Most everyone knows that a low resting heart rate is a positive indicator of a healthy heart, one that is capable of moving blood around the body in an efficient manner . In fact, some world-class athletes, Lance Armstrong for example, have resting heart rates as low as 32-34 beats per minute where the average person is more likely in the 60s or 70s.
However, as I have recently discovered, resting heart rate is only a small part of the story.
“When you lose flexibility and responsiveness, you die. Perhaps the most spectacular example of this is heart-rate variability, i.e. the amount by which the heart rate alters from beat to beat. This is, possibly, the single most sensitive indicator of a healthy heart, and a loss of beat-to-beat variability is one of the most powerful single indicators of the risk of dying of heart disease.” From The Great Cholesterol Con by Dr. Malcolm Kendrick (Amazon link).
I read that statement and it stopped me in my tracks. What? I thought a healthy heartbeat was even and steady, like a metronome. With a little research, I quickly discovered that my simple perception of how a heart works was completely wrong. A heart that beats like a metronome is indeed as Dr. Kendrick describes – a powerful indicator for death.
Hopefully this post will be a useful introduction to heart rate variability (HRV). All comments, corrections, and criticisms are 100% welcome.
How the Nervous System Controls the Heart
The autonomic nervous system (ANS) is a subcomponent of the central nervous system and is responsible for involuntary functions. It is composed of two branches, the parasympathetic (vagal) nervous system and the sympathetic nervous system, each working in opposing directions to maintain a balanced state of control over internal organs. Parasympathetic restricts or slows down how organs function and the sympathetic increases or speeds up the same organs.
Specific to the heart, parasympathetic input modulates heart rate on a time scale of a few seconds (mostly in tune with respiration) while sympathetic input tends to act on a slower time scale, typically around 30 seconds . During inhalation, the sympathetic nervous system takes control and increases heart rate to take advantage of the extra oxygen in the lungs. During exhalation, parasympathetic dominates, lowering the heart rate.
At rest, both the parasympathetic nervous system and sympathetic nervous systems are active, but the parasympathetic will overshadow the sympathetic (at least in a healthy, well rested individual). In order to create an ideal state within the body and in response to the many forms of external stimulation, this balance between the two branches of the ANS is constantly being adjusted .
Disease and Decreased Heart Rate Variability
It is only in the past 50 years that doctors have come to understand the variable nature of how the heart beats. Specifically, interest in HRV began in 1965 when researchers found that reduced beat-to-beat intervals indicated fetal distress prior to any meaningful change in actual fetal heart rate .
In 1977 doctors found that people who had had a myocardial infarction (heart attack) and had reduced HRV were at a much greater risk for “post-infarction mortality” . Scores of additional studies have found similar results.
“Diminished heart rate variability is associated with high sympathetic tone and an increased mortality rate in heart failure cases.” 
The following suggests that there is a relationship between HRV, high cholesterol, and cardiovascular health risk.
“A statistically significant correlation between chronic (tonic) distress (expressed by long-term depression of heart rate variability-HRV) and the increase of the serum lipid level was found in the experimental group (39 clinically healthy subjects with hypercholesterolaemia; 16 women and 23 men; mean age 42.4 +/- 2.45 years).” 
Diabetes researchers have found a similar pattern.
“Cardiac autonomic impairment appears to be present at early stages of diabetic metabolic impairment, and progressive worsening of autonomic cardiac function over 9 years was observed in diabetic subjects.” 
“The importance of our findings is that abnormal HRV identifies people with cardiovascular disease, irrespective of diabetes status, that may have autonomic neuropathy. HRV analysis is easily implemented by primary health care providers and has the potential to lead to improved health care by reducing inequity in rural areas and specifically addressing cardiovascular complications associated with diabetes.” 
What about Cancer?
While research looking at the relationship of HRV and cancer seems to be more limited, there have been studies using HRV as a way to estimate remaining lifespan in terminally ill cancer patients [9,10].
Factors that Modify Heart Rate Variability
Can we do anything about it? From everything I have read, vigorous exercise appears to be the single biggest factor in improving (increasing) HRV.
“For men, total leisure-time physical activity and both moderate- and vigorous-intensity activity were associated with higher HRV independent of age.” 
Exercise appears to be positively correlated with improved HRV, but what type of exercise? When reading the word “aerobic” below, think of activities like CrossFit or Mark Sisson’s Primal Blueprint for Fitness – vigorous exercise that raises the heart rate.
“In sedentary, healthy young adults, aerobic conditioning but not strength training enhances autonomic control of the heart.” 
“There were significantly higher levels of vagal HRV indices in the most active group compared with the least active group. Regression analysis revealed that the number of bouts of vigorous Physical Activity (PA) undertaken was the best predictor of the vagal HRV indices assessed. This study suggests that vagal modulation is enhanced with high levels of PA and that it is the number of bouts of vigorous PA that is most closely associated with cANS function.” 
Here are the key results of a study that looked at a six month aerobic workout program.
“Before exercise training, the older subjects had a 47% lower HRV at rest compared with the young subjects …. Training decreased the heart rate at rest in both the older (-9 beats/min) and the young groups (-5 beats/min). Exercise training increased HRV at rest (p = 0.009) by 68% in the older subjects … and by 17% in the young subjects…. “ 
Another study I looked at compared high-intensity sprinting to aerobic exercise requiring similar energy expenditure. The study demonstrated that “Parasympathetic reactivation is highly impaired after Repeated Sprints (RS) exercise and appears to be mainly related to anaerobic process participation. “  In other words, after highly anaerobic exercise, it takes a long time for HRV to recover.
The caveat to this study is that authors only observed parasympathetic reactivation for 10 minutes following completion of exercise. This time scale may not have allowed enough time for full parasympathetic reengagement following high-intensity sprints.
Interestingly, there are some relatively easy things that can be done to reactivate the parasympathetic nervous system. Splashing cold water on your face or a cold shower will do the trick [16,17]. I guess those people who jump into freezing cold water aren’t so crazy after all.
Stress, especially chronic stress, can be a huge factor in decreased HRV. And significant overtraining/exercising (think chronic cardio) will present itself similarly to chronic stress. Tracking daily changes in HRV can be a great indicator of an overtrained or stressed condition.
So, exercise, but don’t overtrain, and make sure to get plenty of sleep .
This advice seems pretty similar to that found in Mark Sisson’s Primal Blueprint (Amazon link)
Finally there were a number of studies I looked at that were quite interesting, but a bit beyond the scope of this post. Here are a few:
- A low carbohydrate diet affects autonomic modulation during heavy but not moderate exercise
- Dietary Fish and Omega-3 Fatty Acid Consumption and Heart Rate Variability in US Adults
- Dietary effects on heart rate variability in salt-sensitivity of blood
- Heart rate dynamics during three forms of meditation