Measuring Outcomes After Critical Illness

Abstract

Outcomes after critical illness remain poorly understood. Conceptual models developed by other disciplines can serve as a framework by which to increase knowledge about outcomes after critical illness. This article reviews 3 models to understand the distinct but interrelated content of outcome domains, to review the components of functional status, and to describe how injuries and illnesses relate to disabilities and impairments afterward.

Keywords: Critical illness; Disability; Functional status; Survivorship.

Figure 1. Critical care publications from 1970 until 2013

Panel A demonstrates the overall number of publications in critical care (solid line) and the number of randomized trials in critical care (dashed line). Panel B demonstrates the number of publications focused on outcomes among survivors of critical illness. As with panel A, the solid line represents the overall number of publications and the dashed line represent the number of randomized trials. While both panels demonstrate that the number of publications and randomized trials have increased over time, the scale of the Y-axes should be noted. The number of overall publications is approximately 40 times larger than number of publications focused on outcomes for survivors of critical illness. From Turnbull AE, Rabiee A, Davis WE, et al. Outcome Measurement in ICU Survivorship Research From 1970 to 2013: A Scoping Review of 425 Publications. Crit Care Med. 2016 Jul;44(7):1267–77. doi: 10.1097/CCM.0000000000001651.

Figure 2. Continuum of Outcome Domains for Survivors of Critical Illness

Outcomes for survivors of critical illness can be divided into five interrelated domains. Panel A demonstrates each of these domains, providing a conceptual definition for each. Panel B applies these domains and concepts to survivors of critical illness. Each domain considers a specific aspect of patient outcomes. As outcomes move from left to right in the figure, however, information from the previous domains are integrated into subsequent ones, increasing the complexity of each outcome domain. At higher levels, individual and environmental characteristics also play a role. Relationships between non-adjacent domains are possible. Data from Wilson IB, Cleary PD. Linking clinical variables with health-related quality of life. A conceptual model of patient outcomes. JAMA 1995;273:59–65

Figure 3. Conceptual Framework of Functional Status

Functional status is an overarching term for what activities people do in the normal course of their lives to meet basic needs. It is comprised of four components: functional capacity, functional performance, functional reserve, and functional capacity utilization. Functional capacity is one’s maximum potential to perform an activity and represents what one “can do” (top horizontal line). Functional performance represents what one “actually does” in day-to-day life (middle horizontal line). The difference between functional capacity and functional performance is functional reserve. Functional capacity utilization is the amount of one’s functional capacity that is used to achieve functional performance. Because functional performance represents the activities that one does, it represents the ability to live independently. If one’s functional performance falls below a certain threshold (red horizontal line) one moves from being able to live independently to being disabled. Adapted from Leidy NK. Functional status and the forward progress of merry-go-rounds: Toward a coherent analytical framework. Nurs Res. 1994:43:196–202; with permission.

Figure 4. Comparison of functional status between two patients

Panel A depicts the functional status of a 35-year old fit woman and Panel B depicts the functional status of a 67-year old sepsis survivor. Note the overall difference in the magnitude of functional capacity. Although functional performance differs slightly between the two, panel B is much closer to falling below the threshold for dependence (red line). The woman in panel A has greater functional reserves, indicating she is capable of performing strenuous tasks. In contrast, the man in panel B has very little reserve. Moreover, he is using much more of his functional capacity (high functional capacity utilization) and is therefore exerting himself more to perform his daily activities. High levels of exertion are often not sustainable for long periods of time and can lead to a decrease in the frequency or overall cessation of activities needed to live independently.

Figure 5. The Disability Process

This figure illustrates the disability process. Illness and injury (pathology) affect the structure and function of different body systems (impairments) that result in reduced ability to perform physical or mental actions (limitations). When placed into a specific environmental context, these limitations result in the inability to perform socially defined roles and tasks (disability). Above the chevron diagram are the conceptual definitions for each component. Below the chevron diagram is the application of these concepts to a survivor of critical illness. From Brummel NE, Balas MC, Morandi A, et al. Understanding and reducing disability in older adults following critical illness. Crit Care Med. 2015;43:1265–75. Adapted from Verbrugge LM, Jette AM. The disablement process. Soc Sci Med. 1994 Jan;38(1):1–14; with permission.