Atrial Fibrillation (AF) and HRV
Heart rate variability (HRV) is all about analysing variation in the beat to beat intervals of the heart.We measure the rhythmic increase and decrease in heart rate as we breathe in and out. That’s called respiratory sinus arrhythmia (or RSA for short). It is a natural, healthy phenomenon, in fact the more variation the better!
But some heart rhythms are not healthy, and potentially dangerous. One of the most common heart rhythm disorders is called Atrial Fibrillation (A-Fib or AF). This is when the top chambers of the heart beat faster, and out of sync with the bottom chambers. It’s not life threatening, but AF reduces the heart’s output by about 30% and also churns up the blood, making it more likely to clot, potentially leading to stroke.
Although AF is most common in older people with conditions such as obesity, diabetes and heart disease, it’s also quite common in masters age endurance athletes, particularly those with a long training history. Champion Kayak surfer Oscar Chalupsky recently reported his experience with AF in this podcast.
The new research
Now a new study has come out sponsored by the US National Heart, Lung & Blood Institute. It looks at data from a large study where subjects were followed up for an average of 20 years, allowing them to identify factors such as HRV and resting heart rate which might predict the chances of contracting AF years later.
The researchers looked at the HRV and resting HR of 11,700 45-65 year olds from multiple ethnic backgrounds. They looked at how many people then developed AF during an average follow up period of 20 years. The study captured lots of lifestyle data such as smoking, alcohol use and the presence of existing medical conditions.
What did they find?
The first thing they found was that the likelihood (Hazard ratio) of contracting Atrial Fibrillation reduced with increasing HRV. This is not surprising, since as well as being a generally good thing for health, more HRV has also been shown in previous studies to be ‘cardio protective’ i.e. it protects the heart against potentially lethal arrhythmias.
What is a little more surprising is that there seems to be an optimum amount, above which the risk of AF seemed to start increasing again.
This is shown more graphically in what they found about resting heart rate (expressed as RR interval = 60000/HR):
The optimum resting heart rate is therefore 60000/900 = 67 bpm, relatively high by athletes’ standards. A resting heart rate of 120 bpm makes you about 20% more likely to develop AF. But a resting HR of 40 bpm actually makes you nearly twice as likely to develop AF within the next two decades as someone with a resting HR of 67 bpm!
What does it mean?
I expected the main message of this study to be that higher HRV and lower resting HR (which usually go together) are always better, and this is true up to a point. The authors use this nice diagram to explain how this portion of the findings works, mostly for sedentary and less lifestyle conscious people:
For the increased risk associated with low resting heart rates and high HRV, the mechanisms are more elusive. In the case of Masters athletes, years of training have caused extensive cardiac remodeling. Not only do the chambers of the heart become greatly enlarged, but the muscular walls include more scarring and fibrous tissue, which can interfere with the heart’s normal conduction system. This can lead to spontaneous contractions of the upper chambers (atria) during episodes of AF which are more likely to occur during periods of stress, either mental, physical or chemical, e.g. due to alcohol or caffeine.
AF affects the performance of the heart, especially amongst older, less healthy people. But it also affects a significant percentage of Masters athletes. A large study over a 20 year period shows an interesting correlation between Atrial Fibrillation (AF) and HRV. There are optimum values of HRV and resting heart rate, above and below which the chances of developing AF increase significantly.