Billion heart beats M.V.N. Murthy, The Institute of Mathematical Sciences, Chennai A beating heart is a sign of life in animals. Any other organ may fail, including the brain, but as long as the heart beats the animal is not dead. It is a vital sign of life along with body temperature and respiration. There is more to it than just being a vital sign--- increasing scientific and observational evidence indicates that the number of times a heart beats during the life time, on the average, may be similar in a wide variety of animals, small or big. This is sometimes called the {\it heartbeat hypothesis}. Support for this hypothesis comes from many recent studies in mammals as well as in other animals including insects. To understand this, consider what is usually referred to as basal Heart Rate (HR). This is the number of times the heart beats per minute (bpm) measured when the person or animal is in a state of rest. In humans this is easily measured by holding a major artery, say next to your thumb on your wrist. If you do this many times over, across many persons and average it, you find that it is approximately around 70 bpm for humans. Usually in humans this is done when the person is resting since any physical activity increases the HR. BOX on Life expectancy Life expectancy of an animal population is an average. It is usually defined from birth onwards and reflects the average life time of the population. Many animals die at or immediately after birth whereas some live for a long time. For example, Asian elephants are known to live for up to 80 years. However, the life expectancy of elephant population as a whole may be around 30 years since many die much earlier due to various causes which nowadays also includes conflict with humans. Whales and tortoises are known to live much longer, with a maximum life span around 200 years. The life expectancy is therefore an average over the whole population. In humans the world average is around 67 years at present, whereas an individual may live up to 100 years or more. The average also includes many babies which die at child birth or later. The average for men is less than women though it is known that the average HR for women is slightly higher than for men. These numbers also vary from region to region and from country to country. END OF BOX It is much more difficult to measure HR in the case of animals. Scientists use radio-telemetry to measure parameters on animals. The heart rate is measured over a period of time, say 24 hours, and then averaged to give the number of beats per minute.We will discuss these measured rates and their implications for mammals where extensive measurements have been done. Let us start with some examples. In mice the HR is about 550 bpm and they live on the average for about -2.5 years. There are 365 days in a year and 24 hours in a day with 60minutes per hour so each year equals 525600 minutes. So the total numberof heart beats over the life time of mice is given by HR x Lifetime inminutes = 550x(2 or 2.5)x525600 = (6 or 7)x 10^8,which is close to a billion heart beats (10^9) per life time. On the other hand an elephant has a HR of about 30-35 bpm and its life span, measured from birth onwards, is about 30 years on the average. Therefore the total number of heart beats over the life time for an elephant is around 5-7x10^8. Interestingly, the mouse and an elephant are very different in many aspects; for example, the elephant weighs more than 3000 times the mouse, but they seem to have similar values of total number of heart beats. If this appears like a statistical coincidence, look at Figure 1. In this figure, you can see the plot of HR vs life expectancy. (These numbers may be slightly different depending on the source; these numbers are from Levine, 1997). Life expectancy is simply the average life time of a mammal type measured from birth onwards. While all the mammals fall on a straight-line as shown, the average human HR and life expectancy seems to be way off the chart. Multiplying the HR in bpm with life expectancy the total beats per lifetime may be plotted as shown in Figure 2. As you can see beats per life time is very similar for all mammals. It turns out that the number of heart beats per life time among mammals, not including humans, is remarkably similar and averages to (7.3+-5.6)x10^8. The number of heart beats per lifetime for humans is a little bit more. Notice that these mammals vary in size, weight and many other parameters overs several orders of magnitude. But, for all practical purposes we may say all mammals have close to a billion heart beats per life time!! It is this observation that has attracted much attention from scientists working on the phenomenon of aging in recent times. There is one puzzle in the data given in Figures 1 and 2. While all the mammals have their HR along a line, the humans have a higher HR but tend to live longer than those mammals whose HR is even less than humans. For example elephants live about 30-35 years with a HR of around 30 bpm, but humans with an average HR of around 70 bpm tend to live longer. The world average is around 65 years (also in India) and account for almost 3 billion heart beats per life time. This is nearly three times the number of heart beats per life time compared to other mammals. Therefore, are humans biologically very different from other mammals? It appears that for most of the history of human existence, humans were not very different from other mammals. This is shown in Figure 3 (from Zhang and Zhang, 2009). From the time of early man such as Neanderthals who had a life expectancy of only about 20 years, the number doubled by the 20th century. It is clear that, for a majority of human history, almost until the beginning of the 20th century the life expectancy of humans was hovering around 20-30. This value is also found for tigers, giraffes which have a similar HR to humans. Life expectancy has increased to the present levels only from the late 19th century. This increase is mainly due to the developments in the technology of disease prevention and treatment, especially those connected with the heart. During the early history of humans, several diseases which were fatal are not deemed fatal any more due to a better understanding of prevention and treatment. Discovery of new drugs, methods of resuscitation, invention of pacemakers, have all helped in preventing fatalities due to heart failure and increasing life expectancy. The relation between life span and HR seen above reflects the phenomenon in which HR is a measure of the metabolic activity. Therefore is it possible to extend human life by slowing the Heart Rate? There are some indications but we will have to wait for more detailed studies of large populations and clinical studies. References: Herbert J Levine, "Rest Heart Rate and Life Expectancy", Journal of American College of Cardiology, 30, 1104 (1997) Gus Q Zhang and Weiguo Zhang, "Hear rate, life span and mortality risk", Ageing Research Reviews, 8,52 (2009).