Why does maximum heart rate drop with age?

Researchers at the University of Colorado have new insight into the age-old question of why maximum heart rate (maxHR) decreases with age. This decrease in maxHR not only limits the performance of aging athletes but it is also a leading cause for nursing home admittance for otherwise-healthy elderly individuals who no longer have the physical capacity required for independent living. We say we're just getting old and slowing down, but exactly what is it that is slowing down?

15 oct 2013--Everybody knows that aerobic capacity decreases with age. You know that chart in your gym that shows your target heart rate decreasing as you get older? Well, that's not a senior discount to let the elderly get off easy on their treadmill workouts. It's because older hearts simply can't beat as fast as younger hearts. So the older person who's doing 120 beats per minute is probably working harder—at a higher percentage of maximum heart rate—than the younger person who is at 150 beats per minute.

A new study by a group led by Catherine Proenza, PhD and Roger Bannister, PhD from the University of Colorado School of Medicine reports that one of the reasons for the age-dependent reduction in maximum heart rate is that aging depresses the spontaneous electrical activity of the heart's natural pacemaker, the sinoatrial node.

A dissertation from Eric D. Larson, a graduate from Proenza's lab in the Department of Physiology and Biophysics, is described in the article. Larson said, "I utilized a method to record ECGs from conscious mice and found that maximum heart rate was slower in older mice, just as it is in older people. This result wasn't unexpected. But what was completely new was that the slower maxHR was because the individual pacemaker cells—called sinoatrial myocytes, or 'SAMs'—from old mice just couldn't beat as fast as SAMs from young mice."

The researchers recorded the tiny electrical signals from the isolated cells and found that SAMs from old mice beat more slowly, even when they were fully stimulated by the fight-or-flight response which can be observed in these individual cells. The slower beating rate was due to a limited set of changes in the action potential waveform, the electrical signal that is generated by the cells. The changes were caused by altered behavior of some ion channels in the membranes of the older cells. (Ion channels are proteins that conduct electricity across the cell membrane. Imagine a balloon with little tiny pinholes that open and close to let the air in and out; ion channels are like the pinholes.)

Like most initial discoveries in basic science, this study opens many more questions and avenues for further research. But the significance of the study is that it raises the possibility that sinoatrial ion channels and the signaling molecules that regulate them could be novel targets for drugs to slow the loss of aerobic capacity with age. In the meanwhile, Proenza notes that "although maximum heart rate goes down for everybody equally, regardless of physical conditioning, people can improve and maintain their aerobic capacity at all ages by exercising."

This study will be published in the Oct. 14 Proceedings of the National Academy of Sciences.

More information: Depressed pacemaker activity of sinoatrial node myocytes contributes to the age-dependent decline in maximum heart rate, www.pnas.org/cgi/doi/10.1073/pnas.1308477110

Provided by University of Colorado Denver

Higher Resting Heart Rate Linked to Diabetes and Mortality in Older Age

October 31, 2007 — Higher resting heart rate (HR) was associated with diabetes and mortality in older age, according to the results of a study reported in the October 24 Online First issue of Diabetes Care.
"Given that estimates of autonomic function and fitness are associated with the development of insulin resistance and hyperglycemia in population studies and that heart rate is correlated with these physiologic measures, it is biologically plausible that higher heart rate is associated with the development of diabetes," write Mercedes Carnethon, PhD, from the Feinberg School of Medicine, Northwestern University in Chicago, Illinois, and colleagues. "In a sample of young and middle-aged adults, we tested the hypothesis that a faster resting heart rate was associated with a greater likelihood of experiencing diabetes-related morbidity or mortality in older age (after age 65 years)."
In the Chicago Heart Association Detection Project in Industry Study, resting HR was measured at baseline, from 1967 to 1973. Using Medicare billing records for 14,992 participants aged 35 to 64 years who were free from diabetes at baseline, the investigators identified diabetes-related hospital claims and non–hospital-based diabetes expenses from 1992 through 2002. They also determined diabetes-related mortality from 1984 to 2002 using National Death Index codes 250.XX (International Classification of Diseases [ICD]-8 and -9) and E10–E14 (ICD-10).
Diabetes-related hospital claims occurred in 1877 participants after age 65 years, and 410 participants had some mention of diabetes on their death certificate. For every 12-bpm higher baseline HR, the odds of having a diabetes-related claim was approximately 10% higher (odds ratio [OR], 1.10; 95% confidence interval [CI], 1.05 - 1.16), after adjustment for demographic characteristics, cigarette smoking, and years of Medicare eligibility.
This association became nonsignificant after adjustment for body mass index (BMI) and postload glucose levels at baseline. In adults aged 35 to 49 years at baseline, higher HR was associated with diabetes-related mortality after adjustment for postload glucose levels and BMI (OR, 1.21; 95% CI, 1.03 - 1.41).
"Higher resting HR is associated with diabetes claims and mortality in older age, and is only due in part to BMI and concurrently-measured glucose," the study authors write.
Limitations of the study include measured glucose levels lower than would be obtained with current standard measurement techniques, possible underestimate of the number of participants with fasting glucose levels of more than 11.1 mmol/L who had undiagnosed diabetes, diagnosis of diabetes at follow-up based on Medicare claims data, findings only generalizable to adults who live to older age, reliance on hospital visit diagnosis codes to identify diabetes, inability to evaluate the role of changes in covariates with time on the association between HR and diabetes, and no measurement of physical activity available for statistical adjustment.
"In our study of a large sample of middle-aged adults, baseline heart rate (measured up to 35 years before) was associated with diabetes diagnoses and mortality in older age," the study authors conclude. "Our findings provide further evidence that higher heart rate is associated with adverse morbidity and mortality from a number of causes including diabetes."
The National Heart, Lung, and Blood Institute funded this study.
Diabetes Care. Published online October 24, 2007.