What is deconditioning after acute illness, particularly in older adults or those with pre-existing chronic conditions such as cardiovascular disease, respiratory disease, or neurological disorders?

Defining Deconditioning After Acute Illness

Deconditioning after acute illness is a complex, multi-system physiological deterioration triggered by inactivity, bed rest, or immobilization during hospitalization, resulting in functional losses across cardiovascular, musculoskeletal, metabolic, and neurological systems that can begin within 48 hours of immobilization. 1

Core Pathophysiological Features

Deconditioning represents a final common pathway of organ dysfunction that encompasses both physical and cognitive domains, not merely reversible weakness. 1, 2

Cardiovascular Changes

• Fluid shifts occur rapidly within 48 hours, with extracellular fluid losses up to 600 mL causing diuresis and hypovolemia 1
• Cardiac deconditioning develops with prolonged decreases in cardiac output, reduced stroke volume at any orthostatic stress, and plasma volume reduction leading to secondary cardiac atrophy 1
• Peak oxygen pulse becomes reduced with a characteristic left-shifted heart rate-oxygen consumption relationship, manifesting as increased submaximal heart rate responses and compensatory tachycardia 1

Musculoskeletal Deterioration

• Muscle strength declines by 2-5% per day during immobilization, with marked loss of leg strength that seriously limits mobility 3
• Muscle mass diminishes rapidly with muscle shortening and changes in periarticular and cartilaginous joint structures 3
• Bone metabolism alterations result in increased calcium excretion, bone resorption, and trabecular bone loss (up to 3.1% additional bone mineral density loss) 1

Metabolic and Respiratory Dysfunction

• Early-onset metabolic acidosis develops with a low anaerobic threshold and impaired muscle oxygenation indicative of metabolic deconditioning 1
• Increased insulin resistance emerges after several days of inactivity 1
• Respiratory compromise occurs as abdominal viscera displace upward against the diaphragm when supine, increasing inspiratory muscle effort, decreasing oxygen inspiration, and impairing gas exchange 1
• Loss of endurance and strength in respiratory muscles compounds functional decline 1

Cognitive and Neurological Components

• Cognitive deconditioning is defined by delayed mental processing as part of a spectrum with fulminant delirium at one end, and does not occur in isolation from physical decline 2
• Neurological changes play a role in the pathophysiology beyond simple physical weakness 4

Diagnostic Cardiopulmonary Exercise Testing Patterns

The American Thoracic Society and American College of Chest Physicians define deconditioning by specific objective criteria: 1

• Peak VO₂ is low or at the lower limit of normal in very deconditioned but otherwise normal subjects 1
• Left-shifted heart rate-VO₂ relationship with increased submaximal heart rate responses, normal slope, normal peak heart rate, and consequently little or no heart rate reserve 1
• Reduced peak O₂ pulse 1
• Normal ventilatory response at low exercise levels, but increased submaximal ventilation at any level of VO₂ above anaerobic threshold reflecting increased metabolic acidosis 1
• Significant ventilatory reserve with normal PaO₂ and dead space ventilation (Vd/Vt) responses 1

Clinical Manifestations

Functional Losses

• Mental status changes, degree of continence, and ability to accomplish activities of daily living are commonly affected 3
• Exercise intolerance and fatigue persist beyond the acute illness period 1
• Protracted symptoms including muscle soreness and impaired mobility that persist after the period of inactivity 1

Systemic Effects

• Electrolyte aberrations including increased excretion of sodium, chloride, and potassium 1
• Fluid shifts causing nasal congestion, headaches, and acid reflux 1
• Skin breakdown and nerve compression 1
• Shifts in hormone levels (cortisol, thyroid hormone, aldosterone) 1
• Altered sleep cycles, insomnia, and impaired immune function 1

High-Risk Populations

Older adults, particularly those living with frailty, are at greatest risk for hospital-associated deconditioning. 2 Additional risk factors include: 4

• Advanced age 4
• Poor nutritional status 4
• Impaired mobility 4
• Pre-admission functional decline 4
• Cognitive impairment 4
• Depression 4
• Critical illness requiring ICU care 1

Critical Timing Considerations

Deconditioning can be triggered by as little as 20 hours of bed rest, making early recognition and intervention essential. 1 The decline in muscle mass and strength has been directly linked to falls, functional decline, increased frailty, and immobility. 3

Context in Critical Illness

In critically ill patients, improved survival from intensive care is often associated with general deconditioning, functional impairment, and reduced health-related quality of life after ICU discharge, underscoring the need for assessment and measures to prevent or attenuate deconditioning during ICU stay. 5 Prolonged immobilization is frequently viewed as "inevitable" in critically ill patients, which enhances deconditioning and may further complicate the clinical course. 5