Every time you see a doctor or go into a clinic, the first thing they do is check your heart rate. They’re specifically checking your resting heart rate, which is how fast your heart beats without stimuli like exercise. While you wait for the machine's beep after it checks your heart rate and blood pressure, you don’t even think about the value these metrics tell your caregiver about your health. Our heart rates are influenced by almost everything: our fitness level, hormones, age, and even our mental well-being. The resting heart rate can give an indication to your doctor as a warning sign. If something is going on in your body, then your resting heart rate could be elevated.

Despite the many factors that can influence resting heart rate, the nervous system is its main controller. Specifically, the autonomic nervous system, which monitors and controls things we are not conscious of, like heart rate, muscle contractions during digestion, and breathing. It can be further divided into the sympathetic and parasympathetic nervous systems. While they have big names, their functions are quite easy to comprehend. The sympathetic nervous system controls our fight-or-flight responses.

Whenever we experience a stressor like exercise, work, or even relationships, the sympathetic nervous system prepares the body to move. Meanwhile, the parasympathetic nervous system controls our rest-and-digest responses. You have experienced this part of the nervous system in action after you eat a hefty meal. The parasympathetic nervous system recognizes that you just ate and then makes you feel relaxed and tired so the food can be properly digested and absorbed.

The resting heart rate can be an indicator of the balance between sympathetic and parasympathetic. When the heart rate is low, it’s a sign of good cardiovascular fitness and overall health. When the heart rate is high, however, it can lead to complications like high blood pressure, heart failure, and even cognitive decline. Most of all, an elevated resting heart rate can impact the functioning of the immune system. Variability of heart rate influences the development of inflammation and increases the risk of infections. However, researchers have not fully determined how elevated resting heart rate influences immune system functioning. Fortunately, a group has found a connection between the two by looking at immune cells known as natural killer cells.

Natural killer cells, aside from their cool names, are valuable players in the immune system. They function in innate immunity, which focuses on quick reactions to foreign invaders of the body. However, natural killer cells don’t target bacteria and viruses directly; they target and kill the cells that have been infected by them. Not only do they target infected cells, but natural killers also target cells that have become cancerous. In essence, they act as bouncers for the body, checking every cell to determine if it is healthy or will cause disease. With this cross-checking ability, natural killer cells are pivotal in coordinating other immune responses throughout the body.

To show the relationship between resting heart rate and natural killer cells, seven thousand and five hundred patients were involved in the study and had their resting heart rates measured. The researchers then created four groups based on average resting heart rate: C1 with less than 60 beats per minute (bpm), C2 with 60-70 bpm, C3 with 70-80 bpm, and C4 at over 80 bpm. The activity of natural killer cells from each group was then analyzed by retrieving blood samples and stimulating the natural killer cells with a trigger. The researchers analyzed the level of inflammatory chemicals to determine natural killer cell activity. The higher the levels, the more active the natural killer cells were.

After natural killer cell activation, the researchers discovered that those with higher elevated heart rates, the C3 and C4 groups, had the least active natural killer cells. They also showed that participants in the C4 group had other indicators for poor metabolic and cardiovascular risks, such as higher blood pressure and insulin resistance (a sign of diabetes). Upon further analysis, the researchers believe that a higher elevated heart rate increases the amount of cortisol (the stress hormone) in the bloodstream.

This would mimic a fight-flight response by the sympathetic nervous system, disrupting the immune system as a survival method. While the ideas haven’t been confirmed, this study sheds light on the interaction between the nervous and immune systems by showing the link between heart rate and immune cell function. It also highlights the importance of maintaining a healthy resting heart rate, as it impacts more than people believe. So, the next time you get a physical, the heart rate monitor can give you a heads-up on many things within your cells that you wouldn’t know until it’s too late.

References

Colangelo LA, Yano Y, Jacobs DR Jr., Lloyd-Jones DM. Association of resting heart rate with blood pressure and incident hypertension over 30 years in black and white adults: the CARDIA study (2020).
Nanchen D, Stott DJ, Gussekloo J, Mooijaart SP, Westendorp RG, Jukema JW, et al. Resting heart rate and incident heart failure and cardiovascular mortality in older adults: role of inflammation and endothelial dysfunction: the PROSPER study (2013).
Mao M, Liu R, Dong Y, Wang C, Ren Y, Tian N, et al. Resting heart rate, cognitive function, and inflammation in older adults: a population-based study. Aging Clin Exp Res (2023).
Adam J, Rupprecht S, Künstler ECS, Hoyer D. Heart rate variability as a marker and predictor of inflammation, nosocomial infection, and sepsis - A systematic review. Auton Neurosci (2023).
Morvan MG, Lanier LL. NK cells and cancer: you can teach innate cells new tricks. Nat Rev Cancer (2016).