Just How Fast Is Your Heart Beating?

Just under 2 months ago, I looked at a study attempting to predict cardiorespiratory fitness via non-exercise means.  Of note, the authors linked body mass index and resting heart rate to CRF.  But do we really care as much about CRF as we do our mortality?  In fact, in a prospective cohort study published last month in Heart, the authors concluded that resting heart rate was an independent predictor of mortality, regardless of physical fitness, physical activity and the usual slew of cardiovascular risk factors.

The authors measured resting heart rate prior to testing physical fitness as measured by VO2max (oxygen consumption) in 2,798 Danish men who were then followed for 16 years.  As demonstrated in a figure in the paper, there was an inverse correlation between physical fitness and RHR.  In other words, those who were more fit as determined by a greater VO2max also had a lower RHR.  

To my amazement, the 222 fittest men w/avg VO2max 38L/kg/min also had an average resting heart rate of just 48 beats per minute while the least fit 54 had an RHR greater than 90bpm.  Typically, we declare anyone with heart rate less than 60bpm as being bradycardic, and very rarely, aside from exceptionally fit young athletes, do I see resting heart rates less than 56bpm.

Yet, after taking into account all the usual confounders, the authors noted that compared to those w/RHR less than 50bpm, mortality increased by 16% for every 10bpm increase in RHR.  Those with what I previously considered an excellent RHR of just 51-60bpm had a 20% greater mortality compared to those w/RHR less than 50bpm.  Those with a normal or average RHR of 71-80bpm had 33% greater mortality while those with RHR greater than 90bpm had more than twice the mortality of those w/RHR less than 50bpm.  So just how fast is your heart beating now?

By the way, if you're worried and wondering about what you can do to lower your RHR and thus your mortality, aerobic exercise is the key, that is, after you've been cleared by your family physician to engage in exercise.  So don't just sit there, do something!

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