Heart Beats and Life Span – Do we have a finite number of heart beats?
How many times have you read a story about heart disease that begins with the statement: "the heart is a muscle?" It’s enough to put you to sleep before you hit the period at the end of the sentence. Well, the heart is a muscle, but it is unlike every other muscle in your body. Skeletal muscles, like your biceps, hamstrings or masseters (the jaw muscles you chew with), function very differently than a cardiac muscle.
Whereas the heart pumps non-stop until death, managing over 2 billion contractions by age 70, the skeletal muscle simply can’t do that. Research has established a relationship between the number of heart beats and the lifespan of mammals, birds and other living things. The concept is that our lifespan is limited by the number of heart beats we “use up.” Looked at another way, we have only so much energy to expend, and if we expend too much with too high a heart rate, we die earlier. So, do we have a predetermined number of heart beats in our lifetime? Not exactly.
In fact, this relationship, between lifespan and heart beats, is known as an epiphenomenon (no snoring please). In medicine, an epiphenomenon means that what we are observing, in this case an apparent relationship between lifespan and the number of heart beats, is actually a secondary result of the true limitation of our lifespan. Just because most people who get into car accidents are wearing pants doesn’t mean wearing pants causes car accidents. So just because all mammals have a similar number of heart beats during their life doesn’t mean the number of heart beats is the real limitation to living forever.
Heart beats represent energy expenditure. Life span is not determined by the number of heart beats, but the number of heart beats represents the amount of energy expended by mammals; it is a marker of our energy expenditure of cells, which still makes it useful. Many studies have demonstrated that lower resting heart rates are associated with a longerlife. In patients with congestive heart failure, a higher heart rate is associated with a worse prognosis. The same is true after heart attacks. One of the treatment goals in heart attack and heart failure patients is a reduction of energy requirements of the failing heart, by reducing heart rates to 45-55 beats per minute using beta blocker drugs.
Average heart rates can be discussed as an hourly or daily average. Doctors use 24 hour monitors (called Holters) to determine the average. Daytime heart rates are higher than nocturnal heart rates. There is no single normal heart rate just as there is no single average height. There is a range which depends on age and fitness levels. In some people an average heart rate of 45 might be normal. Average heart rates above 100 should be discussed and possibly investigated further. Though incredibly non-specific, possible causes of elevated heart rates are myriad and include anemia and thyroid disease.
How long do different mammals live?
HR – heart rate, ~ - approximate, BPM – beats per minute
In general, the smaller the animal (as in rodents for example), the faster the heart rate and the shorter the lifespan as they rapidly use their allotted number of beats, or rather energy expenditure. As the above graphs show, humans are the exception, seemingly with more heart beats available to us than other mammals.
So does this mean that if we slow down heart rates it will keep us living longer? Though I wouldn’t dismiss the notion outright, and it is an attractive one, no study has yet been done to prove it. Unlike recommending cholesterol drugs to lower LDL cholesterol, it’s premature and dangerous to apply the same concept to such a complex concept as heart rate. The best way to lower your average heart rate is to simply exercise.