The host response to trauma, shock and sepsis behaves as a complex nonlinear system consisting of a great number of variables and systems of variables. It is the systemic properties of the host response to a physiological insult that manifest as health or illness and determine the clinical outcome in critically ill patients. Variability analysis of physiological rhythms provides a novel technology with which to evaluate the overall properties of a complex system.
Complex systems have properties that cannot be wholly understood by understanding the parts of the system.4 The properties of a complex system are distinct from the properties of the parts and depend on the integrity of the whole. The emergent systemic properties of a complex non-linear system arise from the relationships between the components of the system. A key property of complex non-linear systems is their ability to generate negative entropy (less disorder), known as emergent order, and explains why complex non-linear systems are ubiquitous in nature (e.g. a bee hive, central nervous system). Feedback loops are the building blocks of emergent order, where a complex system through a process of self-organization will settle into a reduced number of stable states.
Biological systems are complex systems that are spatially and temporally complex, built from a web of interconnected feedback loops. Understanding the host response to trauma, shock or sepsis as a complex non-linear system offers new insights to improve the evaluation, monitoring and treatment for our critically ill patients.5,6 Numerous studies have demonstrated the relationship between the immune system, nervous system, endocrine system and behavioral and emotional states.7-10 Research has shown that the immune system is an interconnected self-reactive nonlinear network11-13 and that the inflammatory response is complex and non-linear in nature.14-16 Characteristic patterns of variation over time, namely rhythms, are a defining feature of complex systems. Illness represents a systemic alteration, where abnormal rhythms are associated with illness and can be involved in its pathogenesis.17 Connectivity and variability are a means of assessing the alterations in the web of interrelationships and feedback loops that form the biological system’s emergent order. Connectivity is a measure of the systems interrelationships while variability is a measure of change over time of the individual components of the system.
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