Heart rate is defined as the number of contractions performed by the heart per minute (sixty seconds). One single contraction is called a single beat.

The amazing fact is that the heart of a human embryo begins to beat at approximately 21 days after conception in the mother’s womb.

Heart rate is one of the vital signs of our bodies. Though a heartbeat may seem a trivial body sign, it provides important insight into our health.

The heart rate remains constant while a person is at rest. However, it is greatly influenced by physical activity, emotional states, and other stimuli.

What is a normal heart rate?

The range of a normal heart rate for adults is 60 to 100 beats per minute. This is the rate a heartbeats while the person is resting or relaxed.

heartbeats/rate towards the lower end of the figure scale is considered to signify an efficient heart function. For example, an athlete who is well-trained with better cardiovascular fitness may have a resting heart rate of forty beats per minute.

The heart rate is around 40-50 beats per minute while we sleep. This is considered normal.

It should be noted that heart rate is not a consistently stable value. It increases and decreases as per the body's requirement for nutrients and oxygen.

Since the blood volume remains the same under normal circumstances, the only way a heart can meet the bodily requirements, for example, increased oxygen while exercising, is to increase its rate. This is brought about by an interplay of several factors (discussed later).

Heart rhythm is an essential feature of heartbeats/rate. The heart beats with a specific rhythm, which is common in all. Consistent deviation of heart rate from its usual rhythm indicates serious heart issues and must be addressed promptly.

How is the heart rate controlled?

The human heart has four chambers;

• two upper chambers called atria
• two lower chambers called ventricles

A natural pacemaker controls heart rhythm called the sinus node or the sinoatrial node. It is located in the right atrium, and it functions as a natural pacemaker of the heart.

The sinus node is programmed to produces electrical impulses that normally start each heartbeat-like an ignition. When these muscles arise in the atria, it causes the muscles of the chamber to contract, resulting in propelling the blood into the ventricles.

The electrical impulses reach a cluster of cells called the atrioventricular (AV) node via the heart's electrical conduction system. The AV node acts to slow down an electrical impulse before transferring it to the ventricles.

The slowing down of an electrical impulse by the AV node causes a slight delay. This allows the ventricles to be filled with blood coming from the atria. By the time the next impulse is ready to reach ventricles, they are ready to contract and pump the blood forwards. 

This process goes on smoothly in a normally functioning heart.

If the electrical impulses were to stimulate the atria and ventricles simultaneously, it would lead to an inefficient filling of the heart followed by an equally unproductive blood pumping.

Under normal circumstances, the heart rate is controlled by the pacemaker sinoatrial node.

However, the action potential generated by the SA node is influenced by sympathetic and parasympathetic nervous systems via two nerves;

• The Accelerans nerve is responsible for transferring sympathetic input to the heart. It does so by releasing a hormone, norepinephrine, into the cells of the sinoatrial node (SA node)
• The Vagus nerve caters to parasympathetic input to the heart. This is brought about by the release of acetylcholine hormone onto sinoatrial node cells.

The stimulation of the Accelerans nerve increases heart rate.

While stimulation of the Vagus nerve decreases the heart rate.

At rest, both the cardio-acceleratory (sympathetic) and cardio-inhibitory (parasympathetic) pathways act on the sinus node to generate an autonomic rhythm.

Even while this happens, the vagal tone is dominant that keeps a check on the SA node.

If there were no checks on the SA node, a sinus rhythm of approximately 100 beats per minute would be generated at rest.

Abnormals heartbeats/rate

Abnormal heartbeats/rateis are when the heart beats slower or faster than the normal heart rate. Such heartbeats/rate is called arrhythmia of the heart. They feel like fluttering or racing of the heart.

Arrhythmia is of two types;

• When the heartbeat is slow, it is called bradycardia. A heart rate below 60 beats per minute is bradycardia.
• When the heartbeat is faster, it is called tachycardia. A heart rate faster than 100 is tachycardia.

It is important to note that heart rate may become faster in response to external stimuli or physiological phenomenon. This is completely harmless.

For example, running and drinking tea may raise your heart rate. However, this is a transient rise, and the heart starts to beat normally with time.

Then the 'normal' range for heart rate is also varied between ranges of 60-100.

However, an unusually high or low heart rate that persists may indicate an underlying problem. Figures that indicate arrhythmia and call for a doctor’s appointment include;

• A resting heart rate consistently above 100 beats per minute
• A resting heart rate consistently below 60 beats a minute (especially if you are not an athlete and other signs or symptoms are also present as fainting, dizziness, or shortness of breath)

Resting heart rate is related to the mortality rate in clinical settings. For example, the risk of death and disease increases by 1.22% (hazard ratio) when the heart rate goes beyond 90 beats per minute.  

