Vagal tone is the degree of activity occurring within the parasympathetic nervous system, resulting in changes to the heart rate and other key functions. The vagus nerve, among many other things, inhibits the heart beat to keep the organ beating within a safe range. When the vagal tone increases or decreases, a patient will experience changes in heart rate, and these can sometimes become dangerous or even fatal.
When activity increases, patients experience bradycardia, where the heart slows down, because the inhibitory effect of the vagus nerve is stronger. Patients with decreased tone will develop tachycardia, a fast heart rate, because the nerve is not acting to suppress activity within the heart. This can be the result of a temporary situation, or a long term health problem. Sometimes, changes are not necessarily a sign of disease. Athletes, for example, have a higher vagal tone, and as a result will have a slow resting heart rate.
The vagus nerve experiences reflex reactions in response to certain physical stimuli, like being dashed with cold water or tensing the abdomen. Its goal is to keep the body in a state of homeostasis, where all internal systems are stable and running at a reasonable speed for the conditions. In response to some stimuli, vagal activity can decrease, allowing the heart to speed up. This may increase the flow of blood, facilitating the flight or fight response.
Medical professionals can use a variety of techniques to examine the level of influence being exerted by a patient's parasympathetic nervous system. This can include listening to the heart and running tests with various stimuli to see how the patient responds. If a patient appears to have a disorder involving the vagus nerve, the healthcare professional can look into possible causes and discuss treatment options with the patient. Some patients may be prone to issues like fainting because of changes vagal tone or the intensity of responses to stimuli.
Researchers also believe the vagus nerve can provide important information about stress levels. Fluctuations in tone may indicate rising or falling levels of stress, including chronic stress, where people's bodies may be stuck in a permanent state of heightened awareness. Understanding stress is important for treating conditions like post traumatic stress disorder and can help medical practitioners explore possible methods for treatment and management, including using medications and psychotherapy to help patients reduce stress levels and stay healthy.