What is deconditioning syndrome?

Deconditioning Syndrome

Deconditioning syndrome is a complex process of multi-system physiological deterioration that occurs following periods of inactivity, bed rest, or immobilization, resulting in functional losses across cardiovascular, musculoskeletal, metabolic, and neurological systems. 1

Core Pathophysiology

Deconditioning represents a cascade of adverse physiological changes affecting multiple organ systems that can begin within days of immobility:

Cardiovascular Changes

• Significant fluid shifts occur within 48 hours, with extracellular fluid losses up to 600 mL leading to diuresis and hypovolemia 1
• Cardiac deconditioning develops with prolonged decrease in cardiac output and reduced stroke volume at any degree of orthostatic stress 1
• Reduced plasma volume and secondary cardiac atrophy occur, shifting the left ventricular pressure-volume curve and resulting in compensatory tachycardia 1
• Peak oxygen pulse is reduced with a left-shifted heart rate-oxygen consumption relationship (increased submaximal heart rate responses) 1

Musculoskeletal Deterioration

• Muscle strength decreases by 2-5% per day during bed rest, with marked loss of leg strength that seriously limits mobility 2
• Physical deconditioning and muscle atrophy develop rapidly, with impaired oxygenation of muscles (such as the gastrocnemius) indicative of metabolic deconditioning 1
• Diminished muscle mass, muscle shortening, and changes in periarticular and cartilaginous joint structures occur 2

Metabolic and Endocrine Alterations

• Early-onset metabolic acidosis develops with a low anaerobic threshold 1
• Increased insulin resistance emerges after several days of inactivity 1
• Alterations in bone metabolism occur, resulting in increased calcium excretion, bone resorption, and trabecular bone loss (up to 3.1% additional bone mineral density loss) 1
• Decreased metabolism and appetite develop after several weeks 1

Respiratory System Impact

• Abdominal viscera are displaced upward against the diaphragm when lying supine, increasing inspiratory muscle effort, decreasing oxygen inspiration, and impairing gas exchange 1
• Loss of endurance and strength in respiratory muscles occurs 1

Additional 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, nerve compression, and shifts in hormone levels (cortisol, thyroid hormone, aldosterone) 1
• Altered sleep cycles, insomnia, and impaired immune function 1
• Delayed mental processing and cognitive deconditioning 3

Clinical Presentation

Exercise Testing Characteristics

The American Thoracic Society/American College of Chest Physicians defines deconditioning by specific cardiopulmonary exercise testing patterns:

• Peak VO₂ is low or at the lower limit of normal in very deconditioned but otherwise normal subjects 1
• Early-onset metabolic acidosis with low anaerobic threshold 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
• Normal peak ventilation/VCO₂ ratio and normal slope of the ventilation-VCO₂ relationship 1

Functional Manifestations

• Reduced functional capacity in activities of daily living 4, 2
• Protracted symptoms including muscle soreness and impaired mobility that persist after the period of inactivity 1
• Exercise intolerance and fatigue 1, 4
• Increased risk of falls, functional decline, and increased frailty 2

High-Risk Populations

Elderly individuals are particularly vulnerable to becoming deconditioned, especially those living with frailty 4, 3. Risk factors include:

• Acute illness or injury requiring hospitalization 5, 3
• Chronic disease 4
• Disability 4
• Medications that limit activity 4
• Psychosocial circumstances limiting mobility 4
• Pregnancy complications requiring activity restriction 1
• Critical illness requiring ICU care 1

Critical Diagnostic Distinction

Deconditioning is often difficult to distinguish from early or mild heart disease, making clinical history extremely helpful 1. Key differentiating features:

• Normal arterial oxygen pressure and ventilation/perfusion ratio in pure deconditioning 1
• Changes in cardiopulmonary exercise testing responses to an aerobic training program help distinguish deconditioning from cardiac disease 1
• In the absence of medical history suggesting disease, exercise intolerance may be reasonably attributed to deconditioning alone 1

Special Considerations

Pregnancy-Specific Effects

• Impaired maternal weight gain correlating with low fetal birth weight across all gestational ages 1
• Increased risk of gestational diabetes mellitus (4% increased risk for every day of inpatient bed rest) 1
• Increased risk of venous thromboembolism compounded by pregnancy's hypercoagulable state 1

Post-COVID Context

• Deconditioning represents a final common pathway that can be triggered by as little as 20 hours of bed rest and sudden decline in physical activity from baseline 1
• It remains unclear why limited durations of bed rest and modest physical inactivity in asymptomatic or mild COVID-19 illness result in similar degrees of deconditioning as more severe illness 1

Reversibility Concerns

While the term "deconditioning" implies ease of reversibility, this is misleading—reconditioning is a long process that may be overwhelming, and long-term effects can be devastating without early intervention 4, 3.