Power spectral analysis of heart rate variability has often been used to assess cardiac autonomic function; however, the relationship of low‐frequency (LF) power of heart rate variability to cardiac ...

Human plasma contains several catechols, including the catecholamines norepinephrine, epinephrine, and dopamine, their precursor, l-3,4-dihydroxyphenylalanine (l-DOPA), and their deaminated metabolites, dihydroxyphenylglycol, the main neuronal metabolite of norepinephrine, and dihydroxyphenylacetic acid, a deaminated metabolite of dopamine. Products of metabolism of catechols include 3-methoxytyrosine (from l-DOPA), homovanillic acid and dopamine sulfate (from dopamine), normetanephrine, vanillylmandelic acid, and methoxyhydroxyphenylglycol (from norepinephrine), and metanephrine (from epinephrine). Plasma levels of catechols and their metabolites have related but distinct sources and therefore reflect different functions of catecholamine systems. This article provides an update about plasma levels of catechols and their metabolites and the relevance of those levels to some issues in human health and disease.

This essay continues discussion of a new formulation of homeostasis that uses the concepts of allostasis and homeostats. The new formulation moves beyond Cannon's concept of "homeostasis," which posits an ideal set of conditions for maintenance of the internal environment. The notion of allostasis recognizes that there is no single ideal set of steady-state conditions in life, and different stressors elicit different patterns of activation of the sympathetic nervous and adrenomedullary hormonal systems. Allostasis reflects active, adaptive processes that maintain apparent steady states, via multiple, interacting effectors regulated by homeostatic comparators--"homeostats." "Allostatic load" refers to the consequences of sustained or repeated activation of mediators of allostasis. From the analogy of a home temperature control system, the temperature can be maintained at any of a variety of levels (allostatic states) by multiple means (effectors), regulated by the thermostat (homeostat). Allostatic load and risks of system breakdown increase when, for example, the front door is left open in the winter. Applying these notions can aid in understanding how acute and chronic stress can exert adverse health consequences via allostatic load.

This essay describes the evolution of stress as a medical scientific idea. Claude Bernard, Walter B. Cannon and Hans Selye provided key founding concepts for the current view. Bernard introduced the idea of the internal environment bathing cells - the milieu intérieur - maintained by continual compensatory changes of bodily functions. Cannon coined the word, "homeostasis," referring to a set of acceptable ranges of values for internal variables. Cannon taught that threats to homeostasis evoke activation of the sympathoadrenal system as a functional unit. Selye defined stress as a state characterized by a uniform response pattern, regardless of the particular stressor, that could lead to long-term pathologic changes. "Allostasis" was introduced as a concept in recognition that there is no single ideal set of steady-state conditions in life; instead, setpoints and other response criteria change continuously. Stress is now viewed neither as a perturbation nor a stereotyped response pattern but as a condition characterized by a perceived discrepancy between information about a monitored variable and criteria for eliciting patterned effector responses. Different stressors elicit different patterns of activation of the sympathetic nervous, adrenomedullary hormonal, hypothalamic-pituitary-adrenocortical and other effectors, closing negative feedback loops. This systems concept of stress yields predictions that observation or experimentation can test and that are applicable to normal physiology and to a variety of acute and chronic disorders.