A study showed that the mortality rate of patients with myocardial infarction increased from 15% to 41% of their admission heartbeats/rate was greater than 90 beats per minute.

Normals heartbeats/rate according to age?

A resting heart rate is also called basal heart rate. It is defined as the heart rate measured in;

• Awake state
• At temperate ambiance
• In the absence of any recent emotional stimulation as stress or surprise
• With no elements of physical exertion
• In the absence of any stimulant intake

The normal range for resting heartbeats/rate is 50-90 beats per minute.

Recently, according to a survey, the majority (98%) of cardiologists designated the values of 50-90 beats per minute as a desirable resting heart rate.

Setting such low benchmarks for heart measurement rules out the inclusion of otherwise fit individuals with an abnormal heartbeats/rate.

The average heart rate varies in children and adolescents who tend to have faster heart rates than average healthy adults.

The following table shows the average heart rates at rest in different age groups.

Factors affecting the resting heart rate

Balancing the heart rate is a complex interplay between different influences on the cardiac brain center coming from receptors present in the body like;

•          Proprioceptors

•          Baroreceptors

•          Chemoreceptors

•          Limbic system

Some factors affecting resting heart rate are following;

. Hormones

Hormones like epinephrine, norepinephrine, and thyroid hormones also play their role.

Catecholamines, epinephrine, and norepinephrine constitute the body's fight or flight mechanisms. These hormones are directly involved in increasing the heartbeats/rate by stimulating the specific receptors (beta-1 adrenergic) and increasing the influx of sodium and calcium into the heart cells.

Increased levels of thyroid hormones also increase the heart rate. Their effect is more prolonged as compared to the effect of catecholamine. This is because thyroid hormones stimulate the beta-adrenergic response. In addition, the hormones enter the cardiac cells and bring about changes at genetic levels.

• Minerals

The levels of minerals like calcium, potassium, sodium also affect the heartbeats/rate.

Increased calcium levels increase the heartbeats/rate as well as contractility. This effect is so pronounced that extreme cases of increased calcium levels can result in cardiac arrest.

Sodium and potassium have the opposite effects of calcium. Low potassium levels increase the heart rate and the force of contraction of the heart and vice versa.

Initially, both low and high sodium levels may lead to tachycardia. Persistently high sodium levels lead to heart fibrillation (irregular and rapid heart rate), leading to zero cardiac output and cardiac arrest.

Similarly, severe cases of low sodium levels decrease the heartbeats/rate and may trigger arrhythmias.

Low potassium levels may also lead to arrhythmias (irregular heart rate), whereas high potassium levels result in heart failure due to a weak and flaccid heart.

• Stimulants

Caffeine and nicotine are stimulants of the brain, including the cardiac center. Nicotine is also addictive.

Caffeine increases the rates of depolarization at the SA node. This shifts the electric charge distribution from a negative state towards a positive. The overall result is an increase in firing of the SA node and an increased heart rate.

Nicotine increases the input of sympathetic impulses to the heart, increasing the heart rate. Smokers tend to have higher heartbeats/rate.

Alcohol and cannabis also contribute their fair share to increased heartbeats/rate.

• Oxygen saturation levels

The heart is a muscle dependent on aerobic metabolism for energy. That is, the organ requires oxygen to carry out its function.

In severe insufficiency of oxygen (hypoxia), heart rate decreases due to insufficient nutrient supply. This may be seen in anemia (low hemoglobin levels).

• pH balance

Normal blood pH lies in the range of 7.35–7.45. Acidosis is when pH is lower than 7 (with excess hydrogen ions), while alkalosis is when the pH is higher than 7 (with few hydrogen ions).

Body enzymes work under pH values specific to them. These enzymes play an essential part in regulating and catalyzing chemical reactions in the body.

Variations in pH level affect the rate of chemical reactions in many ways. This can have an impact on heart rate.

• Body temperature

Increased body temperature (hyperthermia) increases the heart rate and vice versa.

When the body temperature falls below a certain level (hypothermia), the body diverts the blood circulation to essential organs of the body while shutting off supply to others. Persistent hypothermia may result in a complete stoppage of the heart.

Hyperthermia and hypothermia, if left untreated, result in cardiac arrest and death as the enzymatic activity of the body fails.

• Emotional state

heartbeats/rate is affected by one's emotional states like stress, anxiety, and surprise. All of these events cause an increase in heart rate. This is a physiological response and is temporary, subsiding as the occasion resolves.

• Fitness levels

Heartbeats/rate is also physiologically low in well-trained athletes and fitness enthusiasts. Low heart rate at rest in athletes is not labeled abnormal provided the individual has no associated signs and symptoms (shortness of breath, dizziness, etc.).

