Extreme physiological state: Development of tissue lactate sensor Anna-Maria Spehar-Deleze, Salzitsa Anastasova, Jonathan Popplewell, Pankaj Vadgama Department of Engineering and Material Science Queen Mary, University of London London E1 4NS, UK Email: (a.spehar-deleze, s.anastasova, p.vadgama)@qmul.ac.uk Jonathan_Popplewell@bio-rad.com Abstract ----- Lactate is one of the most important biomarkers of tissue oxygenation and thus of paramount importance for sports and health care applications. Lactate levels provide information on anaerobic threshold which is very important for tailoring training programs in endurance sports. In this contribution we present an implantable amperometric lactate sensor for continuous in vivo monitoring. A needle based construction is used where a sensing platinum wire is inserted into a stainless steel tube that serves as a combined counter and reference electrode allowing for easy insertion, small size and minimally invasive procedure. The sensing enzyme layer is sandwiched between two polymer membranes which allow high selectivity, a wide lactate linear range and biocompatibility. The sensors have been fully evaluated in vitro and tested in vivo in rats. The measured values of tissue lactate obtained with our sensors were compared with lactate levels measured in blood by the commercial LactatePro analyzer. The obtained concentrations were in the same range, however, no clear correlation between blood and tissue values was found. Cold sterilisation by gamma radiation, required for human studies, is currently being investigated. This work will provide valuable information on lactate levels in different physiological compartments and increase our understanding of physiological processes related to endurance sports. Keywords ----- lactate, blood and tissue, continuous monitoring, biocompatibility, in vivo I. INTRODUCTION Lactate is one of the most important biomarkers for assessing body oxygenation levels, and consequently there is great interest in lactate measurement in sports and health care. Current commercial interest for lactate sensing is driven by sports, fitness, dairy and defence industries. Normal blood lactate levels at rest are in the range 0.5-2 mM, however, they increase rapidly when cellular oxygen supply becomes limited, as when the anaerobic threshold is reached during heavy exercise or in the case of haemorrhagic shock following an injury [1]. Lactate measurement helps identify the fatigue response (especially important for athletes, other sports persons and soldiers) and the designing of individualized training routines for athletes. Lactate measurements are also of clinical significance for many critical care situations, for example, lactate acidosis is especially seen in shock states. Thus, there is an association with peripheral ischemia, triggered by myocardial infarction [2] and, lactic acidosis may be seen in association with diabetic ketoacidosis leading to coma. Lactate was considered for a long time to be a harmful metabolite that deteriorates an athlete’s performance. However, at present there is compelling evidence that lactate is used as an alternative energy source in the brain and central nervous system [2]. The most common cause for lactate build-up is hypoxia (low tissue and blood oxygenation). The standard way for extra-laboratory determination of lactate concentration is discrete blood sampling and measurement by lactate test strip. However, this gives little indication of dynamic change or predictive information, and certainly would be likely to lead to the missing of a precise concentration build up point. Discrete sampling can also be practically inconvenient in the case of endurance activity. Furthermore, interpretation of results is not always straightforward. The old dilemma, do blood lactate values reflect tissue values, has still not been conclusively answered. It has been established that the “apparent” blood/tissue ratio for glucose is not constant [3, 4], and this is likely to be especially true for ionic lactate. Previous research by our group demonstrated that blood and tissue lactate correlated in the normal euvolumic state, and under low haemorrhage conditions in experimental animals, but following more significant blood loss blood lactate values rose more than tissue values [5]. Other studies have reported that concentration of L-lactate in plasma is considerably higher than in capillary blood [6], and in a recent study by microdialysis adipose tissue lactate showed unstable lower tissue levels compared with blood lactate in septic shock patients [7]. In situ implanted vascular sensors are not a practical option due to the risk of thrombosis and embolism, initiated through blood/sensor interactions, and facilitated via local haemodynamic changes. Tissue implantation offers a safer route as any adverse effects will be localised. Lactate measurement in tissue moreover, can also provide more relevant information on local oxygen supply, thus allowing detection of local tissue hypoxia as opposed to a body average value obtained by blood analysis. 2012 Ninth International Conference on Wearable and Implantable Body Sensor Networks 978-0-7695-4698-8/12 $26.00 © 2012 Crown Copyright DOI 10.1109/BSN.2012.32 17