Functions of the Vegetative Nervous System
At every time of the day innumerable bio-chemical processes help in a complex interplay to keep us alive. Catalysts, inhibitors, hormones and other substances are continuously generated, modified and dismantled. Like in an industrial chemical plant, these vital processes can, however, take place only under certain conditions. For our bio-chemical processes, these conditions include blood pressure, temperature, acid-base balance, CO2 and O2 partial pressure. The homeostasis describes a situation the optimum ratio of these factors is achieved. The Autonomic Nervous System (ANS) ensures, inter alia, that the homeostasis is maintained at all times. As the name suggests, the ANS acts autonomously, that is, that we cannot consciously control its function. The autonomic control is reflexively. Let’s compare the ANS with central heating. Once set at a comfortable temperature, we must and do not worry about the heating. Even when the outside temperature drops, the central heating system ensures a constant temperature in the house is maintained. Up to certain limits outside temperature fluctuations are compensated without our help, and even without us being aware. If such disorders exceed a tolerable level (e.g. arctic cold snap), the interior temperature cannot be maintained. If this condition occurs only briefly, the normal temperature will be restored after a certain time after the cold snap. However, if the weather remains too cold for too long, even the house can suffer through the cold damage. Just like the central heating in a house the ANS tries to compensate any disturbances from the outside (and inside) to maintain homeostasis, the inner balance. In our modern lives we are constantly exposed to disturbances, either by our environment, like environmental toxics, or by our own lifestyle through stress or unhealthy diet. To a certain degree our body can cope well with such disturbances. However, if we do not listen to warning signs, or if we even consciously ignore them, we also stress our VNS and reduce its ability to cope with other disturbances. Then, a seemingly insignificant event may trigger our VNS to derail. Since all processes in our body are finely tuned, such a derailment may cause a chain reaction, and then manifest in a variety of symptoms and diseases.
Autonomic Nervous System and Heart Rate Variability
The ANS controls important body functions using two antagonistic branches: The sympathetic nerve activates functions that are responsible for the retrieval of energy reserves and prepares the body for physical or mental performance. The parasympathetic nerve on the other side is activated for recreation and activates processes that build up energy reserves. Our heart is innervated by both branches of the ANS alike. While a sympathetic stimulus leads to an increase in heart rate, a parasympathetic stimulus reduces the heart rate. Since, as mentioned above, the processes run continuously, sympathetic and parasympathetic constantly act on the heart. It is therefore a good sign that the sequence of heartbeats are constantly in motion, as this indicates a well functional ANS. With longer observations of heart rate variability (HRV), we can even recognize patterns that indicate the different programs of the ANS. Depending on how hard the ANS is working to maintain homeostasis, different kinds of those patterns can be identified in the HRV. The HRV can be conveniently determined using a surface ECG. Hence, the HRV is very well suited to detect early changes in the ANS. During therapy, HRV can be used for monitoring response to treatment. Since many processes are slow, it is important to observe the VNS over a longer period. A measurement period of 24 hours has been found to be a good compromise.
Metering Heart Rate Variability
So the ANS responded early to disturbances before we even may perceive a change. Using the HRV we can now estimate the operating point of the ANS and identify negative influences on the organism.
HRV may be derived from the surface ECG. A long-term ECG is often applied in cardiology to detect arrhythmias and conduction disturbances in the heart. The detection of cardiology-related events is a positive side effect of a HRV measurement with ECG. HRV analysis is performed by detecting beats in the recorded ECG and determining the time between consecutive beats.
To quantify these vast sequence of numbers and make them human readable, different mathematical and statistical methods are applied.
Since the 1990s a number of different parameters is used to quantify the autonomic regulation in the human organism and to assess the balance between the sympathetic and parasympathetic nervous system in particular. Due to the increasing importance of HRV in the diagnosis, guidelines were finally determined for measurement and interpretation of HRV to ensure the purpose of comparison a standardization of the parameters in 1996.