By Dr. Christopher Kent
In the world of Newtonian physics, everything appeared predictable and causal. Relationships were linear, with an effect being proportional to its cause. However, there were situations where this did not seem to be the case. Poincare showed that there were stable and unstable types of orbits, and that a tiny disturbance could result in a significant change in the nature of the orbit. There were situations where similar causes resulted in dissimilar effects. 
Lorenz studied computerized weather forecasting, and noticed that starting such a program with only slightly different initial conditions would eventually result in completely different weather conditions. These findings led to the emergence of chaos theory. 
Chaos must be contrasted with randomness and periodicity. Random events are inherently unpredictable. In contrast, periodic behavior is highly predictable, as it repeats itself over time. Systems are close to static equilibrium unless there is an injection of energy to excite the system. Chaos shares characteristics of both randomness and periodicity. Chaos never repeats itself exactly, and although it may appear random, it is bounded, never wandering off into infinity. It has a definite form, and a particular pattern emerges. [2,3,4]
A recent paper by Lopez describes a proposed mechanism for control of vasomotor tone. Lopez wrote, “These postganglionic cells show bursts of activity with a periodicity that is related to cardiac and respiratory cycles, a coordination that might help to optimize the blood supply to every organ. How is this bursting activity controlled? One leading idea is that an oscillatory network in the brainstem entrains the sympathetic neurons, causing them to fire simultaneously.” 
What mediates this process? According to Staras et al  the answer is afferent somatic activity that can “reset” the oscillatory networks. This transiently synchronizes sympathetic neuron firing.
Heart rate variability
Many clinicians view the heart as a periodic oscillator, whose rate varies according to the demands of the organism. However, there is growing evidence that under physiologic conditions, the heart is not a periodic oscillator. [7,8,9,10,11]
Variability in heart rate reflects the vagal and sympathetic function of the autonomic nervous system, and has been used as a monitoring tool in clinical conditions characterized by altered autonomic nervous system function . Spectral analysis of beattobeat variability is a simple, noninvasive technique to evaluate autonomic dysfunction .
Heart rate variability analysis has been used in the assessment of diabetic neuropathy and to predict the risk of arrhythmic events following myocardial infarction . The technique has also been used to investigate autonomic changes associated with neurotoxicity , physical exercise , anorexia nervosa , brain infarction , angina , and panic disorder .
Normative data on heart rate variability have been collected [20,21,22]. This technology appears to hold promise for assessing overall fitness. Gallagher et al  compared agematched groups with different lifestyles. These were smokers, sedentary persons, and aerobically fit individuals. They found that smoking and a sedentary lifestyle reduces vagal tone, whereas enhanced aerobic fitness increases vagal tone. Dixon et al  reported that endurance training modifies heart rate control through neurocardiac mechanisms.
In occupational health, the effects of various stresses of the work environment of heart patients and asymptomatic workers may be evaluated using heart rate variability analysis .
Zhang and Dean  reported the results of an exciting study involving 520 subjects in a singlevisit group, and 111 subjects in a fourweek group. The purpose of the study was to investigate the effect of chiropractic care in a multiclinic setting on the balance of the sympathetic and parasympathetic nervous system using HRV (heart rate variability) analysis. The study demonstrated consistent changes in HRV. The authors reported, “The decreased heart rate and increased total power from the HRV analysis indicated a healthy autonomic nervous system balance after correction of vertebral subluxation.”
Acquired dysautonomia is one of the three elements in the threedimensional model of vertebral subluxation . Skin temperature changes, reflecting alterations in vasomotor tone, are used clinically to assess autonomic changes associated with vertebral subluxations. Heart rate variability represents an exciting, noninvasive technology to assess subluxationrelated autonomic function. It will empower the practicing chiropractor to assess and communicate the farreaching impact of subluxation correction on global health.
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