Abnormalities of heartbeats/rate

heartbeats/rate abnormalities are called arrhythmias. These conditions depict themselves as tachycardia and bradycardia.  

Sometimes you may experience an extra heartbeat called a premature beat.

Heart rate abnormalities can complicate your heart health, for example, instigate a stroke. They weaken the heart in the long run, resulting in heart failure. So a timely diagnosis and treatment are crucial.

Pathological causes of increased heartbeats/rate - tachycardia

Tachycardia is when the heart rate exceeds 100 beats per minute at rest. An accelerated heartbeats/rate is standard in pregnancy, post-exercise, and altered emotional state.

Some disease conditions linked to increased heartbeats/rate include;

• Anemia
• Atrial fibrillation
• Atrial flutter
• Atrial tachycardia
• An early manifestation of circulatory shock
• Hypoglycemia
• Hypovolemia
• Hyperthyroidism
• Hyperventilation
• Pain
• Sleep deprivation
• Supraventricular tachycardia
• Ventricular tachycardia
• Wolff–Parkinson–White syndrome
• Sinus tachycardia
• Drug or alcohol withdrawal syndrome
• High body cholesterol levels
• Uncontrolled diabetes

Tachycardia is also associated with the use of some medicines;

• Tricyclic antidepressants
• Nefopam
• Rarely, Opioids

Tachycardia can cause complications like fainting. Increased heart rate causes the blood to flow too rapidly in vessels. This results in friction within the vessel walls. In case of an already damaged vessel wall, it can result in blood clot formation in the vessels.

Pathological causes of decreased heart rate - bradycardia

Low heart rate typically does not cause symptoms until the measurement drops below 50 beats per minute. The symptoms of bradycardia include fatigue, weakness, dizziness, sweating, and in severe cases, fainting.

Some causes of pathological bradycardia include;

• recreational drug use or abuse
• metabolic or endocrine issues as hypothyroidism
• electrolyte imbalance
• prolonged bed rest
• autoimmunity
• acute or chronic ischemic heart disease
• vascular heart disease
• valvular heart disease
• degenerative primary electrical disease
• use of beta-blocker drugs

Symptoms of heartbeats/rate abnormalities

Irregular heart rate is often asymptomatic. Your doctor may find them on routine examination.

• A fluttering sensation in the chest
• Tachycardia presents as a racing heartbeat
• Bradycardia presents as a slow heartbeat
• Chest pain
• Shortness of breath
• Anxiety
• Fatigue
• Lightheadedness or dizziness
• Sweating
• Fainting or a feeling of near fainting

How to measure heart rate?

Measuring heartbeats/rate is simple. You can measure your heart rate by yourself without any gadget or tool.

All you have to do is to check the pulse of your heart.

This pulse is found anywhere on the body where the pulsation of an artery is transmitted to the surface. Pressuring it with the index and middle fingers against an underlying structure like bone usually makes the pulse palpable.

Some of the areas where a pulse is found include;

• The front side of the wrist on the side of the thumb for radial artery
• The ulnar artery
• The inner side of the elbow for the brachial artery
• The groin for the femoral artery
• Behind the inner bony edge (medial malleolus) of the ankle for posterior tibial artery
• The middle part of the back of the foot for dorsalis pedis artery
• Behind the knee for popliteal artery
• In the neck below the jaw for carotid artery
• The temple for the superficial temporal artery

The apex of the heart on the chest can be felt with hand or finger or auscultated using a stethoscope.

Some pre-requisites for measuring pulse include;

• Make sure you have been sitting for a while without any recent physical activity or exercise.
• Also, you should not consume any caffeinated drink like tea, coffee or cola just before checking your hearbeats/rate.

To measure the resting heart rate;

While you are completely relaxed and at ease, place your index and middle finger on either left or right side of your neck, just below the jaw. Place your fingers firmly yet without any pressure.

 You should be able to feel your carotid artery pulsating. That is the most superficial artery of the neck that gives you an accurate count of heart rate.

You can also check your heart rate by palpating your pulse at the wrist. Place your index and middle finger on the wrist along the thumb side. You should be able to feel the radial artery pulsating against your middle finger.

When you feel your pulse at the carotid or the radial artery, count the pulsations for a full minute. Alternatively, you can count the pulses for fifteen seconds and multiply them by four to calculate the count for a full minute.

Some other methods of measuring heartbeats/rate in clinical settings are;

• Pulse oximetry
• Electrocardiograph
• Laser Doppler imaging of the fundus of the eye

Take home message

One of the vital signs, heart rate, signifies our overall health and vigor. Both physiological and pathological changes affect heartbeats. Often asymptomatic, heartbeats/rate abnormalities may indicate a severe underlying condition. Irregularities of heartbeats merit a medical check-up, especially in high-risk cases and in the presence of associated symptoms.