefore, we used the individual regression line of each patient for calibration to calculate estimated plasma values from the dialysate concentrations. While the estimated values were linearly correlated to the plasma lactate values (r = 0.912), the 95% confidence interval of the estimated values was +/- 2.8 mmol/L. Thus, subcutaneous microdialysis does not allow accurate estimation of the plasma lactate concentration. In 3 of the 8 patients, there was a significant negative correlation between the dialysate/plasma lactate ratio and the plasma lactate concentration. This suggests that besides plasma lactate, other factors such as subcutaneous adipose tissue metabolism and blood flow, may influence subcutaneous sampling and dialysate lactate concentration as well. While microdialysis can be used for on-line sampling and continuous monitoring of the concentration of extracellular substances, for the purpose of plasma lactate monitoring, sampling probes should be designed that permit intravascular placement.

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.

Continuous monitoring of lactate during exercise in humans using subcutaneous and transcutaneous microdialysis.

We have evaluated the possibility of monitoring the plasma lactate concentration in human volunteers during cycle ergometer exercise using subcutaneous and transcutaneous microdialysis. In transcutaneous microdialysis, the relative increase in dialysate lactate concentration exceeded that of plasma lactate concentration by a factor of 6 during exercise due to exercise-induced lactate secretion in sweat. During exercise the subcutaneous microdialysis dialysate lactate concentration underestimated the plasma lactate concentration possibly due to diffusion limitation or adipose tissue lactate production. While it was demonstrated that microdialysis can be used for on-line lactate monitoring, neither subcutaneous nor transcutaneous dialysate lactate concentration were linearly related to the plasma lactate concentration during exercise, and it was found therefore that it was not possible to monitor directly plasma lactate concentration during exercise.

Microdialysis probe for transcutaneous monitoring of ethanol and glucose in humans.

A heated (42 degrees C) microdialysis probe and its application for continuous transcutaneous sampling of ethanol and glucose through cellophane-stripped forearm skin are described. Ethanol and glucose concentration in the dialysate were measured on-line with continuous-flow analysis and compared with blood values in human volunteers after ethanol consumption (n = 4) and oral glucose testing (n = 5), respectively. For ethanol and glucose, the dialysate and blood concentrations were linearly related in each subject (r > or = 0.91, P < 0.005), although the dialysate-to-blood ratio varied among subjects. The recovery in vivo was 22.4 +/- 22.7 and 4.7 +/- 2.3% (SD) of the recovery in vitro for ethanol and glucose, respectively. The dialysate glucose concentration was independent of blood flow. When the probe temperature was increased from 32 to 42 degrees C, the dialysate-to-blood glucose ratio increased, with 2.4 +/- 1.4%/degrees C (SD) in fasting subjects (n = 4), which was similar to an increase of 2.1 +/- 0.045%/degree C in dialysate-to-medium ratio in vitro. The present approach for transcutaneous sampling may possibly be used for other substances of low molecular weight as well.

Continuous monitoring of extracellular lactate concentration by microdialysis lactography for the study of rat muscle metabolism in vivo.

A method is described for the measurement and on-line monitoring of muscular extracellular lactate concentration in both anaesthetized and freely moving rats. This method is based on microdialysis sampling and lactic dehydrogenase-catalysed nicotinamide adenine dinucleotide, reduced (NADH)-fluorescence detection techniques. In vivo calibration revealed a resting extracellular lactate concentration of 1.92 +/- 0.13 mmol/l (+/- SEM) in the gastrocnemius muscle of adult male Wistar rats (n = 6), while the average whole-blood lactate level was 0.76 +/- 0.12 mmol/l (+/- SEM). This measured extracellular lactate concentration was 1.73-times higher than that deduced from the arterial lactate concentration. Blocking glycolysis with iodoacetate reduced the extracellular lactate concentration to 52 +/- 6% (+/- SEM, n = 4) of the resting level. The extracellular lactate concentration in rat gastrocnemius muscle had increased to significantly (P less than or equal to 0.05) different levels, 2.4 +/- 0.03 (+/- SEM) or 4.0 +/- 0.55 (+/- SEM) times the control value, 1 h after aortic clamping (n = 3) or cardiac arrest (n = 3), respectively. Stimulation of the sciatic nerve induced elevations of the extracellular lactate concentration in the tibialis anterior muscle which were linearly related to the recorded isometric force-time integral. We also monitored on-line the changes in extracellular lactate concentration in the tibialis anterior muscle of a swimming rat. Our results indicate that microdialysis lactate reflects also intracellular metabolism. Lactography may be a useful alternative to biopsies and nuclear magnetic resonance spectroscopy in clinical medicine and physiology for the monitoring of metabolism in vivo.