Physiological Reserve and Frailty in Critical Illness

Physicians have long sought to define a ‘physiologic age’ distinct from chronologic age which might account for some of the variance in response to critical illness and injury. This has led to the concept of ‘physiologic reserve’ which might represent a major driver of outcome in patients requiring intensive care. The human body is a complex system that adapts to a multitude of external stressors; however, senescence or illness can reduce inherent adaptive mechanisms, reducing complexity and reducing the threshold for decompensation (i.e. acute illness or injury). This theoretical critical threshold can be considered ‘physiologic reserve’. The phenotypic expression of this process is frailty. Frailty is a condition in which small deficits accumulate which individually may be insignificant but collectively produce an overwhelming burden of disease and heightened vulnerability to adverse events. Frail patients expend a greater proportion of their reserve simply to maintain homeostasis, and seemingly trivial insults can contribute to catastrophic decompensation. While frailty has generally been described among older populations, the concept of frailty as a surrogate of physiologic reserve may have relevance to critically ill patients across a wide spectrum of age. Research is needed to characterize the biological underpinnings of frailty, optimal ways to measure it, and its importance in determining survival and functional outcomes after critical illness. The utilization of ICU resources by older patients is rising, and the prevalence of frailty in those admitted to the ICU is likely to increase.