Stress means different things to different people. In common language, people recognise a state of having too much expected of them, of being under pressure or strain, of being barely able to cope with some external demand which is both excessive and prolonged. It has a number of synonyms, but they all carry the connotation of unreasonable demands being placed on the individual in an emotional, mental or psychological sense.
A similar concept underlies the term stress in the physical sciences, a deforming force exerted on an object or structure that, if not resisted, would damage or destroy the object. The analogy is graphic and obvious. Scientists in the biological and health sciences also use stress in similar manner with subtle differences. They refer to environmental demands that affect an organism which, if not resisted, would damage or destroy it. It can also be applied to whole environmental systems. The common theme is an external factor that, if not resisted, would damage or destroy the system.
Why, in medical parlance, is stress therefore not universally measured, analyzed and treated? Why, indeed, has it often been denigrated or ignored or consigned to a fringe area of medicine? The answer is that doctors tend only to believe that which they can measure, observe and classify. It is this lack of physical presence, this inability to biopsy, photograph, probe or quantify which has led to stress not having been considered a part of conventional medicine for so long.
Despite the ignorance and disinterest that much of conventional medicine has shown to the study of stress, recent developments in physiology and bio-mathematics has led to increased insights into measuring the effects of stress on the body. This can also allow us to estimate not only the impact of stress on the body, but also how much reserve capacity is left to cope with further stress, whether it be physical stress (such as heat, cold or exercise) or psychological stress.
The body has an autonomic nervous system that is a complex series of neural connections connecting all organs to the brain to control their whole internal environment. This autonomic nervous system is both our major defense against stress and the system that demonstrates the principal symptomatic manifestation of stress in its early stages.
The autonomic nervous system is conventionally divided into two parts in a yin/yang balance: the sympathetic, which activates organs, getting them ready to cope with exercise or other physical stress; and the parasympathetic, which controls background "housekeeping" functions in the body. Activating the sympathetic nervous system will speed the heart, constrict blood vessels to less important organs such as the skin, and increase breathing and alertness. This system also dilates the eye, raises body temperature by burning off fat and causes heightened activity of motor nerves, producing, for example, the classical tremor and sweating of anxiety. Palpitations can be sensed and indeed frankly, irregular heart beating can result. Breathing rate increases and this can cause carbon dioxide to be blown off, rendering the arterial blood more alkaline, which in turn makes nerves hyper-excitable, leading to unpleasant sensations such as tingling or numbness, classically around the mouth and at finger tips. There is a resultant constriction of blood vessels to the brain that can cause dizziness and a feeling of faintness. Many of these sensations in themselves make the person feel even more anxious, thereby worsening the initial stimulation of the sympathetic system.
The counterbalancing system to the sympathetic is the parasympathetic system. This system controls internal organs at times of relaxation when, for example, the subject is in quiet sleep or rest. It harmonizes heart, blood vessel and breathing patterns causing, classically, a slow heart rate, relaxed blood vessels and slow, deep breathing. The gut and skin get a good blood supply in this situation and background functions, such as digesting food are facilitated when this system is most active. Much has been known of these systems for decades but they have historically been very difficult to measure in patients, leading to less knowledge of their role in health and disease.
A dramatic advance in the study of stress responses has been that mathematical analyses of biological rhythms have allowed us a window to the working of these autonomic systems. The complex interaction of the heart blood vessels, lungs and central nervous system means that physiological parameters such as heart rate are not stable but fluctuate in a complex but not random way.
Imagine a house with central heating. It needs a thermostat to keep temperature stable at the desired level, and it must cope with windows and doors being opened, letting in cold air. The thermostat must sense a reduction in room temperature and then switch up the heating to correct this. Inevitably, there will be a delay before the temperature returns to the desired level and also, inevitably, the temperature will often overshoot the desired level before the increased temperature is sensed to switch the heater lower again. The net effect is that the temperature, which was stable until a window is opened, initially falls, then rises, then overshoots, then falls again, then overshoots, continuing to oscillate far beyond the time of the initial change in temperature. Depending on the characteristics of the thermostat and the heater, this oscillation in room temperature can continue for long periods, or even permanently. The behavior as measured by the rhythmic fluctuations of temperature depends on the characteristics of the thermostat, such as its gain (the extent to which it senses small changes in temperature to produce large increases in heating output) and its delay (the speed with which it responds to a temperature change).
In the body there are multiple sensors like our theoretical home thermostat. They measure things such as heart rate, blood pressure, body temperature and the biochemical and oxygen and carbon dioxide contents of the blood. There are multiple sensors for each measurement, each with a different sensitivity and a different delay characteristic. The net effect is that each measure (heart rate, blood pressure, breathing rate etc.) is not stable but, rather, oscillates not at one frequency but at several different frequencies simultaneously. These oscillatory rhythms can be deconstructed mathematically to uncover the underlying sensitivities and delays of each sensor and the activity of the nerves that attach to these sensors. These sensors and their nerves are the mechanistic elements of the two halves of the autonomic nervous system described above: the sympathetic and the parasympathetic. By measuring these oscillations we can tell how active these two parts are; how active and how much reserve they have. For example, an active sympathetic will reduce the extent of the classical 4-second heart rate oscillation and relatively increase the amount of cycling at one cycle every 10 seconds. In contrast, the parasympathetic activates the 4-second rhythm and suppresses the 10-second rhythm.
These rhythms are of interest not only in what they tell us about the functioning of the body, but also because they have major medical uses. In patients recovering from a heart attack, in patients suffering from heart failure and in very sick patients on an intensive care unit the amount of the 4-second rhythm fluctuation in heart rate has been shown repeatedly to be a powerful predictor of survival. Those patients with reduced 4-second fluctuations are much more likely to die than those with more evidence of this rhythm. Treatments that increase this parasympathetic 4-second rhythm, such as beta-blockers, angiotensin converting enzyme inhibitors and physical exercise training, have been associated with better survival and those that reduce this rhythm to worsen survival.
In a similar way, natural or life-style induced changes in the activity of the parasympathetic and sympathetic systems can tell us about the physiological health of the individual, how much stress they are experiencing and how much reserve they maintain. We now have the opportunity to open a new branch of medical science: the accurate measurement of stress, stress responses and reserves to cope with stress and to tests. This will help us develop treatments to reduce stress, to avoid its damaging effects and to increase our stress resisting reserves.
This dossier summarizes much of what we have learnt and introduces techniques that may help in alleviating the effects of stress. Much has been learnt, but much remains to be learnt. We have in recent years made a good start in correcting this neglected part of medical science, the science of stress and its treatment.