Anemia, hypoxia and hypercapnia thresholds. Lessons from physiological limits in critically ill patients.

Physiological alterations occur in the critical care medicine and reflect illness. Rendering patients physiologic parameters in the range that is normal for the population is not necessarily good; it may be frankly harmful. We do not currently possess outcome-based tools that allow us to titrate physiological parameters and ensure improved outcome. It is highly unlikely that our practice will evolve to inducing anemia, hyperthermia, hypoxemia, hypercapnia and hypotension in our critically ill patients! However, evolving knowledge of the appropriate thresholds for these parameters in critically ill patients, in addition to greater understanding of the potential iatrogenic illness associated with parameter 'normalization', could lead to provision of enhanced patient care. In the coming years it is possible that we will redefine the 'normal' range for common clinical and laboratory values relating to the critically ill. We may switch to 'context sensitive' interpretation of parameters of illness, and manage critically ill patients accordingly.