Physiological Reserve: Understanding Organ-Specific and Combined Reserve Capacity
Each organ system has its own physiological reserve capacity that collectively contributes to an individual's overall physiological reserve, which determines their ability to respond to stressors, recover from illness, and maintain homeostasis.
Organ-Specific Physiological Reserve
Physiological reserve refers to the ability of an organ to successfully return to its original physiological state following repeated episodes of stress, representing an innate mechanism of adaptability 1
Each major organ system has its own distinct reserve capacity that declines at approximately 1% per year after age 40, creating vulnerability to stressors as we age 2
The cardiovascular system demonstrates reserve through its ability to increase cardiac output approximately 20-fold during acute stress, with tissue perfusion increasing tenfold and both heart rate and stroke volume capable of tripling 2
The respiratory system shows reserve through ventilatory capacity, measured as the difference between maximum voluntary ventilation (MVV) and the ventilation achieved at peak exercise, with a normal reserve being at least 15% 2
Coronary circulation exhibits reserve through its ability to increase blood flow 3-fold in adults in response to increased myocardial oxygen demand 2
Components of Physiological Reserve
At the cellular level, physiological reserve is maintained through excess metabolic capacities in bioenergetic pathways, antioxidant systems, and structural components like mitochondrial DNA copy number and telomere repeats 1
These excess capacities allow intermediary metabolism to instantly cope with added workload or stress, substantiating organ reserve and contributing to cellular defense systems 1
Sarcopenia (loss of skeletal muscle mass, strength and function) represents a decline in musculoskeletal reserve that precedes frailty and can be disease-related, activity-related, or nutrition-related 2
Frailty represents a state of vulnerability and non-resilience with limited reserve capacity across major organ systems, leading to reduced capability to withstand stress such as trauma or disease 2
Integration of Organ-Specific Reserves
The net overall physiological reserve results from the integration of individual organ reserves, with the weakest system often becoming the limiting factor during stress 2, 3
During acute stress, the body's response involves coordinated changes across multiple systems - for example, in the "fight or flight" reaction, cardiovascular, respiratory, and metabolic systems all increase their activity in concert 2
The concept of "demand ischemia" illustrates how organ systems interact - when myocardial oxygen demand exceeds supply due to factors like tachycardia, hypertension, or hypoxemia, cardiac function becomes compromised despite the heart having its own reserve 4
In trauma patients, the concept of "damage control surgery" recognizes the exhaustion of physiological reserves across multiple systems (manifesting as acidosis, hypothermia, and coagulopathy) and aims to prevent this "lethal triad" 2
Clinical Significance and Assessment
Physiological reserve correlates with the ability of older adults to cope with added workload or stress, suggesting a critical role in the aging process 1
No single test can measure an individual's complete physiological reserve due to its complexity, requiring multiple assessment tools 5
Both subjective and objective tools have been developed to measure components of physiological reserve, with objective measures including the 6-minute walk test, gait speed, and various frailty indices 5
Pre-operative assessment of elderly patients should include evaluation of age-related physiological decline, multi-morbidity, and frailty to determine peri-operative risk 2
Factors Affecting Physiological Reserve
Age is a primary factor in declining physiological reserve, with both fit and aged normal subjects showing reduced ventilatory reserve compared to younger individuals 2
Disease states significantly impact organ-specific reserves - for example, patients with pulmonary diseases have reduced ventilatory capacity and increased ventilatory demand, resulting in reduced ventilatory reserve 2
Nutritional status affects physiological reserve, with malnutrition contributing to sarcopenia and frailty 2
Obesity can paradoxically reduce physiological reserve despite appearing to provide energy stores, as seen in "sarcopenic obesity" 2
Clinical Applications
Understanding a patient's physiological reserve is crucial for risk stratification before major interventions like surgery 2
In trauma management, recognition of limited physiological reserve guides the decision to employ damage control surgery rather than definitive repair in severely injured patients 2
For elderly patients, pre-operative optimization should focus on reducing risks by addressing organ-specific morbidity, preventing ischemia, avoiding cognitive disorders, addressing malnutrition, and preventing functional decline 2
When physiological reserve is impaired or exhausted, the ability of cells to cope with stress is reduced, leading to cell senescence, transformation, or death 1
Emerging Concepts
Recent research proposes that physiological reserve, intrinsic capacity, and physical resilience are interconnected determinants of healthy aging 3
Physiological reserve positively correlates with cognitive and locomotive domains of intrinsic capacity, which in turn affects physical resilience and clinical outcomes 3
The biology of physiological health may involve distinct adaptation mechanisms that coordinate interactions between all physiological systems, separate from disease pathogenesis mechanisms 6
Efforts to improve intrinsic capacity and physical resilience in older patients are necessary to promote healthy aging and maintain physiological reserve 3, 7