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Kendrick asks.... why have we ignored heart rate variability?

Studies have shown that people whose heart rate is not sufficiently adaptable have a hugely increased risk of CVD – yet heart rate variability does not figure in any CHD risk calculators. Our clinical columnist Dr Malcolm Kendrick asks why

Studies have shown that people whose heart rate is not sufficiently adaptable have a hugely increased risk of CVD – yet heart rate variability does not figure in any CHD risk calculators. Our clinical columnist Dr Malcolm Kendrick asks why

There is one risk factor for premature death that makes all others fade into relative insignificance – heart rate variability (HRV). It first came to my notice when I saw a graph of 10-year cumulative survival from a study on an elderly population in Finland.

Two groups were compared – those with ‘good' HRV and those with ‘bad' HRV. All patients were over 65; the average age studied was 731. As can be seen from the graph below, by the end of 10 years the chances of being alive were more than 70% for those with a higher level of HRV and under 20% for those with poor HRV.

Perhaps even more telling was the plot of individual data points showing who was alive and who was dead at the end of the 10 years. If you were in the ‘good' HRV group your chances of being alive at the end of 10 years were very high. But, in those whose HRV was classed as bad, only 19 were alive and 77 were dead.

Reinforcing the critical importance of HRV, a large American study has shown that poor HRV was associated with a 530% increase in the risk of dying of a heart attack over the next two-and-a-half years2, which, extrapolated, is a 1,060% increased risk over five years.

There is no other factor that can compare with this level of increased risk. It's a more powerful risk factor for CHD than all of the Framingham risk factors added together, which makes it all the more surprising that it does not feature in any significant risk calculators currently available.

What is heart rate variability?

The measurement of HRV, and what is good and bad, is complex. But the basic concept is very simple. The heart rate is not constant. It ‘hunts' up and down, and changes rapidly in response to various stimuli. For example, if we are challenged physically or mentally our heart rate will rapidly rise. When we relax it falls.

All the time, in fact, our heart rate is altering not just from minute to minute, but from beat to beat. The changes are often small and will not stand out on an ECG tracing. But beat-to-beat variation can be measured and the greater the variation the healthier our cardiovascular system.

As we age, HRV gradually falls3, indicating an unhealthy cardiovascular system. It is also a more general indication of ill-health and has been used to predict outcomes in diabetic neuropathy4.

What causes an ‘unhealthy' HRV?

Unhealthy HRV was, in fact, first noted in the unborn child. It is widely known that if the foetal heart rate flattens out this is a critically important sign of foetal distress4. However, it took a number of years before people began to recognise that more subtle loss of HRV may be a sign of poor health in an older population.

There is no doubt, however, that a wide range of conditions cause a reduction in HRV – conditions ranging from depression, to cancer, diabetes and CHD5.

Can we do anything about it?

If reduced HRV is just a measure of ill-health, albeit a very powerful one, looking for it is important. But what can we do about it? A growing body of evidence suggests stress and depression, through an impact on stress hormone levels and the parasympathetic/sympathetic nervous system, can significantly reduce HRV5.

This suggests that stress, or a poor reaction to various stressors, may indeed be the underlying cause of reduced HRV and that it may be possible to do something about it. A study from 2004 looked at using biofeedback mechanisms to improve HRV6. Sixty-three patients with established coronary artery disease – mean age 67 – were randomly assigned to conventional therapy or to six biofeedback sessions consisting of abdominal breath training, heart and respiratory physiologic feedback, and daily breathing practice. The authors concluded: ‘Biofeedback increases HRV in patients with CAD and therefore may be an integral tool for improving cardiac morbidity and mortality rates.'

It is clear from an increasing number of studies that biofeedback can significantly improve surrogate end-points in a number of areas – BP reduction, HbA1c and HRV itself. The main problem is that such techniques do not require drug therapy and may therefore fail to attract the level of funding available to drug-related research.

I would suggest poor HRV is the single most important measure of overall health there is. Ignoring it is to disregard an entire area where we may be able to treat the underlying drivers of ill-health and premature mortality.

Dr Malcolm Kendrick is a GP in south Manchester

ECG

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