Does your heart rate change from time to time?

Does your heart rate change from time to time?

Our heart rate is not a fixed figure from the beginning of our life to the end of life. It is constantly changing, second to second as well as year on year. Many of you would have noticed your heart pounding when under stress or during exercise. This is one of the commonest short term variability of our heart rate which we are aware. But do you know that your heart rate increases when you breathe in and decreases when you breathe out?

When you breathe in, more blood returns to the right side of heart due to the suction effect of negative pressure formed in the chest cavity. More blood reaches the lungs as lung capacity also increases when you breathe in. For the same reason, less blood goes out of the lungs when you breathe in as more blood is stored in the lungs. This leads to lowering of the amount of blood returning to the left side of heart when you breathe in.

Lower blood volume in left upper and lower chambers mean that lower amount is pumped out into aorta, the large blood vessel which takes oxygenated blood to the whole body. When less blood is pumped out during each beat, the brain increases heart rate by a reflex mechanism to improve the amount of blood reaching the various organs. That is why heart rate is higher when you breathe in. The reverse sequence of events operates when you breathe out so that heart rate comes down during that period. This change in heart rate is technically called respiratory sinus arrhythmia as the changes occur in the rate at which the sinus node fires. Sinus node is the natural pacemaker of the heart, located in the right upper chamber, which regulates the normal heart rate. Sinus node is under the control of the brain through the nervous system.

Increase in heart rate during exercise and emotional stress is also mediated through the firing rate of the sinus node. Exercise increases the metabolic requirement of the body. More blood is required by the exercising muscles both for oxygen and nutrients. Signals from the muscles go to the brain which increases the heart rate as well as the breathing rate, proportional to the grade of exercise. Similar signals are also sent by the brain during emotional stress to increase heart rate to cope up the body to the stress.

These changes in heart rate will be blunted by regular training. Trained athletes have a lower resting heart so that they do not have an undue rise in heart rate while running fast. This is known technically as athlete’s bradycardia. Bradycardia means slow heart rate. Similarly, increase in heart rate with stress may come down as you develop more endurance to stress. At the same time some persons with undue anxiety may develop excessive rise in heart rate with stress, sometimes leading to panic attacks with fast pounding heart beats.

Another important change in heart rate is the decrease in heart rate as we grow up. When we are born from the mother’s womb, our heart rate is around 140  per minute. By adult life, our normal heart rate comes within the range of 60 to 100 per minute. Maximum predicted heart rate during peak exercise is 220-age. That means that the peak heart rate with maximal exercise also comes down with age. For example, peak exercise heart rate in a 20 year old will be 200 per minute while it will be only 160 per minute in a 60 year old.

Heart rate is also dependent on your body temperature. In fever there is a progressive rise in heart rate with rise in temperature. The rise is roughly 10 beats per minute for every one degree Fahrenheit. Heart rate increases during major illnesses as a part of stress response. If there is a fall in blood pressure with any illness, that also usually causes an increase in heart rate by a reflex mechanism involving the brain.