Measures of Adaptability – Heart Rate Variability

Heart Rate Variability

How do we measure adaptability?

Measuring heart function has long been used as an assessment of autonomic activity in the body. Heart rate is the simplest and most widely used measure of cardiac function, in many medical situations it is the first part of assessing a patient. It is also the metric that has been most thoroughly studied. So what insight can measuring the heart give us about the nervous system and adaptability?

The heart has two different nervous inputs, sympathetic and parasympathetic. The sympathetic supply comes from spinal nerves of the upper thoracic spine (top of the back). The sympathetic innervation of the heart causes the heart beat to speed up as part of the fight or flight response. Heart rate will elevate towards 200bpm in order to quickly supply oxygenated blood to the muscles to facilitate the required actions.

If the heart did not have either sympathetic or parasympathetic innervation the resting heart rate would maintain around 105-110bpm, this is an unusually high resting heart rate. Therefore parasympathetic tone is required at rest inorder to facilitate a healthy resting state. The parasympathetic supply comes from the vagus nerve which exits from the upper cervical region (top of the neck). Vagal tone should always be active in resting states.

In people who are near death heart rate is often closer to the 105-100bpm range as there is less parasympathetic supply influence. Testing has shown that vagus nerve activity decreases in these patients, this vagal withdrawal allows for the rise in heart rate closer to its natural baseline.

From this we can make some conclusions regarding heart rate. If a person’s resting heart rate is 90bpm we can make the assumption that they may be experiencing vagal tone withdrawal. The other conclusion is that the high resting heart rate may be caused by sympathetic overactivation. There are studies that have demonstrated a relationship between elevated heart rate and mortality and morbidity. The shift in neural regulation of heart rate is certainly significant in the research.

Sympathetic tone may be high while parasympathetic is low, or vice versa. It is also possible for both to be elevated, or both be lowered. Therefore heart rate alone can’t give a pre/post-intervention metric to measure objective changes to a patient's physiology. As with all physiological measures, the heart rate should express a degree of adaptability in response to internal and external environments. This is where heart rate variability (HRV) testing becomes important.

HRV is the measure of the variation in time between heart beats. Higher variability of this time period indicates good function of vagal tone, and a higher level of adaptability. A decrease in HRV is being increasingly linked to a multitude of different diseases and health conditions, and the research is still ongoing.

It is for this reason we are introducing HRV technology into our practice so we can develop an increasingly accurate measure of our patient’s physiology and their ability to adapt. This data is very important when looking after people with a range of health conditions so that we can make accurate clinical decisions about their health with the most objective data.

Make sure you also read our previous blogs related to this topic, HRV and 3 areas to be aware of, 5 Signs of Life and Importance of Adaptability, and Measures of Adaptability - Thermography.

Source: Textbook of Human Adaptability - Rob Sinnott


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