Acidosis and tissue hypoxia in the critically ill: how to measure it and what does it mean

We routinely monitor blood gases to determine the adequacy of ventilation and the presence of acid-base abnormalities. Changes in the blood are easily assessed, but of more importance is the abnormality at tissue level. Defects in acid-base homoeostasis have major effects on protein function, thus affecting tissue and organ performance. We concentrate on the changes seen in critically ill patients with acidosis because they form a large portion of the workload of the average intensive care unit. In addition, such patients have significant morbidity and mortality. The development of acidemia in the critically ill is often attributed to reductions in oxygen utilization, which in the past has generally been regarded as dysregulation of tissue blood supply. Resulting tissue hypoperfusion leads to anaerobic metabolism and lactic acidosis. Carbon dioxide production increases as anaerobically produced hydrogen ions are buffered by extracellular bicarbonate. The effectiveness of tissue perfusion is the target of much research, and in this review we outline factors that affect tissue acid-base status, techniques to measure tissue acid-base status, and explore the relationship between tissue acidosis and hypoxia in the critically ill. However, things are not always as simple as they may first appear.