Limitations of adaptive physiology: are normal values really normal in the critically ill child?

The management of critically ill children requires an understanding of the pathological processes of disease and the physiological processes of adaptation to critical illness. The concept of stress-related decompensation is deeply rooted in medicine and the notion of physiologic homeostasis has evolved over centuries. The overarching goal of intensive care medicine is identifying and targeting an optimum point between over and under treatment, as there are harmful consequences associated with both ends of the spectrum. Should this optimum point reflect a state of homeostasis as defined in normal health? Or rather mimic the adaptive physiology of compensated critical illness? How do we define these targets in critically ill children? These questions are particularly challenging in children, as there are inherent difficulties associated with monitoring their physiology due to limitations in available technology and patient size. Moreover, normal and adaptive physiologic responses change throughout childhood and thus definitions will vary. Finally, normative values are largely undefined in critically ill children and many of the treatment guidelines in use are not underpinned by rigorous pediatric evidence but rather expert consensus. Ultimately, treatment targets should be dynamic with consideration for the pathology of the disease and the adaptive physiology of the ill individual. In this review we evaluate and summarize the available evidence used to guide the management of critically ill children in the general pediatric intensive care setting. We have concentrated on six commonly manipulated variables that are universally measured at the bedside: oxygen saturation (SpO2), heart rate (HR), blood pressure (BP), hemoglobin, temperature and glucose. We present the available evidence, question the dogma of accepted practice and highlight the research priorities that need to be addressed to ensure that the best care is being provided for critically ill children.