Recent months have been an extraordinary time in intensive care medicine. With many serving on the front lines of the pandemic, critical care teams are confronted not only with the untimely death of coronavirus disease 2019 (COVID-19) patients but also with fundamental uncertainties about the post-ICU recovery of thousands of COVID-19 survivors. On a scale perhaps never seen before, this pandemic has underscored both the magnitude of functional disability acquired during critical illness as well as the speed of its onset, indicating that the path to recovery will be complicated for many survivors of the disease. Although the specific features of functional decline and recovery associated with COVID-19 need to be characterized, insights gained from other critically ill populations are valuable.
In this issue of Critical Care Medicine, Ingraham et al (1) leverage data from the Cerner Acute Physiology and Chronic Health Evaluation outcomes database to examine functional decline associated with critical illness and to evaluate secular trends over the 2008–2016 period. Focusing their analysis on 90 hospitals (of 236) with relevant data, they selected nonoperative patients admitted to the ICU who survived to hospital discharge. Functional status was retrospectively ascertained from the medical records and categorized as fully independent (can perform all basic activities of daily living [ADLs] without assistance), partially dependent (needed assistance to perform some ADLs), or fully dependent (cannot perform ADLs and must be cared for by another person). Functional status prior to ICU admission was inferred from the patient or surrogate reports, and functional status at discharge was abstracted from notes of the healthcare team (providers, nursing, therapists). Of the 129,917 patients included in the analysis, functional status decline during hospitalization was found to occur in 38,116 (29.3%) of patients. When 8-year secular trends were assessed, both the occurrence and the degree of functional decline had increased, respectively by 15% and 4% per year.
The authors should be commended for a perceptive analysis conducted in a large contemporary sample of critically ill patients. Certain limitations of the work should be noted, including the exclusion of a significant number of patients due to lack of functional status data, the exclusion of surgical patients, the lack of any posthospital discharge outcomes, and most importantly, the overly simplistic categorization of functional status using a three-point scale whose face or construct validities are unknown. The study by Ingraham et al (1) elicits some additional questions about the study of functional status changes in critical illness which deserve discussion.
Assessments of self-reported physical function are generally considered in three domains: ADLs (e.g. basic tasks such as bathing, toileting, and dressing), instrumental ADLs (e.g. higher order tasks such as taking medications or preparing food), and mobility (such as walking around outside or climbing stairs). Acute illness engenders a well-characterized temporal progression of disability (2–4), with loss of mobility and higher order activities preceding the loss of basic activities. Even the loss of higher order activities (mobility and IADLs) can have a profound impact on quality of life and well-being. It follows that functional status changes described in this report, based exclusively on basic ADLs—participants who were fully independent and then became partially or fully dependent—represent a very coarse metric of functional decline, as these patients almost certainly lost their ability to execute mobility and IADLs before losing the ability to execute one or more ADLs. Stated otherwise, the reported 29.3% rate of functional status decline identifies the most severe category and very likely underestimates the true extent of functional loss in this population (Fig. 1). In effect, we might regard 29.3% as a baseline metric which very likely would increase if we implemented more granular measures of function capturing the spectrum of functional decline during an ICU hospitalization (Fig. 1).
Ingraham et al (1) identify an increase in both the incidence and magnitude of functional status decline between 2008 and 2016. Yet, this epoch is contemporaneous with the widespread implementation of a number of evidence-based outcome-modifying interventions in the ICU including integrated protocols such as the ABCDEF bundle (Assess, Prevent, and Manage Pain, Both Spontaneous Awakening Trials and Spontaneous Breathing Trials, Choice of analgesia and sedation, Delirium: Assess, Prevent, and Manage, Early mobility and Exercise, and Family engagement and empowerment) (5), which begs the question of why the burden of functional decline seems to have increased during this time. However, the study findings may be more reflective of demographic changes, most notably increasing age, in the ICU population in the United States rather than evolutions in ICU care (6). With advancing age, there is a higher prevalence of vulnerability factors (frailty, multimorbidity) increasing the likelihood of adverse outcomes such as functional decline. A longitudinal assessment of ICU admissions among Medicare beneficiaries between 1996 and 2010 found that both the average age and number of comorbid illnesses increased significantly during this period (7).
Despite the rise in vulnerability factors at the population level, it is important to note that outcomes vary greatly at the individual level. It is well known that some adults with frailty and other vulnerability factors have unexpected capacities to recover after a critical illness (8,9). Likewise, frailty and other vulnerability factors are not unique to older adults and may be observed in certain categories of younger ICU patients (10). Gaining a deeper understanding of factors governing interindividual differences in the propensity to recover in the face of critical illness is likely to yield crucial insights as well as targets for intervention.
Resilience, or the ability of a system—cell, tissue, organ system, individual, organization, society—to recover structure and/or function after exposure to a stressor, is emerging as a highly consequential paradigm to model the response to stressors in clinical care (11). Stressors may be physiologic, psychologic, internal, and/or external, and can vary considerably in severity. An illness or injury severe enough to require ICU admission generally represents a complex stressor. Nearly all patients in the ICU experience disturbances in one or more organ systems (physiologic stressors), limited mobility (physical stressors), and very often they are exposed to psychoactive medications, delirium, restraints, and—particularly in the current pandemic—isolation (psychologic stressors). Faced with the same stressor, patients who exhibit a resilient phenotype will demonstrate a lesser degree of functional decline and a faster recovery when compared with less resilient subjects. Unlocking the biologic factors accounting for these different phenotypes is an important task for critical care research.
To date, studies of resilience in intensive care medicine have focused largely on psychological resilience (12) Moving forward, it will be valuable to study other dimensions of resilience (physiologic, physical, cognitive) in ICU populations. Prospective and longitudinal studies are needed with sampling frequencies that capture the host’s dynamic response to stressors in multiple domains. There is a need to identify modifiable factors that build resilience, with the goal of mitigating or even preventing significant functional decline during an ICU hospitalization. Intensive care is likely to benefit considerably from this novel perspective which considers clinical and biologic factors driving recovery in the face of multidimensional stressors.
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