Hyperlactataemia in critical illness

Hyperlactataemia is a metabolic disorder commonly observed in critically ill patients, although the precise prevalence is unknown. The definition of hyperlactataemia has changed in recent times and it is now generally considered to be clinically relevant in critical illness if the lactate concentration is greater than 2.0 mmol/L.1,2 The previous paradigm considered lactate as a toxin, or a pure waste product, but this has been challenged as we develop a greater understanding of lactate’s production and physiological function. It is probable that lactate production in humans has evolved as a substrate for metabolism within muscle and brain, and as a signalling molecule for altering metabolic processes.3 The work of Brooks et al.,4-7 and detailed reviews in recent publications,3,8 have developed this understanding further and in some respects challenge the previous paradigm. Although it has long been understood that short duration hyperlactataemia occurs during both exercise in healthy athletes and seizure activity, the concept that hyperlactataemia primarily occurs in anaerobic conditions has shifted significantly. We now understand that increased lactate production occurs in response to an increased requirement for energy, and this production occurs simultaneously with increased lactate consumption in certain tissues, such as skeletal muscle.9 The concept of lactic acidosis itself has also changed. Acidaemia related to hyperlactataemia tends to only occur at higher lactate levels, whereas the prognostic implications of hyperlactataemia are evident at lower levels, even in the absence of acidaemia.10,11 It has been suggested that rather than the term lactic acidosis, reference should be made to lactate associated metabolic acidosis, or even hyperlactataemia and metabolic acidosis as two separate entities.10 However, despite the increasing understanding of the role of lactate in the human body in times of stress or increased energy requirements, hyperlactataemia during illness correlates with prognosis in a number of settings, most notably during shock states.10-14 Understanding the clinical significance of hyperlactataemia is aided by the understanding that severe hyperlactataemia and even acidosis in healthy athletes during substantive exercise is not harmful at all.15