Needle Enzyme Electrode for Lactate Measurement In Vivo

Electrochemical lactate needle enzyme electrodes were fabricated based on lactate oxidase with a conventional hydrogen peroxide detection regimen with a linear range up to 7 mM, response time ~ 3 min, and sensitivity ~ 1 nA/mM. A negatively charged inner (sulphonated polyether ether sulphone-polyether sulphone) membrane was applied for ensuring selectivity by limiting oxidazible anion diffusion to the Pt working electrode; polyurethane outer membrane layers were dip coated over the enzyme layer to limit substrate diffusion to the enzyme layer to achieve: (1) stir independence and (2) a low oxygen requirement. Lactate was monitored subcutaneously in rats during controlled haemorrhage and hypovolaemic shock. Tissue lactate showed agreement with blood lactate before haemorrhage and for limited haemorrhage (up to 2 ml blood withdrawal from 16 ml total blood volume) but with blood loss above 3 ml the tissue lactate rise was less pronounced than in blood. Loss of intercompartmental equilibrium due to diffusion limitation is suggested as a factor in causing this difference. An experimental in vitro model was developed which employed the needle lactate electrode within a cylindrical collagen gel to monitor inward diffusion of lactate as a basis for determining lactate diffusion coefficient. The high precision measurement gave a diffusion coefficient consistent with report values 3.54 times 10^-6 cm²/s. The simplified experimental approach could allow lactate transport studies across tissue analogues.